Chapter Xvi Volumetric Methods

8/17/2019 Chapter Xvi Volumetric Methods

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CHAPTER XVI VOLUMETRIC METHODS

A. Fundamentals of Volumetric Analysis

Volumetric or titrimetric analyses are quantitative analyticaltechniques which employ a titration in comparing an unknown with astandard. In a titration, a volume of a standardized solution containing aknown concentration of reactant "A" is added incrementally to a samplecontaining an unknown concentration of reactant "B". he titration proceedsuntil reactant "B" is !ust consumed stoichiometric completion#. his is knownas the equivalence point. At this point the num$er of equivalents of "A"added to the unknown equals the num$er of equivalents of "B" originallypresent in the unknown. Volumetric methods have the potential for aprecision of up to %.&'.

(or volumetric methods to $e useful, the reaction must reach ))'*completion in a short period of time. In almost all cases, a $uret is used tometer out the titrant. +hen a titrant reacts directly with an analyte or with areaction the product of the analyte and some intermediate compound#, theprocedure is termed a direct titration. he alternative technique is called a$ack titration. ere, an intermediate reactant is added in e-cess of thatrequired to e-haust the analyte, then the e-act degree of e-cess isdetermined $y su$sequent titration of the unreacted intermediate with thetitrant. egardless of the type of titration, an indicator is always used todetect the equivalence point. /ost common are the internal indicators,compounds added to the reacting solutions that undergo an a$rupt change

in a physical property usually a$sor$ance or color# at or near theequivalence point. 0ometimes the analyte or titrant will serve this functionauto indicating#. 1-ternal indicators, electrochemical devices such as pmeters, may also $e used. Ideally, titrations should $e stopped precisely atthe equivalence point. owever, the ever2present random and systematicerror, often results in a titration endpoint, the point at which a titration isstopped, that is not quite the same as the equivalence point. (ortunately, thesystematic error, or $ias may $e estimated $y conducting a $lank titration. Inmany cases the titrant is not availa$le in a sta$le form of well2de3nedcomposition. If this is true, the titrant must $e standardized usually $yvolumetric analysis# against a compound that is availa$le in a sta$le, highly

pure form i.e., a primary standard#. he $asic requirements or componentsof a volumetric method are4

&. A standard solution i.e., titrant# of known concentration whichreacts with the analyte with a known and repeata$lestoichiometry i.e., acid5$ase, precipitation, redo-,comple-ation#


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6. A device to measure the mass or volume of sample e.g., pipet,graduated cylinder, volumetric 7ask, analytical $alance#

8. A device to measure the volume of the titrant added i.e., $uret#9. If the titrant2analyte reaction is not su:ciently speci3c, a

pretreatment to remove interferents

;. A means $y which the endpoint can $e determined. his may $e aninternal indicator e.g., phenolphthalein# or an e-ternalindicator e.g., p meter#.

a$le &a? phenolphthalein none>2III# 60?9 methyl red digest5distill >

>9?#/acro2@!eldahl Acidimetric

VolatileAcids

>a? phenolphthalein distillation istillation

Precipitation

=hloride Ag potassium

chromate=hloride g diphenylcar$azo

ne/ercuric>itrate

Complexation or Chelation=a 1A 1riochrome Blue

Black ardness 1A 1riochrome Black

=> Ag p2dimethylamino$enzalrhodanin

eOxidation/eductionissolved ?6 >a606?8 starch /n*II#, I2I# +inkler=a /n?9 auto o-alate Cermanganat

e itr=hlorine5=l?

6

>a606?8 starch I2I# Iodometric


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0?826 >a606?8 starch I2I# Iodometric=hlorine5=l?

6

(e0?9 C C (errous itr.

=? (e>9#60?9#6 ferroin @ 6=r6?D

Volumetric methods may $e $ased on acid5$ase reactions.precipitation reactions, comple-ation reactions and redo- reactions. a$le&=6=6>=6=??#6F. his is a he-adentate ligandwhich $inds very strongly to many metals. (or calcium and total hardnessdetermination, a couple of speci3c dyes are used to determine the presenceof e-cess cation. /any o-idation5reduction $ased volumetric methodsemploy the iodometric method. his involves the o-idation of iodide toiodine and su$sequent titration with sodium thiosulfate using starch as anindicator. /any of these employ a series of redo- reactions. hepermanganate method for calcium is somewhat unique in that the calcium isprecipitated as the o-alate, and it is the solid2phase o-alate group which

participates in the redo- reaction, not the calcium.

