Amalyst, August, 1973, Vol. 98, $9. 563-571 563 Mass and Charge Transfer Kinetics and Coulometric Current Efficiencies Part VIII.* Single-scan Voltammetry of Vanadium(V) - Vanadium( IV) in the Presence of Chromium, Manganese and Iron, and the Kinetic Parameters of the Vanadium System, at Platinum Electrodes Pre-treated by Five Methodsf. BY E. BISHOP AND P. H. HITCHCOCKS (Chemistry Department, University of Exetey, Stocker Road, Exeter, EX4 4QD) Continuing the earlier examination of the vanadium system alone, under various conditions and with various electrode pre-treatments, the effect of neighbouring steel-forming d-block elements has been investigated. Chro- mium(V1) at pH 4.0 suppresses the vanadium(V) reduction wave, and the degree of suppression is quantitatively proportional to the chromium(V1) concentration.Activated electrodes are deactivated by dipping them in a chromium(V1) solution, and remain so even when well washed thereafter, so that chromium(V1) as well as chromium(II1) is adsorbed strongly on platinum. In 2.0 M sulphuric acid, chromium(V1) and vanadium(V) are reduced at the same rate. Manganese(VI1) in acetate buffer gives a fast, well separated wave, but the separation is not as good in 2-0 M sulphuric acid ; slowing the vanadium(V) reduction by using an oxidised electrode effects no improvement : the manganese wave is similarly affected. Addition of chromium(V1) to the manganese - vanadium mixture a t pH 4 suppresses the manganese wave only slightly, even when the vanadium wave is com- pletely suppressed. In 2.0 M sulphuric acid, the manganese wave is un- distorted and chromium and vanadium are simultaneously reduced.Iron (111) in 2.0 M sulphuric acid does not interfere, but the separation of the vanadium and iron waves is not good. Iron(I1) can, however, act as a potentiostatic intermediate. The kinetic parameters of the vanadium system are repro- ducible in acetate buffer, but only when the electrode is fouled in 2.0 M sulphuric acid. Pattern theory and diffusion-corrected Lewartowicz methods give results that agree. The charge-transfer kinetic parameters are shown to be potential dependent in acidic media. The results are compared with those in earlier reports. The generation current efficiency for vanadium(1V) in acetate buffer was computed. IN a previous paper,l the influence of hydrogen-ion concentration on the conditional potential of the vanadium(V) - vanadium(1V) system was examined, and the voltammetry of the system in sulphuric acid at concentrations ranging from 5 x to 2-0 M, and in potassium sulphate - acetate buffer at pH 4-0, was described.Platinum electrodes treated by five different methods, (a) chemically reduced with iron(I1) in sulphuric acid, (6) cyclically anodised and cathodised and reduced, (c) chemically and electrochemically stripped and electrochemically reduced, (d) cathodically reduced and (e) anodically oxidised, were investigated. Method (a) was of little use, while method (e) invariably gave the slowest charge transfer. Method (d) was moderately effective, but the preferred method was (b) because the anodic stripping appeared to remove some adsorbate from the electrode surface.These experiments related t o pure solutions of vanadium in pure supporting electrolyte. The objective was to measure the electrode kinetic parameters with a view to selecting control potentials for potentiostatic coulometry, and to determine the current efficiency under various conditions. With the further objective of application to the determination of vanadium in real samples, such as alloy steels, the influence of neighbouring d-block elements was investigated in order to discover whether they offered any interference, and also whether they could be determined * For Part VII of this series, see p. 553. t Presented a t the Second SAC Conference, Nottingham, 1968. Present address: Ever Ready Co.