Amino Acids as Additives in Copper Electrodeposition

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MINO CIDS IN COPPER DEPOSITION

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MINOACIDSASADDITIVES INCOPPERELECTRODEPOSITION

by

Robert James Gale

A thesis submitted to the Faculty ofGraduate Studies and Research inprtilfulfilment of the requirementsfor the degree of Doctor of Philosophy.

From the Physical Chemistry Laboratoryunder the supervision of Professor

.C.A.Winkler.

McG ill UniversityMontreal Canada August 1972

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@ Robert JamesGale 973


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Department of R.J.GaleChemistry

AMINO ACIDS AS ADDITIVES IN COPPERELECTRODEPOSITION

ABSTRACT

The addition agent behaviours of cystine andmethionine were studied by radioisotopic and galvanostaticoverpotential measurements. There appeared to be majordiscrepancies between the extents of surface coverages

Ph.D.

estimated from overpotential increments by simple site-blockingtheory and from occlusion rates determined by radioisotopictechniques. The mean rate of adsorption of methioninecorresponded closely to the mean rates of occlusion duringelectrodeposition in the c.d. range 0.25 - 3 A dm 2 Therewas a maximum in i t s occlusion rate t 5 A dm 2 Exchangereactions in the absence and presence of oxygen indicatedCu I) complexes to be the major species adsorbed. The ratesof occlusion of cystine indicated a rapid, reversiblereduction to cysteine and i t s subsequent occlusion as Cu(I)complex. Morphologies of the deposits for various conditionsare described and a mechanism suggested for init iation ofpyramidal deposition.

Cu II) and Cu I) complexes of sorne related amino acidswere prepared and their characterizations attempted, with aview to assessing the likelihood that they might function aseffective addition agents.

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ACKNOWLEDGEMENTS

l am pleased to acknowledge the award of ademonstratorship by McGill University during 1968 andthe award of Scholarships during the period 1969-1972by the National Research COuTIcil of Canada.

l m also grateful to Dr.G.Donnay CrystallographyDepartment, McGill University) for the use of faci l i t iesand for considerable personal assistance, that enabledx-ray diffraction experiments to be made with cupricpenicillamine disulphide complexes: and to Dr.R.St.J.Manleyof the Pulp and Paper Inst i tute , McGill University, whovery kindly provided the electron microscope photographs.l also acknowledge gratefully the assistance of Mr.J.Klingerduring the acid-base studies. Finally, l m indebted toJulia Mary my wife, for typing the manuscript of thisthesis.

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TABLE OF CONTENTS

PAGEABSTRACT iACmOWLEDGEMENTS

INTRODUCTION 1

LEVELING ND BRIGHTENING A C T I O N S 5ADSORPTION IN ELECTRODEPOSITION POLARISATION IN THE PRESENCE OF ADDITIVES 2MORPHOLOGY ND ELECTROCRYSTALLISATION THEORIES 26

EXPERIMENTAL ND R E S U L T S 34

PRELIMINARY E XPE R lM E NT S 34Stabi1it ies of cystine and methionine 34Degradation in storage 35

ADSORPTION ND OCCLUSION OF CYSTINE NDMETHIONINE ON C O P P E R 36

Apparatus and method 37Adsorption of methionine on copper 46Adsorption of cystine on copper 58

1 Radioisotopic estimations 582 Colorimetrie estimations 6

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PAGEOcclusion of cystine and methionine incopper c a t h o d e s 64

COMPLEXES OF COPPER IONS WITH AMINO ACIDS 76Cupric-amino acid complexes 77

1. Experimental method 82 Re su l t s 8

i)Acid dissociation constantsKa3 and Ka4 . . . . . . . . . . . . . . 83

ii)Complex f o r m a t i o n 88iii)Elemental analyses 90iv)Infrared spectra of complexes 95v)Cu II) complexes of penicillaminedisulphide 105

Cuprous amino acid complexes 1081. Preparation of complexes 1122. Elemental analyses 112

POLARISATION S T U D I E S 113Apparatus and method 114Results . c 117

POLAROGRAPHY OF C y S T I N E 123MORPHOLOGIES OF ELECTRODEPOSITS 127

Experimental observations 127

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PAGEDISCUSSIO \I. . . . . . . . . . . . . . . . . . 34

MORPHOLOGIES OF COPPER DEPOSITS 135ADSORPTION OF CYSTINE ND METHIONINE ON COPPER 139

Pr.oposed mechanism for adsorption 139OCCLUSION OF CYSTINE ND METHIONINE ON COPPER 144

Proposed mechanism of occlusion 144RELATION BETWEEN ~ ~ S U R E ND CALCULATED EXTENTSOF COVERAGE BY CYSTINE ND METHIONINE 150

APPENDIX l FORTRAN IV COMPUTER PROGR MME FOR pKaTITRATIONS. . . . . . . 56

APPENDIX I I TREATMENT OF WILSON S DISEASE WITHPENICILLAMINE. 157

APPENDIX I I I LlNEAR DIFFUSION TO A PLANAR ELECTRODE 159

CONTRIBUTION TO KNOWLEDGE 162

BIBLIOGRA PHY . . . . . . 67

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INTRODU TION

At present, the deposition of copper from cupricsulphate-sulphuric acid baths is widely used for electro-plating, electroforming, electrowinning and refining, andfor the manufacture of copper powder. Historically,these applications resulted from a number of classicalinvestigations of copper deposition, including those ofGrotthus in 1806, and of Faraday in 1832, the i r s tcommercial electroplating operations by Bessemer, De la Rue,and the Elkingtons in the 1830s (1,2), Jacobi's electrotyping patent of 1840 (3), and the commencement of electrochemical copper refining in 1869 by James Elkington.About the year 1845, i t was observed that bright copperelectrodeposits were obtained 4) whenever a large numberof wax moulds, which had been phosphorized with a solutionof phosphorus in carbon bisulphide, were put into acidcopper baths. Soon, addition agents or additives were addedpurposely to the electrodeposition baths to improve thesurface smoothness, the brightness, the hardness, and thegrain size of deposits, or to increase the limiting currentdensity and to reduce dendritic formations.

Usually, the choice of suitable addition agentshas been made empirically. Many hundreds of addition agents

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have owbeen studied, patented, andapplied to thedepositionof copper. Originally, colloidalmaterialswerefound tooffer the greatest promise as addition agents4,5) and the reducingpropertiesof the additive 6), or

i t s bili tyto formcomplexes with themetal ionswereregarded asmainly responsible for i t s beneficial effects.Theseearlyattempts toestablishprecise classificationofadditives in terms ofmolecular properties suchas structureandfunctional groups were invariablyunsuccessful 7).Discordant effects , often revealedbysubstanceswithslightstructural differences, support the notion thathighly selective interactionsare involved in theseprocesses. This isi l lustr tedbythe relative largedifferences displayedby similar amine acids in theirb ili t iesto increase the cathodicpolarisation 8,9), or

tomodify internaI stressesof deposits 10). Thedata tobe presented in th isthesiswill provide further evidencefor the highdegree of specificitytobe encounte red inthebehaviourof sulphur-containing amineacids as additionagents. In general, each system andadditive has tobeevaluated on an individualbasis. There iss t i l lno simpletheory that cancorrelate stisfctorilythe remarkablespecificitiesand addition agent bili t iesof certainorganic compoundswith theirmolecular structures. Indeed,t iscurrentlyacknow ledgedthat relatively l i t t le i s


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understood of the way in which addition agents may beinvo1ved in the consecutive steps during the e1ectro-crysta11isation of meta1s at sol id e1ectrodes 11,12).This is hard1y surprising, perhaps, whn i t is rea1izedthat even the basic aspects of e1ectrodeposition mechanismsin the absence of additives are controversia1. For examp1ethere is yet no unequivoca1 reso1ution of such prob1erns asthe relative extents to which deposition of reduced meta1ions occurs at the growth si tes from either the solutionphase direct1y or by diffusion as ad-atoms on the crystalplane surfaces 13-18). Simi1ar1y, 1it t1e progress hasbeen made in understanding the factors that govern themorpho1ogy of crysta1s formed by e1ectrocrysta11isation.

Abso1ute quanti tat ive measures for a11 of themanifold physica1 properties of the e1ectrodeposit, andperhaps for those of the e1ectro1yte too, wou d be requiredto assess fu11y the efficacy of a compound as an additionagent. Together with the range of operating conditions,possibi1it ies of chemica1 changes that may occur and effectsdue to trace impurities that may be present, these totalto a formidable set of variables. According1y, the mostconvenient way to categorize addition agents is byreference to their prime function in operation. The majorindustria1 uses of addition agents in copper e1ectrop1atingare for 1eve1ing and brightening e1ectrodeposits, and

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theories to account for these effects have been deve1oped,as out1ined 1ater. I t is not possible within the 1imitedscope of this thesis, to survey a11 the practica1 andtheoretica1 aspects pertaining to the use of additionagents in acid copper deposition, but the reader may bereferred to excellent review art ic les by Kardos and Fou1ke

19) and Krug1ikov 20), and to the many referencesmentioned therein, which t reat the definit ions,terminologies, and effects of additives on 1eve1ing andbrightening. Theories to account for e1ectrocrysta11isationand for the behaviour of additives inhibitors) uponmorphologies of e1ectrodeposits are to be found in thetextbooks of Raub and Mller 12), Bockris and Razumney

14), and Fischer 21).Lit t le is known regarding the dependence of addition

agent action on parameters such as current density, bu1kconcentration of additive, temperature and p of thee1ectro1yte, etc . because few quanti tat ive determinationshave been made of actua1 surface concentrations of additivesduring e1ectrodeposition. Extreme1y low concentrations ofaddition agents are often sufficient to achieve themodification s) that is required to the deposit or to thedeposition process and, therefore, the concentrations aredifficu1t to measure. In part, this thesis seeks toprovide ana1ytica1 data, from which estimates might be

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made of the rates and extents of adsorption of sorne aminoacid addition agents at copper electrodes in acid coppersulphate electrolytes. Both open-circuit and electro-deposition conditions were employed. For particularsystems, the data are applied to examine effects producedby these addition agents on cathodic polarisat ions and ondeposit morphologies, in terms of the appropriate theoriesthat were available.

