Basic Photographic Chemistry

· .A ~OUNTAIN PHOTOBOOK

I·

BASIC PHOTOGRAPHIC CHEMISTRY

)

BASICHOTOGRAPHIC

CHEMISTRY

by

Keith M. Hornsby

{ITHE FOUNTAIN PRESS

I Iin conjunction Vi h

I II 1-: III I T I H J 0 U'R NA L 0 F PHO TOG RAP H Y

Fi1'st Published 1956

Copyright 1956 by the Fountain Press46-47, Chancery Lane, London, W.C.2

U.S. Representatives:Rayelle Publications, 76, West Chelten Avenue, Philadelphia 44, Pa, U.S.A.

P1<INTED ~N GREAT BRITAIN BYTHI! MARSHALL PRESS LIMITED, 7. MILFORD LANE, STRAND,LONDON, W.C.2.

~- -- ----=------ -"-----= --=- -- -

CONTENTS/ "1'/'I~r Page

II( REWORD 9

CIIEMICAL PRINCIPLES .. 11

III Elements 11ulen y 12

'olllUon 12Chomical Principles 13

,Ids and Bases 141\, Ids and Alkaline Salts 14()"Idalion and Reduction Reactions 16()'f(I\nic hemistry .. 17(;lflllpS and Radicals 19

'I IIII~ EMULSION 23I hI Binder 23I IIf Sensitive Salt 25• I II itivity .. 26( ••oIClation 26( ,I'lnlin Modifiers 28

'1i111111s rs 291\11 It isors 29

IIII!, SE SITISING DYES 31MI'T!lcyanines 33I h I\. .tion of the Sensitising Dyes 35III1lIra nsitisers 36

III E DEVELOPING AGENTS 37I'!ty I 0.1 and Chemical Development 37• IIIlpl Inorganic Developing Agents 38(Ii I inlc Developing Agents 39I' fll'c't of Nuclear Substitution 41

lilt tl 1I nts in the Amino Group 421\ 1111 I ar Developing Agents 44lit VIloplng Agents in Practice 45I 'Wild tnry Developers 47

Chapter5 THE DEVELOPMENT PROCESS

Physical and Chemical DevelopmentThe Effect of pH .. . . . .Sodium SulphiteReduction of Fog ..The Metol-Hydroquinone DeveloperDeveloper Types .. . . . .Fine Grain Developers ..Other Developing Agents ..Phenidone DevelopersPyro Developers . . . . . .Concentrated Developers .. . .Special Development Techniques ..Physical Development TechniqueForced Development . . . .

6 FIXATION AND WASHINGStop-Baths ..Fixation ..The Thiosulphates as Fixing AgentsCompletion of Fixation .. . .Fixing Bath AdditionsHardening Fixing BathsWashingHypo Eliminators ..

7 INTENSIFICATION A D REDUCTIONChromium IntensifierSulphide IntensifierMercury IntensifiersMiscellaneous Intensifiers . . . .The Quinone-thiosulphate IntensifierPhysical Intensifiers . . . .Reduction ..Photographic Effect of Intensification and Reduction

8 TONING PROCESSESSilver Salt Images ..Direct Sulphide Toning ..Selenium and Tellurium ToningMetallic ToningMetallic FerrocyanidesTwo-stage MethodsOther Metal Salt ImagesDye-Toning.. ..Dye-Toning Technique

Page494950515556575962636364646566

696971717273757676787879798182828383

85858788898990919293

f h"I,lrf) OLOUR DEVELOPMENT

rimary Colour DevelopmentSecondary Colour Development

olour Developing Agentsolour Couplersolour Coupling Reaction

Stability of DyestuffsTreatment after DevelopmentColour Development Accelerators

10 I TEGRAL TRIPACKSTripack Principle ..tripping Tripacks ..olour Transparency Tripacks

Kodachromellford ColourTripacks Containing Non-Diffusing Couplersolour Accuracyolour Maskingoloured Colour Couplers ..

L I MISCELLANEOUS PROCESSESPrint-Out EmulsionsReversal ProcessingProcesses Not Using Silver Salts ..Blue-Print ProcessesOiazonium Salt Processes ..Semi-Wet Developed MaterialsDry Development MethodThe Carbon and Carbro ProcessesImbibition ProcessesDye-bleach ProcessesImage Diffusion Processes ..

l:l SOLUTION PREPARATIONI~quipment ..The BalanceMixing VesselsM asuring VesselsStorage of SolutionsWaterThermometers

Page9696979799

104105107108

109109110110111112113115117120

125125126128129130130131132133134135

137137138139140140140141

Page

143143144144

APPENDIX: COMMON RADICALSMonovalent RadicalsBivalent Radicals "Trivalent Radicals, ,

INDEX 145FOREWORD

As FAR as simple books are concerned, photographicchemistry is probably the most neglected of all photo-graphic subjects,

In writing this book it has been my aim to present to theIndent or technically minded photographer-whether amateur

III professional-a relatively simple outline of the basicpi mciples of the chemical reactions involved in the photo-Waphic process, It seemed desirable to supplement the usuallyrn.nty outlines given in most general photographic text-books

.u.d fill the gap between these books and the advanced books1111photographic science, which are written for the photo-/{,.Iphic scientist and are therefore worded for the scientistIiilher than the photographer.I make no apology for introducing chemical terminology

1111 a popular book in order to avoid the ambiguity and.Iurnsiness whioh would be associated with any attempt tor-xplain a scientific subject wholly in everyday language, Ihave, however, endeavoured to explain the unfamiliar terms,"lid photographers do not, in general, appear to object toII'iLI'nand use scientific expressions,

I~xperience has taught me that personal notes on technique!IIII'll find thei.r way into the margins of practical books, or.t 1'0 jotted down on scraps of paper or committed imperfectly10 memory, To discourage these habits I have set asideoveral pages at the end of this book in the hope that

ludividual observations based on practice will be given more11I'I'manent and more readable form,

I would like to take this opportunity of thanking Mr.rt hur J, Dalladay , the Editor of the British Journal of

f'l/OtagraPhy and his staff for their invaluable help in theprtparation of the material as it appeared in serial form inllinl journal.

KEITH M, HORNSBY,

1II

IIili

Chapter I

CHEMICAL PRINCIPLESThe Elements

THE bricks which the chemist uses to build his complicatedchemical compounds are the elements, which are thesimplest form of matter. Almost ninety of them exist in

nature, and during the nineteenth century, those which had110 in discovered were carefully classified according to theirch .mical and physical properties. They were also givenymbols which in many cases were the initial letter of the

l'lc'ment's name, with, in most cases, a second letter to' differ-«ntiate between two elements with the same initial letter. Thecommonest elements usually have only the initial letter, andIh less common have the second letter. We all know that anuom of hydrogen is represented by the letter H and oxygenhy O. Barium and aluminium have the symbols Ba and AI., me symbols are derived from the Latin names for the

I lern nts. The symbol for silver is Ag from argentum andIron is Fe from ferrum.

When two or more elements combine to form a compoundIII symbols show the ratio in which the elements are present.Thus hydrogen and oxygen combine to form two compounds

water and hydrogen peroxide. In water, two atoms ofhydrogen combine with one atom of oxygen to give theInmiliar H20. Hydrogen peroxide is a compound of twouoms of hydrogen and two atoms of oxygen, and has the

mbol H20,.The combination of two or more atoms is termed a

tuolcoule, and the group of symbols which represents a mole-1111 is the [ormula, Atoms are the smallest of the units,IllIl rarely exist alone. The inert gases exist in the monatomic111I m, but the majority 01 gases form diatomic molecules, forI ample, nitrogen=-Nj. oxygen-O 2' and hydrogen-H 2'

These simple formul<e-H.O for example-are termed"III/)il'icalformulCl'-they show only the relationship betweenIhll numbers of various atoms in a molecule. More complexr umpounds-c-par'ticularly organic compounds-have a moreelc'lail d formula, the structural formula, which is a picture

11

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12 BASIC PHOTOGRAPHIC CHEMISTRY

of the way in which the individual atoms are arranged. Thelater section on organic chemistry will illustrate this type offormula.

ValencyWhen the atoms of different elements combine to form a

compound, they do not always combine atom for atom. Eachelement has a valency number which is the number of atomsof hydrogen which will combine with, or replace, one atom ofthe particular element. Chlorine, for example, is monovalent,as one atom will combine with one atom of hydrogen to givehydrogen chloride H-Cl. Sodium also has a valency of one.as one atom will replace one atom of hydrogen from hydrogenchloride, to give sodium chloride-NaCI.

Oxygen, by combining with two atoms of hydrogen, H20,has a valency of 2, and calcium is also bivalent as it willreplace these two hydrogen atoms to give CaO. Nitrogenand aluminium, by combining with, or replacing, threehydrogen atoms have valencies of three, and carbon has avalency of four.

Some of the elements have a variable valency. Iron, forexample, forms three oxides-the normal FeO, and also twoothers which contain more oxygen, Fes04 and Fe.Os' Photo-graphically these variable valency elements are of greatimportance, having been used at one time as developers,and currently in most processes where the silver image is tobe altered-for example, intensifiers, reducers, and toningsolutions. These applications will be more fully discussed inthe appropriate chapters.

The properties of a compound differ greatly from those ofthe parent elements. The familiar sodium chloride-e-commonsalt-is a hard crystalline material which is essential to life.Its parent compounds are quite dissimilar. Sodium is a softwhite metal and chlorine is a greenish gas. Both are verycorrosive and poisonous.

SolutionWhen sodium chloride crystals are added to water they

disappear. The water dissolves the salt to give a solutionin which the crystals lose their large rigid form and split intothe single sodium chloride molecules. Some of these singlemolecules change still further; they dissociate into ions

NaCI ~ Na+ + CI-sodium sodium chloridechloride ion ionmolecule

I

I II,I,MTCAL PRINCIPLES 13

The signs on the ions tell us several properties of the ion.p sitive sign tells us that the ion is a cation and a negative

I harge that it is an anion. The number of signs indicate thev d incy of the ion (e.g., H+, Na+, Al +++ and Cl", S- -).'1'110 presence of the sign also differentiates an ion from anct m.

A sodium ion Na+ differs from a sodium atom Na as it hasone less electron and therefore carries a positive charge.Conversely, a chlorine ion CI- has one more electron thanIII' chlorine atom CI and so carries a negative charge. Notrll compounds which dissolve in water dissociate. It is onlyIho salts, bases and acids which do this.

Also, not all compounds which dissociate do so to the same,I gree. The double arrow in the equation indicates that there, an equilibrium between the dissociated and the undissoci-.u d molecules. The degree of dissociation depends mainlyupon the particular compound, but it is also dependent uponIII dilution. In general the weaker the solution, the greaterIhe percentage of dissociated molecules.

An ion need not consist of only one atom. Sodium nitrate,Jor example, has a negative ion with four atoms:

NaNOs ~ Na+ + NO,-sodium sodium nitratenitrate ion ion

Water itself is dissociated to a very small degree givinghydrogen and hydroxyl ions:

H.OH (HoD) ~ H+ + OH-

In a litre of pure water the amount of hydrogen ions is verymall-about O.OOOOOOlg.This figure is called the hydrogen

10'1'1 concentration, and is extremely important. It is anr-xpression of the acidity or alkalinity of a' solution. To, press it as the actual concentration in grams per litre isuiconvenient. so it is expressed in a way which gives an easyII(.(ure,known as the pH. The way this figure is obtained

1-- For water this gives aI garithm of concentration of H+.

II/{ureof seven, but no knowledge of mathematics is necessary10 appreciate the meaning of pH.

I

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14 BASIC PHOTOGRAPHIC C}ffiMISTRY

II

Acids and Bases

Acids and bases are substances which alter the pH whenthey are dissolved in water. Hydrochloric acid, for example,dissociates to give

HCI ~ H++ CI-and obviously gives a solution with a higher concentration ofhydrogen ions=-asolution with a low pH.

Sodium hydroxide is a base, or alkali. It also dissociateswhen dissolved in water:

NaOH ~ Na+ + OH-In this case it is the hydroxyl ion concentration of the waterwhich increases, causing the hydrogen ion concentration todecrease. The pH is increased.

Acids which dissociate almost completely when dissolvedproduce a large increase in hydrogen ion concentration. Theseare the strong acids, for example, hydrochloric and nitricacids. Other acids dissociate less and are, therefore, weakeracids.

Most of the hydroxide bases which dissolve in water dis-sociate strongly. These are the strong alkalis, for example,sodium and potassium hydroxides. Ammonium hydroxidedoes not dissociate to the same extent-it is a weak alkali.

Acid and Alkaline Salts

If the salt of a strong acid and a strong base, 'or a weakacid and a weak base, is dissolved in water, the change in pHwill not be very great. But if the salt of a strong acid and aweak base-ammonium chloride for example-is dissolved inwater the solution will be slightly acid.

Conversely, if sodium borate (borax), the salt of a strongbase-sodium hydroxide-and a weak acid-boric acid-isdissolved in water, the solution will be alkaline. As there aremany weak acids, many sodium and potassium salts arealkaline.

When hydrochloric acid is dissolved in water we have onehydrogen ion and one chloride ion. If this solution is mixedwith an equivalent solution of sodium hydroxide we have thereaction

HCI +(H+ + CI-)

hydrogenchloride

NaOH -+(Na+ + OH-)

sodiumhydroxide

NaCI(Na+ +

sodiumchloride

+ H.OCI-) (H+ + OH-)

water

I II ICMJCAL PRINCIPLES 15

'l'h ' hydrogen atom of one molecule of hydrochloric acid isI(placed by a monovalent base. It is a monobasic acid.

Sulphuric acid dissociates in the following wayH2SO, ~ 2H+ + SO.--

'I'wo sodium atoms are needed to replace the two hydrogenj toms. Sulphuric acid is dibasic, Phosphoric acid-H .PO,-I tribasic.

These polybasic acids need not be completely converted totII' corresponding salts. The conversion proceeds in stages

H2SO, + NaOH -+ NaHSO. + H.Osodium

bisulphate

NaHSO. + NaOH -+ Na2SO. + H.Osodiumsulphate

H.PO, + NaOH -+ NaHPO. + H,Osodium

dihydrogenphosphate

NaH.PO, + NaOH -+ Na2HPO. + H.Odisodiumhydrogenphosphate

Na.HPO, + NaOH -+ Na.PO. + H.Otrisodiumphosphate

These acid salts have a pH between that of the acid andt It normal salt. The salt, despite its name acid salt, neednot necessarily have an acid reaction. It is acid because itvontains a replaceable hydrogen atom.

When two ionic solutions are mixed, the solid obtained ifth! water is removed is not necessarily a mixture of the two

C nn ponent salts. In cases where no gaseous or insoluble"' l rials are formed the solid will probably be a mixture of( v ral compounds. For example-solutions of sodium

r hloride and potassium nitrate are mixed and "the resulting11111 lion is evaporated to dryness. The solid will be a mixture

of odium chloride, sodium nitrate, potassium chloride, andpotassium nitrate. The proportions of the individual com-pWl nts will vary with the ionic concentrations, and thenlubilities of the possible salts. In general the salts of lowernluhility will be preferentially produced.


When two of the ions produce an insoluble salt, or a gas orvolatile liquid, these compounds are no longer present in theionic system, and therefore alter its equilibrium, and willusually predominate in the final reaction products. Forexample:

CuSO. + Na.S ~ CuS + Na.SO.copper sodium copper sodium

sulphate sulphide sulphide sulphate

Copper sulphide- is the insoluble material in this reaction andas long as copper and sulphide ions are present the reactionwill proceed to the right. It cannot, under normal circum-stances proceed in the reverse direction. Similarly where oneof the products is removed as a gas the reaction cannot bereversed.

Oxidation and Reduction ReactionsReactions of this type are very important in photographic

processes. In the simplest forms the superficial meaning iscorrect. Copper when heated in oxygen is osidised to copperoxide-

2Cu + O. ~ 2 CuO

which, if then heated in hydrogen, is reduced back to copper. heat

CuO + H. ~ Cu + HaC

Oxidation and reduction always accompany each other. Anoxidising agent is one which is reduced relatively easily-conversely a reducing agent is easily oxidised.

Oxidation and reduction is not necessarily accompanied bytransfer of oxygen or hydrogen. Ferric chloride is reducedby, or alternatively will oxidise, finely divided silver

Ag + FeCls ~ AgCI + FeCI.The silver is oxidised to silver chloride-it loses an electron ;

whilst the ferric chloride is reduced to ferrous chloride bygaining an electron. Iron is one of the elements which hasvariable valency, and the majority of inorganic oxidising andreducing agents depend. upon variable valency for theiraction. We can write the equation

Ag + Fe+ + + =<= Ag+ + Fe+ +silver ferric: silver errousatom ion ion Ion

which implies that the reaction can proceed in either direction,depending upon conditions.

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('lmMICAL PRINCIPLES 17

As we have seen, ferric chloride will oxidise finely dividedsilver, but the now obsolete ferrous oxalate developer reducessilver chloride, the ferrous ion being oxidised to the ferric.

The brief explanations given should help the reader toIollow most of the following text, apart from organicchemistry, but for those who wish to study somewhat deeper,Electronic Theory and Chemical Reactions is recommended, '"in addition to the normal chemistry text-books.

Organic Chemistry

Organic chemistry is the chemistry of carbon compounds.In simple carbon compounds, such as sodium carbonate-Na.C03-carbon behaves in a similar manner to otherclements with a valency of four. But carbon compounds canbe prepared in which carbon plays a part unlike otherclements.

The feature of carbon which is remarkable, is its abilityto combine with- itself. Diamond and graphite are nothingmore than carbon atoms arranged in the form of giganticthree- or two-dimensional molecules. Other elements may beincluded in these large structures, and to understand theproperties of these compounds it is necessary to know notonly the proportions in which the various elements are present,but also the way in which they are arranged. This is whythe ' picture' or structural formulas mentioned earlier are soimportant. They show the properties of the compound.

The simplest organic compounds are the hydrocarbons,and, as their name implies, consist of carbon and hydrogen.The carbon atoms may be arranged as a chain

-d-d-d-d-I I I I

or the terminal atoms may join up to form a ring

•R. W. Stott, Longmans, Green & Co. Ltd.


The links which join the carbon atoms to each other, or toatoms of other elements are termed bonds. These bonds mayeach be connected to a single atom of carbon or hydrogenH H H H

H-b-t-~-t-H~ ~ ~ ~

when the compound is said to be saturated. If all the bondsare not used, then two bonds instead of one will join twoatoms.

to give an unsaturated compound.lhe same division may be used with the ring or cyclic

compounds

H,H,C./' ~H"C""H

'H WI H H IwC" ~;C'H

CH

saturated unsatu rated

but with an important difference. The saturated hydrocarbonsare very resistant to chemical attack, but the unsaturatedchain compounds, because" all the bonds are not fully occu-pied, are weak, and readily attacked by reagents. The cyclicunsaturated compounds, such as benzene in the above figure,are not readily attacked, and are almost as stable as satur-ated compounds. The alternate double and single bond struc-ture is now known to be inaccurate. All the bonds are of thesame type and _are called aromatic bonds, as the compoundsof this type are the aromatic compounds. The straight chaincompounds are termed aliphatic and the non-aromatic ringcompounds are alicylic,

------- - - ~ - -------"---- -~- --- - - - --- -- ---- --

, III ~IICAL PRINCIPLES 19

I'll simplify structural Iormulze the I11dividualatoms- are11111 n rrnally drawn in aromatic compounds. Instead, aunpl ring is drawn to represent the benzene nucleus

HI

H-C~C"C,H, II

WC~C/_C-HI'r

II IIi1st two rings represent naphthalene.

o=

H H

H_C~C"C/C""CHI II I

H/C""c /C" C~c..HH H

Groups and Radicals

Wh n a number of atoms arranged in a definite systemI'HIIlS a part of a more complex molecule, it is called a groupIII a radical, for example

-0IIC-HH

methylgroup

phenylgroup;c-~

-C C-H~C=<H H

H H HC- C- C-H, , ,H H H

propylgroup

1111\ woups being joined together via the unoccupied bonds

II

II-~ -0 methyl benzene = phenyl methane = toluene

II


The symbols for these groups may be further abbreviatedHI

-C-H = -CHs -Me

~. Other elements than carbon and hydrogen may be presentIn, or even constitute, a group. Those which are mostcommon in photographic chemistry are the

amino group-NH2hydroxyl group-ORcarboxylic acid group-CO. ORsulphonic acid group-SO,Rnitro group-NO.

The above groups are all groups which can replace onehydrogen atom, and are therefore similar to the mono-valent atoms, which can also be considered with the groups.These examples show how the groups are attached to thehydrocarbon radical

H-~-CO _OH).I acetic acidH

CHs- CO.OH

H H 1H-t-t-OH

I I J ethyl alcoholH HC,HsOH

I Ibenzene sulphonic acid

Ot-!

6 hydroxybenzene or phenol

-- ~-- - - ------------ _. ----- -~

, III',MICAL PRINCIPLES 21

The halogens-chlorine, bromine and iodine-are the.ummonest single valency element substituents, and replaceIi drogen in the same way as monovalent groups.

CH3-CI methyl chloride

Bro brornob en zene

The bivalent atoms and groups can be introduced inovcral ways, oxygen for example can give the ether group,

, as inC2HS- 0 - C.Hs diethyl ether

III an replace two hydrogen atoms from the same carbon.uorn to give the carbonyl or ketone group

o\I

-C-lilt i .h is itself a bivalent radical

oII

CH.- C - CH3 dimethyl ketone or acetoneitrogen, being normally a tervalent element, can be intro-

ollie d into the aromatic radical to give, for example

H,H'C/S::~C-H

II I or

H,C.'N~CH O pyridine

N

( r-li compounds in which all of the atoms actually in theIlilH are carbon, are termed carbocyclic, whilst pyridine andimilar compounds with one or more other elements in the

IIIIKH are termed heterocyclic.'111se groups can give to organic compounds properties

uui lar to the inorganic acids and bases. The hydroxyl groupI lh equivalent of the inorganic hydroxide, and compoundsI uulnining this group-the alcohols and phenols-react WithI u lioxylic acids to give compounds like the salts calledr 1(,I's

propionicacid

methylalcohol

methylpropionate

water

22 BASIC PHOTOGRAPHIC CllEMISTR Y

Once again we have a reversible reaction. The reactionto .the right is known as esterfication and that to the leftis hydrolysis.

The amines are also basic and react with carboxylic acidsto form the acid amides which have the carbonamide group-ing-NH-CO-

R-NH-CO-Rl

The capital lefters Rand R' denote a radical of a specifiedtype, and together with other letters are frequently used tosimplify structural formulas. In this case they could bealiphatic (alkyl) groups or aromatic (aryl) groups.

Sulphonic acids also form compounds-the sulphonic esters-with hydroxyl bases

R-O-SO.- Rl

and sulphonamides with. the amino bases

R-NH-SO.-Rl

The preparation of organic compounds with various groupsin the desired positions is a very specialised field. Substitu-tion is not always straightforward, and it is frequentlynecessary to adopt roundabout methods to produce therequired compound.

Chapter 2

T.HE EMULSION

'THE photographic emulsion consists of three main parts-the binder, the sensitive salts, and the' finals.' Thefinals are small quantities of substances which improve

III emulsion, and are normally added immediately prior toroating.

The Binder

In all normal emulsions the binder is gelatin, but manynth r materials have been used in the past. They all fallinto the class of materials known as colloids. These arematerials which either consist of very large mole-I III s which do not dissolve to form a true solution; or theynro insoluble materials-even metals-which stay suspendedin a liquid because they are divided into extremely smallpnrticles, The colloids used in emulsions are all large-mole-I 1110 materials. Some can form viscous solutions known assot in the fluid state, or gels in the solid or semi-solid state.Colloids which have been used in emulsions include variouswater-soluble gums, starches and proteins such as albumen"lId gelatin. Gelatin has several characteristics which haveIosulted in its great photographic importance.(.1) When dry it is strong and tough, and secures the silver

halide grains :firmly to the support. This toughness alsoprotects the grains from physical damage caused byabrasion.

(I» It also protects the grains from chemical action (an emul-sion made without a binder would be completely foggedin a normal developer).

(I') The change from a sol to a gel in aqueous solution is ata convenient temperature for coating, and is reversible:-

ta.30°cGel ~ Sol

(d) Substances present in gelatin have a favourable influenceon the photographic characteristics of the emulsion.

23

24 BASlC PHOTOGRAPHIC CHE7STRY

The manufacture of photographic gelatin from animal hidesis a very complicated process, and the details are the guardedsecrets of gelatin manufacturers.

The first stage in manufacture, after thoroughly cleaningthe hides, is to remove the inorganic materials which makethe hides hard. This step is relatively simple, and is commonto the manufacture of all types of gelatin for culinary andindustrial use.

The gelatin at "this stage would be quite useless for photo-graphic work as it contains many substances which wouldfog an emulsion. It is the removal of these substances whichis the great difficulty. Some of these substances which causefog when present in large quantities can have very beneficialeffects on the speed and contrast of the emulsion when theyare present in small quantities. The great difficulty, then, isto reduce the amount of these materials present to such a levelthat a fog-free emulsion can be prepared, yet leave sufficientto give a gelatin with the required photographic characteristics.

The process is further complicated by variations in thenatural raw materials. It is, of course, impossible to obtainidentical batches of hides, so the gelatins prepared from eachbatch of hides will vary to some extent. However, by skilfulblending of different batches a remarkable degree of consis-tency is present in the final product.

Gelatins are available in a wide variety of types suited todifferent emulsion characteristics. Some are inert gelatinsfrom which all the substances affecting photographic charac-teristics have been removed or neutralised. Others are blendsin which the quantity of substances increasing speed or con-trast are widely varied. The presence of sulphur compounds-such as allyl isothiocyanate-produces emulsions of greatersensitivity due to the formation of sensitivity centres of silversulphide. Too much of the sulphur compounds, however,results in a foggy emulsion. Proteins such as albumen, or theaddition of bisulphites or sulphur dioxide affect the ripeningof the emulsion.

Of gelatin itself, little can be discussed in simple chemicalterms. The chemical structure which results in the peculiarphysical properties of gelatin is very complex. The gelatinmolecule itself is very large, consisting of many long-chainunits containing both amino and acidic groups. The giganticmolecules of gelatin are composed of large numbers of thesesimpler amino-acid groups-not all of the same type-linkedtogether in many different ways.

