The Effect of Chemisorbed Sulfide Ions on the Gold Electrodeposition from Acidic Thiocarbamide Electrolytes

Introducing sodium sulfide (about 10(-5) M) into acidic thiocarbamide solutions reduces the gold reduction overpotential. The reaction rate passes through a maximum at a potential of 0.1 V. The overpotential depends on the sulfide ion concentration and the time of electrode exposure to solution prior to the beginning of scanning. Transients of potential measured on a renewable gold electrode in thiocarbamide electrolytes containing catalytically active species served as the basis for calculations of the coefficient of trapping of sulfide ions by the growing gold deposit. The kinetics of gold electrodeposition at fixed surface coverages with adsorbed sulfide ions theta is studied. It is shown that at theta = const, the dependence of the reaction rate on the overpotential is described by the Tafel equation. It is shown that with an increase in theta, the effective values of exchange current and transfer coefficient increase from i(0) congruent to 10(-5) A/cm(2) and alpha congruent to 0.25 in pure solutions to alpha congruent to 0.5 and i(0) congruent to 10(-4) A/cm(2) theta >= 0.3 and then remains virtually unchanged. The reaction order decreases in the absolute magnitude, remaining negative. Thus for theta congruent to 0, p(k) = partial derivative log i/partial derivative log c = -1, whereas for theta >= 0.3, p(k) = -0.3. A possible explanation is proposed for the catalytic effect of the sulfide ion adsorption on the mechanism of the gold reduction from acidic thiocarbamide electrolytes.