LEVELING ND BRIGHTENING CTIONS

Leveling and brightening phenomena are fundamentallyassociated ,th those parameters which determine thecurrent distribution and the processes of crystal growth onan electrode. This is because, broadly speaking, thecurrent distribution a t the surface will control the localgrowth rates of the electrodeposit . The current distributionwhich would result i f polarisation were considered to beabsent a t a metallic electrode s known as the primarycurrent distribution {PCD . If the counter-electrode isa t a distance such that the equipotential l ines of theelectric field are able to become independent of the shapeof the electrode prof i le , the PCD will depend directly onthe profile shape {19,20,22 . Dirichlet boundary conditionsthen apply and the local current, i in the solution near

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the electrode is determined by

where X is the electrolyt ic conductivity and I d ~ d n l isthe magnitude of the local potential gradient n is theoutwardly directed normal to the equipotential surfacet the given point). The PCD is identical on different

sized electrodes having a specifie geometrical configuration.Depending on the experimental conditions, the

heterogeneous electrodeposition reaction must produce t anelectrode, to varying degrees, overpotentials due to chargetransfer, diffusion, reaction, and crystallisation, as weIlas resistance polarisat ion 23). As a consequence, theactual current distribution that prevails, called thesecondary current distribution SCD), becomes a funct on ofthe nature of the cathodic polarisat ion and the speciesdischarged, the magnitude of the current density, the scaleof the roughness profile, and whether or not additives arepresent. Variations in local current efficiencies andnon-uniformities arising from polycrystall inity may alsoinfluence the SCD. The former is particularly importantduring the deposition of nickel, owing to the concurrentevolution of hydrogen; i t is of relatively l i t t l esignificance during the deposition of copper. If the

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sizesof individualcrystalscanbe neglectedre lative tootherroughness factors, anexperimental levelingrateofa saw-toothgroovemay be determined incompar isonwithaso-called natural or geometrical levelingra te toobtaina figure for themicro-throwingpower ofa particularelectrolyte. The termgeometrical leveling s used todescribe the levelingobtained from a uniformcurrentdensitydistribution.

D Ehl

Inthediagram, a uniformdepositofthicknessx coversthesurfaces, BandDR andwalls, C and CD ofa groovesection,ABCDE (thetreatment neglectseffectsa t thesharpedgesB C and0). The depositthickness,Y atthe

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bottom of the groove is re1ated to the thickness x by

y = x/sin 62

Consequent1y, a groove of depth h will disappear completelywhen the deposit thickness becomes X l i f

yI = Xl hi e X l = h/[ 1/sin6) - 1]2

True leveling is said to occur when the current densityeffective in the groove, BCD, is greater than on thesurface edges, AB and DE. The micro-throwing power, P,may be defined by the expression

P = yI sin 6X 2

For P>l, t rue leveling will occur. However, deviationsfrom the theoretica1 1eveling expressions are commonplace.Attempts to measure true leve1ing are discussed in depthby Kruglikov 20).

Two theories have been advanced to explain theaction of leve1ing additives: i) Adsorptiontheory,according to which the additive is preferential1y adsorbedon micropeaks of a rough cathode, so that metal deposition

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is inhibited there, and the current density, hence rate ofdeposition, becomes relatively higher in the microrecesses.The general theoretical basis for adsorption of organiccompounds a t electrodes can be found in the textbooks ofConway (24), Gileadi (25), and Damaskin Petri i , andBatrakov (26). Beacom and Riley (27), and others (19),have provided evidence for adsorption a t preferred si tesusing radioisotopic techniques. For example nickel foi lswere plated to thicknesses in the range 0.0005-0.0025 in.on grooved cathodes (depth, h = 0.030in., included angleof groove, e = 90 , from a Watts type nickel plating bathcontaining the radioactive leveler, sodium al lylsulphonate- 5S (27). When autoradiographie film was thenexposed to the flattened foi ls , darker bands appeared onthe autoradiograms in places corresponding to the originallymore elevated ridges of the cathode profile. (ii) Diffusiontheory, based on the observation that leveling additivesare often occluded and consumed fair ly rapidly a t thecathode. The SCD necessary for leveling is thought toarise from non-uniformities in the thickness and compositionof the diffusion layer immediately adjacent to the cathode.The rate of consumption of additive is supposedly greatera t th n sections of the diffusion layer i e at peakson the surface, and diffusion of additive from micro-recesses towards regions of lower concentration a t micro-

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peaks results in a relatively higher seD hence moredeposition, in the microrecesses. Inhibition by adsorbedadditive a t the micropeaks may complement this mechanism.

I t is weIl established that the leveling and thebrightening actions of additives are not correlative.Often applied in conjunction with leveling agents,brighteners usually decrease the degree of leveling.Effective copper brighteners e.g. thiourea, sulphonatedacetylthiourea, acetylcyanamide etc . increase the specularreflectance of the surface by reducing the sub-microscopici rregulari t ies on the crystal planes. A quantitativegeometrical condition for surface brightness is 28)

a = 2H cos ewhere is the wavelength of the l ight incident on thesurface at angle e to the normal, a a constant ~ O . l andH is the height of the surface protrusions. For a mirrorl ike brightness, the protrusions should be about 1/20 thewavelength of l ight i .e . for = 500 nM H 25 nM.

Randomization of crystal growth and grain refinementdecrease of crystal size) are thought to be contributory

factors to brightening action. Grain refining has beenattributed to increased nucleation on the mid-planes ofcrystallographic faces, brought about when inhibitors are

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adsorbed on the edges, kinks, grain boundaries, etc Highly specific mechanisms, invoking current-density-sensit ive, si te-sensi t ive, or shape-sensitive adsorption

29), seem most probable when strong brightening occurswith additives present in low concentrations IO-SM).Diffusional disparit ies, as postulated in leveling action,could be responsible for the action of those brightenersthat are used in much higher concentrations e.g. thiourea,coumarin) .

DSORPTION IN ELECTRODEPOSITION

Few quantitative determinations have been made ofthe adsorption of organic substances during electrodeposition

19,26). Highly sensitive analytical techniques arerequired since extremely low concentrations of additivesare able to cause profound changes in the electro-crystal l isat ion processes. For example, Bockris et al 30)have shown that bulk concentrations of as l i t t l e as10-9 M n-decylamine were able to modify the morphologicalgrowth forms on a copper single crystal a t a deposition

-2current density of S m cm . Ideally, the surfacecoverages of a single species should be measured duringelectrodeposition. In practice, data have to be interpretedcarefully for, as Kardos and Foulke caution 19), the

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\ additive m y be electrolyt ical ly and/or chemicallydecomposed into two or more products with differentadsorptivit ies. Gileadi, Duic, and Bockris 31) haveemphasized the usefulness of electrochemical andradioisotopic methods in combination to invest igate thesurface coverages of species adsorbed t electrodes.Radioisotopic analyt ical techniques are invaluable inaddition agent studies because of their high sensitivityand non-interference with the electrodeposition conditions.Certain electrochemical methods are sufficiently sensi t iveto determine fract ional monolayer quanti t ies of adsorbedcompounds, but they often involve electrolytic reductionof the additive. In the presence of excess metal ions,such as are found in electroplating solutions, i t m y notbe possible to distinguish from the total reductionreactions that occur the faradaic yield due to reductionof the additive. I f decomposition of an additive issuspected, double radioactive tagging with two-channelcounting e.g. l4C and 3H) can sometimes be used to tracethe ultimate disposition of various fragments of theadditive molecule.The electrocapil lary method applied to the adsorptionof neutral additives, t ideally polarised l iquid metalelectrodes such as mercury, provides the thermodynamicallysoundest approach to studying adsorption t electrode-

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solution interfaces 24,25,26). At constant temperatureand pressure, the Gibbs-Lippmann equation has the forro

dcr = -qmdE - I r. 1 d ~ 1i ~where dcr is the change in surface tension, r. 1 is thesurface excess quantity of component i in the liquid phase,

d ~ . 1 is the change in the chemical electrochemical)potential of component i in the liquid phase, qm is thesurface excess charge on the metal electrode, and E isthe electrode potential . From the fundamental electro-capil lary equation, i t follows immediately that

=

= ~ ]E ~ i T P = CThe electrode potential a t which ocr/aE) T P = 0 is the. ,potential of zero charge PZC), sometimes denoted Ez. ThePZ will be a function of the nature of the metal and ofany adsorption of specifical ly adsorbed ions and/ororiented additive dipoles. The metal-solution potentialdifference is not necessarily zero a t the PZ when q = 0mon account of adsorbed solvent or additive dipoles,chemisorbed ions, and other factors.

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The second derivative of the interfacial tensionwith respect to the electrode potential is called thedifferential capacity, C. I t is found experimentally thatneutral additive molecules reduce the surface tension mosta t the peaks of the IIbell-shaped electrocapil lary curvescr versus E i e at potentials a t or near the PZC. The

maximum adsorption is often sl ight ly negative to the PZC,but depends on the nature of the additive molecule. Onmercury, for example aliphatic molecules have theiradsorption maxima at potential values negative to the PZC,while aromatic molecules have their adsorption maxima onthe positive side. The PZC was displaced to positivevalues by adsorption of aliphatics and to negative valuesbyaromatics 25).

At sol d electrodes, the adsorption behaviour ofan organic compound also can be influenced by the metallographic history and the surface heterogeneity of themetal. If the electrodes are non-polarized, the surfacecoverages may be modified by competitive adsorption ofreaction intermediates, gases, water molecules, or ions.Of course, the capil lary electrometer cannot be used tostudy the adsorption of organic additives a t solidelectrodes and, generally, the number and precision ofexperimental techniques that are available for such studiesare considerably restricted. The major experimental methods

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with their l imitations are described in references 24)and 26).

