Studies on the oxidation properties of oxidizing agents have suggested two possible oxidation mechanisms: one containing a one-electron oxidation step and the other a two-electron step. The latter has been proposed in numerous reaction systems 1'2 and one of the present authors found that a two-electron oxidation step is involved in the oxidation of vanadium(II) by nitrate ion 3. The various oxidation states of vanadium ion in aqueous solution and the strong reducing ability of vanadium(II) are available for clarifying this problem of oxidation mechanism.
A kinetic study of the oxidation of vanadium(II) by chlorate ion shows that the rate is first-order with respect to each reactant; the rate constant extrapolated to 0.1 tool 1-1 hydrogen ion at 25Β°C is about 22 1 mol-1 s-1, and no vanadium(IV) is found in the products 4. It has also been pointed out that two chromium(II) are oxidized to chromium(Ill) with chlorate ion by way of two successive one-electron oxidation steps s. We attempted to distinguish which type of oxidation occurs in the case of chlorate ion and vanadium(II). An electrochemical investigation of the effects of oxidizing agents on the current in the reduction of vanadium(IV) to (II) should be very useful in examining the oxidation mechanism of oxy-anions. It is polarographically anomalous that the reduction potential of vanadium(IV) to (II) is more negative than that of vanadium(III) to (II), where the wave is fairly well defined. The increase in the reduction current of vanadium(IV) to (II) due to the presence of chlorate ion shows catalytic characteristics, that is~ chlorate ion oxidizes vanadium(II) to produce vanadium(III) or (IV), which is re-reduced to vanadium(II) at that potential. The kinetic data could be obtained by measurement of the decreasing diffusion current of vanadium(II) on the addition of chlorate ion. The second-order plots were linear for more than 60~ of the reaction, giving the rate constant 24.4 1 mol-1 s-1, which is in agreement with the value obtained by Gordon and Tewari 4. The rate constant could also be calculated by applying Koutecky's equation to the catalytic current, giving a smaller value: 19 1 mol-1 s-1.
Kalousek's polarograms 6 of catalytic current are shown in Fig. 1, indicating the increase in the current at the reduction potential of vanadium(III) to (II) and (V) to (IV). The fact that no increase in the reduction current of vanadium(V) to (IV) on the addition of chlorate ion was observed by d.c. polarography supports the conclusion that the oxidation of vanadium(IV) by chlorate ion is not significant. Hypochlorate ion, one of the reaction intermediates of chlorate ion, would not give