1111'; EMULSION 25The Sensitive alt

I'he individual crystals of silver halide consist of many thou-nnds of silver and halide ions arranged in a crystal lattice,

which, however, is not quite the perfect lattice picturedh re.!.2.

The silver halide grain is always prepared with an excess ofhalide present. This makes sure that the outermost ions are.ilways halide ions, which, being negatively charged, form a1\ igatively charged" sheath" around the grain. At normalIt.mperatures this lattice vibrates, and some silver ions areu-leased from their original positions and wander about in the, rystal.

When light strikes the grain. some of it is absorbed, con-verting bromide ions to bromine atoms. In doing this, an«loctron is released, and this also moves about in the crystal.It is now that the gelatin impurities, which contain sulphur,Ii their work. When the emulsion is prepared, these sulphurcompounds react with silver salts to form minute specks ofliver sulphide, mainly on the surface of the grain in norxpal

r-mulsions. These sulphide specks-or sensitivity specks-takeup the free electrons and therefore become negatively charged.

,.

26 BASIC PHOTOGRAPHIC CHEMIST

In this condition they attract the wandering positive l' verions, converting them to silver atoms and neutralisirig thecharge. Further electrons and then silver ions are attracted ina similar way to produce more silver atoms near the sensitivityspeck.

When sufficient silver atoms are present in one portion ofthe crystal (the number has been estimated at about twohundred), the protective sheath of negatively charged bromideions is disturbed. If the grain now comes in contact with adeveloper ion which is also negatively charged, the developingagent ion will not be repelled by the group of silver atoms-called the development centre-as at this part of the crystalsurface there is no repelling negative charge. The developingagent can now reduce the rest of the silver halide in thegrains' to metallic silver.

SensitivityA large silver halide grain will, because of its larger pro-

jected area, absorb more light than a small grain. The numberof silver atoms produced will therefore be greater than in asmall grain if the two are exposed to light of equal intensity.A development centre will, therefore, be more easily pro-duced. In other words, a large grain will be made developableby a lower light intensity than a smaller grain. Its sensitivityis greater.

GradationIf the grains in the emulsion could be all of the same sensi-

tivity, then the emulsion would have the maximum possiblecontrast. Although light of different intensity may fall upondifferent parts of the emulsion, the emulsion would not dis-criminate between the different intensities. A given lightintensity would either make all of the grains developable, orwould not make any developable.

In practice, emulsions made in the normal way have grainsof a variety of sizes. The large grains will absorb the neces-sary light to form a development centre at a lower intensitythan the small grains. As the intensity of light increases, thenumber of developable grains will also increase. This emul-sion will respond differently to each light intensity, and willgive an image with gradation. The emulsion maker can con-trol the gradation (and incidentally the speed) of an emulsionquite easily, within limits.

If we follow the course of typical emulsion technique therelevant points can be illustrated.

I liE EMULSION 27

OUTLINE OF THE PREPARATION OF A SLOW TO MEDIUM SPEED

SILVER lODO-BROMIDE EMULSION.

I. Swell 40 gm, of gelatin in200 c.c. water for 3 hrs.

2. Heat to 50 degs. C. onwater bath.

3. Add 20 gm. ammoniumbromide and 2 gm. potas-sium iodide in 200 c.c.water.

4. Add, with constant stir-ring, 30 gm. silv-er ni tra tein 100 c.c. water at 45degs. C.

5. Continue heating at 50degs, C. 10-60 mins.

6. Stir in 20 gm. gelatin andcontinue stirring untilcompletely dissolved. (Thisgelatin may be inert.)

7. Cool the emulsion asrapidly as possible andleave in refrigerator over-night.

8. Cut the emulsion intosmall pieces.

9. Wash to remove unwantedsalts - e.g., ammoniumnitrate and the excess bro-mide which would hinderthe after-ripening stage.

Type of gelatin (slow, medium,fast; soft, medium, hard) willdepend upon type of emulsionrequir-ed.

Choice of halides will dependupon type of emulsion.

The time of addition of thissolution will depend upon thecontrast required-1 min. fora slow contrasty emulsion inwhich all the grains are nearlythe same size, up to ~O mins.for a soft, fast emulsion witha great variation in grain size.First ripening stage. The timevaries according to the gelatinand the emulsion type.This addition is to enable theemulsion to be set to a firmjelly for washing. The prepara-tion of chloride emulsions,which are frequently unwashed,ends here.

This is to enable the washingoperation to be performedefficiently.There are many possible waysof telling when an emulsion' iswashed. When ammoniumhalide is used (as in this case)the washing is continued untilther-e is no reaction with anammonium salt test. Othermethods are to determine theamount of free bromide in theemuls-ion shreds, or to measurethe electrical conductivity.


10. Digest or after-ripen the This operation increases theemulsion at 60 degs. C. speed and contrast of the emul-

sion. Heating is usually con-tinued until the emulsion showsvery slight fog in the case ofnegative emulsions ; or isstopped short of fog for posi-tive emulsions. Unless previousinformation on the particularemulsion is available the timemust be determined empiric-ally.

The proportions of the different constituents may varygreatly from the example given. Some very high-speed emul-sions have only a small amount of gelatin present during pre-cipitation-about 1 per cent.-to enable the grains to increasein size rapidly as a high concentration of gelatin slows downthe first ripening process.

In the case of negative and high-contrast emulsions thesilver nitrate is frequently converted to silver hydroxide byadding ammonia, and dissolving in excess ammonia. Thismethod also results in more rapid ripening, but all traces ofammonia must be removed during the washing stage.Ammonia emulsions are usually prepared at lower tempera-tures than neutral or acid types.

Although ammonium bromide was specified in the example,it is not essential for ammonium salts to be present. Paperemulsions are usually prepared by using sodium, potassiumor cadmium halides, frequently in slightly acid solution.

The chemical sensitisers (sulphur compounds and preciousmetal salts) referred to later, are usually added before or dur-ing after-ripening.

After the addition of the "finals" the emulsion will beready for coating.

Some of these compounds are added to the majority ofemulsions. Others only to special types.

Gelatin Modifiers

Chrome alum, or an organic hardening agent, is a necessityin all emulsions except those which are to remain soft(physically) for special purposes. Usually only a smallquantity of hardener can be added to the emulsion beforecoating, further hardening after development being the prac-tice. This small quantity of hardener, however, does make

THE EMULSIO:s' 29development much more simple, as the softening of the emul-sion due to developer alkali is reduced.

Among the organic hardeners which are commonly used arealdehydes such as formaldehyde and glyoxal, often togetherwith other compounds which either increase their efficiency orreduce their tendency to fog the emulsion.

Spreading and wetting agents, such as saponin and varioussulphonated fatty alcohols and oils, are added to make coatingeasier, particularly when a non-absorbent base, such as glassor celluloid, is used.

Stabilisers

To digest the emulsion it is necessary to remove almost allof the excess halide by washing. In this condition the emul-sion will become foggy when stored at normal temperatures,and particularly so at high temperatures. This is in part due1(' continued digestion. Further additions of potassium bro-mide will make the emulsion more stable.

The addition of bromide will also alter the characteristicsof the emulsion to some extent, usually reducing the sensitivityand the contrast. Organic stabilisers have been developedwhich give stability without adversely affecting the emulsioncharacteristics. These are usually heterocyclic compoundswhich are capable of forming silver salts of very low solu-bility, normally less than that of silver bromide.

Sensitisers

These can be divided into two groups-the "chemical"sensitisers and the " optical" sensitisers.

As we have seen, the formation of the latent image dependson the presence of sensitivity specks-particles of impuritiesin the structure of the grain.

Following the isolation of one of the compounds in gelatinwhich produces these specks-allyl isothiocyanate-manyorganic and inorganic sulphur compounds have been investi-gated with a view to increasing emulsion sensitivity.

Another method which appears to be used extensively isthe inclusion of salts of gold, platinum and palladium, fre-quently together with a sulphur compound such as potassiumthiocyanate. It seems possible that small metallic specks canbehave in an analogous manner to sulphide sensitivity specks.

The colour sensitivity of an emulsion can be increased bydyeing the halide grains. As we have seen, it is the absorbed

light which produces photochemical action in the grain to givea latent image. Normally the silver halides reflect all butthe blue light. By adding certain dyes, the grains can be madeto absorb light of all colours, and so be more sensitive. Thesedyes are added in very small quantity to the emulsion, anddo not colour it appreciably.

Attempts to explain the mechanism of optical sensitisationin electronic terms, on the lines of the Gurney-Mott latentimage theory, have not yet attained the perfection of the lattertheory, and cannot therefore be readily summarised.

Until 1945 no information on the preparation of commercialtypes of emulsion was available. As a result of the investiga-tions on German industries carried out after the war byBritish and American teams, such information is now avail-able to the public in reports available from the StationeryOffice".


REFERENCES

1 MOlt and Gurney-Electronic Processes in Ionic Crystals, also Proc. Roy.Soc., 1938 ; 164A, 15!.

2 Berg, W. F.-Discovery, Oct., 1950; Vol. II, pp. 314-318.3 B.I.O.S. Final Report No. 1355.

II

~I"

Chapter 3

THE SENSITISING DYES

UNTIL 1873 the sensitivity of photographic materials waslimited to the blue, violet, and ultra-violet spectralregions. In that year, however, Vogel found that

collodion dry plates manufactured in England by Wortleywere sensitive to green light. He rightly attributed this to thepresence in the emulsion of the yellow dye, corraline, whichWortley had added to reduce halation. The green sensitivityof these plates was very slight, but two years later, WaterhouseIound that eosin was a more efficient green sensitiser. 1884 sawtwo important advances-Eder's use of erythrosin, and Vogel'sdiscovery of the sensitising action of cyanine blue. Erythrosinis closely related to Waterhouse's eosin, and was used com-mercially for the production of the cheaper orthochromaticroll-films until 1939. The discovery of cyanine blue's sensitis-ing action, however, was the first step in the researchesIinanced by most of the photographic manufacturers whichhave resulted in an enormous number of cyanine sensitisersbeing disclosed in patents.

The cyanines and their related dyes are now useduniversally to the exclusion of the commoner dyes, as the sen-sitising efficiency of the cyanines is so great. Generallyspeaking, the cyanines have no practical application apartfrom their use as photographic sensitisers They can be used(or dyeing textiles, but are not fast to light.

The structure of the cyanines is

•

.,z\ . rZ2iy- N=C - (CH=CH\-CH= C-N-X

I ~n

A

Z' and Z2 represent the atoms necessary to complete hetero-cyclic nuclei, X and Yare hydrocarbon groups, and A is ann id radical: n is 0, 1, 2, 3, 4 or 5. Among the heterocyclicnuclei in common use are:

31

-.

32

ex)8-

BASIC PHOTOGRAPHI CHE~IISTRY

quinaldine ((0,-

C- benzoxazoleN"v

• lepidine(XS,

C - benzthiazole

N"v

and their substitution products, i.e.,

aDO,C - 5-phenylbenzoxazole

N~

CH:DS

, C - 5-6 dimethylbenzthiazoleCH N~

The number of - (CH=CH) - groups represented by thesymbol n depen~s on the sensitising range required. Forexample, 1 : l'-dlethyl pseudocyanine iodide (n = 0)

sensitises with a maximum at 570m~L' whereas pinacyanol or1 : l'-diethyl-2 : 2'-carbocyanine iodide (n = 1)

E" - _ __ _ _ -_______ __~_

THE SENSITISING DYES 33

WCH-CH=CH-W

/" IC1.HSI C2HS

sensitises with a maximum at 640mj.t.The nature of the heterocyclic nucleus also influences the

sensitisation range. For example carbocyanines-that isdyes with one - (CH=CH) - group-s-which have two benz-oxazole nuclei-the oxacarbocyanines-sensitise with a maxi-mum at 515mj.t, with benzthiazole nuclei-the thiacarbo-cyanines-the maximum is at 582mj.t; the parent carbo-cyanine, as we have seen, sensitises with a maximum at640mu; and when both nuclei are lepidine, 1: l'-diethyl-4 : 4':carbocyanine-commonly called kryptocyanine-is pro-duced, and this dye sensitises in the near infra-red with amaximum at about 745m~L. The presence of substituents inthe nucleus also shifts the maximum of sensitivity towards thelonger wavelengths.

As the inclusion of more - (CH =CH) - groups extends thesensitisation to longer wavelengths, the majority of currentsensitisers for the infra-red contain several of these groups-the practical maximum being five in the pentacarbocyanineswhich sensitise up to 1200mj.t.

The use of cyanines containing two different nuclei usuallyresults in a band of sensitivity roughly midway between thebands of symmetrical dyes containing each nucleus. Thus,lhiacarbocyanine sensitises with a maximum at 582mj.t andIho maximum sensitisation from kryptocyanine is at 745mj.t.A carbocyanine containing one benzthiazole and one lepidine1111 I us should, and in fact does, sensitise with a maximum,d about 660mj.t.

MerocyaninesThe most effective of the cyanines are those which contain

n azole nucleus, and until the mid-thirties the fastestorthochromatic and panchromatic contained these dyes. Thena new class of sensitisers was discovered independently byKendall, of Ilford, and Brooker, of Kodak. As these dyescontained half of a cyanine dye, they were named the mero-cyanines. Unlike the cyanines in which the quaternary

nitrogen atom having an acid radical attached to it )N<acid

is an essential feature, the merocyanines contain only


trivalent nitrogen in the cyanine type nucleus, the othernucleus containing essentially a carbonyl group >C=O.As there are many heterocyclic nuclei containing this groupthe number of merocyanines available is very great. Twotypical ones are

1-phen y1-3-methyl-4 (3'-ethylbenzthiazoylidene) -5-pyrazoloneand

5-( l'-ethylquinaldin-2-ethylidene) -rhodanine.While these merocyanines are, in themselves, useful sen-

sitisers, those containing a >C=S grouping, like therhodanine compound above, can be converted to more com-plex dyes, for example by condensing with a furtherrhodanine nucleus to give the dye

which is itself capable of further similar condensation.Alternatively the simple merocyanine can be condensedwith a salt of a basic nitrogenous heterocyclic compound toproduce a dye similar to a carbocyanine, having a rhodaninenucleus in the methine chain

-' .-- -

1111';SENSlTISING DYES 35

The Action of the Sensitising Dyes

According to the Grotthus-Draper law, in photochemicalreactions it is only the absorbed light which 'has any chemicaleffect, and it is therefore not surprising that silver bromide,wnich is yellowish and therefore absorbs only blue, violet,and ultra-violet radiation, is only sensitive in this spectralregion. The optical sensitisers extend the sensitivity ofphotographic emulsions by enabling the halide grains toabsorb light of other colours. The verification of the rela-tionship between absorption and sensitivity was not possiblefor some time, as dye solutions have absorption curves differ-cnt from those of the dyed grains, and the absorption curvesof the dyed grains could not be measured satisfactorily, be-ause the light used for measurement produced photolytic

silver which altered the absorption. By using Hardy's rapidworking photoelectric spectrophotometer, Leermakers, Carroll,and Staud' were, in 1937, able to prove that optically sen-sitised emulsions do absorb light to which they are sensitive.

The reasons why some dyes sensitise and others do not aren t fully known. The first requirement of a sensitising dye isthat it should be adsorbed on to the surface of the halidegrain, and so dye it and enable it to absorb light which theundyed grain does not absorb, Relatively few of the avail-nble dyestuff types are adsorbed in this way without possess-ing undesirable properties also (fogging, desensitising, etc.),nnd those that are free from these disadvantages are notn cessarily efficient sensitisers. Kendall' has proposed a/{ neral formula for sensitisers in which an anionic oxygen,sulphur, selenium, or nitrogen atom is linked to a cationico ygen, sulphur, selenium, or nitrogen atom via a conjugatedchain of an uneven number of unsaturated carbon atoms

+0'1 {OS l S~e f -(C=C)n-C= ~


This formula, however, gives no clue to the efficiency of asensitiser, and many dyes which fit the formula are photo-graphically inert. In the same paper, Kendall says that atthe moment it is not possible to forecast whether a dye willactually be a sensitiser, even though it may possess therequired features. One of the main difficulties is the lack ofknowledge of the way in which the light absorbed by the dyeor superficial silver halide-dye complex produces the energynecessary to produce a latent image in the grain. It is un-likely that the dye undergoes a permanent change, andtherefore the energy produced when the dye absorbs lightmust be transferred to the halide crystal, or be lost.

The arrangement of the sensitiser ions or molecules on thecrystal surface appears to have a strong influence on theefficiency of energy transfer, and the action of supersensitisersfurther emphasises the importance of this energy transferprocess.

SupersensitisersIn 1920 Bloch and Renwick' published an account of the

action of auramine, a dye almost devoid of sensitising action,on optically sensitised emulsions. When incorporated in verysmall quantities it produced greatly enhanced colour sensi-tivity. Up to that time it had been assumed on the evidenceof practical tests that sensitiser mixtures were generally lessefficient than the individual sensitisers.

This action of auramine was apparently neglected until1937 when Mees patented a number of sensitiser combinationswhich produced greater sensitivity than the individual sen-sitisers. Now the number of patents claiming supersensitis-ing action is as great as, if not greater than, the number ofpatent specifications which describe normal sensitisers. Manypatentees, in fact, claim both sensitising and supersensitisingaction for their inventions.

Supersensitisers do not necessarily have any sensitisingaction when used alone; some in fact are not even coloured,so their action would seem to be connected with increasingthe efficiency of the transfer of ·energy from the light absorb-ing compound, rather than a simple increase in lightabsorption.

REFERENCES:Leerrnakers, Carrol and Staud. J. Chern. Pill's. 1937:5:878Kendall. Cllem. and Ind. 1950; February.Block and Renwick. Photo Jour. 1920:60:145.

-

- - -.----..

Chapter 4

THE DEVELOPING AGENTS

To the chemist, the developing agents are reducing agents.It is unfortunate that to the photographer, a reducer isa substance which dissolves metallic silver. It is just the

opposite of a reducing agent-an oxidising agent. However,all reducing agents are not developers. Sodium sulphite andordinary' hypo' are reducing agents, but will not develop.Stannous chloride and sodium llydrosulphite are also reducingagents, but they reduce the unexposed as well as the exposedsilver halide grains. Therefore they are not developers.

The standards for the photographer are that the exposedgrains should be reduced as completely as possible, but thatthe unexposed grains should not be reduced. Other factorswhich have some importance in certain cases are the contrast,grain size, and colour of the image. But these factors dependmore upon the constitution of the developing solution thanupon the reducing agent alone. Some types of developingagents, of course, are more suitable for particular developmentprocesses, and this does influence the choice of developingagent.

Physical and Chemical DevelopmentThe action of developers can be roughly divided into two

types-physical development, and chemical development.The basic difference between them is that the physicaldevelopers contain a soluble silver salt which is depositedfrom the solution on to the development centres, whereas thechemical developers merely reduce the silver halide adjacentto the development centres. The majority of developers inurrent use combine the two types of developing action, for

although they contain no soluble silver salts, they containsufficient silver halide solvent to dissolve some of the finegrains, and deposit silver on to the exposed and partiallydeveloped grains.

In the days of the wet plate and the collodion dry plate itwas customary to use a developer of the physical type con-taining pyrogallol, an organic acid, and silver nitrate (although

37


the latter was frequently present in the emulsion). Silver isreadily deposited from this solution by reduction of the silvernitrate by the pyrogallol. Any metallic silver present willact as .a.prec~pitation centre. The small groups of silver atomscompnsmg the development centres will do this, but it is neces-sary for the precipitation centres to be much larger than for, chemical' development, i.e., the exposure must be increased.

In 1861, an English photographer's assistant, Wardley,found that the acid and the silver nitrate were not essential,but that pyrogallol alone would develop the latent image ina dry collodion plate. This idea was elaborated by otherworkers, who made other additions to the solution, and whoused different developing agents, to give the developer typeswhich we use to-day.

Simple Inorganic Developing AgentsAlthough all the developing agents in common use are organic

compounds, there are three types of inorganic compoundswhich will, under certain conditions, develop a latent image.

In the first group are metallic ions which can form stablecompounds in different degrees of oxidation-that is, metalsof variable valency. Some salts of vanadium, chromium,molybdenum, tungsten and iron have developing properties,but only iron salts have had any practical use. >I< For manyyears before the general use of alkaline organic developers,the ferrous oxalate developer was very popular. The activeagent is the complex ferro-oxalate cation Fe(e.O.)., and itsaction can be summarised by the equation ;-

Ag + Fe(C.O.h- - -+ Ag + Fe(C.O.).-silver ferro-oxalate silver fer ri-exalaeeion ion atom ion

This developer was studied at some length by Sheppardand Mees.'

The statement above that sodium hydrosulphite is toopowerful a reducing agent to be classed as a developer is truefor normal emulsions, as hydrosulphite does not discriminatesufficiently between the exposed and unexposed grains. Itwill, however, develop silver iodide emulsions which are noteasily developed with normal developers. The addition ofbisulphite and bromide has been claimed to reduce its potencyto such an extent that ~lormal emulsions can be developed.

• Two recent Kodak patent specifications (B.P .. ?17,040 and B.P. 720,235)describe the use of developing solutions contammg ferrous titanous andvanadous salts as the active reducing agents. It appears that an ~ssential featureof these methods is the continuous electrolytic regeneration of the developingsolution which implies that the method is unsuitable for small-scale work.

rus DEVELOPING AGENTS 39

Such solutions could have no practical use because of theinstability of sodium hydrosulphite. .

The third group of inorganic developers also has no practicaluse, but they are of interest as they contain the basic groupsof the main types of organic developing agents. There arethree of these compounds, hydrogen peroxide HO-OH,hydroxylamine HO-NH., and hydrazine H2N-NH2• Hydrogenperoxide and hydrazine will only develop in strongly alkalinesolutions, and both hydroxylamine and hydrazine form smallbubbles of nitrogen in the emulsion.

Organic Developing AgentsIf an organic radical-usually a phenylene radical-is

present between the two active groups in the above threeompounds, we have the three basic organic developers-the

dihydroxybenzenes, the aminophenols and the aminoanilines(or phenylenediamines).

OH

HO-OH --'> 60H."d

o-dihydroxybenzene(pyrocatechol)

OH

HO-NH2--> 6NH

'and

o-aminophenol

NH2~6NH

'

oOH

p-dihydroxybenzene(hydroquinone)

o-aminoaniline(o-phenylene diamine)

oNH2

p-aminophenol

and oNHZ

p-aminoaniline(p-phenylene diamine)


The para compounds are much more extensively used thanthe ortho compounds as they are more powerful and efficient.The dihydroxy benzenes require a more alkaline developingsolution than do the compounds containing an amino group.

It will be noticed that the meta or 1:3 compounds are notin the above table. They are not developers.

There have been several attempts to define developingagents by means of a simple formula and rule. The mostgenerally applicable of these is Kendall's rule. The formulais:-

rx - (C = C) - rxln

in which (X and (Xl are the active developing functions-either

amino or substituted amino -N<~or hydroxyl substituted

hydroxyl-O-R and n is zero or a whole number.Where n =0 we have hydrazine, hydroxylamine and

hydrogen peroxide.Where n =1 we have the ortho compounds

0(

IO~dand where n =2, the para compounds

0<I

C(""r(.,::>(

~,

In the case of meta compounds n = 1t and therefore doesnot fit the rule.

Other organic radicals than the phenylene radical havebeen used in developers but apart from the polynuclea.raromatic radicals which will be dealt with later, none havehad any practical application.

The majority of developing agents are based upon the sixtypes above, and are usually these developers with sub-stituents in either the nucleus or in the amino group.

TilE DEVELOPING AGENTS 41

Effect of Nuclear Substitution

If the nucleus is substituted by a further active developingfunction (which must be in an ortho or para position to oneof the groups already present) the result will be an increasein developing power, thus

60H

OH is more powerfulthan

OHI :2:3-trihydroxybenzene

(pyrogallol)1-2-dihydroxybenzene

(pyrocatechol)

OH

is more powerful r\than y

NH~p-aminophenol2:4 diaminophenol

(amidol base)Halogen atoms also increase the

OH

developing

OH

is more powerful r\than IV

power

OHchlorhydroquinone hydroquinone

A second chlorine atom in the 5-position

~(I

(IVOH

2 :5-dichlorhyd roq ui none

increases the developing power still further, whilst a bromineatom in the nucleus


bromhydroquinone

results in an even greater increase in developing power.In ad~ition to increasi~g the developing power, halogen

SU?Shtut.lOnhas a fu.r~her Important practical effect. Hydro-quinone IS very.sensIhve to cha~lges in development tempera-ture-.m technical language, It has a high temperaturecoefficient. Chlorhydroquinone is far less sensitive.

The .other type of substituent which increases developingpower IS a lower alkyl group, for example, a methyl or ethylgroup.

OH

c)CHlNHz

3-methyl-p-aminophenol

p-aminophenol

(tHlNH2

2-methyl-p-aminophenol

is morepowerful

than

which ismore

powerfulthan

Acid nuclear substituents generally decrease the developingpower-in some cases even produce a substance which doesnot develop. Examples of such groups are: carboxylic andsulphonic acid radicals, and nitro and sulphone groups.

Substituenrs in the Amino Group

The amino group of the aminophenols, and one of theamino groups of the aminoanilines are frequently substitutedwith one or two methyl or ethyl groups. This substitution,like the alkyl group substituent in the nucleus, increases thedeveloping power-in some cases very greatly.

/TilE DEVELOPING AGENTS

6NH.CH3

p-methylaminophenol(metol base)

is much. morepowerful than

oHsCI.-N-CZHs

p-diethylaminoaniline

is morepowerful than

43

oNHz

p-aminophenol

oNHz

p-aminoaniline

This alkyl group is sometimes substituted in its turn togive the compounds greater solubility in alkali, or to red~cethe toxic effect upon the skin. In the most popular developmgagent which has this substituent-glycin-the methyl. gro~pof p-methylaminophenol is substituted by a carboxylic acidgroup.

6NHCHz·COOH

p-hydroxyphenylamino acetic acid(glycin)

p-diethylaminoaniline is a common colour-coupling developerbut it can cause severe dermatitis. If one of the ethyl groupsis further substituted with a hydroxyl group

to give p-(hydroxyethylochylamino)aniline the toxic effect onthe skin is reduced. .