For the present study, the simplest radioisotopicprocedure was chosen to t ry to establish the nature, thequantit ies, and the behaviour of species that are adsorbedon polycrystalline copper foi ls , or which affect thecathodic reactions during copper deposition from an acidcopper sulphate electrolyte with added amino acids. Touse this method, the electrode must be placed in anelectrolyte with a radioactive additive, withdrawn a t anappropriate time, and washed to remove residual electrolyte.n removal of the electrode the adsorption conditions are

disturbed and when the electrode is washed, traces of theadsorbate may be lost . The precision of this technique istherefore limited 26). However, i t was considered to beadequate for the purposes intended. Generally, fewerassumptions are needed for interpretat ion of data soobtained than for those from measurements of the usualelectrochemical parameters.

In sorne respects, gelatin would have been aninterest ing addition agent with which to make the presentstudies. t was one of the ear l iest additives used toharden copper electrodeposits and i t s behaviour has beenstudied perhaps more extensively than that of any otheraddition agent e.g. 32-40). In recent years, however,

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t has come into disfavour for practical applicationsbecause t s degradation products cause brit t leness of thedeposits 1,41). Moreover, t is not readily availablein radioactive form. On the other hand, cystine andmethionine have been shown to behave as cathode polarizingagents during copper electrodeposition in much the sameway as gelat in 42), and cystine, a t least , has foundpractical application as a nickel brightener 43), whilet s reduction product, cysteine, has been used in the

electrodeposition of rhenium 44), and manganese andmanganese-molybdenum alloys 45). Because of theiravailability with various radioisotopes and their relativelysimple molecular structures, cystine and methionine werechosen for much of the present work.

While no studies of the adsorption of cystine ormethionine at copper electrodes were found in the chemicall i terature, pradac and Koryta 46) have measured thecoverage-time curves of cystine- 5S bulk concentration10-4 M in N H2S04) on pre-heated platinum and goldelectrodes. Their investigations, by radioisotope andcyclic voltammetry techniques, were instigated becauseprevious attempts by others to establish a standardreduction-oxidation potential for the cystinejcysteinecouple a t these solid electrodes had been unsuccessful.They found that the extent of open-circuit adsorption on

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Pt increased slow1y, to reach about 2X10-10 mole cm- 2 in15 minutes a factor of 3.0 was assumed for the surfaceroughness). The corresponding coverage figure for goldwas slight1y lower than for p1atinum, but the adsorptionoccurred much more rapid1y. No desorption of theradioisotope 35s was found from either p1atinum or golda t 0.4 - 0.65 V versus normal hydrogen e1ectrode NHE) ina cystine-free, deaerated e1ectro1yte. They conc1udedthat cystine probab1y dissociates to cysteine, which thenforms a strong chemisorption bond to p1atinum and a weakerbond to gold.

Disparities between the potentia1 of the cystine/cysteine couple and the theoretica1 Nernst relation havebeen surnmarized by Leyko 47), and Cater and Si1ver 48).The reaction,

2RSH RSSR + 2H+ + 2e

would give, rep1acing activit ies by concentrations,

E = Eo + RT 1n[RSSRj [H+] 22F [RSH] 2

Leyko was unab1e to account for the potentiometricbehaviour of a cystine/cysteine system a t copper e1ectrodesi a cysteine-cupric cysteinate equi1ibrium were assumed,

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similar to a cysteine-mercuric cysteinate couple proposedby earl ier workers for measurements at mercury electrodes.Determination of the cystine/cysteine standard potentialfrom heats of combustion data gave a value +0.025 V vs. NH

49). This is in good agreement with the results from arecent kinetic technique which similarly does not rely uponsolid electrodes. I t involves, instead, analyses of thethiol-disulphide exchange between glutathione GSH) andcystine or oxidised-glutathione GSSG) and cysteine. Fromknowledge of the standard oxidation-reduction potential ofGSSG/GSH, a value of +0.02 V vs. NH was computed for thecystine/cysteine standard potential 50). Hence, cysteinemay be oxidised to cystine by cupric ion in agueoussolution and the cuprous ion so formed may then complexwith cysteine, under certain experimental conditions, in thereaction

2Cu 2+ + 4RS- RSSR + 2RSCu

A polarographic investigation of cuprous cysteinatecomplexes in ammoniacal medium by Kolthoff and Stricks 51)indicated that the formation of complexes between cuprouscysteinate and cuprous ions might be feasible [Cu+ RSCU)etc . ] but that complexes between cuprous cysteinate andcysteine should not be possible.

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Miller and Teva 52 ) have studied the reductionreactions of cystine by potential sweep oscil lopolarography.They suggested that mercury cysteinate was formed slowlyfrom cystine by a surface process the rate of which wascontrolled by the reactions

RSSR Hg RS)2Hg

RS)2Hg + Hg 2RSHg

On the other hand a fast diffusion controlled reactionoccurred betweenmercury and cysteine to forro the cysteinate

RSH + Hg RSHg + H + e

To resolve the potentiometric behaviour of the cystine/cysteine couple tsolid electrodes t is necessary thatthe electrode processes and the identities of complexspecies such as those postulated above be dete rmined forthe experimental conditions.

As a mean s of investigating the adsorptionbehaviour of additives under conditions of copper electro-deposition the determination of occludedmaterial shouldindicate the extent to which an additive is irreversiblyadsorbed to the electrode. If however, the additive

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species undergoes electrochemical reaction a t the surface,both the reactant i t se l f the additive) and products of thereaction have to be monitored, since they may ei ther beoccluded or diffuse back into the bulk electrolyte. Slightamounts of additive may also be occluded into deposits bymechanical trapping in crevices, holes, or grain boundaries(53)

Kochergin and Khonina (54) have used theradioisotopic technique to measure the amounts of 35Soccluded in copper deposits from the additives cystine- 35 sand thiourea-35 s as functions of the temperature andcurrent density. The electrolyte comprised

-1cystine in CuS04.5H20 (200 g L and H2S0410.534 g L

150 g L at40C. The temperature dependence experiments showed thatsulphur was occluded preferentially a t about 50C in therange 20 - SOC, but no explanation for this result wassuggested. The sulphur content of the electrodepositedcopper was found to decrease, from 0.06 to 0.02% by weight,

-2with increase of the current density, from 1.2 to 7.5 A m A k d d 3 5 S 14c d t fmar e ecrease ln or ra 101S0tOplC conten 0deposits with increasing current density has often beennoted e.g. 12), 55)).

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POLARISATION IN THE PRESENCE OF ADDITIVES

Despite numerous studies of changes in thepolarisation caused by the presence of additives a t anelectrode during deposition, the manner by which thepolarisat ion becomes modified is s t i l l a matter of greatinterest 8,9,19,20,42,56). For a particular currentdensity, many organic compounds which are effective asaddition agents increase the cathodic polarisation abovethat obtained in the pure electrolyte. Lahousse andHeerman 57) have recorded cathodic overpotential plotsfor the steady s tate and transient galvanostatic depositionsof copper with a number of thiocompounds, includingmethionine and cystine. As previous data had already shown

9,42,58), both of these additives strongly influencedthe current densityjoverpotential characteristics and thesurface roughness of copper deposits on polycrystallinefoils. -6At bulk concentrations of 2xlO M however, theadditives were found to lower sl ight ly the overpotentialsfrom those found in the absence of additives. Of theadditives tested, only methionine had, a t aIl concentrations,normal forms for the transient curves i e curves similarin form to those obtained in the absence of additive. I twas suggested, therefrom, that methionine bonds to thesurface more weakly than does cystine.

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Various approaches have been taken in attempts tocorrelate the surface coverage of an addition agent to themagnitude of the polarisation increment I ~ n l Volk andFischer 59) have determined differentia1 capacities a te1ectrodes during nickel deposition with the additives2-butyn-l 4-dio1 and 2-propyn-1-o1. The strongest decreaseof differentia1 capacity thus the strongest increase ofsurface coverage occurred a t the lowest current densities.Krug1ikov 20) states that in genera1 I ~ n l shou1ddecrease with increasing current density for a constantthickness of the diffuse layer but that exceptions areknown to this behaviour. He suggests that the decrease insurface coverages a t high current densities may be causedby desorption of the additive due to the negative potentialsor to i t s consumption by occlusion e1ectrochemica1reduction etc An attempt has been made by Krug1ikov et al

60) to account for the rate a t which thiourea is occ1udedat a copper rotat ing disc electrode by app1ying the conceptof a simple b10cking action of the adsorbed additive.

This theory has been deve10ped by Sukava and hisco1leagues 61,62) to correlate the I ~ n l values and theadditive adsorbability. They proposed that i f a fract ione; of the surface becomes blocked by the additive the t ruecurrent density i will be increased on the uncoveredsurface to


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c

l = i1 e)

The assumption is then made that the measured overpotential,n, is made up entirely of the charge-transfer overpotentialfor cupric ion discharge and that the la t ter obeys theTafel equation for the experimental conditions i .e . thecontribution of the mean outer Helmholtz plane potential ,

is neglected. For conditions of high ionic strength,such as prevail generally in acid copper sulphateelectrolytes, wil l be small and will vary l i t t l e athigh values of n 24). In the absence of additive

n = a b ln i

In the presence of additive

nI = a b ln l= b ln [ l / l el]

or e = 1 e x p [ - ~ n / b ]I f the experimentally determined Tafel slope, b,

remains constant in the absence and presence of an inertadditive, i t seems reasonable to assume that , for liquidmetal electrodes, the kinetic parameters of the metalcharge-transfer reaction remain uninfluenced by the

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addition agent. For example, the transfer coefficient wasfound to be unaltered for the discharge of Zn II} t ahanging zinc amalgam electrode in the presence of n-amylalcohol 63)

Sukava and Chu 6l) have measured ~ n l for a seriesof straight-chain, carboxylic acid additives during copperdeposition from acid copper sulphate electrolytes and havecalculated the e values based on the above expressions.They have then proceeded to deduce standard free energiesof adsorption by application of a Langmuir isotherm,

e1 e) = c exp AG OJRTa

in which e is the fractional surface coverage, c is thebulk concentration of additive, and ~ G is the standardfree energy of adsorption. Adsorption of each additivemolecule was considered to displace n water moleculesadsorbed t the electrode surface

A + n H 0soIn. 2 ads. A + n H 0ads. 2 soIn.

