Ultramicro chelometric titrations with potentiometric end point detection

YOI,. I, 1’.2GI:S E-201

Ultramicro

Chelometric metric

Titrations

End Point

with

(l%i)

Potentio-

Detection

Chelometric tibrations with EDTA1 ha\re been sucwssfully applied t,o the ultrarnirrodeternlirlatioll of certain metal ions using various metallochromic~ indkators for end point detection.” These indicator methods, in addition to being tedious :Llld often disturbed by t,he presence of caert)aill other trace metal ions, fsilcd for the aualysis of Furthermore, a certain metal ions swh as barium or strolltium. given metallochromk indivatjor is applicable on a macrowale only to a rest,ri&ed set of metal ions and conditions of pII and buffer type. In contjrast, the mercury electrode has been employed for potentiometric~ end point detect)ion in macwtitratiol~ s of approsimatcly 30 different The purpose of this study uw to adapt this met,al ions with ED’I’A.’ mode of pot,ellt,iometric~ end point dctwt~ion for the analysis of jvholc and fractional microgram amounts of metal ious. The rcsulk indic-ate that. this procedure is a general ogle’, applicable for most metal ions over a wide range of pII using diffrwllt buffer solutions and several t,ypes of chrlolw. In &ending the method from ma(w) to mkro analysis, many details which were of little sigllifkallw and elrell overlooked in macro titrations were found to be of utmost importance. X detailed deswiption of these factors with srqgpwtvd prwautiolw is given.

Functioning of the Mercury Electrode A merwry electrode, in contact with a solution containing metal ions (t)o be tit’rated) and a small added quantity of mewury(TI) * On leave from Sationnl lkse:trc~l~ (‘c~rltw, I~:g~y)t. 1s:(

chelonat,e, HgYzen, exhibits a potential c~orresponding to t,hc following half cell: Hg/Hg;2--7L,MY”-“,

M+z

where Y-” stands for the completely dissociated chelon. The potential for this indicator electrode may he fourrd by combining the Nernst equation for the mercury elect’rode L” = I&,

+ 0.0’2% log (Hg + +)

with bhe equat,ion for t,he stahilit,y chelonate K IIgY

=

(1)

constants of a 1 : 1 mercury(D)

[HgY2-n] [Hg++][Y-“1

(2)

and 1: 1 met’al chelonate [Mu-n] K Ivy = ~+j[y-r,] This yields at 25%. I<‘IIg -- p”‘Hg + 0.0296 log

[M+“] [HgY2-“]Kb,\r [MY?“]Kn,y

(9

From t’his equation it may be seen that the potential of the electrode bears a linear relationship wit’h log M+*, and consequently the electrode may serve as a pM indicator electrode in the presence of fixed concentrations of the metal chelonate and mercuric chelonate. The mercury electrode is therefore analogous to the glass electrode which exhibit’s a pot’ential proportional to pH. Just as acids may be t,itrated with base using a glass electrode, metal ions may be titrated with chelons (such as EDTA) using a mercury electrode. The high reversibility of mercury, its large hydrogen overvoltage, its noble potential, and t,he high stabilit,y of mercury chelonate render this electrode system superior to any other. Detailed interpretation of pot’ential-pH diagrams as described in detail elsewhere6 permits reliable predic%ion of t>he success of a given metal titrat)ion. Such diagrams, obtained experimentally, illustratje t’he caombined effect, of the pH, buffer type and c*oncentrat,ion, the iilhercnt st)ability of t,hc metal chelonate, as well as the effect of imMTCROCIIEhIIC.~~T~

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purities such as the halides. The ext,ent) of the potcilt,iometric+ cild point break can also he determined from such diagrams. Usable end point breaks for macroscale titrations require at least’ 100 my. intcr(SW refcrcwe (i and 7 for dc\-als on the potentialLpH diagram. tails.) Syst~ems exhihitiiig such iiitei7xls arc also satisfactory for ultramicrodetcrminations.

