ANALYST, NOVEMBER 1986, VOL. 111 1239 Differential Pulse Cathodic Stripping Voltammetric Investigation of Cr04*-, Mo042-, W042- and V03- M. Rasul Jan* and W. Franklin Smytht Department of Chemistry, University College Cork, Cork, Ireland The differential pulse cathodic stripping voltammetric behaviour of Cr042-, Moo4*-, W042- and V03- has been investigated and applied to the determination of trace concentrations of these oxyanions. Detection limits of 5.6 X lO-7,6.1 x 10-8, 1 . 1 x 10-7 and 8 x 10-8 M and quantitation limits of 1.87 x 1.1 X 10-6 and 2.7 x M have been calculated for the cathodic stripping voltammetric determination of Cr042-, W042-, Mo042- and V03-, respectively. The effects of equimolar and lower concentrations of selected cationic and anionic interferents on their differential pulse cathodic stripping voltammetric behaviour have also been examined.These reveal that certain heavy metal cations such as Pb", Cull, Znll, Cdil and Agl can compete with Hgl for the appropriate oxyanions and that anions such as S*- and I-, which form partially insoluble mercury salts, can compete for sites around the mercury drop. Keywords: Differential pulse cathodic stripping voltammetry; oxyanions 2.03 X The polarographic behaviour of Cr042- and Mo042- has been described in detail elsewhere,, with the reduction of MoVI receiving the most attention.1-18 These studies were usually conducted in hydrochloric and sulphuric acid solutions and showed multi-step reductions. The catalytic effect of MoVI in the concentration range 4 X 10-7-4 x M on the reduction of nitrate can be used quantitatively for the determination of the former.5 The stripping voltammetric behaviour of Cr042-, MOO^^-, W042- and V03- has received some attention in recent years.Vydra et aZ.19 summarised the literature in this field. The d.c. cathodic stripping of these ions in 0.05 M potassium nitrate at a hanging mercury drop electrode, after the formation of the corresponding Hg22+ compounds at a positive potential, has yielded quantitative methods for the determination of Mo042-, W042- and V03- in the range 3 X 10-6-10-5 M . ~ O The reaction Mo042- --+ Mo02.2H20, carried out on a mercury electrode, has been proposed for the determination of M0042- in 3 M sodium chloride down to a concentration of 5 X 10-6 M by chronopotentiometric stripping.21 The selective adsorption of W042- on the hanging mercury drop electrode in the zero current state has been recommended for the stripping determination of this anion in the concentration range 1 x 10-7-2 x 10-6 M, using a plating potential of +0.2 V and a solution buffered at pH 3.6.22 Owing to the importance of these anions in environmental chemistry, this study was carried out to develop more sensitive voltammetric methods by which the oxyanions could be determined.The differential pulse mode was chosen for the stripping investigations. The selectivity of these methods was also evaluated with respect to the determination of the oxyanions in the presence of other cations and anions. Experimental Apparatus All experiments were performed using a Princeton Applied Research (PAR) Model 174 A polarograph with a PAR Model 303 hanging mercury drop electrode (HMDE).Voltammo- grams were recorded on a PAR Model RE 0074 X - Y recorder. The electrode system was constructed with a platinum wire auxiliary electrode and a saturated calomel electrode as a reference electrode, together with the hanging mercury drop electrode. Oxygen-free nitrogen was used for de-gassing the system and a Kent Model EIL 7055 pH meter was used for the pH measurements. Reagents All solutions were diluted with de-ionised water unless stated otherwise. All glassware was washed with Decon 90 and thoroughly rinsed with de-ionised water. Solution preparation Standard stock solutions and interfering cation and anion solutions were prepared using the method reported earlier.23 Procedure Solutions (10-5 M) of oxyanions were prepared in a 0.05 M potassium nitrate supporting electrolyte, and 10 ml of this solution were taken for the sample cell.The solution was de-gassed by bubbling oxygen-free nitrogen through the solution