B. Acid/Base Titrations!. A"#A"I$IT% & ACI'IT%

a. Environmental Significance

Alkalinity is a measure of a waterGs a$ility to neutralize strong acids.It re7ects the waterGs $uHer capacity or resistance to a drop in p uponaddition of acid. =onversely, acidity is a measure of a waterGs a$ility to

neutralize strong $ases.Alkalinity is important in assessing the need for additional $uHeringor p control with p2sensitive operations. (or e-ample, the alkalinity of awater must $e known in order to calculate lime and soda ash doses forprecipitative softening. 0pecies responsi$le for either alkalinity or acidity canaHect rates of corrosion, the speciation of metals and organic compounds,the rates of certain types of reactions, and numerous $iological processes.


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Alkalinity and acidity might also correlate with other properties of a watersuch as hardness and 0.

. S!ecie" Re"!on"ile for Al#alinit$ an% Aci%it$ in &ater"

Alkalinity and acidity can $e interpreted in terms of concentrations of speci3c constituents only when the chemical composition of the water isknown. 0pecies that impart alkalinity to a water are $ases. 0pecies thatimpart acidity to a water are acids. In unpolluted fresh waters, hydro-ide,car$onate and $icar$onate are the most important $ases. herefore, totalalkalinity is often interpreted as the sum of the num$er of equivalents of these $ases minus the hydrogen ion concentration#. hus, alkalinity can $ee-pressed in terms of eq5 or meq5, $ut not moles5 or mmoles5.

Alktot J E=?82F * 6E=?826F * E?2F 2 E*F&


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Alktot J E=?82F * 6E=?826F * EB?#9F * EC?926F * E80i?9F *E/g?2F * E?2F 2 E*F

&


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determined $y the addition of an acid or $ase until a p change occurs,$ecause the 3rst drop will always result in some change. Instead, samplesare commonly titrated to a pre2determined p. By convention, the p isusually a$out 9.; for the alkalinity titration and L.8 for acidity. owever,these are not purely ar$itrary choices. itration to p L.8 corresponds to the

point where the quantity of $ase added equals the sum of the strong acid,car$on dio-ide and $icar$onate originally present. Because this prepresents the appro-imate point of color change for the indicator,phenolphthalein, the amount of titrant required to reach this p is oftenreferred to as the phenolphthalein acidity. 0imilarly, titration to p 9.;corresponds to the point where the quantity of acid added equals theamount of strong $ase, car$onate and $icar$onate originally present. 0incethis p represents the appro-imate point of color change for the indicator,methyl orange, the amount of titrant required to reach this p is oftenreferred to as the methyl orange alkalinity.

/ethyl orange is an azo dye that changes color from yellow to red as

the p is lowered $elow a$out 9.;. It is also known as p2dimethylaminoazo$enzene2pG2sulfanilic acid.

Chenolphthalein is a polyphenolic compound which loses $oth awater molecule and a hydrogen ion at high p. As the deprotonation occursthe color changes from colorless to $right red.


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ii. =hoice of itration (lasks It is good practice to have as little space$etween the sample and the $uret and p electrode# as practical. +ithconventional2sized electrodes, the 6%% m tall form Berzelius $eaker is agood choice. (or miniature com$ination electrodes and for the colorimetricmethod, an erlenmeyer 7ask &6; m or 6;% m# is recommended. In

general a magnetic stirrer is recommended for proper mi-ing and electroderesponse, however the speed should $e kept low to minimize e-change withthe atmosphere. A ru$$er stopper with holes for the electrodes# and $uretmay $e used to further reduce atmospheric contact. (or the colorimetricmethod, manual agitation of the erlenmeyer 7ask via a swirling wrist motionmay $e used.

iii. 0ampling Crocedures

&. andle carefully to avoid loss of car$on dio-ide6. Analyze promptly after opening to avoid loss of volatiles car$on

dio-ide, ammonia, hydrogen sul3de#8. =ollect samples in polyethylene or glass $ottles and keep cool to

avoid changing gas solu$ilities.