Th~se two compounds show, in a very pronounced way,the different effect of nuclear and amino group substitutionby the same group. A hydroxyl substituent in the nucleusus.ually results in an increase in power-s-the hydroxyl sub-stituent in 'p-die.thylamin~aniline reduces the power. Usuallya carboxylic acid group m the nucleus seriously reduces, ifnot destroys, the -developing power, but glycin is a veryuseful, though somewhat slow acting, developer.

Binuclear Developing AgentsMany years ago, developing agents containing a naphthalene

nucleus were popular. Among them were Eikonogen

~OH

S03Na~Sodium salt of l-amino-2-naphthol-6-sulphonic acid and

Diogen,


'~OH

S03Na~S03Hmono-sodium salt of l-amino-2-naphthol-3:6-disulphonic acid.

Inclusion of sulphonic acid groups in the molecule are~ecessary as l-amino-2-naphthol is not soluble enough for useIn an aqueous developing solution. It will be noted thatthese developers contain sulphonic acid groups but despitethis are efficient presumably due to the use of a naphthaleneinstead of a benzene nucleus.

Many other binuclear compounds have been found to havedeveloping properties, but have had no practical application.The two nucl~i may be as the above ones-naphthalenesystems; or biphenyl may be the aromatic radical.

00Another type has one or both of the developing systemsjoining two radicals-e.g., hydrazobenzene below.

THE DEVELOPING AGENTS 45Also of purely academic interest are developing compounds

such as phenylhydrazine

ONH-NH2

phenylhydroxylamine

ONH-OHand hydrazobenzene.

These compounds behave as substituted hydrazines andhydroxylamines.

Developing Agents in PracticeThe developing agent types which have so far been men-

tioned can obviously be expanded into a very large numberof relatively simple, distinct developing agents. The majorityof these would be efficient, yet very few have found their wayinto the darkroom.

Metol and hydroquinone are by far the most widely usedof the developing agents. Their manufacture is not difficult,therefore they are cheap. Their efficiency is high, especiallywhen they are used together. Also-and this is a mostimportant property-they are very versatile. Both metoland metol-hydroquinone mixtures are used with mild alkalisin the majority of fine grain negative developers wheremaximum emulsion speed is essential (D.76, D.23, DK.20,etc.). 'When a carbonate alkali is used, developers for almostevery purpose can be prepared-fast working negativedevelopers; lantern slide, positive film, and paper developers;high contrast developers for aero and X-ray films. Hydro-quinone with a strong alkali (sodium or potassium hydroxide)is used for process and line work. Information from a largechemical manufacturer shows that about four times as muchhydroquinone as metol is used; but that compared "nth thesetwo developing agents the amounts of other developers soldis negligible.


Pyrogallol-at one time the only developer used for nega-tives-is now rarely used apart from press work. It has oneproperty which is sometimes desirable. When used alone orwith metol in a developer containing little sulphite, a yellowishstain image is produced together with the silver image. Thistreatment can be used in cases of known under exposure, togive increased contrast.

Amidol is used mainly in print developers. The developercan be easily prepared as only three ingredients are normallyused=-amidol, sulphite and bromide. Its main disadvantagesare that it does not keep in solution, and it stains the handsand equipment badly.

p-aminophenol has two main uses-in concentrateddevelopers, and as a substitute for metol in cases where theuser is prone to metol poisoning. Developers made withp-aminophenol, bisulphite and a caustic alkali can be preparedas a stock solution forty times as strong as the workingsolution. These very concentrated solutions keep well insmall stoppered bottles (Azol, Certinal, Koclinol, Rodinal, etc.).

Some people have skins which are very sensitive to metol,and are unable to handle developers which contain it. Insuch cases p-aminophenol is recommended as a substitute.This works well with paper developers, but is not so efficientas a negative developer. However, by using a developingtank it is unnecessary to touch a negative developer; with aprint developer there is no convenient way of entirely avoidingcontact with the solution.

The one-time popularity of p-phenylene diamine for fine-grain development appears to have faded somewhat. Evenwhen used in combination with glycin it is necessary toincrease the exposure two or three times to secure adequateshadow detail. A further fault which is sometimes attributedto p-phenylene cliamine developers is that the tone separationof the resulting negative is not good. In general it is betterto use an inherently fine-grain emulsion and a normal fine-grain developer, rather than attempt to obtain very fine-grain results on a normally coarse-grain emulsion.

Apart from its use in p-phenylene-diamine fine-graindevelopers, glycin's main use is in print developers.

While it is not the purpose of this book to discuss theaesthetic points about glycin developed prints, it can be saidthat developers containing glycin (frequently together withmetol and hydroquinone) do produce prints of a good colour,

THE DEVELOPING AGENTS47

even though the development time be cut short, or undulyprotracted.

Proprietary Developers

From time to time new developing agents, or compound~ddevelopers, come on to the market-particular~y fine-gra~ndevelopers. In many cases the extra:ragant claims made inadvertisements are not borne out by mdependent labora~~rytests. Four proprietory developers which are on the Brit.ishmarket are worthy of mention as they have been proved inpractical work. , . , f Ilf d

These are the developing agents Phemdone rom orLtd and' Meritol ' of Messrs. Johnson & Sons Ltd., and the

aCked developers 'Promicrol' and 'Microdol' o.f May~ Baker Ltd., and Kodak Ltd., respectively. Mentol,. asis well known, is an addition compound of p-phenyl~ne dla:neand pyrocatechol.2 It can be used with no alkali o~her ansulphite to give a very .:fine-grain d~veloper, or wit'h otheradditions to obtain maximum emulsion speed. .

Phenidone is a more recent addition to the range of avail-able developers. It is I-phenyl-3-pyrazohdone

H2.C--CO

I I 'HC" NH

N/oand is a general purpose developing agent, particularl~ .usef~~as a replacement for metol in M.Q. developers. In ~ ISf r~ eit is more efficient than metol, only about 7 per cen . 0 ewei ht of hydroquinone being required as a~al1lst up ~o 40

crgcent. of metol. Packed developers. conta~l11ngPh~l1ldone~re available, the most noteworthy bemg '~'bcrophen , ~ finezrain developer which gives increased emulsion speed wltho~t~oarser grain than M.Q.-Borax developers. ~enerally spea.k-in heterocyclic compounds such as Phenidone are mo~eex~ensive than the customary types, but the efficlenc~ of ~~~s

articular compound is great enough to make up or I~~isadvantage, as usually only half a gram or less IS used perlitre.

48. BASIC PHOTOGHAPHIC CHEMISTHY

Nothing definite is know b tpacked developers Both n a ou .the constitution of the twofine-grain develop~rs so compames have recent patents onconnection. we can reasonably expect Some

The May & Baker patent in th .Covers the use of 2-(~-h clr e names = Field and ] ohn!

y oxyethyl) ammophenol sulphate.OH

ONH-(2H,-OH

As would be expected from thegroup, this is not a powerful de rre.sence of the hydroxylsuggests glycin as an accel ve opmg agent. The patentThe grain size is claimed to b:r:i::' :nd fia carbonate alkali.dlamme-sulphite develo os as. ne as a p-phenylenespeed is slight comparel~~thb~~ ~:Q~~s~r: effective emulsion

The developing agents clai d J h x type of developer.substituted p-phenylene diarnf!le m. th e Kodak patent- are

Ines WIt the structure

,~C2, Hq-NH- SO.2-CH,3

yVxNHZ

In this formula R represent Iethyl, and X or Y or both must ban a kyl radical-normallymethoxy (CH3-0-) or ethoxy (C

2H _'b~tlkOxy group, such as

These substituted p-phen I d·5 .•much more active than th ene lammes are claimed to bephonamide grouping (_N:r~~~nt compou':1d. The sul-sol:rbility, and reduces the toxic eff o~) also mcreases theclaims regarding emulsion speed oec of.tJ;1ecompound. Nothe paten.t. r grallllness are made in

1 Sh . REFERENCES:Pr~ces:r)pard and M~es-Illvestigation On the Theory of the

; ~hnson & Sons Ltd.-B.P. 466625• K~J.: l:~~e(W~~~brrgi~~&GIJOhO&),B·.P. 644,249.

. ass Vittum). B.P. 651,749.

Photographic

Chapter 5

THE DEVELOPMENT PROCESS

WE have already briefly discussed the latent image and itsformation. Many theories have been proposed to ex-plain the precise function of the groups of silver atoms

on the surface-and to some extent in the interior-of thegrain, in starting the development reaction.' Electron micro-scope photographs have shown that it is at these minute specksof metallic silver on the exposed grain that development starts.Without these specks-or development centres-the silverhalide grain will not be reduced at all in a normal developer.As this book is concerned only with photographic chemistry,we will leave the photographic physicists and physical chemiststo argue out the exact part played by the development centre.

Physical and Chemical Development

There are two possible ways in which the visible imagecan be formed-that is, in which silver can be reduced adja-cent to the silver latent image speck. In the so-called, chemical' development process, the developer ion, which isnegatively charged, is repelled by the protective sheatharound the silver grain, but at the development centre on thesurface the developer ion is not repelled. It can give its freeelectrons to the silver speck. The silver speck is then nega-tively charged, attracts the wandering silver ions in thehalide crystal lattice, and neutralises them to form moresilver; the cycle can be repeated.

In ' physical' development, on the other hand, the presenceof the halide grain is unnecessary. Fixed emulsions can bephysically developed provided the exposure is sufficient andthe fixing bath does not dissolve the latent image. The silverspecks merely serve as precipitation centres .. The developingsolution consists of a developing agent and a soluble silversalt. The solution is very unstable and must be mixedimmediately prior to use. This solution will deposit silver

49


freely on certain substances, and the minute silver specksserve as excellent precipitation centres.

, Normal' development is probably a combination of thesetwo reactions, as developers usually contain substances whichdissolve silver halides and give, therefore, an appreciable con-centration of dissolved silver in the emulsion layer. Most, ifnot all, of the methods which restrict the process to eitherparticular type are inefficient, requiring increased exposure,and usually increased developing time also. Apart from thephysical development method which will be given later, weshall only concern ourselves here with normal developmentmethods.

The active organic reducing agents were discussed pre-viously, but not the conditions which control their action.In general, the nature of the developer is governed as muchby these conditions as by the choice of developing agent.The popular combination of metol and hydroquinone can beused in all types of developer solution, from the energetic andcontrasty document or process developer to the other extreme-the slow, soft working fine grain developer. The mostimportant condition governing the type of developing actionis the pH-or degree of acidity or alkalinity-of the solution.

The Effect of pH

In general, the developing solution must be alkaline for .theefficient reduction of silver halide, and this table illustratesthe way in which developer type and pH are related.

pH VALUES OF DIFFERENT DEVELOPERS

Caustic Process Developer 10.5-11M.Q.-Carbonate Print Developer 10.0-10.3M.Q.-Bora;x F.G. Negative Developer 9.2Metol or P.P.D. Extra F.G. Developer) 69-90Amidol Print Developer r' .Acid Amidol ' Depth' Developer 4

Amidol is the only common developing agent which canbe used in acid solution, and-then only in special applications.Metol is next on the list, and can be used as a low contrastfine grain developer at a pH of 6.9," but development is slowand there is some loss of emulsion speed. Hydroquinonewhen used alone in the developer will only reduce in the

THE DEVELOPMENT PROCESS 51

presence of a strong alkali. In fact, Levenson3• andFrotschner consider it questionable whether hydroquincnewill develop at all unless a catalyst is present; but in thepresence of air and strong alkali, oxidat~on products areformed which are suitable catalysts. Metol is a very effectiveagent for activating hydroquinone, and this action will bediscussed later.

It is immaterial whether a large quantity of a mild alkalior a small quantity of a strong alkali is used, provided thepH is correct. In practice, however, it is usually desirableto use as large a bulk of alkali as possible, as this results in abetter buffered developer. That is, the developer will be lesssensitive to the addition of acids or alkalis, and inaccuraciesin the amount of alkali added will be less serious. The changedue to the acid liberated during development will be mini-mised. The alkalis in general use at present, in order ofdescending pH are: sodium hydroxide (caustic soda);sodium phosphate, tribasic; sodium carbonate;. amn:oniumhydroxide; sodium metaborate (Kodalk); sodium bib orate(borax). Sodium sulphite has a slightly alkaline re~ction andin some fine-grain developers it serves as an alkali. It alsohas a very good buffer action when use~ with stronger ~lkalIS.

rganic amines (e.g., triethanolamme) are occasionallysuggested as mild alkalis. Potassium salts are n:equentlyincluded, but offer no advantage apart ~rom increaseds lubility which permits a higher concerrtrat ion to be used.It has been stated that the presence of both sodium andpotassium ions in the developing solution reduces fog, butthere is no general acceptance of this statement.

Sodium Sulphite

Our simplest developer, then, is a soluti?n in. water of .ad vcloping agent with sufficient .alkah to aC~lva~eIt; but thisc1 v I per will be of little practical use,. as It .WIlll?e unstable:~IIUinefficient. The main reason for instability IS that thenolution will quickly absorb oxygen. from t~e ai~, which willoxidiso the developing agent. ThIS reaction IS known asuu loxidation. * Although many organic and inorganic reduc-in r agents such as hydroxylamine, sodium formaldehyde

• Tho term' autoxidation' is, strictly s~aking,. incorrect, It is, however, con ..y III ru to use it for oxidation by the air, to differentiate this re~tlOn fr<;>IDthe normal",Idllli n of a developing agent during use-e-oxidation by Sliver halides.


sulphoxylate, cyclohexanone- and even hydroquinone areused in special developers, sodium sulphite, because of itsmany advantages, is used exclusively in black and whitedevelopers.

Apart from technical advantages, the low price of sulphitecannot be overlooked from the user's point of view.

A small quantity of sulphite will reduce the autoxidationrate greatly. For- example, one part of sulphite to twentyparts of metol in solution is sufficient to reduce the rate ofoxygen absorption to one-tenth.' An increase in concentra-tion will not reduce the rate appreciably, but as the sulphiteis itself oxidised, a very large excess is usually included inany developer which is to be kept.

Another advantage of sulphite over substances such ashydroxylamine, is that it is too mild a reducing agent tohave any reducing effect on the photographic emulsion.

The reaction between hydroquinone-which we will takeas a typical developing agent-and silver bromide can berepresented by the equation

hydroquinone silverbromide

quinone silver hydrogenbromide

The double arrows indicate that quinone and hydrogenbromide will, under certain conditions, convert silver to silverbromide. As the reaction which we wish to take place is thereaction to the right we must avoid conditions which makethe reaction go to the left. Without outside influence thereaction would, as we say, reach an equilibrium with some ofthe silver bromide reduced, and some in its original state.As our reaction takes place in an alkaline solution, the hydro-gen bromide (or hydrobromic acid) will be quickly neutralised.But this alkaline solution will cause the quinone to react toform undesirable complexes, which stain the gelatin and tanit, in addition to restraining development.


It is in this role of a remover of development oxidationproducts that sodium sulphite fulfils its second function inthe developer. It converts the quinone (or other oxidationproduct) to a stable, unreactive sulphonic acid salt:

6 + H,O + No,SO, ~

o 6S~No '. +NaOH

OH

quinone sodiumsulphite

sodiumhydroquinone

sulphonate

sodiumhydroxide

water

The term 'unreactive' just used is, of course, relative.Although under normal conditions of temperature and pHmost of these sulphonic acid salts will not act as developers,sodium metol sulphonate will develop to some extent, but theactivity is low compared with metol.

The reaction between quinone and sodium sulphite israpid, and is effective in keeping down the quinone con-centration. Our development reaction can then proceedsmoothly to reduce the silver bromide.

The third important function of sulphite in the developeris that of a silver halide solvent. For a developer to functionat all silver ions must be present. They may be the ' inter-stitial ' or wandering silver ions of the halide crystal lattice,or they may be present in the solution. Silver halides arenormally considered to be insoluble in water, but they dodissolve slightly and dissociate to give a solution containingsilver ions:

Ag Br ~ Ag+ + Br-silver

bromidesil v er

ionbromide

ion

For the silver ion to become a silver atom, its positive chargemust be neutralised. It must acquire an electron.

Ag+ + e Agsilve r electron silver

ion atom

S4 BASIC PHOTOGRAPHIC CHEMISTRY

The developing agent also ionises in solution:

(5OH

hydroquinone quinoneion

hydrogenions

and ~ we see from the equation, the developing agent ion isnegatively charged. It is a donor of electrons.

V!e ~ave, then, a s~stem of ions with opposite charges,which IS u~stable. If It were not for the protective actionof. t~e gelatin, all of the silver grains would be reduced indis-criminately, The gelatin slows down this reaction to sucha~ extent that unless the developer is too energetic the grainsWIth no development centres will not be reduced. As we have:,aId p~evlOusly, there are many attempts to explain the wayIII which the dev~lopmen~ ce~tres work, but it is generallyaccepted that theIr function IS to accelerate this reductionand overcome the protective action of the gelatin. Theunstable system then becomes stable:

0-00 + 2AS + 0-- + 2Ag

0-0

quinone silver quinone silverion ions molecule atoms

and H+ -I- Br- .....>.HBr...-

hydrogen bromide hydrogenion ion bromide

molecule

. This explanati~:m shows that an adequate supply of silverions must be available. If no SLIverhalide solvent is presentthe development process will proceed very slowly, as waterItself has such a slight solvent action that the supply of silver

THE DEVELOPMENT PROCESS SSions for the' physical' reaction will be inadequate, while thesupply of interstitial silver ions 'within the crystal would belimited by, among other causes, the rate at which bromineatoms could escape from the crystal. Other more powerfulsolvents than sodium sulphite are occasionally used, such asammonia, thiocyanates, and even thiosulphates. If toomany silver ions are in solution the protective action of thegelatin will be overcome, and silver atoms will be depositedmore or less evenly throughout the emulsion layer. TIllSdeposition of silver where there has been no light action iscalled fog, and we have already discussed some of its causesin the chapter on emulsions.

Reduction of Fog

The amount of fog which can be tolerated depends on theuse of the emulsion. In developing negatives it is usual totolerate a fog density up to 0.2 in order to keep the maximumspeed of the emulsion. In the first development of a reversalprocess the fog is often greater, as a large amount of solventis used to make sure that all of the exposed grains aredeveloped. As this first image is always removed, the fog isunimportant, providing it is not so high that the reversalimage is reduced in density. With printing materials, how-ever, it is essential that there is no visible fog as this wouldreduce the brilliance of the highlights.

The low pH fine grain developers do not develop an objec-tionable amount of fog for negative work. But for printmaking, and where more vigorous negative developers areused, a fog reducing agent must be included in the developingsolution. The high pH developers will overcome the pro-tective action of the gelatin more readily than the lessenergetic developers.

Since the earliest days of the use of alkaline developers ithas been customary to add a soluble bromide to the developer,to reduce fog. The presence of potassium bromide not onlyincreases the strength of the protective sheath around thegrain, and therefore tends to restrict the access of thedeveloper, but also suppresses the ionisation of dissolvedsilver bromide and so reduces the rate of 'physical'development.

Unfortunately the bromide does not restrict its action tothe unexposed grains, but also hinders the development ofthe grains containing development centres. It is for this


reason that the fog developed by negative developers istolerated. The fog could easily be reduced to a negligibleamount, but it would be at the expense of emulsion speed.When prints are made the loss in speed is relatively unimpor-tant, and is willingly sacrificed to obtain minimum fog. Alsowe find that the emulsions which are used for print makingare not in themselves so liable to fog.

Of recent years other' antifoggants' than bromides havebeen more extensively used. They have a similar structureto the emulsion stabilisers previously mentioned, and theiraction is also similar. The best of these materials have theadvantage over bromide that for a given degree of fog reduc-tion their effect on the latent image is less. When stalebromide paper is used-sometimes unavoidable in the amateurdarkroom-these compounds are especially valuable inreducing fog due to bad storage. Compounds suitable forthis purpose are heterocyclic nitrogen or sulphur organiccompounds such as 6-nitro-benzimidazole and 2-mercapto-benzthiazole.

There are innumerable materials for this purpose describedin patents taken out by the photographic material manu-facturers in recent years. Some of these compounds such aschlorobenztriazole, some quinoline derivatives, and quininewill modify the image colour to blue-black. The effect is notentirely predictable, for in one recent patent it is claimed thata 4-hydroxy quinoline will modify the image colour to warmblack. 6

Some of the other developer additions which are intendedto increase the development rate will be described in con-nection with their main application-s-colour development.

The Metol-Hydroquinone Developer

It was mentioned earlier that metol is an efficient activatingagent in a hydroquinone developer. This effect has beenthoroughly investigated by Levenson.! The majority of hispaper is devoted to the results, both to the image and to thesolution, of varying the concentration of the individual com-ponents of the developing solution. He found that if the pHof his solution were below 9.2, hydroquinone was ineffectivein the absence of metol. But the presence of hydroquinonein a metol developer even at this pH was effective in increasingthe development rate. He proposed the following explanation.Metol is the active agent in reducing the silver halide, and is

THE DEVELOPMENT PROCESS 57therefore oxidised. The metol oxidation product reacts withsulphite very slowly, but reacts with hydroquinone muchmore rapidly. The metol oxidation product is regenerated tometol, the hydroquinone being oxidised. The oxidationproduct of hydro quinone then reacts quickly with sulphiteand is removed as the sulphonate.

Developer TypesAlthough a great deal of information on this subject is avail-

able in books and almost every issue of popular photographicmagazines, some of the normal developer types will now beexamined with reference to the points already discussed,starting with the most energetic developers-the processsolutions.

The main requirements are the maximum possible contrastand freedom from fog. As:the process emulsions haveinherently fine grain and high resolving power, no emphasison these points is necessary when compounding the solution.The main points require a high pH solution with a relativelyhigh quantity of restrainer. Owing to the high pH the mixedsolution will not keep under normal conditions, and it istherefore usual to prepare two stock solutions containing thedeveloping agent and preservative, and the alkali andrestrainer respectively.

To ensure rapid development and good tone separation thequantity of developing agent is normally high. The sulphiteconcentration is not critical, for, as we have seen, in its useas a preservative a high concentration is unnecessary. Theother function of the sulphite in this :type of solution is theremoval of developer oxidation products, and here again amoderate sulphite concentration is adequate. Hydroquinoneis the developing agent normally used in process developers,as it is very efficient at high pH values.

A developer recipe which fully illustrates these character-istics is Kodak D.153. For the purpose of these illustrations,working solution recipes will be given throughout, althoughin many cases concentrated or two-part stock solutions areused.

D.153HydroquinonePotassium metabisulphitePotassium bromidePotassium hydroxideWater to

12.5 gm12.5 gm12.5 gm25.0 gm

1 litre


For high, but not extreme, contrast, many of th.e proc.esscharacteristics are retained, but a carbonate alkali ill highconcentration replaces the hydroxide. This gives a highcontrast developer which is used for contin~ous tone. pr?cesswork as well as for X-ray, aerial, and techmcal applications.The absence of halftone--desirable in linework and pboto-mechanical reproduction processes-would in the~e cases bea disadvantage. A further advantage of this type ofdeveloper is that -it will keep fairly well in a tank.

Kodak D.19b is a high contrast solution, and has, as wouldbe expected, high concentrations of develop~g agents andbromide, as well as alkali. The use of metol with the hydro-quinone is advantageous at its lower working pH.

D.19b

Contact Bromide PapersPapers and Negatives

Metol 1.1 gm 0.55 gmSodium sulphite (anhyd.) 37.5 gm 18.75 gmHydroquinone 8.5 gm 4.25 gmSodium carbonate (anhyd.) 32.5 gm 16.25 gmPotassium bromide 1.4 gm 0.7 gmWater to I litre lItre

The non fine-grain negative developer normally has an evenlower content of all substances except sulphite, and themetol-hydroquinone ratio is higher to ensure development·

MetolSodium sulphite (anhyd.)H ydroquinoneSodium carbonate (anhyd.)Potassium bromideWater to ...

The general purpose developers fall into the next group.They are used at varying dilutions for different materials-the more concentrated solutions, used for contact papers,differ only slightly from the previous group. The ~rincipaldifference is in bromide concentration. Slow emulsions areinherently less foggy than negative materials, therefore thebromide can be reduced. At dilutions suitable for enlargingpapers and negatives, the reduction in. energy of the deve-loper is sufficient to make the low bromide content adequate.Kodak D.163 is a popular general purpose developer.

D.163

2.2 gm72.0 gm8.8 gm

48.0 gm4.0 gm1 litre


efficiency at lower pH values. These developing solutionsare not suitable for paper developm.ent as .they do ~otproduce images of good colour, or of high maximum den~ltyunless the development time is protracted, w~en the li~e-lihood of fog and stains is increased. D.61a is a ne~ativedeveloper recommended for use in deep tanks. It will benoted that the metol-hydroquinone ratio is I to 2 comparedwith about 1 to 8 for D.163. The sulphite-carbonate ratioof 7t to I is aiso very different from the D.I?3 ratio of lit~emore than 1 to 1. The still lower workmg pH of thisdeveloper, which reduces the development rate, enables alower bromide content to be used.

D.6IaMetol 0.75 gmSodium sulphite (anhyd.) ... 22.2 gmSodium bisulphite 0.5 gmHydroquinone 1.5 gmSodium carbonate (anhyd.) 3.0 gmPotassium bromide 0.4 gmWater to ... 1 litre

The sodium bisulphite in this recipe is presumably used togive a better buffered solution, as the alkali content is solow.

Fine Grain DevelopersThe developing solutions with a lower pH than D.6Ia

generally fall, with the exception of amidol developers, intothe class known as fine grain developers. The developedimage consists of much larger grains than those of theunexposed emulsion. It has been suggested' that the heatproduced by the reduction of silver halide to silver, is suffi-cient to cause local disruption of the gelatin surroundmgthe individual grains. This allows individual grains to moveslightly and to form ' clumps' of metallic silver which givethe image its grainy appearance. If this theory is correct,then a reduction in development rate would also reducethe rate at which heat is generated. This would permitthe heat to be dissipated more easily and prevent local hightemperatures, which in turn would reduce the clumping ofgrains, and therefore the graininess of the developed image.

Other factors alter graininess, but the rate of developmentis the most important. Even the energetic high alkalidevelopers will produce finer grain if they are dil.uted byupwards of ten times, with increased development times.