Observations on the Working Methods The purity of the reagents, as well as of t’he water used in preparing the sol&ons, is one of the most dec%iw fartors in t)he suwess of -411reagents wcrc ailalyt’ical grade c-hemultramicrodeterminatiolls. icals or better; in some casts spec~trowopic~pure c~hrmic*alswere used. Ordinary distilled water cannot usually lx used in t,he microgram chelometric dcterminatiolls of the vations either potciltiometrically 01 wit,h visual indicaators. First, the blank \vill he stn-era1 times greater than the amount of t,he metal cation to lw drtermined and, second, some metallochromica indicators cmploycd ill standardizing the solutions will be blocked by t,raws 0f cwtain met al ions, especially copper and iron. Incidentjally, the mercury electrode has the great) advantage of not being blocked hy swh metal ions. Of equal import’ance, hut often overlooked, is the method of (*leaning the glassware and type cont#ainer used for t,he storage of stock solutions. In order to obt’ain thv required sensitivity and response for the mercury clectrodc, several factors which do not notiwably affect, the macro potcntiometric titration, q :111dI\-hivh itlflwnce in a most deleterious way the elevtrodo response ill the Illtrarl~ic~rodettr~lli~latiolls must, be considcrcd. FLh of thrsc factors;, their cffrvt 1 and the manner of removing theso effects was studied ; t#his work is drsc~rilwd MOW. Hy applying t,hc precautions suggested, the determination of microgram quantities of metal ions WLS readily awomplishc:d. the results being of surprisingly high reprodwihility and accuracy. (a). Effect of Halide. Traces of halide ions affect the merwry electrode potential through the formation of insoluble mercurous halides. This cffrvt is illustrated in Figure I. In this experiment an ordinary calomel clwt rotlc \I-it,h a11 ngar bridge (fkle orifice) was c:mployctl for the tlctcrillitlatioll of a lop4 AI wpper solution at, pIl 10. The three vurvw wcrc ol)tainc,tl at \wious time itlttrvals after the insert~ion of the c*alonic~l ~lwtrod~~: (w77~ (z after 2 niitrutcs, curate h

186

F. S. SADEK

AND

C. N. 11EILLEY

after 5 minutes, and c~uvc c after 30 minutes. The diffusion of the chloride from the calomel elect,rode causes the end point break to become more drawn out with a corresponding greater error in the

1

MICROLITERS

Fig. 1. IMcct

of chloride

EDTA

on titration

~~nw~s.

detjerminatjion of the equivalence point. The effect of halide is even more severe in acid medium. In some determinations the use of chloroacetic acid was abandoned because of the presence of chloride as impurit,y. (b). Effect of Oxygen. Titrations of microgram amounts of different cations in alkaline medium are affected seriously by the presence of oxygen which tends to oxidize mercury under t,hese conditions. This mixed pot,ential difficulty can be alleviated by bubbliug nitrogen With this prethrough the solution after each addition of titrant,. caution good end point,s mere obt,ained for the titration of barium and lead in alkaline range. Oxygen had no effect, on titrations carried out under acidic> conditions where the mercury elwtrode potentials are more positive t,han the reduction pot8ellt,ial of oxygen. (c). Effect of Buffer-Type and Concentration. 13uffers always decrease the pM break at the end point because of their complexing 3lICROCHEMICAL

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action on the metal ions to be determined. However, buffers must be present to maintain the desired pH during the course of the t’itration and to keep the metal ions from precipitating. In this last respect a buffer should be selwtcd whicBh exert,s complexing ability sufficaient to keep the metal ions in solution but yet not, cause a considerablr decwnsc in the pM break. Occasionally mixtures are employed. 111 the vase of lead, amilloethanol was employed as a buffer to maintain the proper pII and a minimum quantitjy of tSartaric avid was added to keep the lead in solution. :2minoethanol, being less \-olaClc than ammonia, \vas ~~mployed to minimize> loss of buffer (aapacity as ilitjrogclr i,q bubbltd through the solutioll. (d). Surface Area of Mercury Electrode. ‘l’hc titration ~11 was wlrstructed in a way to minimize the surfaw area of the mercury. This feature dccrcaws t hc introdwt.ioll of titratable species present, OII the mcrcwry surfaw and dwrcasrs the quantit,y of solution that, might swp betwwl the mercury and glass vontailw. (e). Rate of Reaction. Rwvtions which proceed reasonably rapid ill mncsrodetcrminat ions will oftell proceed quite slowly in the very dilute c,o?r~t~llt’ratio?ls prcscnt in the determination of microgram amounts. 111these CMJS one must wait after ewh addition of tit,rant; otherwise the resulting potent iomctric titrat,ion curve \vill be clistortcd and drawl out. (f). Concentration of Mercury Chelonate. The concentration of With larger conrelrmercury chelonate was found to be not critical. tration t,hc cxt,wlt of the end point break remained the same although the absolute potrlltials ww more positive. A c~onccntration of ‘2 X 1OF dl mrrcwry cshelolratc ill the working solution was suffic~irntJ tjo stabilize t)lie potential readings. (g). Treatment of Glassware. Glassware for ultramicwdetcrminations must 1~ of I’yres and pre-treated to decrease the effect, of the ion eschangt~ property of the glass walls and to remove ana reacti\-c m&l iolw prwellt. 011 the surface of the glass.” The glassware QXS t,rcated with hot dilute nitric acid; and, after rinsing wit,h waler’, was treated \\-ith a strong, alkaline EDT,4 solut,ion (about .!Yj&), heated t,o boiling, a~rd left ovcr~light. .!fter remo\-ing the alkaline EDTA solution, the glawvarr ~-as rinsed WVcral times with demineralized KdtcT. Then all glasswaw (piprts, buwt, titrating cell, the caalomel elwtrodc (outsitltl), the nitrogen tube, as well as the titrating t,cst tubes I\-ith their stirring rods (whicah are usrd ill stalldard-