iv. =olorimetric vs Cotentiometric /ethods

In the vast ma!ority of cases $oth the colorimetric and potentiometricmethods are accepta$le. owever, there are certain circumstances whereone might $e preferred over another. (or e-ample, some highly colored ortur$id samples may mask the color of indicators, and render it impossi$le touse the colorimetric method. If free residual chlorine is present, it will have to$e removed $y addition of & drop of %.&/ sodium thiosulfate solution prior tousing the colorimetric method. his is necessary to avoid $leaching of theindicators. +ith some very dilute waters, this type of sample pretreatmentmay eHect the results and the potentiometric method is recommended. (orgreatest accuracy, the potentiometric method is recommended. Nnder idealconditions this method allows one to graphically determine a sampleGs


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equivalence point. ?therwise, one must rely on some predeterminedendpoint which has $een found to correspond to the equivalence point inmany samples of the same alkalinity. Nnfortunately, the potentiometric2graphical method is more time2consuming, and it requires a great deal moreanalyst e-perience.

he potentiometric method also suHers from some disadvantages. (ore-ample, it may not $e a convenient method for rapid on2site analysis.Cotentiometric determination relies on the proper operation of a sensitiveinstrument i.e., the p meter#. arsh conditions in the 3eld or the lack of electrical power may preclude its use. 1ven in the la$oratory thepotentiometric method may $e su$!ect to certain types of interferences. +ithsome waters, surfactants and precipitates which coat the p electrode willimpede its response. hese su$stances must not $e removed as they maycontri$ute to acidity or alkalinity. Instead, the electrode should $e cleanedfrequently, or the colorimetric method should $e employed.

%. Anal$tical Proce%*re"

&.Alkalinitya. =l standardization

&. Add &; ml of the primary standard sodium car$onatesolution to the titration vessel with a$out


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$. 0ample itration Alkalinity#&. =hoose a sample volume that is e-pected to contain less

than ;% mg of alkalinity as =a=?8. (or most waters, &%%ml is a convenient volume.

6. Add 6 drops of either methyl orange or $romcresol green2

methyl red indicator solution or insert electrodes andtitrate with the =l titrant to the endpoint. he 3rstpotentiometric endpoint is p L.8 and from this thephenolphthalein alkalinity Alkph# is calculated. he secondendpoint Alktot# depends somewhat on the alkalinityrange. 1-perience has shown the following ps may $eused to esta$lish an endpoint in lieu of a potentiometricdetermination of the in7ection point4

Alkalinity Cotentiometric

=olorimetric

mg5# p# from greenish $lueto#

8% 9.) light $lue lavender&;% 9.< light pink;%% 9.8 red

=aution4 1-cessive agitation or prolonged titrations maylead to loss of volatiles e.g., car$on dio-ide# or signi3cantuptake of atmospheric car$on dio-ide. Avoid heating thesample a$ove room temperature $y a magnetic stirrer.

6. Aciditya. >a? 0tandardization

&. Add &; ml of the primary standard @C solution to thetitration vessel with a$out a? stock using 6;% ml $eaker and notestarting point. As $efore, you must keep the solutioncovered with a watch glass during titrations. itrate, whilestirring, until the endpoint is reached.

8. epeat this procedure steps a2$# until 6 titrations give

results that agree within 6'. inse the $uret and 6;%2ml$eaker thoroughly with distilled water.

9. =alculate the normality of the titrant, >t, from theconcentration of the primary standard @C solution, >ps,the volume of the @C solution used, Vps, and the volumeof sodium hydro-ide titrant used, Vt. It should $e close to%.%6>.


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>t J >psVps5Vt

$. 0ample itration Acidity#&. =hoose a sample volume that is e-pected to contain less

than ;% mg of acidity as =a=?8. (or most waters, &%% ml

is a convenient volume.6. Add 6 drops of phenolphthalein indicator solution or insertelectrodes and titrate with the >a? titrant to theendpoint. he potentiometric endpoint is p L.8. (or somehighly acidic waters e.g., acid mine drainage# the volumeof titrant required to reach p 8.D should also $e recordedfor determination of mineral acidity.

e. Reagent"

&. Alkalinity

a. Crimary 0tandard 0odium =ar$onate 0olution.&. ry at least &% g of anhydrous sodium car$onate at 6;%

degrees = for at least 9 hours and allowed to cool in adesiccator charged

with "rierite".6. are a piece of weighing paper.8. ?pen the desiccator and place a$out 6.; g sodium

car$onate on the paper, close the desiccator and quicklyre2weigh paper * primary standard to the nearestmilligram. 0u$tract com$ined weight from the tare weightand record.