BASIC PHOTOGRAPHIC CHEMISTRY60

The usual way to compound a fine gr~in developer i.s,however, to reduce the activity by reducing the pH stillfurther. This necessitates a change of alkali as D.61a withonly 3 grammes per litre of carbonate is 1;11elow limit fora carbonate developer. With less carbonate the developerwould probably be unstable. Borates are frequently used,in conjunction with high sulphite concentrations. Thesesolutions have adequate stability.

Not only does' the high sulphite concentration assist inproducing a well buffered solution, it also increases thesolvent power of the solution. This appears to be desirablefor fine-grain developers aajhe developing rate is increased ,without increasing the grain size. The solvent action is, ofcourse, very slight in comparison with a fixing bath. Toohigh a solvent action would give low image density and highfog.

Semi-fine grain developers of the M.Q. Borax type are'published by most of the sensitised material manufacturers.They are usually based on the original Kodak D.76 recipeformulated by Capstaff :

D.76MetolSodium sulphite (anhyd.)HydroquinoneBoraxWater to

2 gm100 gm

5 gm2 gm1 litre

Other more recent Kodak developers which give finer grain.at the expanse of emulsion speed are DK.20, D.23, and D.25.DK.20 uses the proprietary alkali Kodalk (sodium meta-borate) .and the inclusion of a small quantity of thiocyanateaugments the solvent action of the sulphite, but also increasesfog to some extent-hence the inclusion of potassium bromide.

DK.20

MetalSodium sulphite (anhyd.)KodalkPotassium thiocyanatePotassium bromideWater to

5 gm100 gm

2 gm1 gm0.5 gm

1 litre


D.23 and D.25 are the latest solutions devised by Crabtreeand Henn at the Kodak Research Laboratories. D.23 is thesimplest developer yet recommended for general work,

D.23MetalSodium sulphite (anhyd.)Water to ...

7.5 gm100 gm

1 litre

and is an excellent compromise between fine grain ::ud .highemulsion speed. It produces negatives with clean hlghl~gh~and apparently higher contrast than normal. ThIS ~sbecause of the long straight-line part .of the ~h~racteTl~ticcurve, which does not flatten off at high densities to grvethe poor highlight tone separation characteristic of many finegrain developers. D.25 is a lower pH developer of tlle. sametype, but here the loss in emulsion speed must be consideredwhen making the exposure.

D.25MetolSodium sulphite (anhyd.)Sodium bisulphiteWater to ...

7.5 gm100 gm15 gm

1 litre

D.25, to develop with sufficient energy for practical pur-poses, must be used at 25 deg. C.

The extra exposure needed with these developers variesto some extent with the emulsion used, but for DK.20 abouthalf a stop extra is recommended, and for D.25 about onestop extra. In practice DK.20 can be used with J?-0rma~yexposed film as the latitude of modern emulsions WIll e,\sllyaccommodate this very slight underexposure. D.23 givesthe normal emulsion rating, although laboratory tests showthat there is a loss of speed, but so slight that the averageshutter and diaphragm calibration inaccuracies exceed it.

Although all the developers quoted are from the Kodakrange, it has not been the writer's intention to bias the readertowards developer recipes originating 'in the Kodak Labora-tories. They were selected merely to illustrate the dev~lopertypes which can be prepared using metol and hydroqumoneas developing agents. The majority of the larger manufac-lurers publish a similar comprehensive range of developerrecipes.

Other Developing Agents

It has already been stated that the use of developing agentsother than metol and hydroquinone is very restricted. Thefigures are, presumably, based upon total usage, .th~ ma;iorityof which is in the motion picture and photo-finishing indus-tries. When individual treatment, as against mass production,is the rule, special developers. to meet individual requirementsare frequently used. Fine grain and paper-print developersoften specify developing agents other than metol and hydro-quinone. Objectively, there may be no advantage over thedeveloper types quoted above, but as the small user will haveno scientific method of assessing the efficiency of a developer-if indeed such a method exists-his preference may liein the direction of the unusual in his choice of developers.

Amidol is one of the most powerful developers known.Without the addition of alkali, a developer of the D.163type can be prepared


D.170Sodium sulphite (anhyd.)AmidolPotassium bromideWater to

25 gm4.5 gm1 gm1 litre

which is suitable for negative and print development. Owingto its poor keeping qualities in solution, and its tendency tostain the hands-and anything else which comes in contactwith it-the use of amidol is restricted to the occasional user.Amidol-developed prints usually have strong blueish blackimages which appeal to many users. As a negative develope~,amidol has no particular virtues. It does not develop parti-cularly fine grain images, although the negatives are crisp ingeneral appearance.

Chlorhydroquinone is more expensive than its parenthydroquinone which it frequently replaces. It is somewhatmore active, and does not suffer from one of the disadvan-tages of hydroquinone-great sensitivity to temperaturechanges. It is specified in some :fine grain developers andprint baths, but generally its doubtful advantages do notjustify the higher price.

Glycin, p-phenylenediamine and o-phenylenediamine areusually used in the 'fancy' fine grain developers, aboutwhich there is always a great deal of controversy-based

THE DEV~LOPMENT PROCESS 63

usually on unreliable information. Admittedly, these deve-lopers do frequently give fine-grain results-but usually at theexpense of good gradation or emulsion speed. The writer'srecipe for fine grain is an inherently fine grain emulsion anda D.76 or D.23 type of developer.

Phenidone DevelopersIn general, Phenidone can replace metol in all types of

M.Q developers. The quantity, however, is much reducedcompared with metol. The following universal developerillustrates this difference, and also the inclusion of an organicanti-foggant to reduce the objectionably high fog level whichPhenidone developers give with some materials. It will beseen that the developer (which is given at bromide paper andnegative strength) is not unlike the previously quoted D.l6.3diluted for the same materials. .

ID-62Sodium sulphite (anhyd.) 12.5 gmHydroquinone 4 gmSodium carbonate (anhyd.) 15 gmPhenidone ... 0.125 gmPotassium bromide 0.5 gmBenzotriazole 0.05 gmWater to 1 litre

The Ilford recommended recipe for a fine grain developerusing Phenidone is also very similar to the corresponding M.QBorax type. Again there is less Phenidone than metol, andwhilst the sulphite, hydroquinone and borax concentrationsare unchanged, the solution contains boric acid, to reduce theactivity but still have a well-buffered developer, and bromide-an unusual ingredient in a low pH negative developer.

Sodium sulphite (anhyd.) 100 gmHydroquinone 5 gmBorax ... 2 gmBoric acid 1 gmPotassium bromide 1 gmPhenidone 0.2 gmWater to 1 litre

Pyro DevelopersWhilst pyro developers for normal negative development do

not have any particular advantage over M.Q developers, thestaining type of pyro developer giving high contrast without


therefore useful for under-exposedexcessive graininess, andnegatives, is of interest.

PyroMetolPotassium metabisulphiteSodium carbonate (anhyd.)potassium bromide ...Water to

3 gm2.6 gm7 gm

18 gm3.2 gm1 litre

Concentrated Developersf tr ted developer containsTh most popular type 0 concen a. . lk li

e th d loping agent With a caustic a a 1.p_aminoph~nol as ~ eved d in the diluted solution andNo appreClable fog IS pro uce . .'therefore bromide is omitted. A tYPical recipe IS .

Stock WorkmgSolution Solution 1 : 19

p_aminophenol ~Cl . ... 50 gm 2.5 gmPotassium metabisuiphite .. , 150 gm 7.5 gm

d id 70 gm 3.5 gmPotassium hy roxi e 1 litre 1 litreWater to ... .,. ... 'f it is stored in

The stock solution will keep for manYdyear~l ~ Ib~t once the1 ith bber bungs or waxe COl{ ,

full bott es WI ru 'd't h uld be used fairly quickly other-bottle has been opene ISOwise it will deteriorate.

Special Development Techniques. 1dInt methods which have been

Of the many S)?eCla. eve op:eare suitable for normal use,proposed at vanous~~;~thf~ethod. For this the developerexcept perhaps the rt containing the developing agentsis split int? two ~a t;;-~~;er the alkali. The method is toand sulphlted an 1 ing agent solution-which is usually of asoak up the eve ?PI normal-into the emulsion, andhigher concentrah0!l ~an lkali when it will be developed.then put tge .film l~fUe o~;ortunity for control of contrast,. asThhet~eth? !~~~Sb~th can usually be varied within quite wI~det e irnes in . 11 It' th result It has been sailimits without ~ras~f s~bjec~:mfhiS emethod will enable .thethat for hars y 1 ed without blocking up the high-s,hadowsi~b~e;~~6p!e;~~~nt present in the emulsion ~Ii.ringlights. e. . tl lkali-is soon exhausted 1D thethe developmg ~~~lo~~ne~~ s~ops, whereas in the light density~~~~: f~:a~~~~loping agent is used less rapidly, and can there-

T}IE DEV,ELOPMENT PROCESS6,

fore develop more fully, and give adequate shadow deta.'The distorted characteristic curve will result in poorer qualllfor the majority of subjects. I~

Somewhat similar results can be obtained by dipping tbnegative in a vigorous de:veloper for about a minute, Q I

then leaving It undisturbed III a water bath for several minut~'1repeating the process if necessary. Again the developrnent e.~,the dense areas ceases before development in the shadows ijtlto local exhaustion. II

Physical Development TechniqueAt one time extravagant claims were made in respect

fine grain for physical methods of development. That'~methods in which the image silver is provided by add' IS,silver salts to the developer, and depositing this silver on ~\development centre, rather than reducing the emulsion sil" \halide. A great deal of work on physical development"" elcarried out by Odella and further developments were made;\Baur and Imhof" and by Turner.'" }. The origin~l O~ell .method used an amidol developer aFurner's modification IS 111 the use of metol and hydroqUino:qwhich can be kept in solution, whereas the Odell fOTrn/''ailed for the addition of solid amidol immediately before 11Slq

Th first step in this method is to prevent the elllu!.1e,h lid from participating in the development process. It 'WOnstated earlier that a fixed emulsion containing a latent irn a~could be developed, but as silver halide solvents also atl gthe very fine photolytic silver deposit of the latent image Ih.kmethod is unreliable. Instead, a potassium iodide for~l I

is used, which converts the silver bromide or iodobronjjsilver iodide, which is not reduced in normal develop--,

Physical Development ForebathPotassium iodideSodium sulphite (anhyd.) ...Water to

The time of treatment in this bath is from 30 second3 minutes. The short time gives higher emulsion speedcoarser grain. The finer grain obtained with the largelmersion is at the expense of a reduction of emulsion 91to one-fifth. The writer believes that in the case of the Iimmersion times, conversion to silver iodide is incon, Iand some normal development takes place.

The physical development ·working solution is obtaUtel1diluting a stock hypo-sulphite-silver nitrate solution


adding the developing agent stock solution to give thefollowing:

TURNER-ODELL PHYSICAL DEVELOPERMetol 2.2 gmSodium sulphite (anhyd.) 22'.0 gmHydroquinone 4.5 gmSodium hydroxide 4.5 gmSodium thiosulphate 30.0 gmSilver nitrate. 3.0 gmWater to 1 litre

Physical development is of academic rather than practicalinterest, but the method has been recommended where goodtone separation is required.

Forced DevelopmentGreat caution must be exercised when assessing the some-

times fantastic claims made for special developers or develop-ing techniques in respect of high emulsion speed. The effectof such treatment is not to increase the actual emulsion speed,but make an under-exposed negative-where the majority ofgradations are on the flat toe of the characteristic curve-ofa higher contrast suitable for printing. While this methodof forcing development may give a printable negative whenthere has been gross under-exposure, it cannot be claimedthat the quality of the print will be equal to that obtainedby using normal exposure and development. A similar effectcan also be achieved by using an intensifier such as quinone-thiosulpha te.

One way of working is simply to develop for a greatlyextended time-with or without an increase in temperature.The inconvenience of this method is sometimes avoided in thethe commercial 'high speed' developers by formulating arapid working solution.

The M.Q Borax and metol-sulphite fine-grain developers arefrequently used for forcing development without too greatan increase in grain. The characteristic curves show how the, extra speed' is achieved.

In the figure, A is the characteristic curve of HP3 35-mmfilm developed in D.23 to a y of .8 which takes about18 minutes. B is the curve of the same material developedto the yoo-about 1.2-of this material. This takes aboutone hour, Further development will produce only an increasein fog. The distance x represents the exposure range of anormal (32:1) subject; y represents the same subject, with


one-eight of the exposure given to x. Both the negativedensity ranges a-a' and b--b' will be about 1,0, but whereasthe whole of the range x lies on the straight part of the curve,only the highlights of y will be on the straight line. Theshadows will be on the toe. The resulting print, although ofadequate density range, will have high contrast highlightsand shadows with poor gradation.

2.

A

•......_-----..,.:t

The portion of curve A marked c-c' which will be obtainedIr m under-exposure and normal development will have anegative density range of only 0.6, and, in addition to havingIhe bad qualities of b-b', will not yield an acceptable print.

'1he method therefore has its use in cases of unavoidableunder-exposure. and it must be conceded that the harsh high-liKhlS and blocked-up shadows may increase the dramaticquality of some subjects.


Johnson's' Capitol' developer has the remarkable propertythat forced development does not produce extreme contrast,although the results are rather grainy if development is pro-longed to obtain maximum speed. The constitution of Capitolis not known.

N.B .-The developer recipes included in the text are toillustrate the particular developer types. In some cases theworhing solutions quoted are not stable and more concen-trated or even divided solutions must be prepared for storage.

REFERENCES1 Mees, C. E. K. Theory of the Photographic Process, pp. 305-328.2 Henn, R. W., and Crabtree, J.1. Photo Jour. 85B.: p. 109.3 Levenson, G. 1. P. Photo Jour. 89B.: p. 17.• B.P. 626,998.6 Mees, C. E. K. Theory of the Photographic Process p. 387.6 B.P. 637,847.7 Lowe, E. W.: What YOIl Want to Know About Developers, p. 71.'Odell, A. F.: The New Photo Miniature, Vol. 2.• Baur and Imhof: Kinotechnik, September, 1933.

10 Turner, P. K.: Brit. Jour. Phor., Vol. 85 (July, 1938), p. 405.

Chapter 6

FIXATION AND WASHING

AFTER development, the gelatin emulsion layer still con-tains unreduced silver halide, in addition to the silverimage. If this were not removed, it would, on exposure

10 light-and particularly if the developer were not washed out-gradually darken.

Development is usually followed either by a rinse in water,or treatment in an acid stop-bath. The purpose is the samein both cases-to remove, or render inactive, the developingreagents present in the emulsion layer. This prevents con-lamination of subsequent treating baths.

Rinsing requires no special explanation. The unuseddeveloper and the soluble- development by-products rapidlydiffuse out of the gelatin layer. A very prolonged wash wouldbe necessary to remove all traces of developer reagents, buta rinse of about one minute in running water is normally quitead quate.

Stop-BathsThe stop-bath, on the other hand, prevents continued

·1 velopment and contamination of the fixing bath by neutral-ising the alkali in the developer. Not only is 'further develop-ment prevented, but there is less discoloration of the gelatinclue to aerial oxidation of the developing agent.

The pH of the stop-bath should be low enough to neutraliseth alkali in the developer, but not so low that the emulsionis damaged, A pH value between 4 and 6 is normally used.Although a solution with this pH could be obtained by usingi~ very dilute solution of a strong acid, this solution would ber pidly neutralised. What is required is a solution which isonly slightly acid, but whose pH is not greatly raised by theaddition of alkali. There are two ways of obtaining such asolution.

In the case of a solution of a weak acid such as acetic acid,only a small proportion of the acid is present in the dis-HO iated form. As soon as alkali is added, which neutralisesIh hydrogen ion, further molecules dissociate to maintain theIt drogen ion concentration. So although only a little acid is

69


present in an active form, there is a large reserve of. acidityready to restore the pH as the active. form is ?eutralised. .

Buffer solutions, and acid salts such as sodium bls~lphateand potassium metabisulilh.ite, f~lfil a similar purpose in pro-viding a large reserve acidity without having a strongly acidsolution.

When thick emulsions are being processed (negative, X-rayand colour materials) the stop-bath used after a carbonatedeveloper should not be too acid. If it is, then the. carbondioxide evolved when the carbonate is neutralised Will formbubbles instead of dissolving in the solution and then diffus-ing out of the film. With. a milder stop-bath .the neutralisingaction of the stop-bath Will be slower, .allowmg some or t.healkali to diffuse from the layer before it IS neutralised. Carbondioxide is sligh tly soluble in water, and the slo~er rate. ofgeneration of this gas permits It to diffuse out in solutionbefore it forms bubbles. A brief rinse between developer andstop-bath will also help to prevent bubbles by removing partof the alkali. .'

When developers using phosphate alkalis are used, eitheracid phosphate or citric acid stop~baths must be used or ascum will be formed on the emulsion. If a phosphate stop-bath is used, a short wash isc necessary before fixation for thesame reason.

For black-and-white work a solution of acetic acid is com-monly used for a stop-bath.

SB.IAcetic acid (glacial) 17 ccWater to 1 litre

For a hardening stop-bath, a solution of ch:rome alum isaffective and cheap, although chrome alum solutions once usedwill not keep well.

SB.3Potassium chrome alumWater to

When a less vigorous stop-bath than plain aceticrequired sodium or potassium dihydrogen phosphateused.

30 gmI litre

acid iscan be

AGFACOLOR STop-BATHPotassium dihydrogen phosphate ...Water to

or alternatively the plainwith sodium acetate,

50 gmI litre

acetic acid bath can be buffered

FIXATION AND WASIDNG

BUFFERED ACETIC ACID STop-BATH

Acetic acid (glacial)Sodi urn acetate (cryst.)Water to

71

10 .cc25 gm1 litre

FixationWhilst a stop-bath or rinse, though desirable, is not abso-

lutely necessary, it is essential to remove the residual ~alidefrom the emulsion if permanent results are to be obtained. *Many substances al'e known which will dissolve silver halides-alkali thiosulphates, thiocyanates and cyanides, thiourea,thiosinamine, potassium iodide and ammonium hydroxide. Inpractice, however, only the thiosulphates and cyanides arenormally used. The other materials are either too slow intheir action or they soften the gelatin excessively.

The alkali metal cyanides are excellent fixing agents, butas they are extremely poisonous they are not normally used,except in process laboratories where iodide emulsions are ~sed.Silver iodide does not dissolve easily in thiosulphate solutions,and so iodide emulsions fix very slowly in ordinary fixingbaths. Cyanides on the other hand are good solvents f.orsilver iodide and fix iodide emulsions quickly. Another dis-advantage of cyanides as fixers is that they attack the silverimage-particularly the low densities. For line work t~is isunimportant and may even be an advantage, but the quality ofcontinuous tone work would suffer if the low densities wereremoved.

This leaves the thiosulphates as the only practical fixingagents.

The Thiosulphates as Fixing AgentsSodium thiosulphate-commonly known as ' hypo '-is used

for all normal photographic fixing. It is an efficient silverhalide solvent, and is both cheap and innocuous. The.unmonium salt is sometimes used where speed of fixing is ofparamount importance, but higher cost and Instabilitypreclude its more general use.

The old theory of thiosulphate fixation was based upon theassumption that first the sparingly soluble silver thiosulphatewas formed:-

• Recently Kodak have taken out patents which describe methods of stabilisingtho silver halide WIthout actually removing It. As the reagents which are used are'lOt harmful to the image. the long wash which is usually necessary to removeIho hypo from the emulsion is avoided.

72 BASIC PHOTOGRAPHIC CHEMISTR

2 Ag Hal -I- Na2S.O. ~ Ag.S.os -I- 2 Na Halsilver sodium silver sodiumhalide thiosulphate thiosulphate halide

In the presence of excess thiosulphate, double salts of sodiumand silver thiosulphate were then assumed to be formed,

Ag.S.O. + Na2S20. ~ Ag2S.03• Na.S.03

Ag.S.O a : Na.S.03 + Na.S.03 ~ Ag.S.03 • 2 Na.S.O.and so on, whose solubility increases as the ratio of sodiumto silver thiosulphal.e increases.

More recent work, however, has indicated that the solventaction is more likely to be due to the formation of complexsalts. In these salts the silver is present in the complex cation,and therefore does not form silver ions in solution. There isstrong evidence to show the existence of several of theseargentothiosulphate ions-

(AgS.os)-' (Ag[S.osJ.)- - -, (Ag[S.oSJ3)- - - - -The sodium salt of the monobasic ion is only slightly soluble

in water, but the salts of the polybasic ions are readily soluble.

Completion of FixationAlthough the above equations indicate that thiosulphate

solutions should dissolve a large quantity of silver halide, inpractice it is not possible to utilise more than a small part ofthe potential solvent action. Not only does the fixing timeincrease greatly as the concentration of silver in the solutionrises, but the presence of appreciable quantities of silver saltscan cause other troubles. If the concentration of silver saltsis above a certain limit (depending on the constitution of thebath and the material being processed) the silver salts willbecome adsorbed either by the silver grains, the gelatin, or thebase material, if this is permeable.

The result is, that although there is no usual indication ofthe presence of silver salts in the emulsion, they are presentand are not removed by washing. On storing, the clear por-tions become discoloured due to the reduction of the complexsilver salts, and the liberated thiosulphate attacks the silverimage.

The usual rule of thumb for negative fixing is to allow theemulsion to remain in the bath for twice the clearing time.This is a very practical rule. 'When a fresh bath is used thenegative can be removed from the fixer as soon as all cloudi-ness disappears. But as soon as any appreciable concentrationof silver salt is present, a longer time is necessary.

lllXATION AND WASHING 73With papers, the olearing time cannot be noted, and an

arbitrary fixing time is chosen. This is many times longerthan the clearing time.

Paper fibres absorb silver-containing salts very readily whenthe silver concentration is high. Obviously, the solution whichis present in the emulsion layer during fixation does containa higher proportion of silver than the bulk of the fixing bath.At this concentration, the paper would retain considerablequantities of silver salts, resulting in rapid fading and dis-coloration. But if the emulsion is left in the fixing bath, mostI these salts containing a lot of silver will diffuse out of themulsion and the paper, and will be replaced by salts contain-

ing less silver. These latter salts can be removed by washing.The surest way to ensure complete fixation-apart, of course,

from the extravagant use of a fresh solution for each negativeor print-is to use two fixing baths. The first bath convertsthe silver halide to thiosulphate complexes and removes thebulk of them from the emulsion. It can contain a much higherproportion of silver salts than a normal single-bath fixer, asthe material is then transferred to a comparatively fresh secondbath. As this second bath never has to remove more thann. very small proportion of the silver salts, its silver concentra-t ion is always low. It can, therefore, remove the last traceso( silver salts, or at least convert them to a form which canb easily washed out. When the first bath is discarded, it isr placed by the second bath. A fresh solution is then used fort he second bath. In addition to being very efficient, the two-bath method uses far less thiosulphate than the single-bathmethod, as the discarded solution-the first bath-can con-lain at least two or three times the silver concentration of asingle-bath solution without giving trouble.

Fixing Bath Additions

Even when a rinse or stop-bath is used, small quantities ofdeveloper are bound to be carried into the fixing bath. Aplain thiosulphate fixing bath will rapidly become discoloured.There are two causes of this. Firstly, the developing agentswill be oxidised rapidly as the concentration of preservativeis very low, and secondly, the sodium argentothiosulphates areslowly reduced to colloidal silver by developing agents-oven in a neutral solution. Both causes of discolorationan be prevented by acidifying the fixing bath.

BASIC PHOTOGRAPHIC CHEMISTRY74The majority of acids will precipitate sulphur from

thiosulphate solutions :-

acidSPa - - --+ SO. - - + S

thiosulphate sulphite sulphurion ion

Sulphurous acid does not decompose thiosulphate and istherefore the acid' which is used for this purpose. It is addedas an acid sulphite as sulphurous acid itself is not stable.Sodium or potassium metabisulphites are commonly used.These react with water to form bisulphites

Na2SZOS +sodiummetabisulphite

Hp --+ 2 Na HSOawater sodium

bisulphite

which in solution can exist as a mixture of normal sulphiteand sulphurous acid :-

2 Na H~Oa ~ NazSO. + HZS03

Another convenient way to produce sulphurous acid whichis frequently used is to acidify a solution of sodium sulphitewith a weak acid (usually acetic acid)

Na2SO. -I- CHa· CO . OH -?; CHs : CO . ONa -I- NaHS03

the bisulphite in turn yields sulphurous acid.

An acid fixing bath is somewhat slower in its action than aplain fixer, and in addition the safe 'limit of silver concentra-tion is lower. But in practice, a plain fixing bath becomesdiscoloured long before the silver content becomes high enoughto cause trouble. The life of the acid fixer is, therefore,greater.

Typical of non-hardening acid fixing baths is:-

KODAK F.52

Sodium thiosulphate (cryst.)Potassium metabisulphiteWater to

250 gm25 gm1 litre

nXATION AND WASHING 75

Hardening Fixing Baths

The majority of developing solutions are alkaline, and there-fore soften the gelatin. A hardening step is therefore desir-able in the processing procedure. As some of the hardeningagents function efficiently at the working pH of the acid fixingbath, they are often included. Chrome and potassium alumsare the usual hardeners.

Potassium alum is the more frequently used as it is morereliable, but chrome alum is a better hardener. Fresh chromealum baths are purple by transmitted light. Heat or age mayturn them green when they do not harden, but old solutionsmay not harden even though they are still purple. Potassium(or potash) alum is colourless, and is not affected by heat orage to the same extent.

A chrome alum fixing bath which is recommended for usein hot weather is

KODAK F.16Sodium thiosulphate (cryst.)Sodium sulphite (anhyd.) ...Potassium chrome alumSulphuric acid (concentrated)Water to .

240 gm15 gm15 gm2 cc1 litre

The chrome alum and sulphuric acid are kept separate fromthe hypo and sulphite until the solution is required, as theworking solution keeps for only about a day.

When potassium alum is used as a hardener, it is convenientto add a concentrated acid hardener stock solution to a plainhypo solution.

KODAK F.53Sodium sulphite (anhyd.)Acetic acid (glacial)Potassium alumWater to '" ...