(G.B.) Ltd., Central Research Laboratory, St. Ann’s Road, London, N15 3TJ. @ SAC and the authors.564 sequentially in a mixture. behaved as expected. BISHOP AND HITCHCOCK: MASS AND CHARGE TRANSFER [Analyst, Vol. 98 Chromium and manganese gave surprising results, but iron EXPERIMENTAL Apparatus and procedures have been describedY2 the electrode pre-treatments are given in detail and the reagents and supporting electrolytes and their standardisation or treatment are set out in Part VII.1 Of the latter, the 2.0 M sulphuric acid and the saturated potassium sulphate - acetate buffer adjusted to pH 4.0 have been chosen for this further study, because the behaviour of the system is least complicated when the vanadium is present entirely as the cations, V02+ and VO,+, or as the tetrahydrogenpentavanadate ion, H,(V501s)3-.RESULTS AND DISCUSSION EFFECT OF CHROMIUM(VI)- Hypothetically (because the charge-transfer process is extremely slow and the exchange current is minute) the separation of the conditional potential of the chromium(V1) - chromium(II1) system from that of vanadium(V) - vanadium(1V) is adequate to permit the sequential determination of chromium(V1) and vanadium(V) . The vanadium reduction is kinetically slow,l and the chromium reduction much slower, so perhaps the sequence vana- dium(V) then chromium(V1) could be realised in practice instead. In the presence of iron(III), the faster reduction of iron(II1) would produce iron(II), which would behave as a potentio- static intermediate and so restore the order to chromium(V1) first and then vanadium(V).0.4 - 0.3 - 0.2 - 0.1 - 0.0 - - 0.9 0.6 0.3 0.0 -0.3 Electrode potential versus S.H.E./V Fig. 1. Effect of chromium(V1) on the reduction of vana- dium(V) in saturated potassium sulphate - acetate buffer at pH 4.0. Before each curve was recorded, the electrode was activated by method ( b ) : 1, supporting electrolyte alone; and 2, supporting electrolyte + 1.5 x M vanadium(V). Portions of a chromium(V1) solution were then added to give the following [Vv] to [CrvI] ratios: 3, 92; 4, 46; 5, 31; 6, 19; and 7, 14. Ramp speed - 1.0 mV s-l However, experimentally, in the pH 4 buffer supporting electrolyte the addition of small amounts of chromium(V1) to a vanadium solution caused the vanadium(V) limiting current at freshly activated reduced electrodes [methods (b) and (c)] to decrease.This remarkable behaviour is shown in Fig. 1. Over a limited range of chromium(V1) concentrations, the decrease in the vanadium limiting current is directly proportional to the chromium(V1) content of the solution. In such solutions, if an electrode was re-used without intervening reactivation the vanadium limiting current progressively decreased until the vanadium(V) wave disappeared entirely. For the curves in Fig. 1, the vanadium concentration, although small, is much larger than the chromium concentration ; when equal concentrations of vanadium(V) and chromium(V1) were present no reduction wave appeared for either species. The reproducibility is such that, for the particular conditions and concentrations used,August, 19731 KINETICS AND COULOMETRIC CURRENT EFFICIENCIES.PART VIII 565 chromium(V1) in the concentration range to 1 0 - 4 ~ could be determined to within 5 5 per cent. by the suppressive effect. The possible development of the method to trace levels is to be pursued. This behaviour cannot be ascribed to formation of a conventional platinum oxide film, because no maximum was detected at 0.6 V, and all the scans are negative to this potential. Kolthoff and El Din3 claimed that a solution of chromium(V1) in mineral acids to 1W2 RI in hydrogen-ion produced a monolayer of chromium( 111) hydroxide on a platinum cathode used in the solution. The monolayer of, presumably, solvated chromium oxohydroxide had the power of suppressing some electron-transfer reactions, but no report was made on the behaviour of vanadium.Kolthoff and El Din did not examine the process at pH 4, but the present work indicates that the film thickness or coverage is dependent both on the chromiuni(V1) concentration