Straight l ine plots passing through the origin wereobtained from Bockris-Swinkels isotherm plots of f e}against c when n was taken to be 2 Typical net freeenergies of adsorption a t 25 calculated from this model

24


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are -3.8 Kcal 1mole for propanoic acid, -5.6 Kcal mole 1for pentanoic acid, -7.4 Kcal mole-1 for heptanoic acid,and -6.3 Kcal 1mole for hexanedioic acid.

This treatment does not attempt to take intoaccount certain considerations that may have importantand seriously complicating effects on the adsorptionbehaviour. i) Since adsorption equilibrium for theadditive may not always be achieved under depositionconditions, i f diffusion, or occlusion, or a slow chemicalreaction controls the adsorption mechanism the kineticsof the adsorption process might require detailed examination;

ii) i t might not be t rue, as the model assumes, that aI lof the surface s i tes at polycrystal l ine copper areequivalent for adsorption of the additive and for nucleationby discharged cations; and i i i) in certain regions, theforro of the adsorption isotherro for the additive duringdeposition might be strongly potential dependent. In viewof these possible complications, i t would seem desirablethat the validity of the blocking theory should beexamined by applying simultaneously sorne other experimentalmethod to estimate surface coverages by an additive duringelectrodeposition e.g. differential capacity, in s i turadioisotopic techniques, etc . ) . This would seem to bepart icularly relevant in view of the observation that theblocking theory :indicated increased values for l with

5


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increasing currentdensity 61,62), whereas othermethodshaveoften indicated the opposite trend, namelylowercoverages thigher currentdensities. These issueswillbe discussed l terin relation toexperimentaldata formethionine andcystine,wheresurface coveragesbytheseadditives, estimated fromexperimentallydeterminedocclusion rates are comparedwithcoverages calculatedfrom n valueswith the straightforwardblockingapproximation.

MORPHOLOGY NDELECTROCRYSTALLISATIONTHEORIES

andm easu rementsof themorphologicalcharacteristicsof deposits are of fundamental importanceinelectrodeposition theory, as areflectionof themannerinwhich the structureof the depositdepends uponthe conditions of electrolysis. Experimental data showthat themorpho logyof the depositmay dependonthenatureof themetal, the currentdensity, the substrateepitaxy, the types of anions andcationspresent in theelectroly te, andthe influenceof additiveswhenthesearepresent 14,18,21,24,30,64-74). hemorphologies producedfrompurifiedacid copper electrolyteswith increaseofcurrentdensity usuallyprogress through stagesof layerandridged-typegrowths, pyramidal growths, platele ts and

6


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c

t r ~ ~ c t e d pyramids, cubic b1ocks, and fina11y to po1ycrysta11ine topographies e.g. 18,30,66,67,71,74,75,76).Additiona11y, spiral forms are often observed which arethought to arise from screw dislocations present in thecrysta11att ice 77,78), as we11 as whiskers, powders,and dendrites which are formed under special conditionsof deposition.

The ear1y genera1 theories of crystal growthproposed by Kossel, deve10ped by Stranski, and uti1isedto discuss e1ectro1ytic crystal growth 79), have provideda framework for the deve10pment of e1ectrocrysta11isationtheories by many workers, inc1uding Vo1mer 80), Conwayand Bockris 14,81), and Pangarov 82).

In the c1assica1 mode1 for crysta11isation thegrowth of f1at crysta1s is ensured by surface diffusionof the species over the atomic planes and theirincorporation t kink si tes . For meta1 deposition bythis mechanism an ion is discharged at a f1at portion ofa crystal face and the so-ca11ed ad-atom is then transportedby diffusion to a 1edge or crystal step where i t isincorporated into the 1att ice. A kink s i te is sometimesca11ed the ha1f-crysta1 position and a1though an atom therehas on1y one-half of the coordination number of an atomin the bulk of the crystal , the energy for an atom in thisl t ter position, from coordination considerations a1one,

7


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is equivalent to that for the energy at the kink si te .This is because the factor 1/2 arises when considering thenet nearest neighbour interact ions of atoms in the bulkcrystal, as each bond is considered twice (24). In supportof th is model for electrocrystallisation, Conway and Bockris

81) have attempted to determine the relative activationenergies for cation discharge at various si tes of a metalsurface. Figure l depicts a solvated cation attempting todischarge a t a) a plane surface, b) an edge, and c) akink si te . I t is apparent that, of these three si tes,displacement of water molecules of solvation should occurmore readily, and the cation approach an atom of thesurface more easily, for direct transfer of the cation tothe planar surface s i te (a). However, as the figurei l lustrates si tes d) and (e), which have not beenconsidered energetically in previous calculations, possiblyare even more accessible to approach by a solvated cationundergoing discharge a t an atom of the crystal surface.A discharge mechanism a t such si tes would support analternative model that has been proposed to account forelectrocrystallisation, in which cations are assumed toundergo simultaneous discharge and la t t ice incorporationat the half-crystal positions. I t should be borne in mindthat these a priori models neglect entirely any role thatadsorbed solvent molecules or anions may have a t the surface.

/\

8


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\

c

FIGURE

l lustra tionof ctiondischrge a tvrioussurfces i tes

9


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a) SURF CE c) KINK

b) 1NTERN L EDGE

cf) EXTERNAL EDGE e) SI NGLE TOM


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The la t ter may be able to facil i ta te electron transfer bythe donor-bridge mechanism i .e . a mechanism similar to theouter-sphere complex electron path for homogeneous electrontransfer reactions between solvated metal ions.

Unlike the crystal l isat ion of ionic salts thegrowth rates of crystals in electrodeposits are alwayscontrolled through the input of electr ical energy. Thus,two considerations of Strickland-Constable 83) areapplicable to electrocrystallisationi (i) theories thatinvolve restrictions of the crystal growth due to surfacediffusion mechanisms, or la t t ice energy requirements, mayfrquently be unable to describe adequately the high crystalgrowth rates observed during electrodepositioni (ii) thegrowth rate of a fIat crystal face will depend only on therate of ini t ia t ion of new layers. Whatever the model chosenfor electrocrystallisation, i t might be expecte4 that , asthe current density is increased, a stage will be reached,possibly heralded by the onset of pyramid forms, when thesurface diffusion or lateral growth processes are supersededby outward growth or, effectively, random deposition at aIlsi tes . uch of the experimental evidence has been interpretedwith an emphasis on lateral rather than on verticalgrowth processes, whereas morphological observations haveindicated that the range of current densi t ies for practicalcopper deposition is conducive mainly to outward growth of

30


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the crystals, exemplified by pyrarnids, cubes, blocks, etc Bockris e t al 30) were the f i r s t to measure the

growth rates of layers and pyrarnids. They used {100} facesof copper single crystals and highly purified 0.25 MCuS04 ,-20.1 M H2S0 4 electrolyte. At 5 m cm , the horizontalgrowtt rates of the visible layers were ~ 2 x l O 6 cm s l and

-6 -1those for the sides of pyrarnids were ~ l x l cm s Fromthe horizontal growth rates of the layers macrosteps),which are generally thought to arise from a bunching actionof atomic microsteps, i t was calculated that the majorityof the crystal growth occurred on the plane areas betweenthe layers. The distances between the macrosteps decreasedas the concentration of an additive n-decylarnine) in thesolution was increasedt The number of pyrarnids increasedfrom about 105 to about 106 per cm2 when the current densitywas increased from 15 to 20 m cm 2 I t was suggested thatthe pyrarnids were init iated when the substrate orientationapproached the perfect 100) plane, because then aninsufficient number of microsteps would be available asnucleation si tes .

On the other hand, Jenkins 76) has emphasized that

tFootnote: Fischer and collaborators 75) have proposed thatthe formation of growth layers from microsteps needs thepresence of traces of adsorbable foreign material.

31


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c

preparation of a nominal crystallographic face isnotlikely to be more accurate than O.lo and th tth is smallam ount of disorientation produces about 105 and about 10 7atomic steps per cmalong the [I ll] and [321] axesrespectively. The formation of pyramids has been observedon copper 100) planes from perchlorate 76) and sulphate18) electrolytes tcurrent densities in the range

-2 -24-400 cm Even t4 cm , the pyramids presentwere calculated to account for approximately 20 of theto t lvolume ofmaterial deposited. From considerationof current densityjoverpotential data for the {Ill} and{32l} faces t was argued th tas the degree of sensitiv ityto the irmicrostep densitieswas minimal m ost likely mechanism for crystal growthwas direct addition of atomsto random sites Jenkins proposed further that there wasno evidence thtstructural defects played a significantrole in either the crysta l growth or dissolution processesof these experiments. However, i-n curves tthese lowcurrent densities were slightly snsitive to the substrateorientation and to themetallographic preparation of thesingle crystals. Hayashi et l 84) have reported th ttheconcentration of free acid in the electrolytes and themagnitude of the current density can influence the relativecathodic overpotentials tdifferent crystallographicfaces.

32


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I t should be emphasized that i f incorporation of adischarged cation occurs in more or less random manner, asJenkins has suggested, fIat crystal faces must resultprovided each arriving cation i s eventually located in anatomic monolayer in formation on the substrate. At

-25 mA cm , the mean growth rate of atoms a t an ideal (100)la t t ice plane may be estimated as roughly 2 atoms si te- l 1s(d

lOO= 0.36l5nM), i f aIl atoms in the plane are taken to be

equivalent si tes . The horizontal growth rate of pyramidsreported by Bockris et al (30), lXlO-6 cm s- l wouldrepresent about 28 atoms si te- l s- l of direct , randomdischarge, which indicates that discharged atoms may not beparticularly mobile after arrivaI a t a s i te on a pyramideThis calculation assumes that pyramids have epitaxy similarto that of the substrate, and this has yet to be studiedexperimentally. I t is interesting to note that pyramids

-2formed in the c.d. range 4-400 cm (18,78) were foundto decrease in size with increase in c.d One possibleinterpretat ion of this observation is that the increase inc.d. makes the init iation of pyramidal forms increasinglycompetitive with their growth.

