Enhancement of the Sensitivity of Tungsten,'~~"Determination by Catalytic Current Polarography

Bharathlbai J. Basu. D.K. Padma* and S.R. Rajagopalan

Materials Science Diuision. National Aeronautical Laboratory. IJANGAl.ORE - 560 017.

• Department of Inorganic and Physical Chemistry. Indian Institute of Science.IJANGALOUE - 560 012.

A new method based on catalytic current in differential pulse polarogrms in a medium containingchlorate and 8-hydroxyquinoline (oxine) is described for the determination of tungsten.Experimental conditions such as pH and the composition of supporting electrolyte are optimizedto get a linear calibration graph and also to lower the detection limit. The influence of possibleinterferences on the catalytic current has been investigated. The mechanism of the electrodeprocess Is studied using DPP diagnostic criteria.

Polarographic methods for thedetermination of tungsten are few innumber in the literature. In 4 - 10 Mhydrochloric acid. W(VI) gets reduced toW(Vl which gets further reduced to W(IIl)11). A medium containing phosphoricacid and sulphurlc acid has been usedfor the differential pulse polarographic(DPP) determination. of tungsten innuclear fuels (2). Catalytic reductionof W(VI) in the presence of hydrogenperoxide or chlorate has also beenreported [3-7) but the shape of the wavewas often unsatisfactory. Recently. areport appeared for analysis of W(VI)-2- mercapto-benzo-thiazole complexbased on the catalytic hydrogen currentin polarography (8). In the present work.the differential pulse polarographicbehaviour of W(VI) in a mediumcontaining 8-hydroxyquinoline (oxine)and sodium chlorate has been reported

J. Electrochem Soc. India, Vol 43 - 1 (1994) 19 - 25

and the most suitable conditions for thedetermination of tungsten are described.The mechanism of the electrode processis examined using DPP diagnostic criteria.

EXPERIMENTAL

Apparatus and Reagents

The polarograph and the cell assemblywas described earlier [9J. For DPPmeasurrnents. the pulse duration was0.04 s and the pulse amplitude was 50mV. The rate of flow of mercury was1.862 mg/s. The droptime wasmechanically controlled. All thepolarograrns were recorded at 25' C.

All che.nicals were of analytical reagentgrade. 11 standard solution of tungsten(1000!l g/ml) was prepared from sodium

© ECSI, Bangalore

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20 Enhancement of the Sensitivity of Tungsten Determination· Basu. et. at.

tungstate and diluted as required. Oxinesolution (0.10 M) was prepared in 0.25M hydrochloric acid. Sodium chloratesolution (1.0 M) was prepared In water.

Required volumes of individualcomponents were added to the cell. pHwas adjusted and test solution wasprepared in the cell. Solution wasdeaerated and polarograrns recorded.

RESULTS AND DISCUSSION

Differential pulse polarograms oftungsten.

The peak current in DPP of W(VI}wasOf poor sensitivity in oxtne medium at

c:!..:::0U

- 0.6 - 0.7 - 0.8 -09E IV Versus set

Fig. 1 Effect of Chlorate on DPP of W(VI)in oxine mediumDPP conditions: t = 0.5s: t:.E = 50 mV:scan rate = 2 mV Is. (W(VII = 3.0pprn: pI! 1.7 ± 0.1(A) 0.005 M oxine; (U) 0.005 111exine + 0.05 M sodium chlorate.

J. Electrochem Soc. India

pH 1.7 (Fig. lA). In presence of bothchlorate and oxtne at the same pH. thereduction peak Increased In height (Fig.IB). Enhancement of the current wasobserved to be about>30times.

c

rl100M

(1)

Effect DC Oxine on DPP of W(VI} inchlorate mediumDr!' conditions: t = 0.5 s: t:.E = 50mV:scan rate = 12 mV Is: pH 1.7 ± 0.1(A) O. 10M sodium ch lo ra tc(0) W(V!) = 5.0 pprn in 0.10 M chlorate(e) 0.005 III oxine in O. 10M chlorate(J)) W(VI) = 5.0 ppm in 0.10 Mchlorat<+ 0.005 M oxine (E) Same as D Nouthat the firsl peak height is recorderat different sensitivity

Vol 43 • 1 (1994

Enhance~nt of the~sitivity of Tungs~ Determination- Basu;·et.a/.

