Determination of pH in Reversed Micelles

Abstract

Abstract A new method for the determination of pH in reversed micellar systems was proposed. The degree of dissociation (α) of Phenol Red in the systems was observed spectrophotometrically. The value of pH can be obtained by use of the Henderson-Hasselbach equation with the aid of pKa of Phenol Red measured by means of 31P-NMR in the systems containing phosphate buffer solutions. The method was applied to systems consisting of sodium octanoate, 1-hexanol, and water and some buffer solutions other than phosphate buffer solutions, and found to be effective within a wide pH range. Analysis of the fluorescence spectra of 8-anilino-1-naphthalene-sulfonic acid indicates that Phenol Red molecules are present at the interface between the water core and the 1-hexanol phase in reversed micelles. The hydrolytic reaction of p-nitrophenyl acetate was also studied kinetically and found to change slightly with pH.