188

IIll’ -

c-z_

GILMONT

CALOMEL

ULTRAMICRO

CELL

CORK STOPPER

I\\

BURET

WITH

3 HOLES

ARM

(1m/=S”PPORT

i 0 9 a,

NITROGEN

r

MERCURY PLATINUM

Fig. 2. Tit,ration

BUBBLER

(STIRRER)

POOL WIRE

cdl.

izing viswlly the different metal solutions) were t’reated with Desitote (Beckman Instrument Co.) and, after drying, rinsed again wi-ith demineralized water. The insulating Desicote layer covering the platinum electrode was removed by rubbing the platinum wire with a long needle whose tip was covered wit’h a small ball of cotton wet with chloroform. The buret tubing was treated with chromic-sulfuric acid mixt.ure and, after washing, was left for several days filled with alkaline EDTA solution. It was then rinsed several times with demineralized water, treated with Desicote, dried and washed. Then the buret was refilled with mercwy and rinsed several times with t,he titrant. MICROCIIEI\IICAL

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~I,TRh.\lI(:I6O

CHISI,O~II’TI11(’

SPECIAL

TITIL1TIOKS

189

EQUIPMENT

1. Ultramicrohurct (Gilmont) : Total displacement 0.1 ml., subdivided in 0.0001 (I ,‘IO,OOO) ml., direct reading, ac*cwrac*yO.OZc/;, of t’otal displacement). With fine orifice. 2. Titration cell : Illustrated ill Figure 2. Madrl of Pyrex glass. X Calomel reference electrode; The elwtrode (with side-arm salt bridge) is prepared as usual except the usual KC1 solut,ion is replaced entirely with agar ngar containing 1X0,. 4. Pot)cntiomet)cr: I,eeds and Northrop pII meter type 7(X4. mult,iplier: The sensitivity of the type i1Xi-k 5. Sensitivity meter was increased to l-t0 mv. full wale by the following simple device. A 120 ohm resistor (momlted in an Amphenol MCWl plug) is inserted in the “Auto. Temp. Camp.” jack and t*he “Temp. Camp.” swit,ch is thrown to ‘LAuto” position. 6. Polyethylene bottles of different sizes were used for all buffers, titrants, and standard solutions. The titer of dilute solut,ions st)ored ewn in pre-&abed Pyrex bott,les changes rapidly. 1 REAGENTS

AND SOLUTIONS

1. Demincralizcd Tvater: The usual laboratory distilled wat,er is passed through a Bantam demineralize1 apparat,us and stored in Pyrex carboys pre-treated as described above. 2. Ethylenediamine t,etraacctic acid (EDT:\) standard solution : The solid disodiunl dihydrate of ethylerlediamine tetraacetic acid is dried according to Blaedel and Knight.’ :3.i23 grams are weighed and diluted to 1 liter. This solution is 0.01000 AT. Immediately aft,er preparing, the sohltion is transfcrrcd to a polyethylene bottle. The dilute solutions are prepared as nccdrd by dilution. Thcsc dilut’ed t,it,rants were rwhwkrd by standardization ngaillst zinc nitrate standard solutiolw with Eriwhrome black T (F241-CI203) and pH 10 buffer. X Metal salt solut,ions were prepared by wtighillg the approximate amounts of salts (using high purity c~hcmic&) to prcparc 0.01 32 solutions and standardized against EDTA solut~ion. 4. Indicsators for visual standardizntiol~ of solutions: (a) Eriochrome black T. I%%-CI%OX Solid illdi(aator, finely ground with sodium chloride in t,ht> proportion of 1 :-LOO. (b) PAN, 0.01% ill ethyl al(*ohol. (c) I’yroc*atwhol viol&, O.OY~~, in demineralized