50 gm75 cc

100 gm1 litre

This solution is added to a plain hypo solution in the pro-portion of 50 cc to each 100 gm of hypo, giving a workingsolution :-

Sodium thiosulphateSodium sulphite (anhyd.)Acetic acid (glacial)Potassium alumWater to '"

240 gm6 gm9 cc

12 gm1 litre


Washing

After fixation the film or paper must be thoroughly washedto remove the fixing bath reagents and any silver complexes

. which have not diffused into the fixing bath. In the case ofplates and film, whose base is non-absorbent, the operationis simple. Only the thin gelatin layer needs washing, and sixminutes in running water is sufficient to reduce thethiosulphate concentration to about five parts in ten thousand-quite a safe figure for normal work. It is very important tochange the wash water quickly. A large, deep tank is veryinefficient as the water cannot be renewed quickly. A small,tank or a shallow dish in which the water can be rapidlycirculated is best. It is better still if a spray of water isdirected on to the emulsion and allowed to run to wastewithout having any sort of container. .

If an efficient running-water washing device cannot beused, the best method is to drain and refill the tank or disha dozen times, allowing each change of water to remain abouthalf a minute.

With papers, washing is more difficult. The fibrous paperbase holds the salts much more strongly than does the thingelatin layer. At least an hour in running water is recom-mended, but even this may be insufficient in the case of thickpapers. If really permanent prints are required, then a hypoeliminator is necessary.

Hypo Eliminators

These are divided into two types-those whose action ismainly physical in releasing the salts from the gelatin orpaper, and those which oxidise the thiosulphate to relativelyharmless salts. Ammonia is the most common reagent in thefirst group, and hydrogen peroxide and potassium perman-ganate are useful oxidising agents. The permanganates havebeen used for many years, but they have the disadvantage thatthey stain due to the formation of manganese dioxide.

Kodak recommend the use of a mixture of ammonia andhydrogen peroxide

KODAK HE.!WaterHydrogen peroxide (!O vol.)Ammonia solution (.880)Water to

500 cc125 cc

9 cc1 litre

FIXATION AND WASHING 77The paper is washed for thirty minutes after fixation, then

treated in HE.! for six minutes, and again washed forten minutes.

Hypo eliminators will only remove the last traces of hypoafter a good wash, to ensure permanent results. They are nota substitute for washing.

The importance of fixing and washing is often under-estimated. Just as much care should be taken in these stepsas in development, or the results will not be permanent.

'I

Chapter 7

INTENSIFICATION AND REDUCTION

ALTHOUGH intensification and reduction are normalpractice in some process methods, in general thesesteps are used only to correct faults in negatives (and

to a very small degree, in prints).There are three methods of intensifying an image :(a) changing the light absorption power of the silver image,(b) converting the silver to a more opaque silver salt,(c) adding material to the silver image.It is probable that the common chromium intensifier works

to some extent, if not completely, by method (a); but mostintensifiers actually add material to the image. In group (b)we have sulphide toner intensifiers, and in group (c) the silverand mercury intensifiers, and stain-image methods.

Chromium IntensifierThe silver image is bleached in a dichromate solution

acidified with hydrochloric acid, and is then redeveloped.During the bleaching operation it is unlikely that the di-chromate serves any purpose other than as an oxidising agentwhich breaks up the silver grain structure. There is noevidence that chromium metal or salt is deposited in situwith the grain, although the density increase is sometimesattributed to a chromium deposit." The disruption of thegrain structure gives a more disperse image on redevelopment,which has greater light-scattering power than the originalimage, and therefore a higher density. Any normal developercan be used for the redevelopment stage.

A typical chromium intensifier bleach isIN.4

Potassium dichromateHydrochloric acid (conc.)Water to

9.0 gm6.4 cc

1 litre• If the bleach solution is not thoroughly washed out of the emulsion. an

overall-not imagewise--deposit of a chrorruum salt will be produced on develop-ment. This will give an increase in density. without affecting printing qualities-other than increasing prinung exposure.

78

INTENSIFICATION AND REDUCTION 79

and the bleaching reaction is2 Ag + K~Crp7 + 2 HCI -+ 2 AgCI + 2 KCrO. + HPand not2 Ag + K2CrP7 + 2 HCI -+ 2 AgCrOs + 2 KCI + HP

Sulphide IntensifierThe conversion of the silver image to a silver sulphide

image is a useful, though not widely used, method of intensi-fication. The silver sulphide image does not necessarilyappear to be more dense than the original silver image, butowing to its brown colour, its printing density is greater.

Mercury IntensifiersAlthough mercury salts are extremely POiSOllOUS,these

intensifiers are very widely used. Two types are used-the single bath and the two bath, the latter being morepopular.

A mercuric halide is the active bleaching agentAg + HgCI2 -+ AgCI + HgCI

and is usually used in the presence of excess halide to ensurecomplete conversion to halide. The insoluble mixture ofsilver and mercurous chlorides is darkened with sodiumsulphite, a developer, ammonia or silver potassium cyanide.In this order the intensification is progressively greater. Theplain sulphite reduces all the silver chloride and half of themercurous chloride, the rest of the mercury passing intosolution.

Developing solutions should give a denser image than plainsulphite solutions, but the writer has found that an M.Q.carbonate negative developer gives less intensification than a10 per cent. sulphite solution. This is presumably due to therelatively high sulphite content of the developer, causingsulphite reduction to be predominant; but due to otherfactors--e.g., the low concentrations of active ingredients andthe presence of bromide-reduction is not as complete as with10 per cent. sulphite. The obsolete ferrous oxalate developer(recommended by Chapman Jones for permanent images)avoids the use of a sulphite-containing solution for blackening.This developer. reduces all the silver and all the mercury.Ammonia darkens only the mercury salt and gives an opaqueimage of silver chloride, mercury, and the compound Cl-Hg-NH3 whieh is rather unstable. The silver potassium cyanidealso gives a very dense complex image.


The normal typ~ of ?1ercury intensified image -Ag-Hgcan be treated agam with the bleach to give :_

AgHg + 2 HgCI. -+ AgCI. 3 HgCI

which when reduced gives

AgCI . 3 HgCI -+ Ag. 3 Hg

This can b: bleached again-

Ag . 3 Hg + 4 HgCI -+ AgCl. 7 HgCI

and so on.

3

2>-l-v;ZLUo

I

Comparative effect of some Common intensifiers on an HP3·MQ. Borax image.

Frequently, the bleaching solution contains a solublebromide, w.ith the mercuric chloride. This will give a bleachedImage of silver bromide and mercurous bromide, which maybe an advantage as these salts are less soluble than the chloridesand ri~k o~ image attack during the thorough wash afte;bleachl1:g IS reduced. The Kodak recipe for a mercurybleach IS

IN.!Potassium bromideMercuric chlorideWater to

22.5 gm22.5 gm

1 litre

INTENSIFICATION AKD REDUCTION 81

The single solution intensifiers give brownish images whichare not stable. The solution consists of the mercury halide,xcess halide-usually potassium iodide-and sodium thio-

sulphate or sulphite, which reduces the insoluble salts as theyare formed.

Miscellaneous Intensifiers

A great many toning methods can be used to some extentas intensifiers, as the coloured images which they give aremore opaque to printing light than the original image. Themost popular of these methods is the uranium intensifier.This gives an image of uranium ferrocyanide which is reddishbrown in colour. Details of this method will be given whenwe deal with toning.

Lead intensifiers work on a similar principle, and producevery dense deposits of silver and lead ferrocyanides

4 Ag + 2 Pb3 [Fe(CN)al. -+ Ag,Fe(CN). + 3 Pb,Fe(CN)a

These salts are then darkened to give the silver and leadsulphides in 5 per cent. sodium sulphide. The degree ofintensification is greater than for the mercury intensifiers,and the method is normally used only for line work.

, B.J.' LEAD INTENSIFIER BLEACH

Lead nitratePotassium IerricyanideAcetic acid (glacial)Water to

45 gm68 gm19 cc

1 litre

Another possibility which has not been exploited to anyappreciable extent, is to bleach the image to silver halide andthen redevelop in a colour developer. The only requirementof the dye image is that it should be as opaque as possible toblue light. Many colour couplers which would be unsuitablefor colour photographic work could be used. Some of theyellow couplers, which give images which are too orange orbrown for three colour use, would be very suitable as intensi-Jiers. The stain image which is obtained with a sulphite-free pyrogallol developer has been suggested for intensifica-tion. Naturally the silver image is not removed as it is incolour work.


The Quinone-Thiosulphate IntensifierThis intensifier was investigated by Lumiere, and following

work by Crabtree, was reintroduced by Kodak. In thepaper- which describes the method of use, no mention ismade of the constitution of the intensified image, but it appearsto be composed of hydroquinone oxidation products, as it issoluble in an acid fixing bath. The degree of intensificationproduced in the low densities is extremely great, and resultsin a great apparent increase in effective emulsion speed.

As one would expect, the increase in graininess is as greatas the speed increase; but as normally unprintable negativescan yield satisfactory prints, this is tolerated.

The working solution, which must be prepared by carefulmixing of three stock solutions, has a life of only about15 minutes, and contains

IN.6Sulphuric acid (cone.)Potassium dichromateSodium bisulphiteHydroquinone •Sodium thiosulphateWater to

7.5 cc5.5 gm1.0gm4.0gm5.5 gm1 litre

, Physical ' IntensifiersSolutions of a soluble mercury or silver salt, in tile presence

of a developing agent and a weak acid or a silver solvent, areused as intensifiers. The metallic salt is reduced by thedeveloper and the metal is deposited on the silver grainsalready present, which act as precipitation centres. Thegreat advantage of these intensifiers is that their action canbe watched, and stopped when the required density is obtained.

The silver intensifier recommended by Kodak is mixed fromfour stock solutions to the working solution

IN.5Silver nitrate 10 gmSodium sulphite (anhyd.) . . 18 gmSodium thiosulphate (cryst.) 9 gmMetol 12 gmWater to 1 litre

which keeps for only 30 minutes. Fixing in plain hypo afterintensification is recommended with a thorough wash beforedrying.

83INT!;NSIFICATION AND REDUCTION

Reduction

The photographic reducer is, contradictorily, an oxidisingI~cnt. It converts the silver image into either a soluble s~lt

01" into a silver salt which is soluble in a solvent, present Inuithcr the reducing solution or in a separate ba~h. .

Examples of the solutions which dissolve the SIlver directlyII"' dichromate and sulphuric acid, or permanganate andulphuric acid :-

10 Ag + 2 KMnO. + 8 H2SO. -+ K.SO. + 2 MnSO. + 5 Ag.SO.+ 8 H.O

Ccric salts with sulphuric acid behave in precisely the oppositeway to ferrous salts used as developers. They lose an electronto ionise a silver atom and are converted to cerous salts oflower valency

2 Ca(SO.lo + 2 Ag -+ Ag.SO. + c-, (SO.}.eerie silver silver ceroussulphate sulphate sulphate

A.sMees points out', the reactions of photogr~phic reducingolntions on the silver image have not been sufficiently studied.1 he simple equations do not necessarily represent the exactIOllrse of the reactions. .

Th reactions of the persulphate reducer have been st~dledIly both Sheppard and Higson, who find that the SImpleII l book reaction

2 Ag + (NH.).S.oa -+ Ag.SO, + (NH.).SO.sllvcr ammonium silver ammonium

persulphate sulphate sulphate

not a true representation of the reactio~ .. Their argumentsIII to involved for discussion here, but It IS sufficient to sayIh. L both workers come to the general conclusion that firstlyt II( P 'l"sulphate ion is reduced and evolves atomic oXY!5e~.t h active oxygen attacks the NH. group. to form n.ltncIIIIII. It also attacks silver to give the oxide .. The SIlverII Ide and nitric acid react to give silver nitrate and water.

Photographic Effect of Intensification and Reduction

'I'll outline of the chemical reactions of intensification an.dI. cillt'lion which has been given, may indicate .that, their\I I il II is proportional. That is, that each density ~n theIIIflld lvc is increased or decreased by an amount directlyell pond nt upon the original density.


Those who have used these solutions, however, know thatthis is not strictly true. The main reason for this is theeffect on the solution of the by-products, or simply exhaustionof the solution. In most cases these factors will tend to reducethe rate of reaction more in the dense than in the light areas.For example, if two Farmer's reducer solutions are preparedwith different quantities of Ierricyanide, the solution with lessferricyanide will have a smaller effect on the high densities,due to local exhaustion and the restraining action of the by-product, ferrocyanide. In the low densities where the solu-tion will not be exhausted so rapidly, and where less ferro-cyanide is formed, the reaction rate will be maintained longer.-Several of the intensifiers and reducers have a comparable

action. The persulphate reducers are, however, affected inthe opposite way. The dissolved silver accelerates thereducing action, so the reduction is greater in the high thanin the low densities.

REFERENCES1 Photographic JO"l'IIol, 1946, 8SB.• Mees, C. E. K. The Theory of the Photographic Process, p. 546.

Chapter 8

TONING PROCESSES,rHE most common way of altering the colour of a silver,image is to convert It to silver sulphide. ~he colour ofa silver sulphide image varies from p~rplish black to

dilly yellow, depending mainly On the phYSIcal state of the01 iginal silver image. The colour of the l~age can also betll( red by varying the composition of the.tomng ~ath, or baths.

fully developed bromide pap~r print, whIch. has beenIltmoughly fixed and washe.d, will be co~ver~e~ .m a well-I'loportioned toner to a pleasing brown or sepia Image.

Silver Salt Images

The most convenient way of doing this is to conve.rt, therlvcr image to an insoluble silver salt, and then treat It 111.aulution of a soluble sulphide. There are many ways in

whi h the silver image can be converted into .a sui?-bl~ silverrll , Normally a solution of potassium ferricyanide IS u~ed

III oxidise the image, in the presence of potassium bromld.ewhi h precipitates silver bromide. A typical bleach bath IS

T.52Potassium ferricyanide 50 gmI) tassium bromide 50 gmWater to 1 litre

rt cr bleaching the print is washed to remove the. excessIIIII yanide and the ferrocyanide by~product, and IS thentOIlxl in a 1 per cent solution of sodium sulphjde.

'I'h reactions produced by these baths are

Ag + K3Fe(CN)s + KBr -+ AgBr + K.Fe(CN)eliver potassium potassium sbirlvoemr"de potassium

ferricyanide bromide ferrocyanide

2 AgBr + Na2S -+ Ag2S + 2 NaBrsilver sodium silver sodiumbromide sulphide sulphide bromide

III LIl ry, any method of converting the silver image. to,III 11l~ luble silver salt can be used. Among methods which

85


have been suggested is to expose a damp print to the actionof chlorine gas or bromine vapour.t-s-a method which is saidto give excellent results, but which cannot be recommendedas a practical procedure. Other oxidising agents which areused in solution are potassium permanganate with hydro-chloric acid, potassium dichromate with hydrochloric acid,quinone and quinone sulphonic acid with an acid and a halide,and mercuric or cupric salts.

Practical considerations are against many of these methods,as they are either unstable in solution, unpleasant to use, orexpensive.

The toning bath, on the other hand, is not nearly so stereo-typed, and is capable of considerable improvement. Dilutesodium sulphide solution is not stable, and should not be usedin the darkroom. A considerable amount of hydrogensulphide is evolved, which, apart from being extremelyunpleasant-s-even dangerous-will ruin any unexposedphotographic materials.

Toning solutions proposed by Triepel- which are odourlessconsist of an organic sulphur compound in strongly alkalinesolutions. A suitable bath is :

ThioureaSodium hydroxide ..Water to

and the reaction with silver bromide :-2 AgBr + (NH,hCS + 2 NaOH -+

silver thiourea sodiumbromide hydroxide

Ag.S + (NH.).CO + 2 NaBr + HoGsilver urea sodium watersulphide bromide

Most other substitutes for sodium sulphide are not odour-less in dilute solutions, and some of them produce differentlycoloured images due to the precipitation of other substancesin addition to silver sulphide. For example, sodium thio-antimoniate gives an orange-brown image, as antimonypentasulphide is precipitated with the silver sulphide; andyellowish images are produced by polysulphides due to theco-precipitation of sulphur.

When fine-grain emulsions such as chloride and chloro-bromide papers are toned by this indirect method unpleasantyellowish images are produced, due to the silver sulphidebeing in a very finely divided state. The same fault alsooccurs when the paper or solution is contaminated with a

1 gm4 gm1 litre

TONING PROCESSES 87silver halide solvent, or when the image is underdevelopedor incompletely fixed. The processing faults giving a finegrain image, or colloidal or soluble silver, can be preventedwith care, but an inherently fine grain image has to be treateddifferently ..

Direct Sulphide Toning

The method used for fine-grain prints consists of reactingthe silver image directly with sulphur. This can be done inseveral ways. The simplest, but least practical, is to boil achloride print-well hardened of course-in water to which alittle flowers of sulphur has been added. The print tones veryslowly as the silver reacts with the sulphur.

2 Ag + S -+ Ag2SA more practical method is to precipitate sulphur directly

in the gelatin in a molecular state of subdivision. The printis transferred directly from the fixing bath to a dilute solutionof hydrochloric acid, when sulphur is precipitated:

Na.SoG. + 2 HCI -+ 2 NaCI + S + H.SO.sodium hydrochloric sodium sulphur sulphurousthiosulphate acid chloride acid

The sulphur remains in the gelatin when the by-productsare washed out, and if the prints are left in a damp conditionfor twenty-four hours at normal temperature the sulphurand silver react, as in the previous reaction, to give a brownimage.

The normal toning method for chloride prints is the hypo-alum method. In principle it is the same as the acidifiedhypo method, but is much quicker. The solution is preparedby precipitating some of the sulphur from a 20 per centsolution of sodium thiosulphate with about 4 per cent ofpotassium alum. The mixture is boiled for a few minutesand then allowed to cool to about 65 degs C when a smallquantity of a soluble silver salt is added, followed by a solublehalide. The precipitated silver halide assists in producingthe toned image quickly. The well-hardened print, whichneed not be thoroughly washed after fixing, is toned atbetween 50-60 degs C for about 20 minutes. As the bathcontains a considerable proportion of solid matter the printshould be wiped after toning, and then thoroughly washed.Fixing after toning has been suggested, to remove any silversalts which have been taken up from the bath.

The hypo-alum method, and indeed any direct method,

88/

BASIC PHOTOGRAPHIC CHEMIs-IRY

usually gives a less yellow image than the indirect method.This is probably because the indirect methods alter the grainstructure of the image to a much greater extent than do thedirect methods.

Other direct toning methods use alkali polysulphides, pre-pared by dissolving sulphur in a solution of a normal sulphide.The polysulphides readily part with the loosely attached extrasulphur atoms, which react with the silver image. A furthermethod which has .been frequently suggested is to bathe theprint in a sodium sulphide solution, and then transfer it, with-out washing, to an oxidising solution such as potassium ferri-cyanide. The sulphide is oxidised, releasing active sulphur,which reacts slowly with the silver.

Selenium and Tellurium ToningBoth of these methods rely on the properties of complex

s~lphur-selenium and sulphur-tellurium salts to part readily;'I'lth the selenium or tellurium, as in the polysulphide methodJust mentioned. Selenium toning is a popular method, usedeither alone or together with sulphide toning. The bath is ofsodium selenosulphite and is prepared by dissolving seleniumpowder in a boiling solution of sodium sulphite:

Na2SO. + Se ~ Na2SSeO.The toning reaction is quite simple

Na,SSeO. + 2 Ag ~ Ag2Se + Na.SO.. Tellurium does n~t dissolve in sodium sulphite, but is used111 a sodium sulphide solution to form a solution exactlyanalogous to a polysulphide.

S.elenium toning is frequently followed by indirect sulphidetoning to convert any residual silver to sulphide. Verybeautiful and .permanent results .can be obtained in this way,as the yellowish results sometimes obtained with indirectsulphide toning are avoided, and all the silver is combinedwith ~ither su!phur or selenium. The sulphides, selenides andtellurides of silver are much more resistant to chemical attackthan the original silver image, and therefore the prints aremore permanent .

.The colours produced by all these methods are, as pre-viously stated, from purple-black, through brown, to dirtyyellow colour depending upon the factors noted. For brighterimages and other colours different methods must be usedintroducing other metals in place of the silver. '

TONING PROCESSES 89Metallic Toning

In the Chroma tone process for making three-colour prints(fully des~ribe8. in Making Colour Prints by J. H. Coote)the three Images are composed of a nickel-iron ferrocyanidefor the blue-green, lead chromate or cadmium sulphide forthe yellow and a complex nickel-organic salt for the magenta.These three methods cover the range of metallic toning pro-cesses and we will examine them separately.

Metallic Ferrocyanides

Some of the. insoluble m~t~llic ferrocyanides are brightlycoloured when 111 a finely divided state, and the substitutionof the silver image by these materials is, even for mono-chrome work, a popular toning method. A much wider rangeof colours can be. obtained than with sulphide toning methods.Most of th~ tonmg baths are compounded in a similar way,and compnse a.soluble metal salt, a soluble ferricyanide, andusually an acid and a buffer salt which prevents theprecipitation of the metal ferricyanide.

Among the metal ferrocyanides which are commonly usedare:

Image colourRed brown to chalk red.Red brown-richer than copper.Blue-green.Yellowish-usually used with

iron to give greens.Typical of these toners is the following copper toner:

Copper sulphate 3.5 gmPotassium citrate (neutral) 28 gmPotassium ferricyanide 3 gmWater to 1 litre

Two stock solutions are mixed just before use as the mixtureis not stable, even with the citrate present. Without thecitrate, copper ferricyanide would be precipitated immedi-ately. The toning reaction is as follows:

4 K,Fe(CN). + 2 CuSO. + 4 Ag ~potassium copper silverferricyanide sulphate

MetalCopperUraniumIronVanadium

2 Ag.SO. + Cu2Fe(CN)c + 3 K.Fe(CN).silver copper potassiumsulphate ferrocyanide (errocyanide


With uranium and iron toners there is no tendency for themetallic ferricyanide to precipitate from the toning solutions,so the citrate is unnecessary. Two stock solutions are usuallyused to secure the best keeping properties, although theKodak iron toner, T.l1, is a stable single solution toner, butits preparation requires care.

T.llAmmonium persulphate .. 0.5 gmIron ammonium sulphate (ferric alum) 1.4 gmOxalic acid .. 3.0 gmPotassium ferricyanide 1.0 gmAmmonium alum 5.0 gmHydrochloric acid (10 per cent solution) 1.0 ccWater to 1 litre

Each of the solid chemicals is dissolved in a small quantityof water and these solutions are mixed in the order given.The solution should be pale yellow and quite clear.

In the copper toner above, the silver image is dissolved awayas the sulphate, but in solutions where a halide is present,or if the solution is made up in tap water containing halide,some, or all, of the silver will be precipitated as halide. Thisinvolves a further fixing step in hypo before washing anddrying.

Two-stage Methods

In order to overcome difficulties due to unstable singletoning solutions, two-stage methods are sometimes used,particularly with iron toners. The silver image is firstbleached in a ferricyanide bleach, to which it may be necessaryto add a little ammonia to speed up bleaching. On noaccount must a soluble halide be present, or the silver will,as in the first equation above, be precipitated as silver halide.TIm; gives an image of silver ferrocyanide.

4 K.Fe(CN)6 + 4 Ag -+ Ag.Fe(CN). + 3 K,Fe(CN)apotassium silve r silver potassiumferricyanide ferrocyanide ferrocyanide

[he silver ferrocyanide is then reacted with ferric chloride.

3 Ag,Fe(CN)6 + 4 FeCI. -+ Fe,[Fe(CN).Js +silver ferric ferricferrocyanide chloride ferrocyanide

12 AgCIsilverchloride

In this case, the silver chloride must be fixed out beforewashing to render the image permanent.

TONING PROCESSES 91

This two-stage method is limited in use to those caseswhere the metallic ferrocyanide in question is less solublethan silver ferrocyanide.

The method used for the blue-green image in the Chroma-tone process is a modification of this method. The silverimage is first converted to yellowish nickel ferrocyanide inthe following unstable bath, which is kept as two stocksolutions:

Nickel nitrate 25 gmPotassium citrate 150 gmCitric acid .. 15 gmFormaldehyde (40 per cent sol.) . . 50 ccPotassium ferricyanide 15 gmWater to 1 litre

After washing the print is treated for 5 minutes in 5 percent ferric chloride, then in hydrochloric acid before fixingand drying. The colour of the image produced in this wayis distinctly greener than that produced using a plain ferri-cyanide bleach, and probably consists of a mixture of nickeland ferric ferrocyanides.

These ferrocyanide images, unlike the sulphide ones, arefar less stable than the original silver image. They are alldecomposed by alkalis, particularly the uranium and ironimages. In these two cases even slightly alkaline tap waterwill decompose them, and it is often recommended that thewash water should have a little weak acid added to it, toavoid decomposition of the image.

Other Metal Salt Images

This replacement of the silver image by a metallic ferro-cyanide is sometimes used as an intermediate stage in theproduction of highly coloured images composed of otherinsoluble salts. Many metallic ferrocyanides can replace thesilver image, but are not suitable as final images because theyare colourless or only slightly coloured. In some cases theferrocyanide radical can now be replaced by another radicalwhich will give a highly coloured image. For example,cadmium and lead ferrocyanides are relatively useless as finalimages, but can be converted to bright yellow salts by treatingwith sodium sulphide, and potassium dichromate respectively.

Cd2Fe(CN). + 2 Na2S -+ Na.Fe(CN). + 2 CdScadmium sodium sodium cadmium(errocyanide sulphide ferrocyanide sulphide


Pb.Fe(CN). + 2 K2Cra07 -+ K,Fe(CN). + 2 PbCr,07lead potassium potassium, I~adferrocyanide dlchromaee Ie r rocyanide dichromate

Another' way of using the ferrocyanide method of replacingthe silver image with another metallic ion ~s to form a :netal-organic complex, again provided that this complex 1S lesssoluble than the metallic ferrocyanide.

The most common way of doing this is, .like the. yellowimages above, usually used only in colour prmt J?akir:g. Itis to treat the nickel ferrocyanide image (obtamed.m thebleach given earlier) with a solution of the ammonium orsodium salt of dimethylglyoxime.