for 4 min, after which a flow of nitrogen was maintained over the solution throughout the analysis. After bubbling the nitrogen through the solution and a 30 s quiescent time, the solution was electrolysed at an appropriate positive potential (+0.15-0.20 V) for 1-5 min and was then scanned in a negative potential direction at 10 mV s-1 to obtain the DP stripping peak using a medium-sized hanging mercury drop. The interfering ions were injected through the side orifice in the cell using a 100 1.11 micropipette.Limits of detection (LOD) and quantitation (LOO) were determined and calculated by the method of M0rrison.2~ Results Optimisation of DPCSV Parameters for the Determination of CrOd2-, Mo042-, W042- and V03- The effect of initial potential, Eel, plating time and pH on the DPCSV behaviour of Cr042-, M0042-, W042- and V03- were studied. The conditions that gave rise to the most sensitive determination of the oxyanions in 0.05 M potassium nitrate supporting electrolyte were then selected and are presented in Table 1. * Present address: Department of Chemistry, University of Peshawar, N. W.F.P. , Peshawar, Pakistan. t Present address: Department of Pharmacy, Medical Biology Centre, Queens University of Belfast, Belfast BT9 7BL, UK. Effect of Concentration on the DPCSV Behaviour of Cr042-, Mo042-, WO& and VOj- The effect of concentration on the DPCSV behaviour of the oxyanions was studied under optimised conditions.For1240 ANALYST, NOVEMBER 1986, VOL. 111 Cr042- at 10-5 M and lower concentrations, only one peak, at a peak potential of E, = 0 V, is observed. At concentrations greater than 10-5 M more than one peak is observed, showing multi-layer formation phenomena. The optimum concentra- tion range for the determination of Cr042- by this method is 10-5-1.8 X 10-6 M in 0.05 M potassium nitrate (pH 6) supporting electrolyte. For MOO^^- at 10-5 M concentration and higher, two peaks are observed, showing multi-layer formation. One peak is observed at -0.04 V and the other at -0.28 V. At concentra- tions of 1 0 - 6 ~ and lower only one peak is observed for Mo042-, at -0.08 V.This peak is of analytical importance at concentrations lower than 10-5 M. At a concentration of 10-5 M, V03- also shows the multi-layer formation phenomena. At pH 4, 5 and 6, V03- gives a two-peak pattern, one peak with a peak potential of -0.03 V and the other at -0.1 V. The peaks are independent of pH at pH 4-6. The effect of plating time on the V03- signal was studied. Only the first peak shows a proportional increase with plating time. This increase suggests that there is an outer multi-layer formed by HgV03 and Hg(V03)2 around the mercury drop. The monolayer is strongly bound to the mercury drop and is stripped at more negative potentials. At lower concentrations only one peak is observed for the DPCSV of VO3-.Also, the peak potential ( E p ) varies with concentration. As the concentration increases, the peak potential moves towards more negative potentials. Multi-layer formation at higher concentrations is also observed for the DPCSV behaviour of W042-. At pH 6, W042- shows two peaks, one at Ep -0.07 V and the other at Ep = -0.235 V. The latter peak shows no increase with plating time. The first peak is of analytical importance and was chosen for the study. At lower concentrations only the first peak is observed. Effect of Interfering Ions on the DPCSV Behaviour of Cr042- and Mo042- Cr042-, Mo042-, W042- and V03- all stripped at the same potential and so inevitably interfered with each other. The effects of cationic and anionic interferences of equimolar and sub-equimolar concentrations were studied for each oxyanion in turn.These studies are illustrated for Cr042- in Table 2. Among the cations studied, equimolar concentrations of PbII, Zn" and AgI interfere with the DPCSV signal of 10-5 M Cr042-, with recoveries of 86.8,89.7 and 90.3%, respectively. This decrease is due to the competitive complexation of Cr042- with these heavy metals, thus reducing a certain percentage of the Cr04*- available for determination by DPCSV. At a concentration of 10-6 M these cations do not interfere. Equimolar concentrations