Recent observations of morphologies produced on{100} faces of copper single crystals indicate that pyramidformation occurs at current densities and overpotentialsonly slightly in excess of the reversible potential . This

33


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perhaps implies that the surface diffusion processes areslow in these systems. No direct experimental me surementsare available for the rates of surface diffusion duringelectrocrystallisation processes. n unresolved problemis whether the pyramidal growths on copper are consequenceof two-dimensional or three-dimensional nucleation.Undoubtedly, much more experimental evidence is neededbefore even the predominant atomic movements of dischargedcations c n be correlated with theoretical approaches.

EXPERIMENT L ND RESULTS

P R E L I M I N ~ Y EXPERlMENTS

Stabil i t ies of cystine nd methionine

t is weIl known that organic additives in practicalelectroplat ing baths must be replenished from time to time,owing to their occlusion or co-precipitation into themetallic deposits 4,19). In sorne cases, their decompositionmay m ke i t necessary to rernove decomposition products forcontinued satisfactory operation of the process. In certaininstances, the transformed species are thought to enhancethe particular addition agent function sought. Because ofthese considerations, i t seemed desirable to ex mine the

4


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st bi l i t ies of cystine and methionine prior to more detailedinvestigations of their behaviours as addition agents.Not only were their st bi l i t ies to undesirable electro-chemical oxidation or reduction a point at issue, but alsotheir st bi l i t ies to chemical changes that might occurduring storage of electrolytes to which they had been added.

Degradation in storage

Previous studies have shown that cystine is fair lystable to i r oxidation in solution with cupric ion and H2S 4(85,86), although cysteic acid or other intermediateoxidation products will accumulate in these solutions overextended periods of storage. For example, about 5 ofcystine was los t af ter 2 days exposure to pure oxygen t

from a solution comprising 1.600 g L-cystine in 32 ml5N H2S 4 with 10- 4 M Cu2+ (85). In the present studies nochemical changes for cystine dissolved in the standardelectrolyte were found to arise from storage 0-4 weeks), asmight be revealed by precipitates, odours, orirreproducibility in polarisation measurements.

Alternatively, selenocystine [D,L-3,3 -diselenobis(2-aminopropanoic acid)] a t 5.0XIO- 4 M under comparableconditions yielded a reddish-orange precipitate after shortperiods of storage, which i) gave a positive tes t for Se

35


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in organic matter 87); i i) extracted into CS2 andrecrystall ised as a l ight red-brown stain; and i i i) changedto grey when oven heated to 130. From these observations,i t is reasonable to conclude that i t was the red amorphousvariety of elemental selenium produced by acid hydrolysisof the additive. In comparison, hydrolysis of selenocystineby M HCl at 110 yields the black vitreous allotrope ofselenium 88).

Methionine solutions of concentration 2xlO-4 M inlM H2S 4 or in the standard electrolyte, developed a slightodour reminiscent of an alkyl sulphide after a few days.The odour increased n intensity with storage time. Allexperiments with methionine were therefore made promptlyafter the preparation of i t s solutions, to minimise i t sdegradation and possible contxmination by the productstherefrom.

ADSORPTION AND OCCLUSION OF CYSTINE AND METHIONINE ON COPPER

As an essential preliminary to any attempt todescribe their addition agent actions quanti tat ively,measurements were made of the quanti t ies of cystine andmethionine adsorbed on open circui t , and during copperdeposition. The rate of adsorption of cystine on copperfoi ls was also determined, since i t was of in terest to

6


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1\compare this value with i t s rates of adsorption on platinurnand gold electrodes, reported previously as relatively slowprocesses 46). A further point of interest for aparticular additive was the relat ion between i t s in i t i lrate of adsorption and i t s mean rate of occlusion duringdeposition at various current densit ies, since l i t t l e isknown about the relat ion between these two rates.

Apparatus and method

1A standard electrolyte was made to conta in 125 g LCUS 4 5H20 analar g r d e ~ twice recrystallised from t r iple1dist i l led water), 100 g L conc. H2S 4 analar grade,redist i l led) , and t r iple dist i l led water.

To study the adsorption and occlusion of cystineand methionine, thin copper foils 1.0 cm x 1.0 cm x 0.004 cm;Fisher Scientific Co.) were suspended in 150 ml beakers opento the air and containing fresh, unstirred 5 ml portions ofthe electrolytes to which the radioactive additives had beenadded. Whan the cell was on open circui t , only adsorptionof additive on the foi l occurred, whereas occlusion ofadditive in the electrodeposit became possible when the foilwas a cathode during flow of current through the celle Forocclusion experiments, the cathode was located between twocopper anodes of polycrystalline sheet, l cm x 10 cm, each

37


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c

at a distance of 2 cm from the cathode. Before they wereused the cathode foils were degreased, etched in HN03/H2S04 ,washed thoroughly in t r iple dist i l led water, and finallyrinsed with standard electrolyte. The anodes were similarlytreated between each experiment.

The electrolytes were prepared to have a nominalradioactivity of 50 L- I unless otherwise noted), withone of : L-methionine methyl-14C , D,L-methionine-2-14C,D,L-cystine-3-14c aIl from Int . Chem. and Nucl. Corp.),D,L-cystine-I- 14c (New England Nucl. Co.), or L-cystine- 35 s

Amersham Searle Corp.). They were diluted as requiredwith D,L-methionine or L-cystine Fisher Scientific Co.).Bulk concentrations of additives were weighed to betterthan 2 . One supplier quoted nominal tota l activity as 5 . However, transfer of the radioactive compounds, usually< l mg quantities, could introduce error larger than thisin the radioactivity measurements.

After each experiment the foi l was rinsed for about20 seconds in l l i t r e of dist i l led water, dried in air a troom temperature, and i t s radioactivity determined with awindowless S-proportional counter Baird-Atomic Scaler-timer,model 135) with constant geometry and sett ings. The foi lsamples were counted three times on each surface and thecount rates shown in tabulated data and figures refer to

2the mean values per cm. The count rates for the amounts

38


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(

of adsorption were not significant1y inf1uenced by anymoderate extent to which the foi1s were rinsed after theimmersion period. For examp1e, two foi1 samp1es withadsorbed cystine had count rates of 77 and 51 counts perminute CPM) after the usua1 r inse procedure and yie1dedcount rates of 81 and 42 CPM, respective1y, after beingwashed for 1 minute in running disti11ed water.

Some of the foi1s were disso1ved in 2.0 ml portionsof nitr ic acid (10 by volume), after which was added10.0 ml of a solution containing 2,5-dipheny1oxazo1e

1 15.0 g L ), naptha1ene (100 g L ) , and 1,4-dioxane toa total volume of 1 l i t re (a11 1iquid scinti l lat ion gradechemica1s). Samp1es of the solutions were then counted ina Beckman 1iquid scinti l lat ion counter, mode1 LS 150, thathad been ca1ibrated against solutions simi1ar1y preparedfrom non-radioactive foi1s, to which known amounts of theappropriate radioactive e1ectro1yte were added bymicroburette. The Gi1mont microburette used for theseexperiments had a to t l capacity of 0.2 ml and de1ivered0.01 ml amounts with better than 10 reproducibi1ity.The print-out of data from the 1iquid counter provideda standard deviation errors 95.5 confidence 1imit)

for the samp1es and the counter was regu1ar1y checked forconsistency of operation against standard 14c samp1es.Typica1 calibration data for the 1iquid counter are given

39


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in Table l The least-squares gradient used for calibrationwith methionine was 4125 CPM liquid) = 1.OxlO-8 mole crn-2data in tables and figures for the count rates have beencorrected for their background intensit ies). Preparedradioactive solutions of cystine and methionine of variousradio isotopes and from various suppliers revealed areproducibility in count rates better than 25 , whendata were normalised for their radioactivity contents. AlIsurface concentration values were calculated for apparent

geometrical) areas of the foi ls , and with the assumptionthat no degradation of additive occurred. Although thecurrents used for deposition of copper on the foils couldbe maintained to about 2 , undetermined error isintroduced by lack of data for the true surface areas ofthe electrodes. To deterrnine the surface area e.g. by theB.E.T. method, by measurernents of the double-layer capacity,or by determinations of the adsorptions of hydrogen, oxygen,or organic compounds on the metal, etc.) i t i s necessary tornake various assumptions and approximations and, generally,one particular method will predict areas that differ fromthose found by another method. Moreover, the surfacecomprises crystal l i tes, crevices, etc. and i f these arepreferred si tes for deposition, the definit ion of the truesurface area is further complicated, especially during thedeposition reaction. l t is obvious, of course, that the true

40


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c

TABLE l

CALIBRATION OF LIQUID SCINTILLATION COUNTER

Total methionine concn.: 2.0 10-4 M in standard e1ectro1yte)Radioisotope: L-methionine methyl-14C)

Electrolyte added ml) CPM* 20'2.0 x 10-4 Mmethionine, 50 }l L-1

0.002 93.0 7.0 )0.004 194.5 5.0 )0.006 247.0 5.0 )0.008 334.6 5.0 )0.010 401.6 3.0 )0.002 117.6 7.0 )0.005 334.5 5.0 )0.01 565.3 3.0 )0.02 947.7 3.0 )0.03 1566.0 2.0 )0.04 1839.0 2.0 )0.05 2279.1 1. 5 )0.06 2745.3 1.5 )0.08 3297.1 1.5 )0.10 4027.0 1.5 )

2.0 x 10-3 M methionine, 500 }l L-10.01 5014.0 3.0 )0.02 10225.0 2.0 )0.03 14500.0 2.0 )0.04 16851. 0 2.0 )0.05 20895.0 1.5 )0.06 25431. 0 1.5 )0.07 27659.0 1. 5 )0.08 28043.0 1.5 )0.09 33306.0 1.5 )0.10 38690.0 1. 5 )

* Corrected for background41


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area w l l be larger than the geometrical area t From samples counted as foils and as solutions of

foi ls , i t was possible to calibrate the count rates observedwith the foi l samples for both adsorption on open circui tand for occlusion during electrodeposition. The data,shown in Figures 2 and 3, indicate that the counting resul tsfor the foi ls and for the solutions were in fair agreementfor both the adsorption and occlusion experiments.