Ni.Fe(CN). + 4 Na[(CHa)..C a-N.OH.NO] -+nickel sodium dimethylglyoximeferrocyanide

Na.Fe(CN). + 2 Ni[(CHa) ••Cs-N.OH.NO].sodium nickel dimethylglyoximeferrocyanide

This method yields a bluish-red image. An orange-redimage can be obtained by using dibenzildioxime in pl~ce ofthe dimethylglyoxime (diacetyl dioxime). AnotI:er s1ml~armethod to obtain a red image is to treat a copper Image WIthp-dimethylaminobenzal rhodanine.

Dye Toning

For the production of extremely bri~liant colour~d imagesthe easiest method is to dye-tone the Images. ThIS me~hodis normally used for the production of natur~l ?olour prints,but can be used in monochrome work for limited subJ~cts.

Some papers may be badly stained by the dye solutl<;H1sused in this method, and if this is the case then the toningmust be carried out using a temporary support, and subse-quently transferring the gelatin layer which contains theimage, to a final paper support. This may, also. apply tosome of the toning methods given previously in this chapter.

In the dye-toning process, the silver image is converte~ to,or replaced by, a substance which is capable of adsorbing=basic dyestuffs. Many such substances are known, an~ongthem are the ferrocyanides and ferricyanides of valladm:TI,copper, silver and uranium; other salts su~h as copper thio-cyanates (Namias' Mordant), lead, cobalt, tm, chromium salts

• The dye is not absorbed as a sponge absorbs, wa,ter-'!dsorption is a physicalbond between two substances which carry opposite electzical charges.

TONING PROCESSES 93

and silver iodide. This silver iodide is probably the mostpopular as it requires no complex or unstable baths; and theimage, although yellowish, is not as highly coloured as themajority of the other mordants. It is also one of the mostefficient mordants.

The silver iodide mordant was first proposed by Traube in1906,s and has been developed by other workers, notablyMiller.' The main development by Miller was the use of ableach bath containing a very large excess of potassiumiodide over iodine, This does not yield the dense silveriodide image of the Traube bleach, which is granular andopaque, but a more or less transparent image consisting of asilver iodide-potassium iodide complex. The transparencyof this image increases with increase in potassium iodidecontent of the bleach bath, but the solubility of the complexalso increases. In extreme cases it is possible to obtain animage which is completely transparent, but unfortunately thisis at the expense of sharpness, for the more transparent salt,being more soluble, diffuses from its original position in thegelatin.

The Traube image, being in compact crystals, presents onlya small surface to the dye solution. It is impossible, withnormal treatment times, to dye more than a very small pro-portion of the crystal. The Miller image, on the other hand,has a much larger surface area for the same quantity of mor-dant, due to its more finely divided state. The dye can thenreact with the mordant much more readily. A silver imageof much lower contrast and density is therefore needed withthe Miller-type mordant as its dye mordanting efficiency isso great.

The silver iodide technique is therefore capable of producinga wide range of mordants by varying the potassium iodideconcentration. As this is increased the transparency and dyeadsorption also increase, but the sharpness decreases.

Dye Toning TechniqueThe silver image is exposed and developed to a suitable

contrast and density, depending on the type of mordant towhich it is to be converted. The Traube mordant requires afairly hard, dense original image, whereas the more trans-parent Miller mordant will require a soft, light density image.It is important that the emulsion is not hardened with analum for, as mentioned above, many salts including chromiumand aluminium salts can act as mordants for basic dyes, and


therefore cause an overall stain. A very thorough washshould follow fixing, and then the image is bleached, or moreaccurately, converted to silver iodide. Traube's bleachconsists of:

Iodine 15 gillPotassium iodide 50 gillAcetic acid (glacial) 25 ccWater to 1 litre

The Miller bleaches have much less iodine and more iodide.After a brief wash the excess iodine which will have dis-

coloured the gelatin is removed in a bath of sodium bisulphite,which is then removed by a thorough wash.

The mordant images are then dyed up in a 1 : 1000 solutionof a suitably coloured basic dye which has been acidified witha little acetic acid. The acetic acid, besides helping to dis-solve the dye, increases the dyeing rate. The dyeing timedepends largely on the type of mordant which is used, but isnormally from three to ten minutes.

A thorough wash is then necessary to remove the dye whichhas been retained by the gelatin. A small amount of aceticacid in the wash water assists in the removal of the excess dye.

If the Traube mordant has been used the image, althoughbright by reflected light, will lose a lot of its brilliance whenlooked through. It is a common procedure to remove thesilver iodide mordant by fixing in a plain thiosulphate fixingbath. Before this step the dye must be made insoluble, or itwill bleed out. .

The mordanting action of silver iodide is not due to achemical reaction between the mordant and the dye, but toan attraction between the two, partly physical and partlychemical, known as adsorption. If the mordant is dissolvedaway, the dye would be free to wander.

The dye is made insoluble by treating with certain organicor inorganic acids which combine chemically with it. Tannicacid has long been known as a precipitant for basic dyes, butis liable to slain badly unless it is freshly prepared from thepure acid. It has been replaced for this purpose by complexinorganic acids such as phosphomolybdic and phospho-tungstic acids.

After the image has been treated with one of these acids,the mordant can be fixed out, when a brilliant, transparentimage is left.

TONING PROCESSES 95

The ~ore transparenit Miller mordants do not, of course,need this treatment as they are already sufficiently trans-parent.

The. silver iodide mordant has been dealt with to theexclusion ?f th~ other mordants mainly because of its wideru~e, especially 111 the making of colour prints. In generalWith the other mordants, the colour of the mordant itself i~so strong that brilliant prints cannot be obtained. Formono~hrome photography, of course, many interesting andbeautiful results can be obtained by modifying the colour offor .example, a copper ferrocyanide image, by treating withbaSIC dyes.

.Colour. development processes are normally considered:WIth toning .processes, but because of their great importancem commercial colour processes, the next two chapters aredevoted to them.

REFERENCES1 Hickman, K. CD, B.J. Phor., 1923, 70, 123.2 Tnepel, W., 8./. Phot., 1911, 58, 657 (B.P. 24,378/1910).: Traube, A., B.J. Phos., 1906, 53, 94.

Miller, B.P. 100,098.

64 HASIC PHOTOGRAPHIC CHEMISTRY

therefore useful for under-exposedexcessive grall1ll1ess, andnegatives, is of interest.

PyroMetolPotassium metabisulphite ...Sodium carbonate (anhyd.)potassium bromide ...Water to

3 gm2.6 gm7 gm

18 grn3.2 grn1 litre

Concentrated DevelopersThe most popular type of concentrated developer contains

p_aminophenol as the developing agent with a caustic alkali.No appreciable fog is produced in the diluted solution andtherefore bromide is omitted. A typical recipe is

Stock WorkingSolution Solution 1 : 19

p-aminophenol nci ... 50 grn 2.5 gmPotassium metabisulphite .,. 150 gm 7.5 gmPotassium hydroxide ... 70 gm 3.5 gmWater to .. , ... .., 1 litre 1 litre

The stock solution will keep for many years if it is stored infull bottles with rubber bungs or waxed corks, but once thebottle has been opened it should be used fairly quickly other-wise it will deteriorate.

Special Development TechniquesOf the many special development methods which have been

proposed at various times, few are suitable for normal use,except perhaps the two-bath method. For this the developeris split into two parts--one containing the developing agentsand sulphite, and the other the alkali. The method is tosoak up the developing agent solution-which is usually of ahigher concentration than normal-into the emulsion, andthen put the film in the alkali when it will be developed.The method gives little opportunity for control of contrast, asthe times in each bath can usually be varied within quite widelimits without drastically altering the result. It has been saidthat for harshly lit subjects this method will enable theshadows to be fully developed without blocking up the high-lights. The developing agent present in the emulsion duringthe developing step-in the alkali-is soon exhausted in thedense areas, so development stops, whereas in the light densityareas the developing agent is used less rapidly, and can there-

rns DEVELOPMENT PHOCESS 65

fore develop more fully, and give adequate shadow detail.The distorted characteristic curve will result in poorer' qualityfor the majority of subjects.

Somewhat similar results can be obtained by dipping thenegative in a vigorous developer for about a minute, andthen leaving it undisturbed in a water bath for several minutes,repeating the process if necessary. Again the development inthe dense areas ceases before development in the shadows dueto local exhaustion.

Physical Development TechniqueAt one time extravagant claims were made in respect of

fine grain for physical methods of development. That is,methods in which the image silver is provided by addingsilver salts to the developer, and depositing this silver on thedevelopment centre, rather than reducing the emulsion silverhalide. A great deal of work on physical development wascarried out by Odell' and further developments were made byBaur and Imhof" and by Turner.:"

The original Odell method used an ami dol developer andTurner's modification is in the use of metol and hydroquinone,which can be kept in solution, whereas the Odell formulacalled for the addition of solid amidol immediately before use.

The first step in this method is to prevent the emulsionhalide from participating in the development process. It wasstated earlier that a fixed emulsion containing a latent imagecould be developed, but as silver halide solvents also attackthe very fine photolytic silver deposit of the latent image, thismethod is unreliable. Instead, a potassium iodide forebathis used, which converts the silver bromide or iodobromide tosilver iodide, which is not reduced in normal developers.

Physical Development ForebathPotassium iodide ... ... ... ... 10 gmSodium sulphite (anhyd.)...... 25 gmWater to ... ... 1 litre

The time of treatment in this bath is from 30 seconds to3 minutes. The short time gives higher emulsion speed butcoarser grain. The finer grain obtained with the larger im-mersion is at the expense of a reduction of emulsion speedto one-fifth. The writer believes that in the case of the shortimmersion times, conversion to silver iodide is incomplete,and some normal development takes place.

The physical development 'working solution is obtained bydiluting a stock hypo-sulphite-silver nitrate solution and

)

Chapter 9

COLOUR DEVELOPMENT

PROFESSIONAL motion pictures apart, more colour photo-graphs are produced by the colour development processthan by any of the other photographic colourir,tg metho?s.

Although this process has only been used commercially duringthe last twenty years, the first suggestions for its use were ma~eforty years ago. About 1913, attempts to produce colour plC-tures based upon the patents of Fischer and Seigrist wereunsuccessful. In the early thirties both Kodak and Agfamarketed practical materials which, using quite differentmethods overcame the difficulties that had prevented thesuccess of the earlier project. These integral tripacks will befully described in the next chapter.

Primary Colour DevelopmentThe majority of organic developin~ agent? will, when

oxidised, produce colonred substances 1£ used in developerswhich contain no sodium sulphite. These coloured sub-stances--which in general are not ve:y solub.le-v~ill bedeposited in the gelatin layer tog~ther with the silver image.The silver image is then removed in a neutral solvent such asFarmer's reducer, and the coloured image will be left in thelayer.

Among the more common de,;eloI?ing agents, :pyrogallolproduces the strongest imag~, v\:hich lS of ~ yellm~lsh-browncolour. The chemical constitution of the image is obscure,as pyrogallol produces a wide variety of oxi~ation products.

As none of the other common developmg agents areoxidised to brightly coloured images, ne~ substances weresought with this end in view. Homolka in 1906 suggestedindoxyl and thioindoxyl as primary colour de,:e~op~rs. Theseare oxidised to the blue indigo, and the red thioindigo respec-tively. Owing to the instability of the developing solutionsthe results were far from satisfactory. Many other leucobases were tried as developers but with 110 conspicuous success.

The leuco bases are formed from a series of dyestuffs knownas the vat dyestuffs. These dyestuffs are almost insoluble

96

COLOUR DEVELOPMENT 97and are very fast to light, and resistant to chemical action.When used in textile dyeing they are reduced with an alkalinehydrosulphite solution to the soluble, and more or less colour-less, leuco base. The fabric is treated with the leuco basesolution and is then either left in the air to oxidise, or istreated in an oxidising solution to re-form the vat dyestuff.Many of these leuco bases have developing properties, and areoxidised to dyestuffs by the silver halide. But they areinclined to oxidise spontaneously in the air, and thereforestain the gelatin. Attempts to stabilise the solutions havenot been successful. One patentee even suggests that aerialoxidation can be eliminated by working in an atmosphere ofnitrogen, the operators being equipped with oxygen breathingapparatus I

A few patents dealing with primary colour developmenthave been published, but the range of colours which can beproduced by this method is very small, and as secondarycolour development has been very fully developed, theprimary process has little more than historic interest.

Secondary Colour DevelopmentIn this method, the oxidation product of the developer

reacts with a second substance, known variously as a colourcoupler, colour former, or dye former, to form a dyestuff.

It is the greater flexibility of this type of process which hasvirtually excluded primary colour development as a practicalprocess.

The Colour Developing AgentAlthough most of the common developing agents will

oxidise and couple to give coloured substances, the imagesobtained are very dull and lacking in density.

The developing agents generally used for colour develop-ment work are derivatives of p-arninoaniline (p-phenylene-diaminej-s-in particular, substitution of one of the aminogroups with lower alkyl groups, and nuclear substitution bya lower alkyl group.

6...N,C1HS C2H5

p-diethylaminoaniline


C'~NONH'I

C2HS~-amino-5-diethylaminotoluene

Paradiethylaminoaniline in particular is a fairly efficientdeveloping agent, and is readily obtainable pure enough foruse as a developer. The bases themselves are not stable anddeteriorate when kept for any length of time, and thereforethe usual commercial product is a salt-either the sulphate orthe hydrochloride.

An even more stable form of p-diethylaminoaniline has beendeveloped by May & Baker Ltd., and is marketed under thename of . Genochrome.' Advantage has been taken of theanti-oxidation properties of sulphur dioxide, ar:d a loo~addition product has been prepared. When dissolved mwater the addition compound breaks down to some extent togive a solution of the p-diethylaminoaniline base and sul-phurous acid. This solutio~, unlike the solutic:>Ils of thehydrochloride and sulphate, IS very stable, but ~t must bementioned that solutions of the other salts to which a mole-cular equivalent of, for example, metabisulphite, has beenadded, are also quite stable. . .

, These developing agents have two dlsadvantages-t~elrreducing power is not great, ,:"hich makes lonl? ,developmgtimes necessary, and they can give severe dermatitis to peoplewith a sensitive skin, The free base appears to be dissolvedby the natural skin oils, and so is absorbed into the tissues,causing severe irritation, The oil-solubility s~lould, thereforebe reduced, and if possible the water solubility increased.Unfortunately the normal methods of doing ,this reduce t~edeveloping potential beyond practical llJ;llts .. />;. partialsolution is to use p-(hydroxyethyl-ethyl) arninoaniline,

o...N,

CzHS CZH40H

COLOUR DEVELOPMENT 99WhiC~l is l~ss readily absorbed. To obtain a developingsolution Wlt~ the same energy as p-diethylaminoaniline,about three times the quantity of developing agent must beused.

Several more complex p-diethylaminoaniline derivativesh,a:-rebeen patented which, it is claimed, do not give derma-~ltIS,and d~ not decrease, but may even increase, the develop-mg potential, None of these, of course, are generallyavailable.

Halogenated p-aminophenols have been claimed in patentsby bot~ Kodak and Agfa as colour developers; but have not?een WIdely used, perhaps on account of their low solubilityin water.

Colour' Couplers

:rhe, b~sic requirement of a colour coupler is that it con-tams III Its structure a methylene group which is activatedby .a ~trongly electro-negative group, The most commona~tJvatmg groups are carbonyl, nitrile, and other unsaturatednitrogen atom groupings.

,The basic, cou'ple.r groupings-~hich were disclosed byFischer and Siegrist III 1915, and which were greatly developedprior to the Second World War-are:

(1) Phenolic compoundS-blue and green dyestuffs

c5-the phenols are capable of existing in two forms, the enolform shown above, and the keto form whose structure containsa methylene group:

6~c5~6Hz

(2) f3-keto ester derivatives (yellow to orange dyestuffs)R-CO-CH2-CO-Rl


(3) acetonitriles (orange to magenta dyestuffs)

R-CO-CH.-C= N(4) 5-pyrazolones (red and magenta dyestuffs)

H.C- C-Rr ",::C, .•..N

o NrRl

The simpler compounds which contain these groupings donot generally give good dye images. The dyestuffs are oftentoo soluble to remain in the gelatin layer, or they are attackedby chemicals present in either the developer, or in subsequenttreating solutions.

For example, a dyestuff can be obtained by reacting phenolwith p-nitroso diethylaniline (a stable oxidation product ofp-dieth ylaminoaniline)

but this dye is quite soluble. The coupler must thereforebe modified to give an insoluble dyestuff without the coupleritself being insoluble. The most common coupler to do thisis I-naphthol

which was suggested by Fischer and Siegrist, who also proposed

COLOUR DEVELOPMENT 101

using the chlorinated l-naphthols in order to obtai~ a .gre~nerdyestuff. (The dyestuff obtained from I-naphthol IS a bnghtblue.)

2-ch lor-I-naphthol

cOCI

(I

~CI

~(I(I

2 : 4-dichlor-l-naphthol

2 : 3 : 4-trichlor-l-naphthol

Among the many phenolic compounds which have beensuggested as couplers giving blue-green images are

00~Br

~ryvOH

OH OH

2 : 2'-dihydroxybiphenyl

2: 6-dibromo-1 : S-dihydroxy-naphtholene

bis-I-naphthol


and a host of phenols and l-naphthols which contain groupsto stabilise the dyestuffs.

Fischer could not obtain good yellow images. He proposedusing acetoacetic ester

CH3• CO. CH2. CO. OC2HSbenzoyl acetic esteroCO-CHZ-CO-OC2.HS

and their simpler derivatives.

It was not until Kodak patented the anilides of these esters

CH3-CO-CH2-CO-NHOOCO-CH2-CO-NHO

that yellow colour developed images were obtained which hadgood density and hue. Naturally the other manufacturerspatented further developments of these compounds, using, forexample, morpholine derivatives to replace aniline derivatives

O CHz-CHzCH3-CO-CHz-CO-NH N '0, /

CHz-CH2

and furoyl acetanilides to replace the aceto-or benzoylacetanilides

Du Pont

Veracol

Ilford patented the use of monoanilides of malonic ester

CzHsO- CO- CH2.-CO - NHO

whilst Agfa mention terephthaloyl bis-anilides


in their patents.The grouping ....:C - CH. - C _ N is most commonly

used in cyanacetic ester derivatives and in acetonitriles. Themost common magenta coupler, also propose~ by Fischer, isp-nitrophenylacetonitrile (p-nitrobenzyl cyanide)

whilst benzoylacetonitrile

and the anilides, in particular the p-nitro anilide, of cyan-acetic ester

yield clear orange to magenta images.l-phen yl-3-methyl-S-pyrazolone is frequently used as a

magenta coupler as it is readily available

but the dye image is not very brilliant. Much better imagesare obtained when the phenyl group is substituted, usuallywith a nitro group or with halide atoms, or if the methyl


group in the 3-position is replaced by a benzoyl or othermore complex group.

The structure of colour couplers has been dealt with atsome length to show that although the active groups maybe well' wrapped up' in complex molecules, the basic essentialshave, until recently, not changed since Fischer's originalpatents. •

The Colour Coupling ReactionAs we saw earlier, when a developing agent ion and a silver

ion react, the silver ion is reduced to silver metal, and thedeveloping agent is oxidised. Normally we remove thedeveloping agent oxidation product by including a largequantity of sodium sulphite in the developing solution. Incolour development, however, it is the oxidation productwhich we require, so the sulphite is either omitted, or reducedto such a small quantity that it does not react quickly withthe oxidised developer. This is then free to react with thecolour coupler

NR2. NR:

(}2A,.- ()+ 2Ag -+ +H

NHz NHquinone-diimine

ion

(). /R Q H+CHz ~ +

"R'NH

CHR' 'R'leucobase

The leuco base which is formed is readily oxidised to the dyeby oxygen in the air, but a more likely oxidising agent is asecond quinone diimine ion, which is reduced back to theoriginal developing agent


O+<S~O+6+H+~H NH ~ NHzCH C

/ <, I / -, ,

R R R RThe complete reaction can then be summarised by the

equation

0+NHz

R/ +CHz+4AQ'R' .J

developing coupler silver dyestuff silver hydrogenagent ions atom ions

For the reactions which we have discussed to take place,four silver ions must be reduced to produce one dyestuffmolecule. Recent work has shown that rather less silverthan this is needed in many cases. It is not unreasonableto assume that in these cases the leuco base is to some extentoxidised by oxygen dissolved in the water used either for thedeveloping solution, or in other baths, or for washing. Theoxidising agent which is subsequently used to remove theunwanted silver image may also be responsible to some extentfor the oxidation of leuco base molecules to the dyestuff.

Stability of DyestuffsOn the whole, the indophenol and azamethine dyestuffs

produced by the colour development process have the dis-advantage that they are stable neither to light nor to chemicals.There are of course exceptions, but in general any acid pro-cessing solutions must be avoided, and especially acid sulphitesolutions. A neutral or alkaline fixing bath must thereforebe used, which is rapidly discoloured and must be discardedfrequently. * Calcium salt deposits on the emulsion are also

• Some dye images will be decolorised with an acid fixing bath but will regaintheir original density after washing and drying.

W6 BASIC PHOTOGRAPHIC CHEMISTRY

difficult to remove without destroying the dye image to someextent.

The light fastness of the dyes is so low that it is not advisableto keep them exposed to light.

Although most manufacturers-notably Kodak-have pro-duced more stable dyes working along normal lines, by farthe most original developments a e from General Aniline.Various approaches have been made by them to obtain othertypes of dyestuff 'either by using more complex developingagents.' different types of coupler; or both."

From the chemist's viewpoint the patent covering the useof substituted m-arninoanilines as colour couplers is the mostinteresting. The first product of the dye-coupling reactionbreaks down-usually spontaneously-to give a dyestuffwith a different structure, which when treated with an acidyields a brilliant azine dyestuff. The general scheme of oneof these reactions is

coupler

_ C~QNN6N""

orange azine anhydride

NH

- C~ONNH-SOzO

NMez oNMe2

developer blue quinoneimine

HX~

NHz

C~(iN'NHA

VNMe.ma&:enta azine dyestuff

Thes~ recen~ advan?es, which point to the possibility ofproducing a wider vanety of dyestuff types without basicallyaltering the mechanics of the colour development process,show that we can expect still more from this process.

I

COLOUR DEVELOPMENT 107Treatment After Development

The normal processing following development is insufficientfor colour development, and may destroy or degrade thecolour image

Bisulphites and strong acids in particular must be avoided.This rules out the acid fixer. To give a plain fixer the greatestpossible life a thorough wash is essential between develop-ment and fixation. A stop-bath of, for example, sodiumacid phosphate (NaH2PO,) also helps to remove the developingagent.

The essential difference, however, between black-and-whiteprocessing and colour development processing is that thesilver image must be removed after colour development if apure dye image is required. Farmer's reducer is commonlysuggested but does not, of course, keep well. The oxidisingagent can be used in one bath and the solvent in anotherwith a rinse between them. Any acid bleach such l,S per-manganate and hydrochloric acid or dichromate-sulphuricacid will completely destroy the majority of dye images.

The following scheme illustrates normal practice in colourdevelopment processing with colour couplers giving acid-sensitive dyes. When acid baths-which would not normallyaffect the dye image-are used it should be noted that solu-tions yielding sulphur dioxide on acidification are isolatedby washing.

Development 10 minWash 5Stop-bath 1W~ 5Hardener (chrome alum) 3Wash 5Bleach (ferricyanide and bromide) 5Rinse 2Fix 10Wash 10

The fixation step normally follows bleaching as at thisstage both the unused halide and the halide regeneratedfrom the image can be removed together.

An alternative procedure is to introduce a fixing stage inplace of the stop-bath. This would make washing easier.The silver halide grains in the emulsion adsorb the developingagent, which does not wash out easily, and may still bepresent at the bleaching stage, when it is oxidised, the oxi-

dised devel.ope~·coupling with any residual coupler to givean ove:-all stain. If the halide is removed there is nothingto retam. the developing agent, so it is more easily removedby washing,

108 BASIC PHOTOGRAPHIC CHEM.ISTRY

Colour Development coelerators

Owing to the low developing potential of colour developersolu~ions, it i~ .desirable to speed up development as far aspossible. Raising ~he pH of a developer by using causticalkali would do this, but may result in degraded colours.

~here are two methods of increasing development ratewhich can be used in some applications.

<?wing to the. low: sulphite content of these developers,their solvent action IS very slight. This can be augmentedby including an additional halide solvent-thiocyanate forexample-in the developer.

Wh~re the co"?pler is used in the developer solution, the dyedepo~lt m~y hinder the access of developer to the grain.The inclusion of a small quantity of a dye solvent in thedeveloping s,?lution will often disperse this dye slightly, toallow the.gral.n to be more.readily developed without causingenough diffusion to result in a loss of definition. A suitablesolvent is benzyl alcohol.

REFERENCES~ B.P. 625664 (BJ. Phot., Feb. 24, 1950, p. 93).- B.P. 633760 (BJ. Phot., June 30, 1950, p. 331).3 B.P. 649811 (B.l. Phot., June 22,1951, p, 315).

Chapter 10

INTEGRAL TRIPACKS

LIKE so many other colour photographic techniques theuse of a tripack for obtaining colour separation negativeswas proposed by Louis Ducos du Hauron. Du Haurou's

tripack comprised three emulsion layers, each on a separatethin base. The front emulsion was blue sensitive, the secondgreen sensitive, and the third red sensitive. It is unlikely thathe was able to produce a practical tripack, for high speedcolour sensitised emulsions were not available to him, nor wasthin transparent film base. A tripack of this type wasmarketed by Defender up to 1940, but even with high speedemulsions and thin film base it was not a great success. Thedefinition on the rear element (cyan printer) was very poor,and the method has now been abandoned in favour of theintegral tripack.

Tripack Principle

The illustration shows the construction which is commonto all integral tripacks, whether they are of the colourdevelopment or the stripping type.

blue sensitive emulsionyellow filter layergreen sensitive emulsion

red filter layer-optionalred sensitive emulsion

White light passing through the emulsion will be auto-matically separated into the three primaries, by either theinherent or added sensitivity of the emulsion, or by the useof filter layers.

The blue component of the white light will affect the bluesensitive front layer, and will then be absorbed by the yellowfilter layer. The remaining light will pass on, the green

109

110BASIC PHOTOGRAPHIC CHEMISTRY

component affecting the green sensitive layer and the redcomponent, the red sensitive layer.