9. Pently pour the sodium car$onate into a &2liter volumetric7ask. (ill to the mark with distilled water. Qou may wish touse a distilled water wash $ottle for the 3nal few ml.

;. =alculate the normality of the primary standard, >ps, fromthe mass in grams of the primary standard sodiumcar$onate added, mps, and the gram equivalent weight of the sodium car$onate P1+psJ ;6.))#. It should $e closeto %.%; >.

>ps J mps5P1+ps

$. ydrochloric Acid 0tock 0olution a$out %.&>#&. Add ) mls of reagent2grade =l &&.< /# to a &2liter

volumetric 7ask and 3ll to the mark with distilled water.c. ydrochloric Acid itrant a$out %.%;># 2 dilute stock

appropriatelyd. /i-ed Bromcresol Preen2/ethyl ed Indicator 0olution.

&. issolve 6% mg methyl red sodium salt and &%% mg$romcresol green sodium salt in &%% ml distilled water


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6. Aciditya. Crimary 0tandard Cotassium ydrogen Chthalate 0olution.

&. ry at least &; g of potassium hydrogen phthalate @C#at &6% degrees = for at least 6 hours and allowed to cool

in a desiccator charged with "rierite".6. are a piece of weighing paper.8. ?pen the desiccator and place a$out &% g Cotassium Acid

Chthalate @C# on the paper, close the desiccator andquickly re2weigh paper * primary standard to thenearest milligram. 0u$tract com$ined weight from thetare weight and record.

9. Pently pour the @C into a &2liter volumetric 7ask. (ill tothe mark with distilled water. Qou may wish to use adistilled water wash $ottle for the 3nal few ml.

;. =alculate the normality of the primary standard, >ps, from

the mass in grams of the primary standard @C added,mps, and the equivalent weight of the @C P1+psJ6%9.&%#. It should $e close to %.%; >.

>ps J mps5P1+ps

$. 0odium ydro-ide 0tock 0olution a$out %.&>#&. Add ;.L mls of a commercial ;%' >a? solution &).& /#

to a &2liter volumetric 7ask and 3ll to the mark withdistilled water.

c. 0odium ydro-ide itrant a$out %.%;># 2 dilute stockappropritately

8. Penerala. =ar$on dio-ide2free water4 Nse freshly distilled water or distilled

water that has $een freshly $oiled for &; min and cooled toroom temperature. =onductivity should $e less than6umhos5cm.

f. Data Anal$"i"

i. =alculation of Alkalinity If a water contains only hydro-ide,$icar$onate and car$onate as signi3cant $ases, then the amount of acidrepresented here $y * or a proton# required to reach the Chenolphthaleinendpoint is equal to the amount of hydro-ide originally present plus theamount of car$onate originally present4

* * O01 ↔ 6?&


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* * CO213 ↔ 0CO21

&


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Alkmo J ;%,%%%Vmo>t5Vs &


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(igure &ear neutrality p D# only very small amounts of acid O&% 2< /# are requiredto lower the p $y one unit, however, at p 8 one must add &%,%%% times as

much to drop one p unit i.e., to reach p 6#. herefore, this semi2log plotgives us a sloping curve $eyond the equivalence point even though little orno reaction is occurring.

(or curve B, the 3rst plateau corresponds to $oth the neutralization of hydro-ide and the protonation of car$onate equations U&


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(igure &


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(igure &


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&ote that in contrast to alkalinity, = must always $e e-pressed onterms of moles5 or mmoles5, and not eq5 or meq5.

&


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&


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(inally these last three equations may $e com$ined with the charge $alanceequation &


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/C#&


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E*F J = @ &5E*F 1C#&


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Also as previously stated we will assume that the equivalent conductance, P,is equal to the limiting equivalent conductance at in3nite dilution Po. husequation &


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responsi$le Cerdue, &)L;#. In order to minimize o-idation and$iodegradation, these titrations should $e carried out under nitrogen.Another pro$lem arises from the di:culty in accurately measuring high pvalues. his is in part due to the high concentration of alkali metal ions whichinterfere with potentiometric p measurements.

$. Barium hydro-ide method for total acidity.