Some adsorption experiments were made under purifiednitrogen n the ce l l with the apparatus depicted nFigure 4 The gas t ra in comprised two Fisher-Milligan gasscrubbers, containing ammonium meta-vanadate solutions toremove oxygen, and a third gas scrubber containing standardelectrolyte to adjust the moisture content of the gas, andto remove any possible spray from the second scrubber. Thegas issuing from the cell aga n was passed through agasscrubber containing meta-vanadate solution and finallythrough a water bubbler, to prevent any back-diffusion ofair into the cell . The solutions to remove oxygen were eachprepared from 2 g ammonium meta-vanadate boiled with 5 ml

tFootnote: The absence of ideal polarizabili ty may introducediff icult ies n the measurement of areas for some metals onopen circuit . I t s diff icul t , for example, to find a surfaceactive substance which s adsorbed on silver, because of i t shigh exchange current 26).

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FIGURE 2

Relation between counts made on copper foils and onsolutions made from foi ls , af ter adsorption of radioactivecystine and methionine.0 - 1.0 x 10-4 M cystine in standard electrolyte)

14 -1Radioisotope: D,L-cystine-l- C, 50 ] lC L ) ( - 2.0 x 10-4 M methionine in standard electrolyte)R d , t D L th , 2 l4c 50].lC L- l ~ o ~ s o ope: , -me ~ o n ~ n e - ,

4


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90 o

80

0 70...J X......

60w1- >z 502: 0

:::W 40Cf1Z 30 xX>8 0

20

10

100 200 300 400 500 600COUNTS PER MINUTE LIQUID)


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FIGURE 3

Relation between counts made on copper foils and onsolutions made from foi ls , af ter occlusion of radioactivecystine and methionine.

-41.0 10 M cystine in standard electrolyte)14 -1Radioisotope: D,L-cystine-l- C, 5 L

- 2.0 x 10-4 M methionine in standard electrolyte)Radioisotope: L-methionine methyl-14C) , 5 L-1 .

44


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,\,

Cf 40zCf>0I

......o 3>0.J

W>z

2: 2:::Wn... 0

CfZ>8

c05 10 5 20 25

COUNTS PER MINUTE SOUD) THOUSANDS


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FIGURE 4

Schemat i diagram of apparatus for adsorption experimentsin purif ied nitrogen

5


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l )

NITROGENCYUNDER

MAGNETICBAR

PRESATURATER25QQ

3 SAMPLE VESSELSFISHER-MILUGAN

GAS SCRUBBERS


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conc. HCl, diluted to 200 ml, and added to 25 g amalgamatedzinc metal, in accordance with the suggestion of Meites 89).Copper foi l samples were prepared as in the previousexperiments and then suspended in the cell in 50 ml deaeratedlM H2S0 4 solution for a t least two hours, while solutionsof 50 ml standard electrolyte non-radioactive) and 50 ml ofelectrolyte containing radioactive additive were flushedwith streams of purified nitrogen gas. The samples wereattached to a glass rod and magnetic bar arrangement that couldbe moved with the aid of a magnet, so that they could beimmersed into the appropriate solutions as required withoutopening the apparatus. The cell assembly was immersed ina thermostat a t 25.0 O.loC for aIl experiments.

Adsorption of methionine on copperData are shown in Table II and Figure 5 for the

adsorption of methionine-2- l4C on copper foi ls , with nocurrent flowing, from 2.0 10-4 M methionine in the standardelectrolyte. The data shown in the figure for cystine willbe discussed la ter . ) The approximate mean rate of adsorption

-12 -2 -1during the f i r s t seven minutes was 1.4 x 10 mole cm s The amount of adsorption then remained constant temporarilyat about 5.4 10-10 mole cm 2 , but increased again after aperiod of about one hour, as shown n Table III The

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TABLE I I

ADSORPTION OF METHIONINE ON COPPER FOILS

Total methionine concn.: 2.0 x 10-4 M in standard e1ectro1yte)Total radioactivity: 50 L-1 D,L-methionine-2-14c)

Immersion Surface e -2 x 1010overage, mole cm )period

seconds) 1/2 CPM e CPM 20- eCopper foi1s) Solutions ofcopper foi1s)3.2 4.7 0.9 - -5.5 7.1 l 4 - -7.7 10.8 2.2 86.2 7.0 ) 2.1

I l 0 13.4 2.7 99.8 7.0 ) 2.413.4 17.5 3.5 156.9 5.0 ) 3.815.5 18.0 3.6 140.5 5.0 ) 3.417.3 22.1 4.4 - -24.5 24.0 4.8 - -30.0 27.8 5.6 196 .6 5.0 ) 4.834.6 27.9 5.6 - -38.7 28.2 5.6 193.7 5.0 ) 4.742.4 24.8 5.0 - -49.0 23.9 4.8 173.0 5.0 ) 4.260.0 26.9 5.4 - -

* Corrected for background

47


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FIGURE 5

Adsorption of methionine and cystine on copper foi1s.X 2.0 x 10-4 Mmethionine in standard electrolyte)Radioisotope: D,L-methionine-2-14c, 5 L-1 o 1.0 x 10-4 M cystine in standard electrolyte)Radioisotope: D,L-cystine-I-

14c, 5 L-

1

48


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00 8X 0' '\J1 72Uw 6J

X X: Xz 5


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TABLE

ADSORPTION OF METHIONINE ON COPPER FOILS

Total methionine concn.: 2.0 x 10-4 M in standard e1ectrolyte)Total radioactivity: 50 L-

lL-methionine methyl-14C))

Immersion Surface coverage, e -2mole cm ) x 1010period CPM e CPM 2 e

min) copper foi1s) solutions ofcopper foils)30 27.6 5.7 237.6 5.0 ) 5.8

120 52.9 10.7 - -150 68.4 13.6 605.6 3.0 ) 14.7180 67.8 13.5 545.9 3.0 ) 13.2240 82.3 16.4 676.1 3.0 ) 16.4300 80.4 16.0 757.2 3.0 ) 18.4


49


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TABLE IV

DESORPTION OF METHIONINE FROM COPPER FOIL

Adsorption 30 minutes)Total methionine concn.: 2.0 10-4 M in standard e1ectro1yte)Total radioactivi ty: 50 L-1 {L-methionine methyl-14C))DesorptionStandard e1ectro1yte - no additive present.

Desorption periodmin)

1060

180360

Residua1 surface coverage, 9mole cm 2) x 1010

5.34.64.75.7

51


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TABLE V

EXCHANGE OF METHIONINE FROM COPPER FOILS

Adsorption 30 minutes)Total methionine concn.: 2.0 x 10-4 M in standard electrolyte)Total radioactivi ty: 50 L- l L-methionine methyl-14CDesorptionTotal methionine concn.: 2.0 x 10-4 M non-radioactive, instandard electrolyte)

Desorption Residual surface coverage, e -2mole cm ) x 1010period CPM* e CPM* a e

seconds) 1/2 copper foils) Solutions ofcopper foils)0 27.6 5.5 237.6 5.0 ) 5.83.2 - - 225.9 5.0 ) 5.57.7 - - 225.2 5.0 ) 5.511.0 - - 214.5 5.0 ) 5.213.4 - - 217.9 5.0 ) 5.317.3 - - 186.5 5.0 ) 4.524.5 27.9 5.6 168.9 5.0 ) 4.134.6 . 15.9 3.2 142.5 5.0 ) 3.542.4 14.4 2.9 132.1 5.0 ) 3.257.4 17.7 3.5 151. 7 5.0 ) 3.760.0 19.2 3.8 136.6 5.0 ) 3.373.3 13.7 2.7 104.6 7.0 ) 2.584.9 6.5 1.3 53.3 7.0 ) 1.394.9 4.4 0.9 48.2 10 ) 1.2103.9 6.8 1.4 68.8 7.0 ) 1.7112.2 4.1 0.8 39.6 (10 ) 1.0120.0 6.0 1.2 37.1 10 ) 0.9134.2 4.2 0.8 48.3 7.0 ) 1.2

* Corrected for background52


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FIGUR 6

Exchange of radioactive methionine from copper foi1s.Standard e1ectro1yte containing non-radioactivemethionine 2.0 x 10-4 M).Radioisotope: L-methionine methyl-14C).

53


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0r orX'\J12:Uw 5.J02:. . . .J

Z 40:: 3ZWUz 20U 0W 0U 1::>l i

20 40 60 80 100 120SECONDS 1 2


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(-

. TABLE VI

ADSORPTION OF METHIONINE ON COPPER FOILS IN SOLUTIONSFLUSHED WITH PURIFIED NITROGEN

Total methionine concn.: 2.0 x 10- 4 M in standard e1ectro1yte)Total radioactivity: 50 L-1 L-methionine methyl-14C

Immersion Surface coverage, e -2(mole cm ) x 1010period CPM 2cr e

(min) Solutions of copper foi1s)0.5 297.6 5.0 ) 7.21 429.2 3.0 ) 10.42 352.5 5.0 ) 8.65 168.1 5.0 ) 4.1

10 905.6 3.0 ) 22.020 331. 7 5.0 ) 8.030 424.2 3.0 ) 10.3

l 544.6 3.0 ) 13.2l 478.1 3.0 ) 11.6l 567.5 3.0 ) 13.8

60 1000.2 2.0 ) 24.3120 1260.5 2.0 ) 30.6

* Corrected for background contd. next page

54


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TABLE VI CONTD.)