It is possible to obtain emulsions which are sensitive togre~n but not red light, and also to red, but not green light.It IS then not necessary to include a red filter layer in thepack. Sometimes, however, this may be included to givebetter separation in the red sensitive element. All fastemulsions, however, are sensitive to blue light, and as wewant to record the blue light only in the outer layer, its passagethrough to the green and red recording layers must be pre-vented. The yellow filter layer does this, and if the colouringmaterial is carefully chosen, it is possible to remove prac-ticall~ the whole of the blue light without substantiallyreducing the amount of green and red light.

Three types of integral tripack are in current use-thestripping tripacks, the materials containing no colour coupler,and those materials which contain a colour coupler anchoredin the emulsion.

Stripping Tripacks.Thes~ materials have no special significance from the

viewpoint of the photographic chemist. The layers betweent~e. three emulsions are designed to part under certain con-ditions. .At present no materials of this type are availablecommercla~ly, but both Kodak and du Pont are activelyengaged with the problem. The Kodak material has beendeveloped for motion picture use, as the complete mechan-Isabor: of the motion picture processing laboratory is almostessential."

Du Pont marketed for a short time, but then withdrew amodified tripack (the' S.T. Tripack ') with only one strippinglayer.

Colour Transparency TripacksAs the use of these materials is the subject of a book

(C,0lour ~ransparencies, by C. L. Thomson, Focal Press), wewill consIder them only from the chemist's viewpoint.

The type of tripack which contains no colour coupler is, inthe two commercially available brands-Kodachrome andlIfOI'd COlour, processed by reversal to a positive trans-paren.cy. . This processing technique is not straightiorwardand, If reliable results are to be Obtained, must be left to themanufacturer. As the two materia,ls differ considerably wewill treat them separately.

INTEGRAL TRIPACKS 111

Kodachrome

The basic material corresponds to the diagram wit~·. nospecial departure in structure, bu~ includes a red sensitiserwhich is not destroyed in a developing bath. After exposure,the film is developed in a normal reversal first de,:e~oper.This is probably an energetic M.Q. develop.er containing asilver halide solvent in addition t? sulphlt~-probably athiocyanate. The developer is suffiClen~ly aC~Ive to developall of the exposed grains. For a neg.atr:re thIS. woul~ resultin unpleasant graininess, but as this Image IS ultl,matelyremoved large grain size does not matter. The film IS thenwashed thoroughly to remove the developing r~agents, .butis not fixed. It is the residual silver halide which providesthe positive image.

The next step is probably to exp?s.e the film (t~rough theback?) to red light. ~he ~ed sen.s~trve layer, ?Wll1g to thespecial sensitiser use?, I~ still sensitive to red light, and theresidual silver bromide IS made developable. In the. other1:\:'10 layers, however, the red ~i~ht has no effect .. Developmentin a colour developer contalmng. a cyaJ?- colour coupler pro-duces a positive silver-plus-dye Im~ge ill the bottom layer.If the film is now exposed to blue light from the front~ on!ythe outer layer will be affected as the yellow filter layer IS stillpresent. Colour dev~lopme~t in the presenc~. of ~ yelIo:wcolour coupler will give a silver-plus-dye positive Image Inthe outer layer.

Only the residual halide in ~e inte:me?iate layer remains,and this is fogged either by light WhICh IS strong enough. topenetrate the now fairly dense outer layers, or by a foggingsolution. This layer is developed i~. a I?agenta colourdeveloper. Both the negative 8;nd positive silver I~~ges areremoved by bleaching and fixing to leave a positive dyeimage.

Two points which are of the greatest importance in theprocessing of 'both Kodachrome and Ilfo~d Colour, are thatthe re-exposure and development techniques must ensurethat there is no developable material left in a layer that hasbeen treated with colour developer. Nor must the developergive any visible image in an unexposed layer. . Secondly, thecomplete removal of developer reagen~, partlcu~arly col?urcouplers, between each colour developing stage IS essential.

112 BASIC PHOTOGRAPHIC CHElVITSTRY

I1ford Coloui:

The above notes on the processing of Kodachrome are not'official,' but are probably reasonably correct. Even lessinformation is available concerning the !lford processingtechnique. In fact, it is only the study of patent literaturethat enables the writer to deduce the following possiblemethod, which, though not necessarily the exact method used,illustrates the -broad principles of the way in which thismaterial is processed.

In the manufacture of the film, sensitisers which are resistantto developer action are not essential. The main differencebetween !lford Colour and Kodachrome is in the materialsforming the isolating layers between the emulsions. !lfordhave for these layers a very fine grain slow chloride emulsion,which is not sensitive at the light values used for exposure.Mixed with this emulsion is a non-diffusing fogging agent-probably colloidal silver sulphide. During first developmentthese layers develop to form opaque barrier layers. It isthen a relatively simple matter to expose the two outer layersseparately, and without affecting either the other outer layeror the inner layer.

After first development and thorough washing the sequenceof processing steps is probably :_

1. Expose to light through the base.2. Colour develop in cyan colour developer.3. Wash.4. Expose front emulsion.5. Colour develop in yellow colour developer.6. Wash.7. Trea t centre layer in chemical fogging solution.8. Develop in magenta colour developer.9. Wash.

10. Remove all silver and silver salts.11. Wash.12. Dry.

In theory it is immaterial whether the front or back layeris treated first, but no doubt in practice the order has someeffect on the final result.

113INTEGRAL TRIPACKS

Tripacks Containing Non-diffusing Couplers

Fischel' and Siegrist suggested incorporating colour couplersin the three emulsions of a tripack ; but they were um~,bletoproduce satisfactorr results as t~e colour couplers diffusedfrom their appropnate layer dunng devel~pmen~. Even athick plain gelatin layer between the emulsions did not pre-vent this, A further difficulty they must have encounteredwas that some of the couplers crystallise when in?orporatedin a gelatin solution which is then coated and dned.

Several methods were proposed, with little success, toovercome the difficulty of diffusion, mainly by precipitatingthe couplers as insoluble salts in the emulsion. T~e difficultyhere was that if the insoluble salt broke down dunng develop-ment there would still be some coupler diffusion to an adjacentlayer. If the salt did not break down, coupling was extremelyslow,

The first practical solution came from LG. FarbenindustrieA.G., and was finally put on th~ mark~t in 1936 as '.N~wAgfacolor.' Many patents were Issued m the early thirtiesdealing with methods of modifying colo?r couplers so thatthey would not diffuse from the appropriate emulsion layer.Generally, the method was to add to the coupler molecule agrouping which .wou~d becom.e physical~y attached to t~egelatin. If we lm~gme gelatm to <:onslst of long organicchains-usually WIth many cross-Imkages-we can alsoimagine that a large molecule will become entangled in thischain structure. Merely increasing the coupler molecule ISnot the most efficient method. The most successful of thedifferent groupings is a large fatty acid residue containing ,achain of at least five carbon atoms. The structure of thischain would be, for a l2-carbon atom chain :-

H H H H H H H H H H H HcouP,er-t-t-t-d-d-d-d-2-2-2-2-2-H

~?~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~

This method is used for anchoring the coupler moleculesin the Agfacolor process, full details of which are given invarious government reports.s


These couplers also contain groups which make themwate~-soluble to facil.itate their inclusion in aqueous coatingsolut!o?s. The way in which these various groups are intro-duced into the coupler molecule varies with different types ofcoupler, but in one of the blue-green substantive couplers=they are introduced stage by staae and serve as a very usefulillustration :- o

l-hydroxy-2-naphthoicacid

octadecylamine

blue-green couplerlong-chain alkyl group

substantive blue-greencoupler-not readily

soluble

substantive blue-greencoupler-alkali soluble

This type of colour coupler is used in Ansco Color, Geva-color, and Ferraniacolor as well as in Agfacolor.

• Couplers which will not diffuse from the emulsion.


The materials can be developed directly in a colour developerto a colour negative, and subsequently printed upon a similarmaterial on either a film or a paper base. Alternatively theycan be processed to a positive by the reversal method.

Kodak use a completely different method of ensuring thatthe couplers stay in their appropriate emulsion layers. Thefirst patent was issued to Martinez in 1937 but the methodhas been thoroughly developed at the Kodak researchlabora tories.

The coupler is dissolved in either a solution of a resinwhich mixes with the coupler in all proportions, or in a highboiling point liquid or low melting point solid, which with thecoupler forms a thick oil. This is dispersed in the emulsionas minute droplets, so fine that they can only be seen witha microscope. As neither the . oil-former' nor the coupleris water-soluble, tbe coupler cannot readily wander from theemulsion layer. Although in patents it is claimed that theoil-formers themselves restrict diflusion of the coupler, it isfar more likely that their presence only enables the insolublecoupler to be dispersed evenly in the emulsion withoutforming large crystals,

Another statement which is usually made in patents isthat the oil-formers are water insoluble, but water permeable.In the writer's opinion they are not necessarily water perme-able, but are good solvents for the oxidation products ofdevelopment. The solubility of these oxidation products inwater is' not' great, but is much higher in the oil-former-coupler solution. The oxidised developing agent is easilyabsorbed to react with the coupler and give a dye.

Colour Accuracy

Although the dyes produced by the colour developmentprocess are far from ideal, they are quite satisfactory whenused in a reversal film-that is, when the dyes are used onlyonce in the production of a positive image. When a colournegative is used to make prints on to a_ similar positivematerial-that is, when there are two dye stages to reachthe final result-the colour distortion due to the imperfectionsof the dyes becomes very noticeable. Commercial motion


pictures demand yet a third or fourth stage for any specialeffects, and the colour quality then becomes intolerable whencompared with processes in which all intermediate stages arevia silver separations.

500 600 700

To correct these deficiencies-which are illustrated above-masking techniques have been used for many years. Thismethod using separate masks is reasonably practical formotion picture work, where registration of the masking imageis automatic; and also in photo-mechanical processes wherethe number of copies produced justifies the expense of maskpreparation and manual registration. For amateur andsingle-copy commercial work, however, methods usingexternal masks would be too expensive.

During the last ten years, several methods have been pro-posed, by means of which masking images are produced inthe original negative. The earlier methods merely proposeddeveloping the residual silver halide of the negative to givea low contrast silver positive in one or more of the emulsionlayers. This method gives an increase in brilliance in thefinal print, but no greater hue accuracy.

The more elegant methods produce a coloured positiveimage together with the colour negative, which corrects forhue as well as for brightness in the following way.


Colour Masking

As the magenta colour developed dye image is the mostdistorted of the three, this image will be used to illustrate thecolour masking principle. Theoretically the magenta dyeshould absorb all green light and transmit all blue and redlight,

>-t-o;;Z

'"o

400 600 700sooA mf'

although a dye which transmits equal quantities of blue andred light, with strong absorption in the green region wouldgive good results.

>-t-o;;Z

'"o

400 500 600 700

~=»The typical magenta dye transmission curve, however,

transmits most of the red light, and has the necessary green


absorption, but has a secondary absorption peak in the blueregion of the spectrum: .

400 500 600 700

If the characteristic curves of an image made from thisdye, read through blue, green, and red filters, are plotted,curves similar to these will be obtained:

GREEN

>-!;;zwo

Log.E

When a colour negative is printed on to a colour positive,the red filter densities control the blue-green image, the greenfilter densities, the magenta image, and the blue filter densities,the yellow image. The magenta image of the negative shouldonly produce magenta gradations in the positive, but in thecase of this dye there is appreciable blue filter density, there-


fore a yellow image will also be produced .. When the printis balanced to record greys in the subject as greys in theprint, this yellow positive image resulting from the magentanegative image will 'make a rednction in the normal yellowimage strength necessary. The net result in the print willbe that reds will generally be too orange, due to the yellowimage associated with the magenta., while yellows will beweak and greens too blue due to the reduction in the strengthof the real yellow image.

If a positive yellow image is produced together with thenegative magenta image, and of such a strength that themaximum blue-filter density of this yellow positive image isthe same as the maximum blue-filter density of the magentaimage, we will have these curves (ignoring the negligible red-filter densities) :

GREEN FILTER DENSITES

>-l-v;Zwo

Log. E

The resuIt of the negative and positive blue filter images ismerely an over all yellow density which with a suitable positivematerial of higher-than-normal blue sensitivity, is notincon venient.

The excessive blue and green absorptions of the cyandyestuff, although not as serious as the blue absorption ofthe magenta image, can be similarly counteracted by a redpositive masking image corresponding to the negative cyanimage.

The yellow image dye characteristics are normally adequatewithout correction.

Attempts to produce integral masking images by using theresidual silver halide to produce either a silver or a stain

(~


image improve colour brilliance and sometimes colour rendering,but, because of the lack of discrimination between the require-ments of the various layers, are merely compromise methods,and no real solution.

To date, the only effective methods are those in which theresidual non-diffusing colour coupler present in the individualemulsion layers is used to form the coloured masking image.Several methods of using these couplers have been proposed,but the most elegant way is to use colour couplers which arethemselves coloured.

Coloured Colour Couplers

Patents covering the use of coloured colour couplers havebeen obtained by Kodak and LC.Ls Basically, the method isto produce from the parent colour coupler a dyestuff whichis of the hue required for the masking image. In the presenceof colour developer oxidation products, the group attached tothe coupler to prepare this dye is split off, and the couplercouples in the normal way to give the dye image.

Kodak use azo dyestuff coloured couplers whilst I.e.I. havecovered the use of styryl dyestuff couplers. The followingexamples show the way in which the coloured images areformed.

I-phenyl-3-methyl-S-pyrazolone

H2C-C-CHI II 3

C N09 "N/ .

••is coupled with a p-hydroxyphenyl diazonium compound

to give the yellow azo dyestuff


HOON=N-HC-C-CHs\ 1\C N

if "N/owhich is incorporated in the emulsion and which IS splitwhen the oxidised developing agent

couples to give the magenta image dye

Red azo dyes from phenolic couplers

are similarly split at the azo linkage when they couple to giveblue-green dyes, with the oxidation products of colourdeveloping agents

122 BASIC PHOTOGRAPIDC CHEMISTRY

R):{ CH,O~N-b-NE"

The styryl dye -coloured couplers, which are applicable tomagenta couplers, are prepared by reacting the parent colourcoupler-which in this case contains a long chain alkyl groupto restrict diffusion:

with an anil of a p-substituted amino-benzaldehyde to give ayellow styryl dye


and during colour coupling the styryl grouping is rupturedwhen coupling with the oxidised developing agent occurs, togive the magenta image dye

Et2NQN=C--C-CI7H35, I \I

CH3 .s: /NO~ "N

050,NO6

Whilst these methods of incorporating the coloured couplerin the emulsion offer the advantage of normal processing,other methods of using the residual colour coupler to form amasking image can be used, and offer certain advantages,One disadvantage of the coloured coupler method is thepresence in the layers, during exposure, of unwanted dyes,which could reduce sensitivity.' Also, if the masking dyesare formed during or after normal processing, any effect whichthey may have on the unexposed emulsion or upon the latentimage, is unimportant.

The proposals which have so far been made to form maskingimages from the unused colour couplers convert the couplersto azo or styryl dyestuffs of the same type as the colouredcouplers.!

The production of a yellow styryl dye mask for the magentaimage is quite straightforward. After development the filmis treated in a solution of a suitably substituted benzaldehyde,This will react with the residual magenta coupler to give therequired yellow positive mask, but the residual cyan andyellow couplers will be unaffected.

For the production of azo masking dyes a more elaborateprocedure is required. Diazonium salts would react with allthree residual couplers if the film were merely bathed in adiazonium salt solution as in the previous method. Suitable dyeswould be produced in the lower layers but the top layer imagewould be unwanted, To overcome this the diazonium com-


pound is added to the emulsion used for the lower layers insuch a form that it does not diffuse into the top layer. Afurther requirement is that the normally unstable diazoniumcompounds must be used in a stable form.

Whilst this azo dye method offers the advantage of maskingimages in both magenta and cyan layers, it has the dis-advantage of being much more awkward in use.

It should be emphasised that although the improvement incolour rendering -by using a double masked negative material,such as Ektacolor, is great compared with an unmaskedmethod, the colour reproduction is still not perfect. Thehighest accuracy is obtainable only in a six-mask system, atevery stage which involves colour images. The doublemasked systems are, however, a very good compromisebetween high accuracy and simplicity of procedure.

REFERENCES1 Hornsby, K. M., B.J. Phot., 1950, 132.2 F.I.A.T., Final Report, No. 721.3 B.P.s, 586,211,598,657,598,174,599,377,673,091.4 Hanson, W. T., Jour. Opt Soc. Am., March, 1950.6 B.P.s, 675,797,645,170.

Chapter 11

MISCELLANEOUS PROCESSES

Print-out Emulsions

IFthe exposure which produces a latent image is continued,the microscopic silver specks which comprise the latentimage become larger and visible. This' print-out' effect

was at one time widely used for the production of contactprints. Because of the long exposures necessary-about15 minutes in bright daylight-the use of this type of materialis now very restricted. Even though the development step isunnecessary, printing out is generally considered less conveni-ent than the use of slow contact development papers, whichlikewise can be handled without elaborate darkroomequipment.

The sensitive salt used in these emulsions is normallysilver chloride, but they are prepared, not as are developmentemulsions with an excess of halide, but with an excess ofsilver salt. This accelerates the photolytic reduction of thesilver halide (but would, of course, fog a development emulsion)

2 AgCI -+ 2 Ag + CI.

or in ionic formAg+ + e -+ Ag

and2 CI- - 2e -+ CI2

Whilst commercial print-out materials are usually madewith gelatin as the binder, home-made papers usually employcolloids such as starches, dextrin, or albumen. A typicalmethod is to paste 40 gm of arrowroot in 100 cc of cold water,add 700 cc of boiling water and boil the mixture until a clearsolution is obtained. To this solution, whilst still warm, isadded

Ammonium chlorideCitric acid ..Sodium carbonate (anhyd.)Water to

30 gm3gm4 gm

200 cc125


The cold jelly is brushed on to good-quality paper and workedwell in, and when dry is sensitised with a silver solution,

Silver nitrate 160 gmCitric acid .. 45 gmWater to 1 litre

which is also brushed on using the same volume as the saltjelly.

Tbe paper is exposed to bright dayligbt (not direct sun-li.ght) behind a contrasty negative until the print is con-siderably darker than required, as after fixing (in plain hypo)some density is lost. Plain hypo-s-or even slightly alkalinesolutions-must be used as acid bypo would attack the veryfine silver image which is formed. This very fine silverdeposit also results in an unpleasantly coloured image, which~o~mally is toned prior to the fixing stage. Before toningIt IS also customary to precipitate the excess soluble silversalts by bathing in a weak sodium chloride solution.

The following procedure is typical for papers of this type:1. Expose.2. Bathe for 5-10 minutes in 1 per cent sodium chloride

solution.3. Wash for 3 minutes.4. Tone for 5 minutes by immersing in a 1: 10,000

solution of a gold or platinum salt such as goldchloride or sodium chloroplatinite.

5. Wash for 5 minutes.6. Fix for 10 minutes.7. Wash for 15 minutes.

S.ome commercial p,:pers are 'self-toning,' and the abovetoning step, and sometimes the salt bath, can be omitted.

Reversal Processing

~a.ITow-gauge motion pictures are normally processed topositives by the reversa.l method. Many colour materials arealso processed in this way. The method is applicable also tonormal materials.

When .an eXJ?osed emulsion is developed fully and thenwashed, It consists of metallic silver and silver halide; thegreater the exposure, the more silver and the less residualhalide.

MISCELLANEOUS PROCESSES 127

If the developed silver is now removed without affectingthe re.~ainin.g halide, this halide can be developed to producea positive picture :

.'O';ij;jj,),U/l/ after flm development

~;.:::::771~ii/t77

after bleaching

TTT".""''''''!Il~ after second development"17m)/~

The first developer must be very energetic to develop allof the exposed grains. A little fog developed at this stageis unimportant, and to ensure complete development, it iscustomary to include a solvent-such as a thiocyanate-inthe solution. Kodak D.168 is recommended:

D.168Metol 2 gmSodium sulpbite (anhyd.) 90 gmHydroquinone 8 gmSodium carbonate (anhyd.) 44.5 gmPotassium thiocyanate 2 gmWater to 1 litre

After washing for about 5 minutes the silver developed in thefirst developer is removed in a solution of dichromate andsulphuric acid:

Sodium dichromate 5 gmSulphuric acid (cone.) 5 gmWater to 1 litre

A further 5 minutes wash is followed by 3 minutes in a solu-tion of a reducing agent to neutralise the dichromate left inthe gelatin, and enable the silver complexes to be washed out:

Sodium metabisulpbite 20 gmWater to 1 litre

White light may be used after the bleach bath, and if theresidual halide is blackened by a normal developer, it isnecessary to fog it with fairly strong light-usually during

the wash after bleaching. Any normal M.Q. developer of the• universal' or print type may serve for the blackening stage.Alternatively 1 per cent sodium sulphide can be used to givea brown image, or 3-5 per cent sodium hydrosulphite willblacken by reducing the silver, without exposure to white light.

Blackening is usually followed by fixing to remove anyresidual silver halide, and then the usual thorough wash.

Many alternative procedures have been suggested, includ-ing, for example, hardening baths or chemical fogging solu-tions. The greatest departures occur in colour film reversalprocessing. Since in this case the silver produced duringcolour development is removed, it is customary to deferbleaching of the negative silver image, and remove all thesilver at the same time. The Ektachrome technique illustratesthis method :

EKTACHROME PROCESSING PROCEDURE (1950)First development. . 15 minWash 1

*Harden 4Wash 5Colour development 25Wash 1Harden 4Wash 5Clear 5Wash 1Bleach 10Wash 1Fix .. 5Wash 10

• Exposure to photoflood light is made during hardening.


/'

Processes Not Using Silver Salts

There have been several processes using sensitive materialsother thaIl: si.lver halides, but only two are still used apartfrom speciallised photomechanical printing applications theblueprint and diazo processes. The main use for these pro-cesses is for reproduction of drawings, and to a very limitedextent, &"eneral documeIl:t copying. The diazo processes aresuperseding the bluepnnt processes, particularly the drydevelopment types.


Blue-Print Pr~de$~es

Many of the ferric salts of organic acids are 'light-senaitive,the ferric ion acquiring all electron to reduce it to the ferrousstate

Fe+++ + e ~ Fe++

Some of these salts-the oxalates, tartrates, and citrates inparticular-have sufficient sensitivity to be used for contactprinting. In the past, many methods of 'developing '. avisible image have been used, but only one-the conversionof the ferrous ion to prussian blue by reacting with a Ierri-cyanide-vhas survived. The familiar engineer's blue print isproduced in this way. Although commercial blue - printpapers may contain a wide variety of substances to stabilisethe coating or increase the printing speed, satisfactory resultscan be obtained with home-prepared papers.

Any reasonable quality paper is sensitised by brushing witha sensitising solution such as :

Potassium ferricyanideFerric ammonium citrate ..Water to

50 gm100 gm

1 litre

and drying. Daylight or very powerful artificial light mustbe used for the exposure, which changes the ferric ammoniumcitrate to the ferrous compound. The print is ' developed'by immersing in water, when the ferrous salt reacts with thepotassium ferricyanide

3 Fe++R + 2 K3Fe (CN)G ~ Fe++3 [Fe(CN)Gh + 2 K.R

to form the pigment ferrous ferricyanide or prussian blue .Sometimes only the photosensitive ferric salt is coated on

the paper, and a ferricyanide solution is used to develop .it.In either case the print must be washed after development,but for only a short time as the prussian-blue image is decolor-ised in alkaline solutions. Tap water in some districts issufficiently alkaline to attack the image slightly.

The blue-print process will give a positive from a negative,and is occasionally used to print from ordinary negatives,but owing to the time taken in printing, and the trouble ofmaking an enlarged negative for enlargements, blue toningof bromide prints is a more convenient process,

130 BASIC PHOTOG.RAPI-lIC CHEMIST.RY

Diazonium Salt ProcessesIf a substituted aromatic amine is treated with nitrous and

hydrochloric ~cids at a temperature of 0 degs.-5 degs. C. theammo group IS converted to a diazonium chloride group:

RONH2 -+HN02+ HCI - RO~i=NIn slightly alkaline solutions the diazonium salts will reactwith phenols, and other compounds containing reactivemethylene groups, to give azo dyestuffs:

RON:ON + ¢H2 -+ RON==N-~HY y

if the substituent R in the aromatic diazonium compound isdialkylamino, the diazonium compound is light sensitive-the diazonium group being destroyed. The light decom-position product will not couple to form an azo dye.

By normal photographic standards, the diazonium com-pounds are not very sensitive, and, like the blue - printmaterials, the maximum sensitivity is in the violet and ultraviolet spectral regions. An exposure of 10-50 seconds at adistance of 1 ft from 'a high-pressure mercury vapour lampis required to produce a satisfactory print from an averageoriginal on an average paper.

As the reactive material is destroyed by exposure to lighta positive print is produced from a drawing, which is preferablymade on translucent material.

Two types of material are commercially available-thesemi-wet, and the dry developed types.

Semi-Wet Developed MaterialsThe paper is coated with a solution of the diazonium com-

pound, usually with the addition of an organi-c acid, thiourea,and zinc chloride to produce a more stable material.

A suitable recipe would be:DIAZO SENSITISING SOLUTION

p-diethylaminobenzene diazonium chlorideCitric acid ..Zinc chlorideThioureaWater to

10 gm20 gm

7 gm20 gm1 litre


. Only a very thin coating must be applied, so that the paper1~ coloured a very pale yellow. After drying in subduedlight the material is exposed, until the portions which are tobe white are completely bleached. It is then developed byapplying a small quantity of :

DMZO DEVELOPER

BoraxSodium thiosulphateSodium carbonate ..Coupling compoundWater to

20 gm40 gm20 gm10 gm1 litre

The choice of coupling compound depends on the colourof image required:

{

ce-naphthclBlue. . 2 : 3 dihydroxynaphthalene (Dinol)

phloroglucinol~-naphtholI-phenyl-3-meth yl-5-pyrazolone*aceto acetanili de *resorcinol

VioletRed ..YellowBrown

For production work it is usual to apply the small quantityof developing solution required with a small machine:

Although prints produced by this process are dry enoughto handle immediately after development, the' dry-develop-ment' method is far more convenient.