and on the number of scans performed with the electrode. It proved possible to deactivate a freshly activated electrode [method (b), (c) or ( d ) ] simply by dipping it into a 1.67 x M sulphuric acid for 5 minutes. When the electrode was removed, thoroughly washed with water and used to scan a vanadium(V) solution in acetate buffer, the limiting current was 10 per cent. less than that for a similarly activated electrode that was washed but not exposed to the chromium solution. Chromium(V1) can therefore be directly adsorbed on the surface of an electrode, and when a cathodic current is applied in a weakly acidic medium [in which the chromium(II1) hydroxide is presuniably insoluble] a layer of chromium( 111) hydroxide forms on the electrode surface.It is not, however, directly proved that the chromium is reduced completely or even reduced at all: formation of chromium(II1) chromate(VI), which is sparingly soluble, is not unlikely. In the 2.0 M sulphuric acid supporting electrolyte both species are reduced at the same rate ; the chromium(V1) wave coincides with the vanadium(V) reduction wave, as Davis4 has found. Kolthoff and El Din3 found that, in mineral acids more concentrated than 0.1 M, chromium(V1) did give a reduction wave at platinum cathodes, and identified the product as chromium(III), but do not mention the current efficiency. They concluded that this finding supported their contention that the film formed on a platinum cathode at lower hydrogen-ion concentrations from chromium (VI) was the hydroxide, but this interpret ation is not unequivocal.Specific adsorption of chromate or yolychromate ions on the electrode surface, and physical blockage of the surface, without reduction, does not seem unlikely. EFFECT OF MANGANESE(VII)- Scanning a mixture of manganese(VI1) and vanadium(V) in the acetate buffer supporting electrolyte at pH 4 gave a manganese wave with a half-wave potential of +0.962 V, super- imposed on the usual vanadium(V) wave as shown in Fig. 2. The manganese wave is sharp and well defined, the reaction being clearly fast, and is well separated from the vanadium wave.The limiting currents of the individual waves were found to be proportional to the concentrations of manganese(VI1) and vanadium(V) . In the 2.0 M sulphuric acid electrolyte, the vanadium(V) and manganese(VI1) waves were additive, but the separation, as shown in Fig. 3, was not good. As the hydrogen-ion concentration increased, both waves moved in the anodic direction, but the vanadium wave moved considerably more than the manganese wave, as would be expected from a knowledge of the homogeneous reaction mechanisms. Although the vanadium(V) reduction could be slowed down by using an oxidised electrode [treatment (e)] the manganese(VI1) reduction was also slowed down, and separation of the waves was not improved. When separately reduced, the product from manganese(VI1) is manganese(II), and scans of mixtures gave no evidence to the contrary.The mechanism was not fully revealed until the quantitative potentiostatic determinations, to be reported later, were carried out. In the presence of vanadium(V), manganese(VI1) is reduced to manganese(III), and not manganese(I1). Further reduction gives a combined wave representing reduction of man- ganese(II1) and vanadium(V). COMBINED EFFECT OF CHROMIUM(VI) AND MANGANESE(VII)- Addition of chromium(V1) to a mixture of vanadium(V) and manganese(VI1) in the buffer electrolyte of pH 4.0 suppressed the vanadium wave in exactly the same manner as in Fig. 1, but only slight suppression of the manganese(VI1) wave occurred.Addition of M solution of chrornium(V1) in566 0.9 0.6 0.3 0.0 Electrode potential versus S.H.E./V [Analyst, VOl. 98 Fig. 2. Reduction of vanadium(V) and manganese(VI1) in saturated potassium sulphate - acetate buffer a t pH 4.0: 1, 1.1 x M vanadium(V), electrode treatment (c); and 2, 1-1 x 10-3 M vanadium(V) + 0.13 x 10-3 M manganese(VII), electrode treatment (c) . Platinum cathode, scan speed - 80 mV min-1 sufficient chromium( VI) completely to suppress the vanadium wave decreased the rnan- ganese(VI1) limiting current by only 8 per cent. Kolthoff and El Din3 reported that the reduction of manganese(VI1) in M mineral acid was suppressed to the extent of 60 per cent. when a platinum cathode that was completely filmed with chromium(II1) hydroxide was used.They did not extend their measurements to pH 4.0, but the difference is rather striking. In the 2 . 