Radioisotope: L-methionine methyl-14C)

Immersion Surface coverage, e -2mole cm ) x 1010period CPM* 20 e

min) Solutions of copper foils)

2.0 x 10-3 Mmethioninei total radioactivity: 500 llC L- l0.5 2034.2 1. 5 %) 49.32 3150.0 1.5 %) 76.45 2838.2 1.5 %) 68.8

10 4387.6 1.0 %) 106.42.0 x 10-5 M methioninei to t l radioactivity: 5 'IlC L- l

0.5 33.0 10 %) 0.81 7.5 15 %) 0.22 83.9 7.0 %) 2.05 26.3 10 %) 0.6

10 95.0 7.0 %) 2.330 79.1 7.0 %) 1.960 84.6 7.0 %) 2.1* Corrected for background

55


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remained in the electrolytes after they had been flushedfor two hours with purified nitrogen. The difficulty ofrem oving a ll traces of oxygen may be appreciated i f tis realised that for a nitrogen flow ra te of 100 ml min-loxygen levels of 1 ppm in the nitrogen streamwould have introduced to the electrolytes about 0.7 10-10 mole s loxygen I t is possible too that differences in theprepared copper surfaces were suffic ient tomake the ir adsorptive ca.pacities significantly d ifferent.

The dependence of the adsorption rates ofmethionineatcopper fo ils on the bulk concentration of additive wasalso investigated in solutions flushed with nitrogen at-5 -4 -3.0 x 10 M 2.0 x 10 M and 2.0 x 10 Mmethionineconcentrations in the standard electro lyte with constantspecific activ ities of the radioisotope. The results shown in Table VI indicate that the surface coverages ofadditives were increased approximately in a proportionalmanner with increase in the bulk concentrations ofmethionine from 10-5 Mto 10-3M.

Desorption experiments also were made in the cellunder purifiednitrogen. The results shown inTable VIIindicated that no appreciable exchange process had occurredin a solution containing non-radioactive methionine ofequal bulk concentrations in the standard electrolyte afte rfive hours of immersion; in an aerated systemmost of the

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TABLE VII

EXCHANGE OF METHIONINE FROM COPPER FOILS IN SOLUTIONSFLUSHED WITH PURIFIED NITROGEN

Adsorption 30 minutes)Total methionine concn.: 2.0 10-

4M in standard electrolyte)Total radioactivity: 50 L- l L-methionine methyl-14C

DesorptionTotal methionine concn.: 2.0 x 10-4 M non-radioactive, instandard electrolyte)

Desorption Residual surface e mole -2 x 1010coverage, cmperiod CPM* 20 e

min) Solutions of copper foils)30 549.0 3.0 ) 13.360 479.2 3.0 ) 11.6

180 549.0 3.0 ) 13.3300 683.5 3.0 ) 16.6


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radioactivity wou1d have been exchanged within this periodeComparison of the data in Tables IV and VII show that ,af ter 30 minutes adsorption, the coverages obtained fromimmersion of foi1s in the deaerated solutions are 2 - 3times higher than those obtained with aerated solutions.

Adsorption of cystine on copper1. Radioisotope estimations

Adsorption of cystine on copper foi1s from standarde1ectro1yte containing radioactive cystine of 1.0 x 10-4 Mconcentration occurred a t a mean rate of about 1.4 x 10-12mole cm 2 s- l during the f i rs t seven minutes, i .e.essentia1ly the same as that for the adsorption of methioninefrom similar solutions with about twice the concentration ofamino acid Table VIII, cf. Figure 5). The amount of cystineadsorbed then remained constant temporarily, at about5.4 x 10-10 mole cm-2, as with methionine, but after aperiod of about 20 minutes i t too, increased. Comparisonwith the data of Pradac and Koryta 46) indicates that theadsorption of cystine on copper during the ear1y stagesresembled that on p1atinum more c1ose1y than that on golde1ectrodes.

The amounts adsorbed a t longer tirnes are given inTable IX. Neither the adsorption of cystine nor that ofmethionine was res tr icted to mono1ayer coverages. Copper

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TABLE VIII

ADSORPTION OF CYSTINE ON COPPER FOILS

Total cystine concn.: 1.0 x 10-4 M in standard e1ectro1yte)1 d , 50 C -1 ( 1 4C)Tota ra 10act1v1ty: L D,L-cyst1ne--

Immersion Surface coverage, e (mole -2cm x 1010 1period CPM e CPM 20- eseconds) 1/2 (Copper foi1s) Solutions ofcopper foi1s)

3.2 12.7 1.3 82.8 5.0 ) 1.05.5 18.4 1.8 175.1 5.0 ) 2.17.7 23.2 2.3 117.9 5.0 ) 1.411.0 25.7 2.6 216.5 5.0 ) 2.613.4 33.9 3.4 160.9 5.0 ) 2.015.5 39.4 3.9 297.2 5.0 ) 3.617.3 46.2 4.6 281.3 5.0 ) 3.424.5 54.3 5.4 436.4 3.0 ) 5.330.0 51.1 5.134.6 60.4 6.038.7 52.7 5.342.4 63.5 6.349.0 75.4 7.5 419.9 3.0 ) 5.160.0 88.3 8.8 629.9 2.0 ) 7.673.5 98.3 9.884.9 116.2 11.6 625.2 3.0 ) 7.6112.2 161.6 16.1134.2 363.2 36.2180.0 613.5 61.2199.0 1314.5 131.1

224.5 2023.9 201.9* Corrected for background

(59


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TABLE IX

ADSORPTION OF CYSTINE O COPPER FOILS

-4Total cystine concn.: 1.0 x 1 M in standard electrolyte)Total radioactivity when applicable): 5 L- l

D,L-cystine-1-14c)

Immersion Surface coverage, e -2mole cm ) x 1010period Radioisotope method Colorimetrie methodmin)3 6.36 8.8 8.89 9.8 9.9

120 11.6 14.7210 16.1 8.33 36.2 15.5540 61.2 24.8

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foils disintegrated completely when they remained forseveral weeks immersed in solutions of cystine 10-1 Min the standard electrolyte in l ightly stoppered bottles.At the same time the solutions became green and yellowdeposits formed on the foi ls . Obviously, persistent andconsiderable chemical reactions occurred.

2. Colorimetrie estimationsA photometrie method for the estimation of amine

acids, using so-called ninhydrin reagent (tr iketohydrindene hydrate), has been described with detectionl imits 0.02 - 0.40 micromole (90). This technique has beenmodified and applied in an attempt to corroborate theradioisotope estimations of the amounts of cystine adsorbedon copper foi ls .

Strips of copper foil Sem x 20cm, were looselycoiled, cleaned as in the previous adsorption experiments,and immersed for various periods in solutions of L-cystine(1.0 x 10-4 M in the standard electrolyte. Excess solutionadhering to the coils was removed by washing themsuccessively in 3 separate dist i l led water baths, afterwhich the adsorbed cystine was obtained by shaking them inair with 10.0 ml of a NaOH/acetic acid buffer for S minutes.The buffer was prepared by adding glacial acet ic acid toN NaOH solution unt i l the pH was 10.3. These solutions, and

the drainings from two subsequent washes of each coil with

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FIGURE

Calibration data for colorimetrie estimation of cystinewith Triketohydrindene hydrate reagent

6


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9

x- 6Jo2wzl /U

1

01 2 3

OPTICAL DENSITY


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\ results are compared with those obtained by the radioisotopictechnique in Table IX. Although the colorimetrie methodmust be regarded as a highly approxima te one because of thelikelihood of incomplete removal and transfer of adsorbedspecies the results are in fai r agreement with the moresensi t ive radioisotopic measurements.

Occlusion of cystine and methionine in copper cathodes

As outlined previously the method and conditionsfor studying the occlusion of additives on copper foi lcathodes during the passage of current were similar ina l essentials to those used to study their adsorption onopen circui t . To obtain meaningful count rates from thefoi ls with occluded addit ives t was necessary f i r s t toestablish the minimum amounts of copper electrodepositthat would provide constant adsorption and back-scatteringof the S-rays from the radioisotopes. The data in Table Xand Figure 8 i l lustrate that this was achieved after thepassage of 6 coulomb cm 2 for the current densi t ies1.0 A dm-2 -2and 4 0 A dm AlI subsequent occlusionexperiments were therefore made with the passage of thisquantity of electr ic i ty.

Tables XI XIII summarize the experimental datafor the occlusion of cystine and methionine with various

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TABLE X

CH NGE IN COUNT RATE FROM OCCLUDED CYSTINE WITHQUANTITY OF COPPER DEPOSITED

Total cystine concn.: 1.0 x 10-4 M in standard electrolyte)Total radioact ivi ty: 50 L-1 L-cystine-35 s)

Depositiontimemin)

25101520304050607590

405984

178322592710331435823495362137433915

CPM Copper foi ls)4.0 A dm- 2

892132714471547


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Cf

z~ ::JI

W3l::JZ:::2W0..

f)lZ 1:JU

2 4 6 8MINUTES


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TABLE XI

OCCLUSION OF CYSTINE IN ELECTRODEPOSITS ATDIFFERENT CURRENT DENSITIES

-4Total cystine concn.: 1.0 x 10 M in standard e1ectro1yte)Total radioactivity: 50 L-1

Current Mean rate of occlusion, Co mole cm-2 -1s ) x 10 12density D,L-cystine-1-14c L-cystine- 35 s D,L-cystine-3-14cA dm-2

CPM Co CPM Co CPM CoCopper foi1s) Copper Copper foi1sfoi1s)0.25 5875 9.9 - - 0.50 - 5841 19.70.75 3593 18.11.0 3468 23.4 3621 24.4 3329 22.41.5 2381 24.1 - - 2.0 - - 2463 32.82.5 1790 30.13.0 1783 36.03.5 1499 35.4 - 4.0 - 1620 43.64.5 1172 35.5 - 5.0 - 1287 43.3 1247 33.65.5 1091 40.46.0 - 1222 49.4


