Effect of solution pH on the electrochemical behavior of thiocarbamide at gold and platinum electrodes

The effect of solution pH on the electrochemical behavior of thiocarbamide at gold and platinum electrodes is studied by using cyclic voltammetry and quartz microgravimetry. It is shown that with the increasing of pH the half-wave potential of thiocarbamide oxidation shifts toward negative values in both cases. In the cathodic branches of the cyclic voltammograms, up to pH ≃ 4 the potential region of reduction of the thiocarbamide oxidation product remains practically unchanged; this allows concluding that the oxidation product is formamidine disulfide (at platinum) or its mixture with gold complex ions (at gold electrode). Further increasing of pH up to 9.5–10 results in the increase of the contribution of the reduction current of these ions to the cathodic signal. An explanation for this phenomenon is given. At the same time, new signals appear at E < −0.4 V; they may be interpreted as the current of elemental sulfur reduction. Increasing of pH over 10 leads to gold passivation by the elemental sulfur; this manifests itself as the electrode mass increase during the microgravimetry measurements. Introducing of sodium silicate (recommended in the literature as a solution stabilizer) to the solution does not eliminate the passivation of gold.