INTERACTION OF THIOUREA WITH SILVER ELECTRODES

Adsorption and surface reactions of thiourea on silverized silver elec trodes have been studied in 0.1 M HClO4 and 0.1 M NaClO4. Quantitative radiometric data obtained using C-14-labeled thiourea indicate that a dsorption of thiourea takes place in the entire range of potentials ac cessible to surface studies on silver electrodes in the acidic and neu tral solutions. The values of the thiourea surface concentration are v ery high, close to the maximum monolayer packing densities calculated from the molecular size of thiourea (ca. 6 x 10(14) molecules cm(2)). Reorientation of adsorbed molecules from perpendicular to parallel to the surface is a possible reason for the decrease in Gamma at potentia ls located on the positive side of the adsorption maximum. As shown by the surface/bulk-exchange experiments, adsorption is reversible in th e potential range preceding and directly following the adsorption maxi mum. In this range, the surface process can be quite accurately descri bed by the Frumkin adsorption model. When corrected for water displace ment, the calculated value of the Gibbs energy of adsorption points to strong adsorption of thiourea on silver (chemisorption). At the most positive potentials, surface processes become irreversible, ultimately leading to the formation of chemisorbed sulfur-containing species tha t act as the electrode poison. Depending on the solution pH, an additi on of chloride to the cell has different effects on the adsorption of thiourea. As a result of the Cl--caused reorientation of the adsorbate , the surface concentration of thiourea increases in the neutral solut ion. The opposite effect occurs in the acidic solution, where surface blocking by adsorbed chloride plays a dominant role.