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TABLE XII

OCCLUSION OF CYSTINE IN ELECTRODEPOSITS ATDIFFERENT CURRENT DENSITIES

-4Total cystine concn.: 1.0 x 10 M 0.5 M CuS04 , no acid)Total radioactivity: 50 L-1 D,L-cystine-1-14c)

of occlusion, o mole -2 -1 x 1012Current Mean rate cm s )density CPM* Co CPM* 20' o(A dm- 2) Copper foi1s). Solutions ofcopper foi1s)

0.25 6987 11.8 - -0.50 4753 16.0 - -0.75 4226 21.3 - -1.0 3642 24.5 - -1.5 3195 32.3 - -2.0 2864 38.6 - -2.5 2466 41.5 - -3.0 2472, 2216 49.9, 44.8 44000 0.3 ) 44.43.5 2079 49.0 37753 0.5 ) 44.54.0 1900, 1733 51.1,46.7 33530 0.5 ) 45.24.5 1868 56.6 35576 0.5 ) 53.95.0 1545, 1511 52.0, 50.9 29989 0.5 ) 50.55.5 1380 51.1 29001 0.5 ) 53.86.0 1383, 1315 55.9, 53.1 27812 0.5 ) 56.26.5 1370 60.0 29248 0.5 ) 64.07.0 1208 56.9 17260 0.5 ) 40.7* Corrected for background

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TABLE XIII

OCCLUSION O METHIONINE IN ELECTRODEPOSITSAT DIFFERENT CURRENT DENSITIES

-4Total methionine concn.: 2.0 x 10 M in standard electrolyte)Total radioactivity: 50 L- l D,L-methionine-2-14c)

Current Mean rate of occlusion, mole -2 -1 x 1012Co cm s )density CPM* CoA dm -2) Copper foils)

0.25 1368 4 60.50 815 5.50.75 384 3.91.0 383 5.21.5 190 3.82.0 236 6.42.5 174 5.93.0 138 5.63.5 186 8.84.0 230 12.44.5 70 4.25.0 178 12.05.5 101 7.56.0 76 6.1* Corrected for background

contd. next page

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\

TABLE XIII (CONTD.)

-4Total methionine concn.: 2.0 x 10 M in standard e1ectro1yte)Total radioactivity: 50 L-1 L-methionine methyl-14C))

Current Mean rate of occlusion, Co mole cm-2 -1s ) x 1012density CPM* Co CPM* 20 Co(A dm-2) Copper foils) Solutions of copperfoi1s)

3.0 242 9.8 5165.2 1. 0 %) 10.43.5 202 9.5 5143.3 1.0 %) 12.14.0 247 13.3 5894.2 1. 0 %) 15.94.5 274 16.6 5802.2 1.0 %) 17.65.0 311 20.9 6425.4 1. 0 ) 21.65.5 151 11.2 2879.3 1.0 %) 10.76.0 135 10.9 2980.7 1. 5 ) 12.0


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FIGURE 9

Relation between count ra teofoccluded cystine andcurrentdensity, for deposition of copper from electrolytecontaining cystine 1.0 x 10-4M.Radioactivity: 5 L- l oD,L-cystine-1-14C

L-cystine- 35 sD,L-cystine-3- l4C

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' 5tfztf> 4IJ-

' -J '

WJ- > 32:0::W0tf 2J-Z>

U1

1 2 3 4 5 6CURRENT DENSITY AMPS DM - 2 )


89/207

to oxidise on paper chromatographs 94) in the presence ofcupric salts and th is complicates the identif ication ofspecies and the development of a satisfactory experimentaltechnique. I t does seem quite probable, however, that noappreciable degradation of the -S-CH2-CH COOH)NH; part ofthe cystine molecule occurred during i t s occlusion.

When the mean rates of occlusion were calculated,t was found that the relat ions between these and current

density were markedly different for cystine and methionine.For the current density range investigated, the plot ofln cystine occluded per second) against ln current density)is approximately l inear with a slope of 0.5 Figure 10).The amounts occluded a t various current density values weresimilar with and without sulphuric acid in the electrolyte.In corresponding manner, the differences in the ini t ia lpolarisation maxima, obtained in the presence of cystineand in i t s absence, are virtually constant at different acidconcentrations 42).

The mean rates of occlusion of methionine in thecurrent density range 0.25 - 3.0 A dm- 2, shown in Figure I lwere about four times higher than i t s mean open circui tadsorption rate. With further increase in the currentdensity, the occlusion rates increased until a maximum was

-2reached a t about 5 A m Mechanisms are proposed later forthe behaviours of methionine and cystine during their

73


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l

FIGURE 10

Occlusion of cystine in electrodeposits a t differentcurrent densit ies.

-41.0 x 1 M cystine.o tO 5 -1Ra 10ac 1V1ty: L .

o D L-cystine-1-14c, standard electrolyte~ L-cystine- 35 s standard electrolyte D L-Cystine-1-14c, 0.5 M CuS04 no H2S04 added)

74


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w 0:: lO

z 00/)>. JUU

0 0 5c. J

-10 o 10Ln URRENT DENSITY AMPS DM-2)


92/207

FIGURE l

Occlusion of methionine in electrodeposits at differentcurrent densi t ies.2.0 x 10-4 Mmethionine in standard electrolyte)Radioactivity: 5 L- l XD,L-methionine-2- 14c)L-methionine methyl-14C

75


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{\

C\l

x1 20rf)

C\l12:U15w-.J02:-

w10r::z0 5f)-.JUU0

0

X X

1 2 3 4 5 6CURRENT DENSITY AMPS DM-2)


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adsorption and occlusion.

COMPLEXES OF COPPER IONS WITH MI O CIDS

t has frequently been suggested that the additionagent effect of an additive during electrodeposition ofcopper may be attr ibuted to complex formation between theadditive and copper ions in the electrolyte e.g. 95-100).Reference was made earl ier to the importance of complexesin the potentiometric behaviour of the cystine-cysteinecouple a t solid electrodes. Sorne organic additives behavein a synergie manner with halide, thiocyanate, and cyanideanions 55,101-104). For example, Venkatachalam andWinkler 55) have demonstrated that a surface brighteningphenomena may be associated with a 1:1 rat io of cystine:cyanide ion which is occluded into the deposit a t aparticular cystine concentration and current density. Thisevidence supports the premise that copper complexes maysometimes have an important addition agent role.

t was decided, therefore, to examine the conditionsfor complex formation between Cu II) and Cu I) ions withcystine and related amino acids and, wherever possible, todetermine the infrared spectra of these complexes as an aidto their identif ications and to the determinations of theirstructures.

7


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Cupric-amino acid complexes

A1though Cu II)-cystine had been prepared andana1ysed as ear1y as 1901 90), the mo1ecu1ar structure isnot known unequivoca11y. ni t ia l suggestions 105,106)that the comp1ex was monomeric were discounted by Hawkinsand Perrin 107), who have proposed that the major speciesis the dimer, d i - ~ - c y s t i n a t o - d i c o p p e r I I ) di-hydrate:Their ana1ysis estab1ishes stabil i ty constants based on theassumption that a non-weighted stat ist ica1 distribution ofcomp1ex species exists in equi1ibrium. In strong1y acidicsolutions, both the carboxy1ate and amino groups of dibasicamino acids are protonated. Complexes of the amino acidswith cupric ion may then be formed when the protons aret i t ra ted from the -NH3 groups by additions of base in thepresence of cupric ion. With cystine, for examp1e,

H+ -RSSR)CU+ + H+ -RSSR-)H+ + OH- H+ -RSSR)CU RSSR-)H+ + H20

where H+ -RSSR-)H+ represents the zwitterion form of cystine,NH; COO-)CHCH 2SSCH2CH COO-)NH;. Thus, the acid dissociationconstants for the amino groups, Ka3 and Ka4 , will control

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\

the extent of complex formation. Again, for cystine as anexample,

Ka3 = [H+ -RSSR-) ] [H+]/ [H +-RSSR-)H +]Ka4 = [ -RSSR-)] [H+]/[H+ -RSSR-)]

No reliable aciddissociation constants, pKa3 and pKa4 ,were available in the l iter tu refor sorne of the amino acidsthatwere of in terestin the present study, and the Speakmanmethod 108) has been applied todetermine them .

To discuss th ismethod t may be assumed forsimplicity that thedibasicacidmay be denoted by H2AThen

Ka3 = a H ~ r H A T 2AYH2AKa4 = a H ~ 2 - . h 2 - YHA-

f Yequivalents of strongbase, MOH are added to asolution containing the acidH2Aa tmolarityx, electr iclneutra lity requires that

~ + ~ = + ~ + mOH -= ~ + y sinceM =y)

8


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c

The ini t ia l arnount of the acid x will represent theionised and unionised acid present at any instant duringthe t i t ra t ion i . e .

If z y mH - mOH-= m - + m 2-H A=

These several expressions may be combined to give

This equation has the forro

I f for an acid-base t i t ra t ion the values of Mare plot tedagainst those for N, the graph has a slope which representsKa3 and an intercept of Ka3Ka4. The values of M and N may becomputed from a knowledge of the number of equivalents ofalkal i added and the appropriate activit ies. The activit iesof undissociated species YH A may be taken as unity and2the limiting law of Debye and Hckel used ta derive yA2- and

79


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\

the ra tioYA2-/YHA-. W ith the ionic strength, definedby the expression

= O.5Im.z. 2. J J.J.where mi represents themolalityof the ion and zi is thevalence, the limiting law ofDebye and Huckel has thewell-known general form

logYi =

where y. isthe activ itycoefficientof the species andA.is a constant for the solvent tthe specified temperature.Forwater t25,Amay be taken as 0.509.

1. ExperimentalmethodThe method, apparatu s, and experimental conditions

describedby Hawkins and Perrin 107) were u

Documents

Amino Acids as Additives in Copper Electrodeposition