of NaI, Ca" and F- also interfere, with recoveries of 116, 117 and 119%, respectively. At a concentration one order of magnitude lower, this effect starts to decrease. Equimolar concentrations of Br- and 9- do not affect the Cr042- peak recovery, but the DPCSV of Cr042- in the presence of S2- shows an interfering peak at approximately 0 V and a residual HgS stripping peak at -0.7 V (Fig.1). This shows a competition between Hg2Cr04 Table 1. Optimum DPCSV conditions for the determination of C T O ~ ~ - , Mo042-, W042- and V03- Optimum pH using 0.05 M KN03 as Initial potential, Plating time/ supporting Oxyanion E,,N vs. SCE min electrolyte c1-0~~- . . +0.2 5 6 wo42- . . +0.2 2 6 V03- . . +0.2 5 6 Mo042- . . +O. 15 5 6 (or HgCr04) and HgS for sites around the mercury drop and seriously impairs the usefulness of the DPCSV determination of Cr042- in the presence of equimolar concentration of S2-. I- is also a serious interferent in that it strips at a slightly more negative potential, i.e., -0.15 V, when it is present on its own.When it is added as an interferent to Cr042- no separate peak is observed for I- and it adds to the Cr042- peak with a resulting recovery of 172.2%. The effect of equimolar and lower concentrations of selected cations and anions on the DPCSV behaviour of M Mo042- is given in summarised form in Table 3. Among the cations studied, only Pb" significantly interferes at equimolar concentrations, reducing the Mo042- peak by +0.2 0.0 -0.2 -0.4 -0.6 -0.8 Vo I tag eiV Fig. 1. Effect of equimolar concentrations of S2- on the DPCSV behaviour of Cr042-. Scan rate, 20 mV s-l; modulation, 25 mV Table 2. Effect of equimolar and lower concentrations of selected cations and anions on the DPCSV behaviour of 10-5 M Cr04*- in 0.05 M KN03 supporting electrolyte (pH 6) Interferent added CU" .. . . . . Cd" . . . . . . Ni" . . . . . . Pb" . . . . . . Pb" . . . . . . Ca" . . . . . . Ca" . . . . . . Ca'I . . . . . . Na' . . . . . . Na' . . . . . . Na' . . . . . . Na' . . . . . . Na' . . . . . . Zn" . . . . . . Zn" . . . . . . Agr . . . . . . Ag' c1- F- . . F- . . . . . . . . . . . . . . . . . . . . . . I- . . . . . . . . I- . . . . . . . . Concentration of interferenth 10-5 10-5 10-5 10-5 10-5 10-7 10-5 10-7 10-5 10-5 10-5 10-5 10-5 10-7 10-5 10-5 10-6 10-6 10-6 10-8 10-9 10-6 10-6 10-6 10-6 Recovery, % 100 100 100 100 117 114.3 100 116 116 115.1 114.8 101.7 89.7 100 90.3 100 100 119 100 172.2 136.4 100 100 100 86.8ANALYST, NOVEMBER 1986, VOL. 111 1241 15.5%.This could be due to competitive complexation of Mo042- with Pb". I- is the main anionic interference at an equimolar concentration, as it cathodically strips at the same potential. An equimolar concentration of S2- reduces the Mo042- peak by 13%. This is presumably due to the competition of S2- and Mo042- for sites around the mercury drop. S2- strips at a more negative potential (-0.7 V). The same effect was observed for the DPCSV behaviour of mixtures of S 2 - and Cr042-. Equimolar concentrations of F- and C1- reduce the MOO^^- peak by 4 and 19%, respectively. Effect of Interferences on the DPCSV Behaviour of V03- The effect of equimolar concentrations of various cations and anions on the DPCSV behaviour of 10-5 M V03- was studied under optimum analytical conditions and the results are summarised in Table 4.Equimolar concentrations of Ca", Agl, C1-, F- and Br- have no effect on the recovery of the V03- peak. An equimolar concentration of PbII gives a serious interference. In the presence of an equimolar concentration of Pb", the first peak of VO3- is observed with a 40% recovery, whereas the Table 3. Effect of equimolar and lower concentrations of selected cations and anions on the DPCSV behaviour of 10-5 M Mo0,2- in 0.05 M KN03 supporting electrolyte (pH 6) Concentration of Interferent added interferenth Cd" . . . . . . 10-5 Pb" . . . . . . 10-5 Pbl' . . . . . . 10-6 CUT' . . . . . . 10-5 Zn" . . . . . . 10-5 Na' . . . . . . 10-5 Ag' . . . . . . 10-5 Ca" . . . . . . 10-5 I- . . . . . . . . 10-5 I- . . . . . . . .10-6 F- . . . . . . . . 10-5 F - . . . . . . . . 10-6 €3- . . . . . . 10-5 a- . . . . . . 10-5 a- . . . . . . 