A popular method for the determination of total acidity in terrestrialhumic materials utilizes $arium hydro-ide. he addition of an e-cess of Ba?#6 to a sample containing organic matter $rings the p up to a$out &8 where nearly all acidic protons are removed. hehigh concentration of $arium coupled with the high comple-ation constants of $arium2organic salts, leads to nearly complete $inding

$y $arium of the charged functional groups. In the process, the comple-ed organic matter is rendered less solu$le and must $e

physically removed $y 3ltration. hen, the protons originally li$erated from the organic matter upon addition of the hydro-ide may $e

determined $y $ack titration of the residual hydro-ide in the 3ltrate. Nnfortunately, this method has some serious weaknesses for

analysis of aquatic organic matter. (irst, for e:cient separation, it is critical that the $arium2comple-ed organic matter rapidly

precipitate. owever, the greater hydrophilic nature and lower molecular weight of aquatic organic matter as compared to terrestrial

matter makes precipitation of this material less complete. 0econd, any functional groups that are not protonated at the start i.e.,

anionic or comple-ed $y metals# are not likely to $e measured $y this technique. owering the starting p would minimize the

pro$lems of incomplete titration, however, this would introduce additional uncertainties from the di:culty of accurately measuring

high concentrations of hydrogen ions.

C. Complexation Titrations

=omple-ation or comple-ometric titrations are $ased on the

formation of a comple-. =ommonly, a metal is the analyte to which someligand is added. his ligand must $e chosen so that comple-ation is quick,has a well de3ned stoichiometry, and goes very nearly to completion. hemost frequently used ligand is ethylenediaminetetraacetic acid or 1AE??==6#6>=6=6>=6=??#6F. his is a he-adentate ligand which$inds very strongly to many metals giving a &4& stoichiometry. It has $eenapplied to the determination of many metals for e-ample see4 (laschka,&);)X 0chwarzen$ach (laschka, &)


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a$le &a *I# &.i *II# &L.


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0u$stitution of the appropriate acidity constants into equation &


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(igure &


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precipitation might occur if the sample were to $e heated, evaporated or if the p was raised. =ar$onate hardness is equal to the alkalinity in mg5 as=a=?8# when the alkalinity is less than the total hardness. he remaininghardness is referred to as non2car$onate. +hen the reverse is true i.e.,hardness K alkalinity#, the car$onate hardness is equal to the total hardness,

and non2car$onate hardness is zero.

otal hardness J =a2hardness * /g2hardness&


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improves the sharpness of the endpoint. o ensure that some magnesium will$e present, a small amount of /g21A is added along with the $uHer.Because $oth analyte and titrant are added in a &4& ratio, this does not eHectthe 3nal determination. he moles of 1A added as titrant are equal to themoles of calcium and magnesium originally present. his type of procedure is

referred to as a direct comple-ometric titration.In a $ack comple-ometric titration, 1A is added in e-cess and ametal other than the analyte metal is used as the titrant. (or this type of titration it is important that the titrant metal $ind less strongly to 1A thandoes the analyte. A $ack titration is useful under conditions where the metalcannot $e kept in solution in the a$sence of 1A, where the analyte $locksthe indicator, or where comple-ation kinetics with 1A are slow.

+hen high concentrations of interfering metal are present, certaininhi$itors must $e added prior to titration. hese are ligands that willstrongly $ind the interfering metal, such as cyanide, sul3de, andhydro-ylamine. A large interla$oratory study found a relative standard

deviation of 6.)' and a relative $ias of %.L' for a synthetic samplecontaining


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small amounts of < / =l appro- ;%' of conc.# until=a=?8 !ust dissolves. Add 6%% ml distilled water and $oilfor a few minutes to e-pel dissolved =?6. =ool, add a fewdrops of methyl red indicator and ad!ust to the"intermediate" orange color $y adding 8> >9? or


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3. CA"CI5M

A. Princi!le

=alcium can $e determined $y atomic a$sorption spectrophotometry,

$y a redo- titrimetric method and $y a comple-ometric method. hecomple-ometric procedure outlined here is quite similar the the totalhardness method. 0everal modi3cations must $e made however, to makethe analysis speci3c for this one hardness species. (irst, a metal ion indicatorthat comple-es with calcium, $ut not magnesium 1riochrome $lue $lack #is used. 0econd magnesium is partially removed $y using a higher p a$out&6# which results in precipitation as /g?#6. >ote that au-iliary comple-ingagents are not used in this procedure. =alcium hydro-ide does notprecipitate at this elevated p. In addition, 1A $inds preferentially with =a$y virtue of its larger sta$ility constant. An e-tensive interla$oratory studyfound a standard deviation of ).6', and a relative $ias of &.)' for a

synthetic water containing &%L mg5 calcium and L6 mg5 magnesium.