100

k’. 8. SADISK AND C. N. REILLHY

water. (d) Platin fast, blue GGNA, Pr 144, 0.05% in methyl alcohol. For calcium, pH 12. (e) 7-(l-sulfo-l-naphthyl-azo)-S-hydroxyquinoline-5-sulfonic acid, O.OS”~cin water. 5. (a) Buffer pH IO: Dissolve 13.5 grams ammonium chloride in 100 ml. demineralized water, add 88 ml. of concentrated aqueous ammonia, and dilute to 250 ml. with demineralized water. Keep in polyethylene bott,le. (6) Buffer pH 10.7; 0.1 M: Dissolve 3.05 grams of 2-aminoethanol in 400 ml. water, adjust with a pH meter to pH 10.7 by adding nit)ric acid, dilme to 500 ml., and store in polyethylene bottle. (c) Buffer pH 9.2, 0.1 M: Prepare with 2aminoet,hanol as above but adjust, t’o pH 9.2. (d) Buffer pH 4.5, 0.1 M: Dissolve 4.10 grams of crystalline sodium acetate in 200 ml. demineralized water, adjust to pH 5.0 by adding acetic acid, dilute to 500 ml., and store in polyethylene bottle. 6. Mercury-EDTA: 0.0005 A/: Mix exactSly 10 ml. 0.001 M mercuric nitrat,e solution with an exactly equivalent amount of EDTh solution. Neutralize with sodium hydroxide to pH 7. 7. Metallic mercury: Rcdist,illcd mercury is shaken with dilute nitric acid and rinsed several tirnes with demineralized water. 8. Desirotc: Beckman Instrument Company. THE BLANK Precise characterizat,ion of the blank present in each of the reagent’s is a decisive fact,or in the ultramicrodetermination of different cat)ions. For example, suppose that the standard EDTA solution contains t,raccs of magnesiuni and zinc. Three different titers will exist for t,his single standard solut~ioil. In very acid solution (pH 2) the rnagnerium and zinc irnpurities are not complexed by t’he EDTA whereas in moderately acid solution (pH 4.5) only zinc ion is bound. At, pH 10 both zinc and magnesium are bound. Consequently, standardizat,ion of EDTA at, pH 2 wit,h bismut’h or mercury yields a higher titer value than at pH 4.5 (with cadmium or zinc). Furthermore, the titer of EDTA obtained at pH 4.5 will be higher than the t,iter obt,ained at pH 10 (with calcium). The presence of cyanide will increase t.hc value of t,he titer obtained at pH 10. In an analogous way the blank \-alue for the reagents will vary with pH and masking agents. Consequently the blank value of reagents, as well as t,he standardizat,ion of metal ions and chelons, must, be det,ermined under ideutical conditioils. ~lICROCHE:hIIChI.