10-6 s2- . . . . . . 10-5 s2- . . . . . . 10-6 Recovery, % 84.5 100 100 100 100 100 100 350 100 100 100 100 100 99.0 95.9 81.0 87.0 Table 4. Effect of equimolar and lower concentrations of selected cations and anions on the DPCSV behaviour of M V03- in 0.05 M KN03 supporting electrolyte (pH 6) Recovery, YO E, = - 0.03 V E, = -0.1 V Concentration of interferenth CU" . . . . . . 10-5 72.7 85.1 Zn" . . . . . . 10-5 82.8 86.0 Ni" . . . . . . 10-5 91.1 91.8 Cd" . . . . . . 10-5 73.7 78.0 Interferent added CU" . . . . . . 10-6 100 100 Zn" . . . . . . 10-6 100 100 Ni" . . . . . . 10-6 100 100 Cdl' . . . . . . 10-6 98.7 100 Na' . . . . . . 10-5 102.8 100 Cal' .. . . . . 10-5 100 100 Ag' . . . . . . 10-5 100 100 Pb' . . . . . . 10-5 40.0 0.0 Pb" . . . . . . 10-6 100 100 s2- . . . . . . 10-6 100 100 1- . . . . . . 10-5 100 0.0 I- . . . . . . 10-6 100 100 c1- . . . . . . 10-5 100 100 Br- . . . . . . 10-5 100 100 F- . . . . . . 10-5 100 100 S2- . . . . . . 10-5 80.0 87.2 second totally disappears. The disappearance of the second and the decrease of the first peak are presumably due to the formation of lead vanadate [Pb(V03)2] after the successful competition with Hg22+ or Hg2+ for complexation with the VO3- anion. The total disappearance of the second peak and partial disappearance of the first suggest that the PbII competes successfully for V03- in the monolayer state and is partly successful on the multi-layer. The effect of PbII on the DPCSV behaviour of V03- is shown in Fig.2. The interfer- ence effect of PbII is no longer observed when it is present one order of magnitude lower than V03-. Equimolar concentra- tions of &I1, ZnII, Ni" and CdII decrease the VO3- peaks with peak recoveries of 72.7, 85.7% (peak 1 and peak 2); 82.8, 86.01; 91.1, 91.8; 73.7, 78.0%, respectively, owing to the complexation of V03- with these heavy metal cations, affecting both the multi-layer and monolayer states of HgV03 - Hg(V03)2. In the presence of these cations the first peak is generally more affected than the second and this can be explained by the cations preferring the multi-layer of HgV03 - Hg(V03)2 around the mercury drop to the monolayer. The anions S2- and I- interfere with the HgV03 - Hg(V03)2 stripping peak at equimolar concentrations. V03- and S2- compete for sites around the mercury drop to form their respective salts, resulting in a decrease in the HgV03 - Hg(V03)2 stripping peak.In the presence of an equimolar concentration of I- (Fig. 3), the second peak of V03- totally disappears, with the appearance of a reduced I- peak at the expected potential of approximately -0.1 to -0.2 V. I- in this instance effectively disrupts the monolayer of HgV03 - Hg(V03)2 around the mercury drop as compared with S2-. To determine V03- by this method the concentration of PbII and I- should be at least one order of magnitude lower than the concentration of V03-. Effect of Interferences on the DPCSV Behaviour of WO42- The effect of equimolar and lower concentrations of selected cations and anions on the DPCSV behaviour of 10-5 M W042- was studied and is given in sumarised form in Table 5.Equimolar concentrations of CuII, Pb", Na', Ni", Zn", AgI, C1- and F- have no effect on the peak recovery of W042-. 16.0 1 I I I +0.2 0 +0.2 0 - Vo I t a g elV .2 Fig. 2. Effect of equimolar concentrations of Pb'* on the DPCSV behaviour of V03- in 0.05 M potassium nitrate supporting electrolyte (pH 6). Scan rate, 10 mV s-1; modulation amplitude, 25 mV; plating time, 5 min; starting potential, +0.2 V1242 I I 8 - ANALYST, NOVEMBER 1986, VOL. 111 -.+ 1o-5Mvo3- Table 5. Effect of equimolar and lower concentrations of selected cations and anions on the DPCSV behaviour of 10-5 M W042- in 0.05 M KN03 supporting electrolyte (pH 6) Concentration of In terferen t added in terferen t/M Recovery, YO Cu" .. . . . . Pbrl . . . . . . Nar . . . . . . Cd" . . . . . . Cd" . . . . . . Carr . . . . . . Ca" . . . . . . Nirl . . . . . . ZnT1 . . . . . . Agr S2- S2- Br- Br- I- . I- . C1- F- , . