Analysis of Films on Copper by Coulometric Reduction

Abstract

A technique is described for coulometric reduction of films on copper and other electropositive metals which utilizes a granular electrode for rapid and thorough pre-electrolysis of electrolyte to remove dissolved oxygen and traces of plateable cations. The sensitivity of the method has been thereby increased, and as little as a quarter of a monolayer of a reducible film can be detected readily. Results are reported for reversible cell potentials, overpotential as a function of pH and current density, and the effect of oxygen in the electrolyte, for the reduction of films of cuprous oxide, and for reduction of hydrogen ions at a copper surface. Data are also reported on the reduction of thick films of cuprous oxide formed at 125~ and the usefulness of the coulometric method in providing information about the topography of reducible surface films is demonstrated. Other films studied included cuprous sulfide, cupric oxide, and cupric hydrogen phosphate, occurring alone or as mixed films. The effectiveness of various preparatory and rinsing techniques in eliminating phosphate contamination from copper surfaces electropolished in o-phosphoric acid has been evaluated. With the most favorable technique, the residual phosphate appears to be much less than a monolayer.