. Proce%*re"&. Clace e-actly ;% ml of the sample into a &6;2ml erlenmeyer

7ask.6. Add 8 ml of >a? solution. If the p is still $elow &6, add

more $ase.8. Add & scoop of 1riochrome Blue Black indicator %.&2%.6 g#9. itrate with the 1A solution until the color changes

completely from red to royal $lue. /aintain continuousstirring throughout.

C. Reagent"&. 1A itrant 0olution appro-. %.%&/# 2 same as with

hardness determination# dissolve 8.D68 g >a21Asodium ethylenediaminetetraacetate dihydrate# in &%%%ml distilled water. etermine titer $y standardizingagainst the standard calcium solution follow steps &29#.

6. 0odium ydro-ide 0olution &># 2 ilute 9% g >a? to & literwith distilled water.

8. 1riochrome Blue Black indicator 2 Prind together in a

mortar 6%% mg powdered dye Esodium2&262hydro-yl2&2naphthylazo#262naphthol292sulfonic acidF and &%% g solid>a=l to a$out 9%2;% mesh. 0tore in a tightly stoppered$ottle.

9. 0tandard =alcium 0olution ;.%% m/# 2 +eigh %.;%%; ganhydrous calcium car$onate primary standard grade#and place in a ;%%2ml erlenmeyer 7ask. =arefully addsmall amounts of < / =l appro- ;%' of conc.# until


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'. edox Titrations

edo- titrations are $ased on o-idation2reduction reactions $etweenthe analyte and the titrant or some intermediate redo- carrier. =ommono-idants used in redo- titrations include dichromate =r6?D26#, iodate I?82#,

iodine I6#, and permanganate /n?92#. =ommon reducing agents are arseniteAs?828#, ferrocyanide (e=>#


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rate of a$out ;' per minute depending on concentration. (or this reasonmercuric chloride is added g=l6#. It apparently inhi$its the reaction$etween monochloramine and C. Although the reaction mechanisms isnGtknown, it is presumed to act $y formation of an unreactive comple- withmonochloramine.

=om$ined residuals are measured after addition of iodide./onochloramine will react quickly with trace quantities of iodide to formtriiodide which then reacts with the C. (or dichloramine, the reaction ismuch slower, and relatively large amounts of iodide are needed for thereaction to go to completion. Both hydrogen pero-ide and persulfate will alsoo-idize iodide, and can therefore interfere with the com$ined residualchloring determination.

he C reagent is also su$!ect to $ase catalyzed o-idation $yatmospheric o-ygen. (or this reason it is kept in an acidi3ed state, andreplaced every month. 0ince the reaction with chlorine and triiodide is $estwhen carrier out at neutral p, a neutral $uHer is used and the C must $e

stored separately from the $uHer and added at the last minute.

(igure &


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8. (ree esidual =hlorine (=#4 itrate rapidly with standardferrous ammonium sulfate (A0# titrant until the red colordisappears eading A#.

9. /onochloramine /=A#4 Add one very small crystal of Cotassium Iodide @I# to solution from step 8 and mi-.

=ontinue titration until the red color again disappearseading B#.;. ichloramine =A#4 Add several crystals of @I a$out & g# to

the solution titrated in step 9 and mi- to dissolve. Allow tostand for 6 minutes and then continue titration until thered color is again discharged eading =#. (or very highdichloramine concentrations, allow an additional 6 minutesstanding time if color drift$ack indicates incompletereaction. +hen dichloramine concentrations are note-pected to $e high, use half the speci3ed amount of potassium iodide.

0 he various forms of chlorine residual are $est calculated

according to the following scheme. >ote that for a &%% mlsample, 9.%% ml standard (A0 titrant J &.%% mg5l residualchlorine.

0pecies (ormula?=l * ?=l A59>6=l B2A#59>=l6 =2B#59

C. REA/E'TS&. Chospate $uHer solution4 issolve 69 g anhydrous >a6C?9,

and 9< g anhydrous @6C?9 in distilled water. =om$inethis solution with &%% ml distilled water in which L%% mg>a61A have $een dissolved. ilute to & liter withdistilled water and add 6% mg g=&6 to inhi$it $iologicalgrowth and to control iodide interferences in the (=titration.