Sodium chlorate solution at pH 1,7 gave'a })PPwlth a l~fge. broad peakaround

-0.iV (Fig. 2A). Recording a polarogramImmediately after adding tungstate tothis. Medium showed two peaks (-0.35

~ v:\{id'·-O.~oV)lnadcUtion to the chloratepeak (Fig.2B). The peak at-0.5V had$UghUy higher sensitivity but the peakcorrent decreased rapidly with time.Subsequent runs showed lower peakheight. (This may be due to hydrolysisof W(VI)at this low pH to give differenthydrolysed species). On adding oxlneto chlorate medium at pH 1.7. both peaks

/

shifted to more negative potentials (-0.4and -().68 V)and also increased In height(Flg.2E. It should be noted that the twopeaks were recorded at differentsenstttvttes. If the two peaks are recordedat" the same senstttvtty, the first peak

, ~most becomes un-noticeable Fig 2D).The peak heights of these do not decreasewtth time. They are reproducible withtime. W(VI) appears to get stabilizeddue to complexation with oxlne and doesnotlmderg?hydrolysls.

.~:.

i

!IiII

'J.J

I

II

22 Enhancement of the Sensitivity of Tungsten Determination - Basu, et. al.

The effect of oxlne concentration on theDPP of W(VO has been examined in amedium of 0.05 M sodium chlorate atpH 1.7. The height of the second peakincreased with oxme concentration andbecame a constant above 0.005 M oxme.So in further studies oxine concentrationwas maintained at 0.005 M.

In 0.005 M oxtne medium adjusted topH 1.7, the effect of chlorateconcentration on the current of thesecond peak in DPP of W(VII is shownin Fig 4. Initially the current increased

~,0.6 B

o.z

0.2

0.1 0.2 0.3

!0. 0.4

0.2

I'L- ~:----L--'--_.---'---L-..__.!

0.02 00< 006thtoroh conc.ntrQtion 1M

Fig.4. Effect of chlorate concentration onthe peak current of W(VI)S.E. 0.005 M oxrne at pH 1.7 ± 0.1tungsten = 1.0 ppm.A peak current vs Ichloratcl: B. peakcurrent vs [lchlora tel] 1/2.

/. Electrochem Soc. India

rapidly with chlorate concentration, butIncrease was only slight above 0.05 Mchlorate, . (Flg.4A). Further at higherconcentration of chlorate, the Irreversiblereduction of chlorate gave rise to a broadpeak at about -0.72 V (Flg.2A). Thisinterferes' with the determination oftungsten at very low concentrations. Sohigher chlorate concentration has to beavoided. Hence 0.05 M chlorate waschosen as the optimum concentrattonfor the determination of tungsten by thismethod.

So a medium containing 0.05 M chlorate,0.005 M oxine and adjusted to pH 1.7± 0.1 was the supporting electrolyte usein the present method.

Mechanism of the Electrode

Tungstate Is known to be reduced itwo steps in strongly acidic, aqueoumedium. The first is due to reductioof W(vIl to W(V) and the. second is duto reduction ofW(VlIri-W(IIJ) (10). It ireported that the first is an Irreverstblstep and the second is reversible.

The second peak in the DPP of W(VIhas been selected in the present studIt is already seen that this representreduction of W(V) to W(III). W(VI) form1:2 complex of the type W02(OX)2 witHoxine. We found that this complex get,reduced at DME in two steps - first 10WO(OH)(Ox)2 and this gets furtherreduced to W(OH)(Ox)~.