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The dcterruiuatioii of the blailk usiiig nlt~talloc~hlomic indicators is especially difficult bwausc cwtaiu mustituents composing the blauk may ilot, be auw~iablr to tit,ratioii wit,11 auy esistiiig iiidicators 01 they may block the usable 011es;. A-Usetwnusc the six of the blauk is so sinall, ac~~iuatc and ol)jwti\ c clttc~i.millatioll of its magiiitude is cxt~rciuely tlifhcwlt. The nicwru~y clrc~trotlc docks iiot suffer such disadrantages alit1 direct, ol)jcct i\-c cvaluatioii of the btalik iuagtiit udc is easily asccrtaiiied. It VW formtl that the blauk origiirated mniuly from t,he dcmii~ci~alizc~d \wt cl*; the iuaguitudc of tht: total l)lalik at pII 10 was npprosiinal cly 0.02 iuic,logrnnl.‘l111. calculated as ruagiicsiwu. The blnuk at plI 2 mw iicgligiblc. PROCEDURE The pretreated t~itratiou cell is rillsed sc\-cral times with demiueralizcd water aud one or more drops of c*len11n~rcury are ptacwl iii the vrsscl. Thr rnrrcury must just completely cover the plst,iuuul wire. (?IIC ml. of t,he desired collc,elltr.atioll of the sample, two drops of the appropriate buffer (0.1 .I/), mid oiic drop of mercury EJ>TX (0.0005 JI) are added to the ~cssel. The titration cell is cm.ered with a three-hole cork stopper and, after thr tip of the mieroburct~ is iiisert rd iii the solutiou, the titmtioil cell is suspcuded over au opeucd clamp, the weight, of the vessel bciug supported by the t\vo arms. dft~cr iusertiig the calomcl clectrodc and the llitrogcll-hubl~lil~g tube, a slow steady stream of iiitrogcu is allon-ed to pass through the solutioii for 2 niiiiutcs. Thr elcctrodrs arc couiiected to the potmtiometcr aud the titratioii is c~mnwucwl: after each addition of titraiit, :N sec~~iids arc allowed to elapse before each potwtiornctcl readiiig. Iii cases where osggcii interferes (e.g., barium aud lead iii alkaliuc solutious) a11c1iii cast I\-hew the rcnctiou is slow (such as lauthauum aud most \-cry dilute c,ollc.entratiolls) 1 miuutc \vaitiug inlrrvals are en~ployetl up to the cid poiiit aiitl 2 miiiutcs iii thr rcgioii of the eiid poiiit.

from the cell but is used over and over again in subsequent titrations. The tip of the buret, the nitrogen-bubbling tube, and the tip of the calomel electrode are rinsed wit’h demineralized water and dried with narrow strips of filter paper. The buret is refilled with desired titrant, rinsed again with demineralized water, and dried. The cell is nom ready for the next determination.

RESULTS Previous work6J illustrated that a large number of metal ions could be titrated chelometrically on a macroscale using the mercury electrode. While most of t,hese cations could be titrated on the ultramicroscale, only a few of bhese metal ions were selected for t,his study. The metal ions were selected on t,he following basis: to illustrate the applicability of the method wder a wide variet,y of solution conditions (pH and buffer type) and the applicability in cases where the failure of met’allochromic indicat’or was known.

Direct Titration in Alkaline Media Copper. The direct titration of the ult,ramicrogram amounts of copper in alkaline medium were successfully carried out potentiometrically. This procedure indicates the usefulness of t’he mercury clect’rode for cases where metallochromic indicators fail. Murexide, for example, may be employed for the titration of macro and micro amounts of copper but completely fails in the case of ultramicro quantities.

Concentration, Amount Amount

Standard

M -+

present, fig. found, pg.

deviation

TAHLIS I I ktcrminat~ion of Copper IO-4 5 x 1OP 2 x 10-s 6.35 6.35 6.45 6.32

3 .34 3.26 :3.30 :%.35

6.3i

3 36

6.33 6.31 0.051

3.39 3 36 0.038

I .30 I .31 1 .30 1 .25 1 .32 1.30 1 .30 0.025

10-S

5 x 10-C

IO-6

0.57 0.50 0.59 0.56 0.54 0.58 0.54 0.024

0.29 0.31 0.29 0.28 0.29 0.32 0.29 0.017

0.073 0.078 0.076 0.078 0.071 0.071 0.065 0.005

The blank was determined by pip&ing 1 ml. demineralized water into the titration cell; then two drops of buffer (pH 9.2) and one ~IICROCIIEBIIC.~L

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drop mercury chelonate were added. The titrant was added in 0.1 microliter increments alId, after waiting one: minut’e, the pot’ential is read using the sensitivity multiplier. The end point break was approximately 40 ml-. LZ blank wlue of about 0.7 microliter was Copper solutions at Yarious concent~ration levels were found. titrated in the same I\-ay, the results appearing in Table I. The copper solutions were standardized in micro and ultramicroscale with PAX (acetate buffer, pH 4.5) and with 7-(A-sulfo-lt~aphthyl-azo)-8-l~ydr(~x~~(~~~i~~(~li~~c 3-sulfonic* acid (acetate buffer,

O-----t. \ \(c)

IO-’

M

I 0 MICROLITERS

EDTA

Fig. 3. Ultramicro t,ilmtion of copprr.

pH 5.5). On a microscale t,he solutions were also standardized with murexide (ammonia buffer, pH 11). The potentiometric result’s agreed well only with the murexidc standardization. The low copper values obtained in the case of tit)rations in acet’ate buffer is attributed to t.he presence of calcGm1 and/or magnesium in the demineralized water. This emphasizes the importance of determining the titer and blank under idcnt,icul condit,ions. As illustrated in Table I, reliable results of copper present) as lo\\- as 10P6 dl wrc possible.