6. C 0olution4 issolve & g >,>2diethyl2p2phenylenediamineo-alate in distilled water containing appro-imately 6 mlconc. 60?9 and 6%% mg >a61A dihydrate. /ake up to &liter, store in a $rown glass2stoppered $ottle.

8. 0tandard ferrous ammonium sulfate (A0# titrant4 issolve&.&%< g (e>9#60?9#6 .


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9. Cotassium Iodine =rystals

3. O6O$- +iodometric method,

Pas2phase concentrations of ozone are most easily measurediodometrically. A portion of the gas stream is directed to a gas $u$$ler 3lledwith 6' @I solution for an e-act period of time. ?zone reactsstoichiometrically to form an equivalent amount of iodine.

?8 * 6@I * 6? 222222222R I6 * ?6 * 6?2 * 6@ *

&


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2. 'I((O"V-' O7%8-$

A. Significance to Environmental Engineering

issolved ?-ygen .?.# is an important water quality parameter for

natural aquatic systems. A minimum concentration is required for thesurvival of higher aquatic life. In particular, the larval stages of certain cold2water 3shes are quite sensitive. 0igni3cant discharges of organic wastes maydepress the .?. concentrations in receiving waters. his occurs due to therapid, micro$ially2mediated o-idation of these wastes upon discharge. (orthis reason each state has esta$lished am$ient dissolved o-ygen standardsthat must not $e violated. hese standards eHectively limit the organic wasteloading and spatial distri$ution of these loads to natural waters.

Another use of .?. is the assessment of o-idation state ingroundwaters and sediments. As samples are collected deeper into thesu$surface or into aquatic sediments, the .?. often drops. 1ventually the

level is low enough so that anaero$ic process predominate.issolved o-ygen is also a very important parameter in $iological

treatment processes. In a gross sense, .?. concentrations indicate whenaero$ic and anaero$ic organisms will predominate. owever, morecommonly, dissolved o-ygen determinations are used to assess theadequacy of o-ygen transfer systems to aero$ic suspended cultureoperations such as activated sludge. It may also $e used to indicate thesuita$ility for the growth of such sensitive organisms such as the nitrifying$acteria.

(inally, dissolved o-ygen is used in the assessment of the strength of a wastewater through either the Biochemical ?-ygen emand B?# or

respirometric studies. Brie7y, the B? test employs a $acterial seed tocatalyze the o-idation of 8%% m of full2strength or diluted wastewater. hestrength of the un2diluted wastewater is then determined from the dilutionfactor and the diHerence $etween the initial .?. and the 3nal .?.

?-ygen is a rather insolu$le gas, and as a result its is often thelimiting constituent in the puri3cation of wastes and natural waters. Itssolu$ility ranges from &9.< mg5l at %o= to a$out D mg5l at 8;o=. In addition totemperature, its solu$ility varies with $arometric pressure and salinity. hesaturation concentration of o-ygen in distilled water may $e calculated fromthe following empirical e-pression4

&


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where4Cvw J water vapor partial pressure atm#

J &&.L;D& 2 8L9%.D%5k# * 6&


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=oncentration $etter represents the "$uHering capacity" against ano-icconditions, and it is more important in stoichiometric calculations.>evertheless, with most engineering application, the two are essentiallyequivalent. ?ther concerns and relative advantages of the two methods areas follows4

+inkler&. oes not require e-pensive or sophisticated equipment.6. It is little hindered $y surfactants and surface2coating species8. /i-ing intensity is of little concern

(igure &


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9. It is non2destructive. hus, the sample may $e returned or used foradditional analyses.

;. It may $e used with highly2colored waters

. Princi!le"

/anganese is rapidly o-idized $y dissolved o-ygen at high p. It theno-idizes iodide to iodine at low p, and the resulting iodine is titrated withsodium thiosulfate. 0tarch is used to o$tain a sharper endpoint.

(inal acidi3cation is employed to reach a p $elow ; where the properstoichiometry for the iodine5thiosulfate reaction is o$tained, however, the pmust not drop too low so that the decomposition of thiosulfate $ecomesimportant. Nnder acidic conditions, the iodine will react with any residualiodide to form triodide anion I8#. It is actually this species which reacts withthiosulfate giving tetrathionate and three iodide anions.

he alkali azide iodide reagent also contains sodium azide, >a>8. hisis a recent modi3cation designed to eliminate interferences due to nitrite. he azide will react with nitrite to give nitrogen gas and nitrous o-ide,neither of which cause interferences.

