A STUDY OF THE VOLTAMMETRIC RESPONSE OF THIOUREA AND ETHYLENE THIOUREA AT GOLD ELECTRODES IN ALKALINE MEDIA

The voltammetric response of thiourea (TU) and ethylene thiourea (ETU) is influenced by strong adsorption of these sulfur-containing compoun ds. Interpretation of the current-potential curves obtained at a Au ro tating-disk electrode (RDE) is aided by the results of shielding and c ollection data obtained with a Au-Au rotating ring-disk electrode (RRD E). Pulsed amperometric detection (PAD) was applied at the ring of the RRDE to avoid the loss of electrode response to TU and ETU that is ob served for a constant (de) applied potential. The largest response for both compounds is in the form of an anodic peak obtained for the posi tive potential scan in the region from ca. +0.1 to +0.6 V vs. SCE for 0.10 M NaOH, This current peak is composed of contributions from the f ormation of surface oxide, oxidation of TU and ETU adsorbed in the pot ential region below 0.1 V, and oxidation of TU and ETU transported to the electrode simultaneously with the appearance of the anodic peak. T he relative contributions of each component depend on the concentratio ns of TU and ETU, the rotational velocity of the electrode and the pot ential scan rate. Voltammetric data indicate that ETU is adsorbed to a greater extent than TU and, furthermore, that adsorbed TU undergoes d esulfurization with accumulation of adsorbed S degrees in the region f rom -0.6 to -0.1 V.