Reduction step of W(V) to W(IlI) in ac«medium Is reported to be reversible {IojIn the present case, the height of thi'

Vol 43 ·1 (1994

rrmswill!gcl~

5 t I.

-t hcr

[101

. t h i:

(1994

al. Enhancement of the Sensitivity of Tunqsten Determination - 8asu, et. al.23

utM

peak current is unduly high. Alogarithmic analysIs of the correpondingDC reduction step [W(V) -> W(IIl)) in thismedium showed that the electrodeprocess is irreversible with a nn valueof 1.47. This value can also be obtinedfrom the DPP using the equationa n = 90/WI/2 where WIn Is the halfpeak width at low pulse amplitude. TheW of the DPP of tungsten peak was1/264 mVat !'IE = 10 mY. The calculatedvalue of no; thus works out to be 1.4.This value agrees well with the valueobtained by DC polarogram. Since thispeak current corresponds to thereduction of W(V) to W(IlI). the numberof electrons involved in the electrontransfer step is 2.

er)Ieadlis

ofSo

be'asionhis

ue,1.7

sed

ess The dependence of peak potential onpH of the medium has already beenshown to be linear with a slope of -60mV/ pH unit (Fig.3B). Since n = 2. onecan infer from this slope value that twohydrogen ions are participating in theelectron transfer step.

I inoust ion

dueIt is.lble

!(VII

udy

Birke et. al.ts DPP diagnostic criteriaII 1.12) was used to characlerise themechanism of the electrode process. Theratio of anodic to cathodic peak currentwas 0.60. (For a reversible electrodereaction. this ratio is unity). Thedifference in peak potential was less thanthe pulse amplitude. This shows thatlhe electron transfer process is nolreversible. Further the peak current ofthe second step in oxme - chloratemedium is much higher than twice thepeak curr ent of the first step. This

e n ts

a cic

J. Electrochem See :ndia

suggests catalytic nature of the electrodeprocess. This peak current was foundto be a linear fucllon of the square rootof chlorate concentration (Fig. 48). Thisis a characteristic of catalytic electrodeprocess. Hence the electrode processis probably an EC (catalytic) mechanismwith quasi-reversible or irreversibleelectron transfer.

From the experimental observations. thefollowing reactions can be proposed forthe catalytic wave of W(V) to W(JIl) inthe presence of chlorate and oxin e:

WO(OH)(Ox)2 + 2 H· + 2e -> W(OH)(OX)2

+ H20 .... (1)

'l'WO(OH)(Ox)2 + 2 H- + 2e -> W(OH)(Ox)2

+ H20 .... (1)

W(OH)(Ox)2 + Cl03' + 4 H' ->\VO(O )(OX)2 + CI· + 2H

20 ... (2)'---- -"J.

We found that in a medium conlaining0.05 sodium chlorate and 0.005 M oxineat pH 1.7 ± 0.1. the catalytic currentwas proportional to tungstenconcentrations in the range 0.50 - 10ppm. A least- squares fit of the peakcurrent vs tungsten concentralion datawas carried out for a straight line byy-residual minimization. The regressionaJparameters were: slope = 900 nA/pprn;y-intercept = - 47 nA and correlationcoel'fecient = 0.997. The slope of thecalibrat ion graph decreased above 10ppm. At concentrations lower than 0.50ppm. the base current clue (0 thereduction of chlorate interfel-ed.

Vol43 - 1 (1994)

24 Enhancement of the Sensitivity of Tungsten Determination - Basu, at. al.

Procedure of Analysis

An aliquot of the sample solution wastransfred into a .polarographic cellcontaining 2.0 ml of oxine (0.1 M) and1.0 rnl sodium chlorate (1.0 M); the pHwas adjusted to 1.7 ± 0.1 with dilutesodium hydroxide or dilute hydrochloricacid: diluted to 20 ml: the solution wascleaeratecl by bubbling pure nitrogen andDPP was recorded from -0.50 V to -0.90 V. The D1'P conditions used were:pulse lime == 0.04 s: pulse amplitude ==50 mY; droptime == 2.0 s. A standardadditon of W(VI) was made and DPP wasrecorded again. Analysis was carriedout by standard addition procedure.