0 ~ sulphuric acid supporting electrolyte a mixture of vanadium(V), man- ganese(VI1) and chromium(V1) behaved as can be predicted by the foregoing results. The 1.45 1.25 1-05 0.85 0.65 0-45 0.25 0.05 -0.15 Electrode potential versus S. H. E ./V Fig. 3. Reduction of vanadium(V) and manganese(VI1) in 2.0 M sulphuric acid: 1, 0.2 x M manganese(VII), electrode treatment (b) ; 2, 0.2 x 10-3 M manganese(VII), electrode treatment ( e ) ; 3, 1.13 x M vanadium(V), electrode treatment (b) ; and 4, 1.13 x M vanadium(V), electrode treatment ( e ) . Ramp speed - 100 mV min-lAugust, 19731 IUNETICS AND COULOMETRIC CURRENT EFFICIENCIES. PART VIII 567 manganese reduction wave was undistorted, and the chromium(V1) and vanadium(V) waves were superimposed one on the other.EFFECT OF IRON(III)- The behaviour of iron(II1) in acetate buffer was not examined, but in 2 . 0 ~ sulphuric acid a well defined reduction wave of half-wave potential 0-647 V was obtained, and the electrode process was clearly moderately fast. Fig. 4 shows the scans of mixtures of iron(II1) and vanadium(V) at electrodes treated by methods (b) and (d). The benefit of using the anodic stripping pre-treatment instead of the simple reduction1 is again apparent, but the separation of the two waves is not good in this medium. 0.9 0.6 0.3 0.0 Electrode potential versus S.H.E./V Fig. 4. Reduction of mixtures of vanadium(V) and iron(II1) in 2.0 M sulphuric acid [1-13 x M vanadium(V) + 1.03 x M iron(III)] : 1, electrode activation method (b) ; and 2, electrode activation method (d).Scan speed - 100 mV min-1 KINETIC PARAMETERS OF THE VANADIUM ELECTRODE REACTIONS- Redztction of va.nadi.um( V)-The work on pure vanadium solutions1 showed that the most easily reproducible scans were obtained in the potassium sulphate - acetate buffer electrolyte at pH 4.0, and the method of pre-treatment of electrodes for use in this medium is less critical because there is no chemical oxidation of the electrode surface by the vanadium(V). The next most reproducible scans were obtained in sulphuric acid at concentrations of 0-1 to 2 . 0 ~ , in which three reproducible conditions of the electrode surface could be defined as follows- (i) a reduced electrode pre-treated by method (b) or (c); (ii) an oxidised electrode pre-treated by method (e); and (iii) a reduced electrode, with adsorption, pre-treated by method ( d ) ; this condition is stable and is attained by electrodes treated by other methods after a single scan.Values for K,,,,, k and a were readily obtained from the vanadium(V) reduction waves in the acetate medium at pH 4.0, by using pattern theory5 or Lewartowicz’s diffusion-corrected method (but not his dubious second linearisation method) .2 The voltammetric scans in 0.1 to 2.0 M sulphuric acid as the supporting electrolyte (Fig. 3, Part VIP) will not yield a single set of charge-transfer parameters, because k and cc change considerably with change in working electrode potential or current, which is immediately revealed by using sequential points in the pattern theory equations, and is also shown by the curvature of the Lewartowicz plots.Such behaviour emphasises that not one, but three reactions are in progress: reduction of568 BISHOP AND HITCHCOCK: MASS AND CHARGE TRANSFER [Ana/$St, VOl. 98 vanadium(V) , reduction of platinum oxide and a potential-dependent adsorption process. The last two reactions change the nature and activity of the platinum surface and therefore the charge-transfer parameters. Empirical