Acid-Base Equilibrium of Derivatives of Phenothiazine Hydrochloride in Aqueous Solutions

Abstract

The concentration dependence of pH of each of six phenothiazine hydrochlorides (PTZ·HCl) in aqueous solution was measured at 25°. Below the critical micelle concentration (cmc), the pH decreased slowly with increasing concentration. Just above the cmc, the pH decreased rapidly, and then decreased gradually with further increase of the concentration. Below the cmc, the pH change was mainly due to the acid dissociation of monomer PTZH⁺, but slightly affected by dissolved carbon dioxide. On the other hand, above the cmc, not only the monomer but aggregated PTZH⁺ also dissociates, and the acid dissociation constant of the latter was greater than that of the former. Proton balance equation was derived on the system, where two independent weak acids coexist. According to this equation, the concentration dependence of pH was culculated for the coexistence of the monomer and carbon dioxide below the cmc and for the coexistence of the monomer and micelle above the cmc. Measured pH change with the concentration was consistent with the calculated one at two separate regions, below and above the cmc. The difference in pK_a between the monomer and aggregated PTZH⁺ (ΔpK_a) was obtained on each PTZH⁺, which was related to the electrostatic work of micelle formation, and to the surface potential of PTZH⁺ micelle.