Titrations with 0.1 ml. total sample volume..‘s allowed estimation of 0.007 microgram of copper wit)h t,he same awurwy and reproducibility. The method could probably be ext)ended t)o even lowr levels. Figure 3 illustrates the titration of copper at different concentrat,ion levels. Lead. After pipetting 1 ml. of the lead sample into the titration vessel, 1 drop of sodium t,art,ratc is added and the solution is stirred by nitrogen. Then t#hrce drops buffer (pH 9.2) and I drop mercury EDTA are added and the titrat,ion completed as in the case of copper. Blanks containing t’artrate are carried out in t,he same may and subtracted from the results. Careful removal of oxygen is necessary as mentioned earlier. The results for the direct titrat,ion of lead arc given in Table II. Although only 4 differentI concentration levcla were studied, lower caoncaentrat’ionscould probably be estimated w&h good wcuracy.

Concc~ntrat~ion, M + Amount, prcsent~, pg. Amormtj f’ound, pg.

St:tndard

deviation

TABI,IS Jkdnminnt.ion 10-J 21 :30 21 .S!J 21.38 2 1 .:12 21.38 20 !I5 0 23

II of‘ Lcntl 5 x IOF

2 x 10-b

10 5

10.12 If. Ni 10.15 9. !U 10.18 IO Xi 0 13

1 l!l 4.:it 1. I:< 4.16 1.25 1. I6 0.07’7

1.81 1.81 I 95 1 Xl I .i8 1.72 0 .08.i

Calcium. Calcium was titrated using the same procedure as cmployed for copper. The results are represented graphically in Figure 4. The heavy diagonal represent,s the theoretical value and the upper Tht> and lower lines correspond to =r=O.1 microgram deviation. experimental values are well within thr +O. 1 deviatiorr. Barium. Hecause of the low stabilit,y constant of barium EDT& direct tit,rations of microgram amount,s with metallochromic~ indicators are especially difficult.. At pH 10.5 quantities of barium between 5 and 10 micrograms could be determined potentiometritally w&h reasonable accuracy. Oxygen interferes seriously wit,h the tit.ratiorl and must be complet’ely removed by extensive bubbling The end point, “break” at of pure nitrogen through the solution. lo-” M levels does not exceed 40 mv. (SW Fig. 5). MICROCHERlIC.IL

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MICROGRAMS

Cd’

PRESENT

Fig. 4. Results for c:tlcilm.

\a)

IOyg. 90’.

MICROLITERS

Fig. 5. Titration

E DTA

of hillm.

50

(b)

10

lc)

196

F. S. SADEK

AND C. X. ItEILLEY

Back Titration in Alkaline Medium Nickel. The react,ion bet’\zechn nickel and EI)‘I’A proceeds slowly even when present in macro amounts; therefore it) was not surprising that the direct determination in ultramic.ro amounts was impossible. The following indirect procedure is employed. Mix 1 ml. of nickel solution with a small known quantity of EDTA in excess and 1 ml. of pH 9.2 buffer; allow the solution to stand. The standing time depends on the concentration level, t’he cwwentration of EDTA in excess, and the pH. The standing period recluired may be reduced Add 10 drops of mercury EDTA (0.0005 M) by heating the solution. and dilute to 10 ml. Using 1 ml. aliquot, back titrate the excess EDTA with standard zinc solution. The results are given in Table III. TABLE III 1)ctcrmination of Nickel Taken, Pg.

lo-~” Al Found, Pg.

Different Pg.

5.870

5.876 5.867 5.862 5.876

+ 0 006 - 0 .003 -0.008 +O.OOG

5.88

5.96 5.85 0.77 5.93

Determination +O.OS -0.03 +o.s9 +0.05

Taken, Pg. 2.98

of Cobalt 2.94

5 x 10-5&f Difference, Found, rg. rg. 2.97 2.97 3.OG 2.97

-0.01 -0.01 +O.OS -0.OI

2.90 2.29 3 50 2.98

-0.01 -0.65 +0.65 +0.04

Cobalt. The procedure is identical to that of nickel and the results are given in Table III. Direct Titration in Acetate Buffer (pH 4.5) Lanthanum. Dirert titration in the moderately acid range is applicable for the ultramicrodetermination of a large number of metal ions. Lanthanunl was chosen for illustration. One milliliter of met’al ion solut’ion, 2 drops of acetate buffer (pH 4.5), and 1 drop of mercury EDTA a,re added to t,he titration vessel and mixed by RlI(‘KOCHEMIC.4L