equations, from statistical analysis of the results from pattern theory, can be constructed, but would apply only to a single scan: the experi- mental reproducibility of the curves for sulphuric acid media is only about 5 per cent., whereas pattern theory has an accuracy of about 0.01 per cent.in the context, so that the empirical equation for one curve would be invalid for the next. Values of the mass and charge transfer parameters for the reduction of vanadium(V) under a variety of conditions are given in Table I. When the parameters are potential dependent the range of variation is given rather than a mean value, which could be misleading. Also included in Table I are the values reported by Davis.6 These values are academic and are useless in the real coulometric context. They were obtained in mixtures of van- adium(V) and vanadium(1V) in 1.0 M sulphuric acid at very low currents that did not exceed 1 per cent. of the limiting currents. The relatively small potential range covered would be unlikely to change the electrode surface condition much from its state at zero current, and so Davis was able to obtain “linear” Tafel plots that intersected at the zero-current potential.For the same reasons, Davis’ values for a and /3 summed to 1.00 & 0.06, which is certainly not valid in the wide-ranging potentials of coulometry. Fig. 5 shows three voltammetric curves computed by VOLTAMMETRY 9,’ by using Davis’ values for K and cc and assuming no alteration with potential. The difference between these curves and, for example, curve 6 of Fig. 3, Part VII, is very large indeed. Davis’ methods of electrode activation were different from those used in the present work, except for the iron(I1) treatment ( a ) , which in the present work gave a curve very similar to curve 6 of Fig.3, Part VII. The differences between the 0.5 0.4 N I E 0 03 a E 2 0.2 2 \ + > .- U t: 4 4 3 0.1 0.0 I I I 1 1 1.0 0.5 0.0 Electrode potential versus S. H. E./V Fig. 5 . Voltammograms computed from Davis’ kinetic values. Mixed vanadium(V) - vanadium(1V) in 1.0 M sulphuric acid, Davis’ values6 converted to litre units- E’,/V k / l cm-2 s-1 a 1 1.053 4.06 x 0.52 2 1.057 3.78 x 0.74 3 1.072 1.85 x 0.67 Other parameters used were: 6x, 10-3cm; k ~ , 7.72 x lcm-2 s-l; MH, 0 - 5 ; [ V ~ B , M ; WI933, 0; n, 1; km,,, ox, 3.1 x 1 S-l; and kmss, red, 3.1 X lo-’ 1 Cm-’ S1TABLE I VANADIUM CONCENTRATION OF 0.13 TO 1.9 x 10-3 M MASS AND CHARGE TRANSFER RATE PARAMETERS FOR THE VANADIUM(V) - VANADIUM(IV) REACTION AT PLATINUM ELECTRODES AT A Medium Reaction Acetate buffer, VV + e - VIV pH 4.0 Acetate buffer, VIV - VV + e§ 2-0 M H2SO4 VV$ e-VV pH 4-0 1.0 M HSSO, VV+ e-VIV ( Daviss) 2-0 M H2S04 Background cathodic Acetate buffer, Background pH 4.0 cathodic Electrode pre-treatment* or used electrode (4, (4, (4, (4 k/l cm-a s-l (7.1 - 9.3) x lo-’ 2.5 x 10-9f ** 4 x 1 0 q t 1.3 x 10-71:: 2 x lO-’tt 3.5 x l O - S i $ 2.1 x lO--’tt 3.3 x 10-8;: 4.06 X 3.78 x 1-85 x 7.72 x 1.62 x a 0.25 to 0.29 - ** 0.92 0.62 0.31 0.22 0.27 0.22 0.52 0.74 0.67 0.5 0-5 kmaes ox?/ 1 cm-2 s-1 (2.15 - 2.2) X lo-’ ** 3.1 x lo-‘ 3.1 x lo-‘ 3.1 x low6 3.1 x lo-‘ 3.1 x 3.1 x lo-‘ 9 x 10-4 9 x 10-4 * Treatments ( b ) , (G), (d) and (e) are mentioned in the introduction and are described in detail in Part V1I.l Davis’ activation proceduress : X, Oxidation by silver(I1) oxide in 6.0 M nitric acid for a “sufficient” time, then reduction in 1.0 M sulphuric acid a t -0.25 V.Y. Oxidation by silver(I1) oxide in 6.0 M nitric acid for a “sufficient” time. 2, Oxidation by silver(I1) oxide in 6.0 M nitric acid for a “sufficient” time, then reduction in iron(I1) solution in 1.0 M sulphuric acid. cm. t Rmsss OX = Dv6/8%; kmsss red = D v ~ / ~ x ; “apparent” value 8% = 1.0 x $ Results approximate on account of distortion by background wave. fj Positive-going ramp. All other scans negative-going. ** Wave too distorted by background wave to give even approximate values. t t For the low current portion of the wave, 5 to 