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10-5 10-5 10-5 10-5 10-5 10-5 10-5 10-5 10-5 10-5 10-5 10-5 10-5 10-6 10-6 10-6 10-6 10-6 99 100 100 93 100 97 100 101 99 100 46 100 71 100 55 100 100 100 +0.2 0 -0.2 -0.4 Vo ltag elV Fig. 3. Effect of equimolar concentrations of I- on the DPCSV behaviour of 10-5 M V03- in potassium nitrate supporting electrolyte (PH 6 ) . Scan rate, 10 mV s-1; modulation amplitude, 25 mV; plating time, 2 min Equimolar concentrations of Cd" and Ca" slightly interfere, reducing the HgW04 - Hg2W04 stripping peak by 7 and 3%, respectively.Certain anions are the only serious interferences at equimolar concentrations on the DPCSV behaviour of W042-. Equimolar concentrations of Br-, S2- and I- reduce the stripping peak with recoveries of 71, 46 and %YO, respectively. The corresponding mercury salts of Br-, S2- and I- presumably also adsorb on the mercury drop and compete for sites on the surface with HgW04 - Hg2W04, hence giving the reduced recoveries. Analytical Applications This voltammetric method of determination is very sensitive for the determination of the oxyanions investigated. The limits of detection of these oxyanions were determined and the limit of quantitation and relative standard deviation near the detection limit were calculated and are shown in Table 6.The DPCSV behaviour of V03- at a concentration near the detection limit is shown in Fig. 4. This method could be used for the determination of any of the oxyanions studied in mixtures if the concentration of the interferent anion is one order of magnitude lower in concentration. Some of the cations studied do interfere with the DPCSV behaviour of Table 6. Limits of detection and quantitation of the oxyanions by DPCSV using the conditions described in Table 1 Limit of Limit of Relative standard Oxyanion detection/M quantitationh deviation,% Cr04*- . . 5.6 x 10-7 1.87 X 10-6 0.93 Mo042- . . 1.1 x 10-7 1.1 x 10-6 0.85 vo3- . . 8 X 10-8 2.7 x 10-7 0.29 wo42- . . 6.1 x 10-8 2.03 x 10-7 1.15 0 -0.2 -0.4 VoltagelV Fig.4. Repetitive scans at a concentration near the detection limit for V03- by DPCSV in a 0.05 M potassium nitrate supporting electrolyte (pH 6) these oxyanions, such as PbII, CuII, Zn", CdlI and Agl, whereas S2- and I- are anionic interferents. This method could be utilised for the determination of any of the oxyanions at trace levels in order to monitor them in biological or environmental samples, Conclusion The interference effects of the different cations and anions can be classified under the following headings. Complexation Certain heavy metal cations such as Pb", Cu", Zn", Cd" and AgI can reduce the DPCSV peaks of the oxyanions studied by their competition with Hg*+ - Hg22+ for complexation with the oxyanions. V03- is mainly affected by this process.Competition for Sites This is generally observed for those anions which also form partially insoluble mercury salts such as S2- and I-. They compete for sites around the mercury drop and result in decreased peak(s) for the DPCSV signal of the appropriate oxyanions. Anomalous Effects The effect of some cations, e . g . , Na', on the DPCSV behaviour of oxyanions is anomalous. The presence of these cations has enhancement effects on the DPCSV peaks of some of the oxyanions, possibly due to catalysis of the mercury - oxyanion reaction. References 1. Parry, E. P., and Yakubik, M. G., Anal. Chem., 1954, 26, 1294.ANALYST, NOVEMBER 1986, VOL. 111 1243 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14. Wolter, M., Wolf, D. O., and Vonstackelbery, M., J . 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Zelinka, J., Bartusek, M., and Okac, A., Collect. Czech. Chem. Commun., 1974, 39, 83; Anal. Abstr., 1976, 27, 676. Morales, A., Gonzales, B. F., and Diaz, C., Chemist Analyst, 1967,56, 89. Vydra, F., Stulik, K., and Julakova, E., “Electrochemical Stripping Analysis,” Ellis Horwood, New York, 1976, pp. 25& 254. Geyer, R., Henze, G., and Jenze, J., 2. Tech. Hochschule Chem. Carl Schorlemmer, 1966, 8, 98. Lagrange, P., and Schwing, J. P., Anal. Chem., 1970,42,1844. Berge, H., and Ringstorff, H., Anal. Chim. Acta, 1971, 55, 201. Jan, M. R., and Smyth, W. F., Analyst, 1984, 109, 1187. Morrison, G. H., Anal. Chem., l980,52,2241A. Paper A41368 Received October 23rd, I984 Accepted June 26th, I986