Abstract
Potential-time curves are constructed for the Cu electrode in aerated sodium hydroxide (NaOH) solutions devoid of and containing increasing concentrations of S2− ions. In sulfide-free solutions, the steady-state potentials (Est) are reached from negative values indicating oxide film thickening. Lower concentrations of S2− ions enhance oxide film thickening and the potential moves more in the noble direction. Increasing the S2− ion content causes the potential to fluctuate in the active and noble directions during a time span that depends on the S2− ion content. This behavior is attributed to the preferential formation of copper oxide and/or copper films. Higher concentrations of S2− ions, on the other hand, cause the potential to change markedly into the active direction, denoting the formation of Cu2S. Within this range of S2− ion concentration, the Est varies with [S2−] according to: Est = Eo − 0.059 log [S2−] at 25°C, while Eo varies with the pH of the solution to give a straight line with a slope of 29 mV/pH unit. A mechanism is proposed that involves the formation of CuHS as an intermediate step in the process of Cu2S formation.