20 per cent. of the limiting current. $$ For the high current portion of the wave, 80 to 90 per cent. of the limiting current. $3 Scan performed immediately after a previous run without reactivating the electrodes, Electrode originally activated by treatment (b) ..57 0 BISHOP AND HITCHCOCK: MASS AND CHARGE TRANSFER [Afia~?JEt, VOl.98 present and Davis’ results cannot be accounted for by differences of electrode treatment, or of the acidity of the medium, and support the finding that the charge-transfer parameters are highly dependent on the potentials at which they are measured. The oxidation wave of vanadium(1V) was always too close to the wave for the oxidation ‘of water, so that the values of k and cc [= (1 - /3)] cannot be regarded as more than an approxi- mate estimate. Considering the very large difference in potential between the waves for oxidation of vanadium(1V) and reduction of vanadium(V), it is not surprising that considerable changes in the nature of the electrode surface pertain and that the difference in rate constants for the two processes is relatively small in the context.‘COMPUTATION OF CURRENT EFFICIENCIES FROM EXPERIMENTALLY DETERMINED PARAMETERS- Because k and cc change with potential and current in sulphuric acid, the simple computa- tion of generation efficiency for the production of vanadium(1V) as an intermediate is not possible without error when using the fully developed VOLTAMMETRY 9 G/P7 program, without inserting an ecological matrix of parameters, This procedure is now possible with the latest satellite programs to VOLTAMMETRY 9. However, the simple calculation was possible for the acetate buffer medium, because there is no significant change in parameter values with potential in this medium. The background reaction in this instance is not reduction of water, but of un-ionised acetic acid, and parameters had to be faked to simulate this change. This choice of parameters was ratified by matching computer-calculated scans with experimental scans: the fit was excellent, but the parameters listed in Table I have no real significance.The generation efficiency (or, rather, the loss of generation efficiency in parts per million) is plotted against current density in Fig. 6. The sharp decrease essentially marks the diffusion- limited current density. ” 2 4 6 8 10 12 14 16 18 Current density/mA Computed current efficiency loss in the generation of vanadium(1V) in the acetate buffer medium a t pH 4.0: “&”, cm; “[H+]”, 0.051 M ; “ k ~ ’ ’ , 1.61 x 10-l1 1 cm-2 s-l; 2.2 x 10-6 1 cm-2 s-’; k,,,, red, 2.2 x lo-’ 1 cm-’ s-’; K , 8-6 x Parameters in inverted commas are adjusted to give an exact computer fit with the experimental voltammograms Fig. 6. “OIH’’, 0.5; E’o, 0.525 v; [vv]B, lo-’ M ; [VIv]B, 0; n, 1; kmass ox, 1 cm-2 s-l; and a, 0.27. CONCLUSIONS The voltammetric iiivestigation of the vanadium(V) - vanadium( IV) reaction alone1 and in combination with manganese(VII), chromium(V1) and iron(II1) has permitted kinetic parameters to be evaluated, current efficiencies to be computed and a basis to be laid for the potentiostatic determination of vanadium(V) alone or in certain combinations with other elements. The adsorption of chromium(V1) and chromium( 111) on platinum at low hydrogen- ion concentrations has been demonstrated.August, 19731 KINETICS AND COULOMETRIC CURRENT EFFICIENCIES. PART VIII 571 We are deeply grateful to Imperial Chemical Industries for a research grant extending over 3 years. REFERENCES 1. 2. - - , Ibid., 1973, 98, 465. 3. 4. 5. 6. 7. Bishop, E., and Hitchcock, P. H., Analyst, 1973, 98, 553. Kolthoff, I. M., and El Din, A. M. S., J . Phys. Chem., 1956, 60, 1564. Davis, D. G., J . Electroanalyt. Ckem., 1960, 1, 73. Bishop, E., Analyst, 1972, 97, 761. Davis, D. G., Talanta, 1960, 3, 335. Bishop, E., Chemia Analit., 1972, 17, 511. NOTE-References 1, 2, 5 and 7 are to Parts VII, V, 111 and I, respectively, of this series. Received February 5th, 1973 Accepted March 27th, 1973