To provide a simple method for accurately controlling the concentration of hydroxide ions in an alkaline permanganate bath to a constant concentration range without being affected by the concentration of carbonate ions, the concentration of hydroxide ions affecting the ability to etch resin using the bath.
A pH drop curve originated by causing an acid to drop into a sampling liquid sampled from the bath of alkaline permanganate is compared in its shape to the pH drop curves of permanganic acid baths containing carbonate ions at varying concentrations, and a pH end point value corresponding to the determined concentration of carbonate ions is set within the bath. Based on the measurement of the concentration of hydroxide ions taken from the amount by which the acid drops at the pH end point value, the concentration of hydroxide ions in the permanganate bath is controlled to a certain value.
YAMAGUCHI YOSHIHIDE
WATABE MAKIO