>a>8 * * 2222222222222R >8 * >a*

&8 * >?62 * * 222222222R >6 * >6? * 6?&6?6 * 66?&6?6 * &56?6 * 6? 22222222222222222R 6>?62 * 6*

&


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$ecause the reaction with dichromate is slow and the color of trivalentchromium may interfere with the endpoint. In either case, a known amountof the primary standard is added to an e-cess of iodide.

6I?82 * &%I2 * &6* 22222222R


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later. In this case it would $e necessary to add additional 6 ml of themanganous sulfate reagent, 8 ml of the alkali2iodide2azide reagent and 6 mlof the sulfuric acid.

D. Proce%*re"&. o a 8%% ml B? $ottle 3lled with sample add & mlmanganous sulfate solution and & ml alkali2iodide2azidereagent, cap and shake. Be careful to avoid trapping anyair $u$$les. If this occurs, remove cap, add a smallamount of distilled water and re2cap.

6. Allow precipitate to settle to a$out half the hight of the $ottleand add & ml conc. sulfuric acid. e2stopper and mi-.

8. itrate 6%% ml of this sample with the %.%6;/ sodiumthiosulfate solution until a very pale yellow color iso$tained. Add a few drops of the starch solution and

titrate until the $lue color disappears.

E. &in#ler Reagent"&. /anganous surfate solution 2 issolve 9%% g /n0?9.66? in

distilled water and dilute to & liter.6. Alkali2iodide2azide reagent 2 issolve ;%% g >a? and &;% g

@I in distilled water and dilute to & liter. Add &% g >a>8dissolved in 9% ml distilled water.

8. 0ulfuric acid concentrated9. 0odium hiosulfate titrant 2 %.%6;/# issolve a606?8.;6? in distilled water. Add %.9 g >a? and dilute

to & liter. 0tandardize with the primary2standardCotassium Biniodate or Cotassium ichromate solutionusing starch to de3ne the endpoint.

;. 0tarch 0olution 2 issolve 6g solu$le starch and %.6g salicylicacid preservative# in &%%ml of hot distilled water.

9. C0-MICA" O7%8-$ '-MA$'

he chemical o-ygen demand =?# of a waste is measured in terms

of the amount of potassium dichromate @ 6=r6?D# reduced $y the sampleduring 6 hr of re7u- in a medium of $oiling, ;%' 60?9 and in the presenceof a Ag60?9 catalyst.

he stoichiometry of the reaction $etween dichromate and organicmatter is4


43/46

&


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-F--$C-(

ACA, A++A, and +C=(, 0tandard /ethods for the 1-amination of +aterand +astewater, ACA, +ashington &9 ed# pp. 6D826L6, or &; ed# pp.69)26;D, or &< ed# pp. 6ew Qork.

@ramer, \.. &)L6# "Alkalinity and Acidity", =hapter 8 in +ater Analysis4Volume &, Inorganic 0pecies, .A. /inear .. @eith editors, AcademicCress, pp.L;2&8;.

/cWuaker et al. &)L8# 1nviron. 0ci. echnol., &D498&298;.

Cerdue, 1./. &)L;# Acidic (unctional Proups of umic 0u$stances, in umic0u$stances in 0oil, 0ediment and +ater4 Peochemistry, Isolation, and

=haracterization, Aiken et al., eds., +iley, Cu$l., >ew Qork, pp. 9)82;6. and C.. /c=arty &)DL# =hemistry for 1nvironmental1ngineering, 8rd 1dition, /cPraw2ill Cu$l., pp. 6926), &


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0ilica 0i?6#o 9.6Aluminum Al#*8 traceIron (e# total trace=alcium =a#*6 &L

/agnesium /g#*6 L.;

0odium >a#* 6.;Cotassium @#* 6.9

Bicar$onate =?8#2 )%0ulfate 0?9#26 ).;=hloride =l#2 &.;(luoride (#2 trace

>itrate >?8#2 %.L otal 0olids measured# &%<

0peci3c =onductanceumhos5cm#

&


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=onductivity J Alktot

In order to account for changes in ionic conductances with changingconcentrations, the ?nsager equation may $e employed4

A J Ao 2

Documents

Chapter Xvi Volumetric Methods