Study of Interferences

The effect of other ions on the peakcurrent of tungsten was investigated. Itwas found that 20 - fold amounts ofAI(lIl) ancl F'e(IlI) did not interfere withthe determination of tungsten. 10 - foldamounts of Ti(lV) and V(V) also did notinterfere. There was no interference fromequal quantities of Cu(l!)' Pb(II). Cd(II)and Zn(lI). Even though these elementsform complexes with' oxine, thecom plexat ion was negligible at theexperimental pH or measurement.Mo lybde n u mtVl] even in traceamounts - gave rise to a large peak ata more positive potential but it did notoverlap with the tungsten peak: the peakswere well-separated. The peak currentof tungsten was nol affected by theprescnce of equal quantities of Mo(Vi)hut resulted in slight error for theme as u re m c n t of t ung ste n p,:ak bydistorting the base curren t.

J. Etectiocbem Soc. India

Determination of Tungsten in SteelSamples

The slandardised procedure was checkedwith two BCS standard steel samples.The sample treatment was carried outas given below:

An accurately weighed amount (100 mg)of the sample was dissolved in 20 ml of1:5 (v/v) sulph uric acid and 5 ml 1:5Iv/vl phosphoric acid. 1 (02 ml of nitricacid was added to oxidize any carbonpresent, boiled for a few minutes, madeup to 100 ml with distilled water. 1m1 aliquot was used for the determinationof tungsten in the sample. When theiron to tungsten ratio in the sample wasgreater than 20 (as in BCS 281), excessiron was removed by solvent extractionwith methyl isobutyl ketone in 6 Mhydrochloric acid.

Table 1

Determination of Tungsten in Steels

Sample Tungsten. %Certified Found>

Standard

deviation

BCS 485

ncs 2816.48

0.70

5.40

0.58

0.26

0.04

• Mean of 6 replicate measurements

The results obtained for the standardsteel samples are given in Table 1. Therelative standard deviat ion for 6 replicatemeasurements was found to be about5%. The method is suitable in testsolutions containing 0.5 10 10 ppmtungsten.

Vol 43 - 1 (1994)

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Enhancement 01 the Sensitivity of Tungste.' Determination - Basu, et. al. 25

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I. J.J. Llnganc and L.A. Small, J. Am. Chern.Soc .. 71 (1949) 973. 8. D. Dan and J. He, Talanta, 39 (1992) 119.

2. P.L. Ouldlni, D. Ferri and D. Nobill,Analyst, 113 (1988) 1317.

9. Bharathtbat J. Basu and S.H. Rajagopalan,Analyst. 117 (1992) 1623.

3. !.M. Kollhoff and!. Hodara, J. Electroanal.Chern and Interfacial Eleclrochem .. 4 (1962)369.

10.Encyclopedia of Electrochemistry of the.Elements. Volume V. (Ed.) A.J.Bard,Marcel Dekker Inc., New York, (1976)p.76.

4. Y. Sohrin, K. lss hi kt, E. Nakayama, S.Kihara and M. Matsui, Anal. Chtrn. Acta,218 (1988) 25.

II.R.L, Birke, M.II. Kim and M. Strassfcld,Anal. Chem., 53 (1981) 852.

5. E.G. Chtkryzova and S. Ya. Mashtriskaya,Zh. Anal. Kllim., 38 (I983) 865.

12.M.I-1, Kim and R.L. Btrkc, Anal. Chern., 55(1983) 522.

S G. E.G. Chfkryzova and L.G. Kiriyak, Zh. Anal.s Kllirn .. 35 (1980) 492 .

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II

:cl

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)4) J. Electrochem Soc. India Vol 43 - 1 (t994)