Dry Development Method

Instead of using the coupling component in the developersolution, it is coated together with the diazonium compound.From 5-10 gm of the coupler is added to 1 litre of the abovesensitising solution. After exposure the material is merely

• It is advisable to dissolve these compounds in a little warm alcohol before addingthem to the developer.


I'

exposed to moist ammonia fumes, which neutralise the acidof the coating solution, and allow the undecomposed diazoniumcompound to couple with the coupling compound,

Either' of the above methods can be used to obtain photo-graphic prints on other materials than paper--cloth, pottery,wood, and almost any absorbent material can be sensitised.Non-absorbent surfaces can be sensi ised by first coating witha thin layer of hardened gelatin; the sensitising solution isthen applied to the gelatin surface after drying.

There are other methods of using the photosensitivity ofdiazonium compounds, reviewed by Murray- : but as thesehave not been exploited to any appreciable extent they aremainly of academic interest.

The Carbon and Carbro Processes

If a soft gelatin layer coated on a transparent support istreated with a solution of a dichromate, and then exposed tostrong light through a negative placed behind the base, theareas where the light reaches the dichromated gelatin willbe hardened. If the gelatin is dyed or pigmented, the un-hardened gelatin can be washed off to leave a positive reliefimage of dyed gelatin.

An alternative method is to expose a pigmented strippingpaper through the face side, and before hot-water develop-ment to transfer it to a suitable support so that the hardenedgelatin is adjacent to the support. The hot water treatmentwill now remove both the original support and the softgelatin, to leave the hardened image on the new support.

The gelatin layers were originally pigmented with carbonblack-hence the term 'carbon' for this type of process,regardless of the nature or colour of the pigment.

The 'Carbro' process is a development of the carbonmethod in which the dichromated gelatin layer is squeegeedon to a wet, unsupercoated, bromide print. The silver imagereduces the dichromate to harden the gelatin to give the sameresult as exposure to light. After hardening during whichthe silver image is bleached, the bromide and pigmented layerare parted. The pigmented layer is fixed to a new supportand hot-water developed, as in the carbon process. Thebromide print, after washing, can be redeveloped and usedagain.


Imbibition ProcessesRelief images can be used as intermediates in the production

of dye copies by soaking the relief in a dye solution and thenpressing the stained relief in contact with a gelatin blank.The blank is usually treated to retain or 'mordant' theparticular type of dye used. Between 50 and 100 copiescan be made from one relief or' matrix' as it is usually known.

The method has very little application for monochromework, but is very widely used for the production of three-colour prints when several copies are required=-for example,Kodak Dye Transfer and the Technicolor motion pictureprocess.

The matrix material is a silver bromide emulsion, dyedyellow or red to make the image, and therefore the relief, ascompact as possible. The exposure is made through the base,and development is in a ' tanning' developer. It was statedearlier that in the absence of sodium sulphite the hydro-quinone oxidation products would tan and stain gelatin.Pyrogallol and catechol oxidation products also have thisproperty. If a developer is compounded to make the fullestuse of these imagewise hardening properties, the gelatinadjacent to the silver image is hard enough to resist treatmentin hot water, which removes the unhardened emulsion toleave the relief.

This washing stage can follow development, as fixing isunnecessary, but steps between development and etching maybe used to increase hardening.

After drying, the matrices are immersed in suitable dyesolutions, and successively squeezed into contact with atreated gelatin layer on paper or film. To receive the aciddyes which are normally used, the gelatin is treated with analuminium or chromium salt such as chrome alum or alu-minium acetate. The time for the dye to transfer varies withthe dye used, the temperature, and the condition of thegelatin blank, but about 3 minutes at 40 degs. C. should beused as a basis for experiments.

For three-colour prints, suitable dyestuffs are:Yellow-Chlorazol Brilliant Yellow 3GMagenta-Chlorazol Fast Pink BKCyan-Solway Celestol B

Further practical details of Carbon, Carbro and imbibitionprocesses are included in Making Colour Prints by J. H.Coote.


Dye-bleach Processes

The action of the finely divided silver which comprises thcnormal photographic image in accelerating the bleaching ofdyes was disclosed fifty years ago by Schinzel," who proposeda c.omplete monopack sys-tem of sensitised and dyed layersWhICh,after exposure, development, and fixation, was treatedi~ hydrogen .peroxide. In the areas where there was a heavysilver deposit, the- dye was rapidly bleached, as comparedwith the rate of bleaching in the areas where there was nosilver image. This resulted in a positive print from a positiveoriginal. However, the differential between the rates ofbleaching in the presence of silver and in the absence of silverwas too small, with the result that if the highlights werecompletely bleached, the dense areas would also be bleachedto some extent, whereas treatment which did not affect theshad~ws would not bleach the highlights completely. In thisreaction, the dyes were bleached by oxidation.

In 1?18 Christensen3• found that strong reducing agents suchas sodium hydrosulphite or stannous chloride would bleachcertain dyes only in the presence of the silver image, but nocommercial process resulted, as the variables could not at thattime be controlled.

~veral other workers ma?e half-hearted attempts to usethis type of process, but It IS to Gaspar that the credit forovercoming the practical difficulties of the process must begiven. He investigated, with remarkable thoroughness, thebleaching of dyes in the presence of silver to find dyes whichwould react with the bleaching agent in proportion to thea~oun~ of silver in the image. Then he had to find ways ofdispersing these dyes evenly in the emulsion, and finally tow~rk ou~ the .best arrangement of h~s dyed sensitised layers.H~s m<?tl(;ll1picture process was available in England in themid-thirties but was not a commercial success, possibly due tothe lack of an adequate camera material or colour camera .

.G~par found that the most suitable dyes were those con-taming the azo group -N =N- in conjunction with a reduc-ing ble~ch or a bleach containing thiourea (NH2-CS-NH2) anda red~clllg agent. Th~ azo ?roup. is ruptured by the bleachingsolution where there IS a SLIverImage, to produce two moreor less colourless compounds, one of which should be solubleand re~dHJ: removed from the emulsion so that subsequentrecombination to form the dye is prevented. It is alsoessential that the dye should not. diffuse from the emulsion


layer during coating or processing. The efficiency of thebleaching process can be increased by adding compoundssuch as azines and cinnoline to the bleaching solution.

Gaspar is at present using the process for the productionof prints on a white opaque base from transparencies, butliford, and Bauchet also, appear to regard this process as theanswer to the problem of making colour prints from trans-parencies, whilst General Aniline are also interested in it,· 6

Image Diffusion Processes

In the last few years three different modifications of thesame principle have been marketed for the prepara.tion ofdirect positives. If an exposed emulsion and a materialcontaining substances which act as efficient developmentcentres (colloidal silver or insoluble metallic sulphides forexample) are placed in intimate contact and are treated witha vigorous developer containing a high concentration of ahalide solvent such as hypo, two reactions will occur. First,the latent image in the emulsion will be developed, and thenthe undeveloped silver halide will be dissolved by thedeveloper, and will then diffuse into the layer containing thesilver precipitation centres where the silver will be depositedto produce an image of opposite character to the image in theemulsion layer.

exposed area-all silver developsunexposed area-silver dissolves anddiffuses into transfer layer

---- emulsion layer

transfer layer

dissolved silver precipitated on developmentcentres

no dissolved silver-no image in transfer layer

In processes such as ' Copyrapid ' and' Ozarapid;' a. d~u-ment paper is exposed in the normal way for reflex printmg.


It is then fed into a small developing machine face to facewith a sheet of transfer paper. The two papers pass throughthe developing solution separately and are then passedbetween two driven rubber rollers which squeeze themtogether. After about a minute the two sheets are separatedand the transfer paper contains a positive image correspond-ing to the original.- The Polaroid Land Camera uses a similar system, but thenegative paper, the transfer paper, and the developer are allcontained in the camera back. The negative paper isexposed in the normal focal plane of the camera. When thewinding knob is turned the negative paper is brought face toface with the transfer paper which carries a foil' pod' con-taining a viscous developer. The papers then pass betweentwo rollers which burst the ' pod ' to release the developerand also press the two papers together with the developerpaste between them. This also brings the papers into asecond compartment in the camera back which is opened toremove the two papers which are torn from the roll andseparated. This gives a positive print and also a papernegative from. which further copies can be made later.

The Gevaert Diaversal materials have both layers on thesame support, the transfer layer adjoining the base and theemulsion on top. After exposure and development the toplayer which contains the negative image is removed leavingthe positive image in the layer still attached to the base.

REFERENCESJ Murray, H. D .. Phot, J., 1933 (April), Supplement. p, 6.2 ts.), Phot., 1905, 52, 608. .3. B.P. 133.03.4.... .4 e.i. Phot., 1954,101,509.o B.i .Phot., 1955, 102,271.

Chapter 12

SOL UTION PREPARATION

ANY photographer with an experimental outlook. will findit necessary to prepare his own working solutions frombulk chemicals. A wide range of packed developers,

fixers, toners, and so on, is available and the busy professionalmay find that it is not much more expensive to use these thanto buy a balance and weigh out his own chemicals. Otherworkers, however, prefer the flexibility of making up theirown solutions, and as only the commoner solutions are avail-able in the packed form, it may be necessary to make upsolutions from time to time. Once the equipment has beenbought, it is then more economical to buy bulle chemicals andwork with these. As an example of the relative cost, twolitres of a stock solution of a popular M.Q. developer cost,as a packed developer, 3s. 9d., whereas the bulle chemicalsfor the same quantity of solution cost only Is. 7d., although,of course, the weighing time-about 10 minutes-and thecapital cost of the balance should not be ignored if an accuratecomparison of the economics of the two methods is considered.

Equipment

The basic essentials for solution preparation are a balanceand weights, mixing vessels, measuring vessels and storagevessels. Chemicals and sensitive materials are not cheap, soit is false economy to use low grade equipment for solutionpreparation. If you cannot afford to buy the necessaryitems, it is cheaper in the long run to use ready packedchemicals, than to risk spoiling your materials. Not that Itis necessary to buy the most elaborate equipment in thephotographic supplier's show~oom. Quite frequently adequateequipment can be boug~t 111 general hardware or surgicalshops at a much lower pnce than from laboratory furnishers.

There can be no doubt at all that the metric system shouldbe used when equipping a darkroom from scratch. Anycalculations for altering concentrations or individual con-stituents or for increasing or decreasing the total volume of

137

BASIC PHOTOGRAPHIC CHEMlSTRY138

solution to be made are very simple with the decimal system.This certainly cannot be said of the avoirdupois system withits pounds, ounces, grains, gallons and pints! A furtherconfusing point is that recipes originating in the United Stateswill use a 16 oz pint against our 20 oz one, but a litre is thesame the world over.

The Balance

The choice of balance depends on the type of work to hedone as well as on the price which can be paid. For theexperimenter, the student's type of chemical balance isdesirable-the' Microid ' supplied by Griffin and Tatlock isa good example. The capacity of this type of balance is about250 gm, and weighings to within 0.002 gm can be made. The, Duorider ' type makes weights below 1 gm unnecessary. Amore robust balance with an accuracy of 0.1 gm and a capacityof 2 kg is made by Towers, of Widnes. 'With this balance theriders can be used for weighings up to 200 gm. Only forquantities over this amount are separate weights needed.These types of balance, costing upwards of £5, are luxuriesfor the ordinary photographer.

For routine preparation of processing solutions there areseveral types of small spring and lever balances availablewhose accuracy of ±0.2-0.5 gm is adequate - for example,the typical letter scale, of which small models with a capacityof about 50 gm can be bought for a few shillings. Formaterials present in small but critical quantities-such aspotassium bromide and potassium thiocyanate in developers-a 10 per cent. stock solution can be made, and measuredout into the developer. For instance, 0.75 gm potassiumbromide would not be weighed accurately on a letter scale,but if 10 gm is weighed out and dissolved in 100 cc of water,the requisite 7.5 cc of solution can be measured with sufficientaccuracy. Another method of increasing the effectiveaccuracy of a balance is to make up large quantities of solu-tions which will keep. To make up 250 cc of the workingsolution of D.163 the following quantities must be weighed:

MetolSodium sulphite ..HydroquinoneSodium carbonatePotassium bromide

0.14 gm4.7 gm1.06 gm4.06 gm0.18 gm

SOLUTION PREPARATION 139

for which an accurate balance is needed. If two litres ofthe stock solution are made, the quantities will be :

Metol 4.4 gmSodium sulphite .. 140 gmHydroquinone 34 gmSodium carbonate 120 gmPotassium bromide 5.6 gm

-quantities which can be weighed with sufficient accuracyon a simple balance. For routine work an accuracy of± 10 per cent. in chemical quantities is adequate for printdevelopers and toning solutions; negative and colourdevelopers should if possible be prepared with errors notgreater than ±5 per cent.

The hypo concentration in fixing baths can vary ±20 percent. without harm, and can be measured by volume if thebalance will not accommodate the large weights of hypousually used. If the bath is used in a process which is carriedto completion-i-e.g., fixing or bleaching-a-the accuracy ofpreparation is usually less critical than for processes whichare stopped before completion-negative development forexample.

Mixing VesselsLaboratory beakers and flasks are the most convenient of

mixing vessels, but are too fragile for darkroom use. Themore rugged glass measuring jugs supplied for photographicand culinary use are almost as convenient, and will outlastthe more fragile beakers. Stoneware' acid jugs' are alsoexcellent for making up solutions. but as with other opaquevessels, dirt or chemical contamination may not be noticed.When stoneware vessels are used it is advisable to avoidusing the same vessel for different solutions. as minute cracksin the glaze can result in contamination. Plastic vessels arenow readily available. and are both unbreakable and easilycleaned. They do. however. become stained with somesolutions. and although safe, look dirty. Polythene vessels,bottles, funnels and so on are strongly recommended.

Stirring rods should be of glass or plastic. A small pieceof surgical rubber tubing over the end of a glass rod willprevent damage to the mixing vessel.

Pressed or welded stainless steel vessels are excellent.though expensive. Some cheap equipment is soft solderedand must be avoided.

For large quantities, mechanical stirrers save much of the

140 BASIC PHOTOGRAPHIC CHEMISTRY SOLUTION PREPARATION 141

labour in mixing, but in choosing and using a stirrer, it shouldbe remembered that the purpose is to agitate the liquid todissolve the solid. Stirring which is too vigorous, or whichis continued longer than necessary aerates the solution, andthis may spoil it.

The writer uses a simple stirrer from a surplus rim-drivegramophone motor and as: I reduction gear which cost about22s. Od. together. In conjunction with a 2 in plastic pro-peller made from odd scraps of Perspex, volumes up to fourlitres can be stirred very effectively.

Measuring VesselsIf graduated jugs are used the mixing vessel is also the

measuring vessel. Otherwise the same remarks as those madeabout mixing vessels apply. Laboratory measuring cylinders,like other laboratory glassware, except in the small sizes, areout of place in darkrooms. Storage bottles marked with adiamond or a file at the appropriate point are quite convenient.

Storage of SolutionsThe most convenient way of storing solutions is in glass

bottles. Half-gallon bottles or winchesters are the largest sizewhich are convenient to handle, and can be easily obtained.Aspirators, in glass or stoneware, are more convenient forlarger quantities, as they have a tap at the bottom for drawingoff solutions. The smaller ones can be kept on a shelf overthe workbench, where they are most accessible.

Rubber stoppers are the best to use with glass bottles;glass stoppers frequently stick, plastic caps break, and metalones are dangerous.

Another point to remember is that fine grain developers andother pH sensitive solutions should not be stored in bottleswhich have recently contained strong alkalis or acids. Wash-ing is not enough to remove these solutions. If in doubtleave a ' dummy' developer or a 2 per cent. sulphite solutionin the bottle for a week, then wash and use.

WaterThe majority of municipal water supplies in this country

are suitable for photographic use. Dirt can easily be removedwith a small filter-the' Steralic ' for example .

.Dissolved salts and .gases present a different problem.High oxygen content can be cured by boiling, and then coolingbefore using for solutions. This will also reduce the hardness

to some extent. Calgon-sodium hexametaphosphate-willprevent calcium deposits from developers and other alkalinesolutions if added to the water before any of the developerconstituents. I gm per litre is sufficient-even for very hardwater.

If any substances are present which are harmful, andcannot be removed by boiling or by calgon treatment, theonly course is to use distilled water. If tap water is suspect,test it by making two batches of the solution in question-one with tap water and the other with distilled. If there isno difference in performance, the tap water can be usedsafely. If there is a difference, distilled water should beused. It is advisable to buy distilled water from a chemist'sshop, as some so-called ' distilled water' from other sourcesis far dirtier than tap water.

Thermometers

Although not strictly under the heading of solutionpreparation, a few notes on thermometers may be useful.Small wide-range ones are almost useless as they cannot beread accurately. The best type would have a range of 15-25° C spread over two inches, but the standard size laboratorythermometer if used with care can be accurate enough forgeneral work. Mercury thermometers are the most reliablebut are banned from some photographic laboratories becauseof the possibly adverse effect of mercury or its vapour onphotographic materials in the event of breakage. Spiritthermometers are quite safe in this respect, but if used above50° C (in emulsion making for example) the spirit oftenevaporates and forms an additional clear section at the highend of the scale, which therefore results in a low reading.The best answer to the problem is to use spirit thermometersin the darkroom and check them weekly against a mercurythermometer, but this is rather an elaborate procedure forany but large establishments. Finally, users of deep tankswill find the floating thermometers invaluable. They aresomewhat like fishermen's floats, and avoid losses of thermo-meter and solution if the thermometer slips out of theoperator's fingers into the tank.

AppendL"C

COMMON RADICALSMonovalent Radicals

aceto -CO.CH, methyl -CH,

aldehyde -CHO c6allyl -C,H~ «-naphthylamino -NH2

amyl -C.Hll CD-« ~-naphthylbenzoyl

nitro -N02

benzyl-CH2O

nitroso -N=O

<:phenoxy

00biphenyl -0phenylbutyl -C~Ho

carboxylic propyl -C,H7acid -CO.OH

Cyanide} -C=N/CH,

nitrile iso propyl -CH"CH,

ethoxy -OC2H. stearyl -Cl1H,.ethyl -C2H.

hydrazine -NH-NH, styryl -CH:CHOhydroxy -OH

mercapto -SH sulpho Isulphonlc J -SOsH

methoxy -OCH, acid143

144

amido

azo

benzal


Bivalent Radicals

-NH- 6-N-=N-o-phenylene

•H .o m-Ph"y,,,,6

ether

carbonamido _-CO.NH-

-0-p-phenylene Q

hydrazo

keto "\carbonyl f

methylene

benzo

-NH-NH-

-co- sulphone -502-

thio -5-

vinyl -CH=CH-

Acetoacetanilide, 102Acetoacetic ester, 102Acetonitriles, 103Acidity, 13Acids, 14

Acid salts, 14Agfacolor, 113Alicyclic compounds, 18Aliphatic compounds, 18Alkali in developer, 51Alkaline salts, 14Alkalinity, 13Amidol, 41, 46, 50, 62o-aminoaniline, 39p-aminoaniline, 392-amino-5-diethylamino

toluene, 98o-aminophenol, 39p-arninophenol, 39, 46, 64Ansco Color, 114Antifoggants, 56, 63Aromatic compounds, 18Autoxidation, 51Azo group, 134

Balances, 138Bases, 14Benzene, 18Ben zotriazole, 63Benzoxazole, 32

INDEX

-CH2-

Trivalent Radicals

Io methineH

-C-I

Benzoylacetanilide, 102

Benzoylacetic ester, 102Benzthiazole, 32Benzyl alcohol, 108Bleaching, 107Blue-print materials, 129Borax, 51, 60Bottles, 140Bromhydroquinone, 42Bromide, 55

Capitol developer, 68

Carbon process, 132Carbro process, 132Chlorhydroquinone, 41, 62Colloids, 23Colour-

accuracy, 115couplers, 99

coloured, 120non-diffusing, 113

development, 96, 104, 106,111, 112, 121, 128

masking, 117processing, 107, 111, 112, 128

Contrast, 26, 28Copyrapid, 135Crystal lattice, 25Cyanacetanilides, 103Cyanide, 71Cyanines, 31

unsymmetrical, 33145

146

Cyclic compounds, 18Cyclohexanone, 51

Developers-colour, 97concentrated, 64fine grain, 45, 46,.50, 59general purpose, 58glycin,46hydroquinone-caustic, 57high contrast, 58metol,61M.Q., 56-60negative, 58, 59physical, 37, 65process, 45, 57pyro, 45, 64, 133staining, 64tanning, 133viscous, 136

Developing agents, 37inorganic, 38colour, 97

Development, 49centres, 26, 37colour, 96,104,106,111,112,

121, 128effect of pH on, 50forced, 66physical, 65reactions of, 52, 104, 106, 121reversal, Ill, 112, 127, 128tanning, 133two bath, 64water bath, 64

Diaminophenol, 41Diaversal, 136Diazoniurn salts, 130


Dibromodih ydroxynaphthalene:10)

Dichlorhydroquinone, 41Diethylaminoaniline, 43, 97Digestion, 28Dihydroxybiphenyl, 101Dinol, 131Diogen,45Dissociation, 12Dye-bleach process, 134Dye-line materials, 130

developers, 131development, 131dry development, 132semi-wet, 130

Dyestuffs-acid, 133azamethine, 105azine, 106, 135azo, 120, 134cyanine,31indophenol, !O5sensitizing, 31, 36styryl, 122vat, 96

Dye Transfer, 133

Eikonogen, 45Elements, IIEmulsion, 23

preparation, 27stabilisers, 29unwashed, 27

Ektachrome, 128Ektacolor, 124Eosin, 31Erythrosin,31Esterification, 22

INDEX

Ferraniacolor, 114Ferrous oxalate developer, 38Fine grain developers, 45, 46, 50,

59Fixation, 71

before development, 65reactions, 72two-bath, 73

Fixing baths-acid,74for colour materials, 107hardening, 75plain, 73

Fog, 28, 35, 55, 63

147

Gasparcolor, 134Gelatin, .23, 24, 27, 54Genochrome, 98Gevacolor, 114Glycin, 43, 46, 62Gradation, 26Groups, 19, 142

p-Hydroxyethyl ethyl amino-aniline, 43, 98

Hydroxylamine, 51Hydroxyl groups, 39, 41, 43Hypo, 37, 71, 135

eliminators, 77

Ilford colour, 112prints, 135

Image diffusion, 135Imbibition printing, 133Indigo, 96Indoxyl,96Intensification, 78

chromium, 78colour development, 81effect on characteristic

curve, 80lead,81mercury, 79quinone-thiosulphate, 82silver, 82sulphide, 79uranium, 81

Ions, 12developer, 54silver, 25, 53, 55, 72

Kendall's rule, 40Kodachrome, 111Kryptocyanine, 33

Latent image, 25Lead intensifier, 81Lepidine, 32

Hardeners, 28, 70, 75chrome alum, 70colour processing, 70, 107emulsion, 28.fixing baths, 75formalin, 29glyoxol,29stop bath, 70

Heterocyclic compounds, 21, 29,32, 47

Hydrolysis, 22Hydroquinonc, 39, 45, 52, 53,

57,60,133

148

Masking, 117 .coloured couplers, J.20

Matrix, 133Measuring vessels, 140Mercury intensifier, 79Mercaptobenzthiazole, 56Meritol, 47Merocyanines, 33Methylaminophenols, 42Metol, 42, 45, 50, 61Microdol, 47Microphen, 47Mixing vessels, 139Mordants, 92

copper ferrocyanide, 95imbibition, 133Miller, 92Traube, 92

Naphthol couplers, 100Nitrobenzimidazole, 56

Oil formers, 115Organic compounds, 17Oxacarbocyanines, 33Oxidation, 16

developer, 51prevention of, 51, 57

Ozarapid, 135

Pentacarbocyanines, 33pH,13

effect on development, 50


Phenidone, 47, 63Phenols, 99 .

o-phenylene diarniue, 39, 62p-phenylene diamine, 39, 46, 62Pigment processes, 132Pin cyanol, 33

Polaroid Land camera, 136Potassium thiocyanate, 60, 108Precipitation, 16

of emulsion, 27Preservative, 51, 57Print-out materials, 125Promicrol, 47Pseudocyanine, 33Pyrazolones, 100, 103Pyrocatechol, 39, 133Pyrogallol (pyre), 37, 41, 45, 63,

97, 133

Quinaldine, 32Quinone, 52

thiosulphate intensifier, 82

Radicals, 19, 142Reducers-

cerium, 83dichromate, 83permanganate,83persulphate, 83

Reduction (Chemical), 16, 49.Reduction (photographic), 83Reversal, 126

processing, 127colour materials, 110, lIS, 128

Ripening emulsions, 27

INDEX 149Saturated compounds, 18

Sensitivity, 26, 28optical,35

specks, 25

Sensitizers, 29chemical, 29dye, 29, 31precious metal, 29

Silver-bromide, 27, 52, 53, 54chloride, 27,halide crystal, 25intensifier, 82iodide, 38, 71ions, 25, 53, 55, 72nitrate, 27

solvents, halide, 55, 108sulphide, 29, 85

Sodium biborate, 51, 60carbonate, 51, 60formaldehyde sulphoxylate,51hydrosulphite, 38, 97hydroxide, 51metaborate, 51phosphate, 51sulphite, 37, 51, 133thiosulphate,

Solu tion, 12

preparation, 137

Spreading agents, 29

Stabilisers, 29

Stirrers, 139mechanical, 140

. Stop bath, 69, 107

Storing solutions, 140

Structural formula, 17

Sulphonates, 53

Su persensitisers, 36

Symbols, 11

Technicolor, 133

Temperature coefficient, 42

Thermometers, 141

Thiacarbocyanines, 33

Thioindigo, 96

Thioindoxyl, 96

Thiourea, 86

Toning, 85

blue, 89, 92copper, 89dye, 92hypo-alum, 87iron, 89metallic, 89nickel, 89red, 89, 92selenium, 88sulphide, 85

direct, 87thioantimony, 86thiourea, 86yellow, 89, 91

Tripacks-principle of, 109separate, 109stripping, 110transparency, 110

ISO BASIC PHOTOGRAPHIC CHEMISTRY

Unsaturated compounds, 18 Washing, 76emulsions, 27

'Vater supplies, 140filtering, !40testing, 141

Valency, 12

Vanadium toner, 09

NOTES IS!

158 NOTES

• ©~@pb~.~, "..........

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