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C:HI~~I,OhlE:TltIC:

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bubbling nitrogen through the solution. (Xot’e: n’itrogen is employed here only for stirring the solution as oxygen does not interfere.) The react,ion between EDTA and lanthanum proceeds quite slowly, and the waiting intervals before the potentiomet,ric readings must be approximately 1 minut,e prior to the end point, break and 2 minutes

Concentration, Amount Amount

Standnrd

TABLE IV Iktrrmination of Lnnthanun~ 31 10 -4 5 x 10 -5

present, pg. found, pg.

deviat’ion

1:3.01 12.!)7 12 99 12.97 13 .o:i 12. 99 1x0s 0.043

ti.52 6.50 ti.48 6.58 ci 50 6.54 6.5-l 0.0x

2 x 10 5

IO-6

2.77 2.73 2 .8:3 2.73 2.81 2.70 2.72 0.048

1 .4:3 1.14 1 .‘&I 1 .4:3 1.40 1.40 1.40 0.027

in the region of the end point’. Incidentally, many chelon reactions appear to take place more rapidly in alkaline medium than in acetate buffer; a detailed kinet,ic study may prove interesting. Manganese. Trials t,o determine manganese by direct titration in acetate buffer were unsuccessful. This failure is attributed to the low effective stability constant of manganese-EDTA at this pH.

Back Titration in Acetate Buffer (pH 4.5) Aluminum. Aluminum is of special interest because of general lack of success of t,he microtitrations using met)allochromic indicators. Although the direct titration of aluminum with potentiometric end point was successfully accomplished in the microgram amounts, the back titration procedure was the m&hod of choice because it was less sensitive to solution conditions. In either the direct or back titrat)ion t,he key to a successful determinat,ion lies in the proper method of neutralizing the solution.” III the direct determination the hydrolysis polymers of aluminum are first fragment,ed by boiling t.he solut,ion in the presence of nitric acid. Then t,he pH is adjust,ed t,o 4-4.5 by addition of pH 5.0 acetate buffer. Alkaline solutions (ammonia or sodium hydroxide) must never be used for neut,ralizat*ion because the extensive hydrolysis of aluminum in the vicinity where the base drops into the solution. Such hydrolyzed species are extremely stubborn in returning to UII-

hydrolyzed species and low results are usually obtained. In the back Litration, neutralization with alkaline solutions will cause hydrolysis of aluminum, even when bound to EDTA, with similar erroneous results. The results for t’he hack titration are given in Table V. TABLE

V

IMermination of iiluminum Taken, rg.

IO-4 nr Found, ia

2.696

2. 736 2.696 2.io4 2.i36

IMfercnrc, PR.

Taken, PY.

+0.04 fO.OO -0.08 +o 04

1 .35

5 x IO-6 nr IMference, Found, Pg. Pg. -0.12 +o.ot3 -0.12 to.11

1 23 I .43 I 23 1.46

Direct Titration in Acid Medium (pH 2) In such acid solutions only few metal ions form st’able chelonates with EDTA, and such metal ions therefore may be titrated selectively in the presence of metal ion forming less stable EDTA chelonates. Such titratable metal ions include Hg, Bi, Th, Zr, Hf, and Fe. Iron gives rise to mixed potential readings but nevertheless can be titrated in the same mamler with reproducible results.

Conrmtration,

dl -+

Amount present, pg. Amount, found, pg.

Standard

deviation

TABLI: I Mermination IO-” 22.28 22.30 22.28 22.28 22.28 22. OR 22.16 0.10

VI of Mercury 5 x IO-5

2 x IO-5

11.14 11.12 II .I2 11.12 11 .I2 II .20 11.12 0 OR3

4.30 4.:35 4.25 4 30 4 30 4.40 4,2!1 0.055

IO-5 2.10 2.08 2.04 2 11 2 I7 2. I4 2.06 0.055

Because the particular metal ions which compose the blank do not form stable chelonat’es at pH 2, the blank present under these conditions was found to be negligible. The mercury electrode exhibits more positive potentials in the more acid solutions.6 Consequently, t’he effect of halides is more severe in this pH region. MICROCHEMICAL

.JOKJRNAL.

