Coulometric reduction of cuprous sulphide films on copper

Abstract

Thin films of cuprous sulphide formed on copper, showing first- or second-order inference colours, can be reduced quantitatively in a deoxygenated phosphate buffer solution, pH 7, at a current density of 300 μA/cm 2. Sulphide reduced is equivalent to charge passed for films having up to 15 μg/cm 2 of sulphur; for those having between 15 and 50 μg/cm 2 the method underestimates the sulphur, through loss of electrical contact between the cathode and unreduced film material. For films having more than 50 μg/cm 2 the method is not quantitative. 1. 1. First- and second-order sulphide films formed on copper can be reduced quantitatively by the coulometric method in a deoxygenated phosphate buffer solution at a current density of 3OO μA/cm 2. Under these conditions there is a clear separation between sulphide reduction and hydrogen-ion reduction, since the polarization characteristics of the reactions are different. 2. 2. The equivalence of charge passed through the cell and mass of sulphide reduced is nearly exact for films containing up to 15 μg/cm 2 of sulphur. Between 15 and 50 μg/cm 2, the method somewhat under-estimates the sulphur present, because of loss of electrical contact detween the cathode and unreduced film material. Above 50 μg/cm 2 the method is not quantitative. 3. 3. The mechanism of reduction is probably the formation of specks of metallic copper in the outer layers of the film; these may grow to colloidal and in some cases microscopic size.