VOL.

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ISSUE

2

Mercury. One milliliter of sample, 2 drops of L;1o N nit,ric avid, and 1 drop of mercury EDTh I\-ere added to the tikation vessel and stirred by means of nit,rogen. The solution was titrated directly with EDT,\. Cn,ution was taken to minimize leakage of cxhloride from t,he czalomcl electrode hy tit rat,ing reasonably cluickly. The result,s are given ill Table \‘I. Bismuth. The titration of bismuth ~-as identical to the titration of mercwy and the results are given ill ‘l’ahlc 1711. TABI‘b: Iktrrmimttion 10-J 2 I ti:< 21 71 22 00 21.42 21 .iI 21 57 0.22

DISCUSSION

VII of Rismllth 5 x IO-5

2 x IO 5

IO 5

1.81 1. (i4

2. -I!) 2 4; 2. -iti 2.55 2.4% 2.33 0 056

11 IO II 1-I 11.1-l IO. 86 1 I .oo II I4 0 II

1

78

4.92 -1 x:4 4 86 0.11

OF RESULTS

The results given in the tables and figures for the microgram and fractional micsrogram determinat)ion of different metal ions hy the prwedures outlined illustrate a surprisingly high order of reprodwihility and awuracay. Mwh of the suwess is attributed t,o careful adherence to the suggeskd prwautions and full caharacsterizat)ion of the blank. This study represents the first application of ehelometrk titrat)ions to the determinat)ion of miwogram amount,s of metal ions using a general elevtromctric end point met’hod. Xikelly and Cooke5 ivere able to Ctratc copper amperometrkally in cbonrentrations down to 1O-7 dl hut t’heir cell required 100 ml. sample volume. Furthermore, their twhnique required Glut the met,al ion king tit’rated bc reducible at the mercury pool; in contrast t,he mercury elertrode method dewrihed in this paper has general applkability. The suww of visual ult,ramicrodeterminations depends upon the al-ailability of a suitable met)allochromic* indicator, proper illuminat,ion, and the trained eye of t)he analyst. Eye fatigue and the difficulty of deteckg extremely small valor changes arc major problems in the

200

F. S. SADRK

AND

C. N. RETI,I,EY

visual det,ermination and are completely eliminated in the potentiometric method. Because of the non-availabilit’y of a metallochromic indicator of genwal usefulness, t’he characterizat’ion of the blank by visual determinations is somewhat difficult. In contrast the mercury electrode is sensitive to all metal ions which may be complexed under the solution conditions employed and this is a great advantage. Certain extensions of this work appear promising. EDTA itself is an unselect’ive chelon but in conjunction w&h the judicious choice of other chelons such as the polyamines, and the proper use of t’he pH effect, selective titrations can he achieved in many situations. In very acid solut,ions, for example, mercury could be selecbively titrated in many pharmaceut)ical materials or t)horium in the presencseof t,he rare eart,hs. Likewise bismuth could he selectively titrated in t’he presence of lead and zinc. In fact, bismuth was titrated in the presence of cadmium with satisfactory accuracy. Triebhylenet,et,ramine complexes cobalt, nickel copper, zinc, cadmium, and merrury much more tightly than the rare earths, the alkaline earths, lead, aluminum, or bismut’h. APplications of this reagent are t,herefore obvious. Examples such as these indicate t,he unusually wide scope of application of the proposed met’hod.

SUMMARY Ultramicro chelometric determinations at concentrations well below the range of conventional elect’rometric methods are reported. For example, the titrabion of 0.006 microgram copper in a total volume of 0.1 ml. is entirely feasible. The mercury indicator electrode is highly sensitive and easily employed without the fatigue experienced when visual indicators are used. The determination of microgram amounts of different cations with pot,ent,iometric end point detection requires careful adherence to precautions that may be overlooked for titrations on a macroscale. A full description of these precautions is given. The procedure is a general one, applicable to the determination of microgram amounts of many metal ions over a wide range of pH, using different, buffer solutions and several t’ypes of chelons. This research was supported by the United States Air Force through the Office of Srientific Research of the Air Research and Development Command. AIICROCHERIICAT,

JOURNAL,

VOL.

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UI,T11AMIClto

~:HI:T,O~II~:TILI~‘

TITII.\TIONS

201