Potentiometric p Ka determination of standard substances in binary solvent systems

Abstract

A new method has been developed for the determination in binary solvent systems of acid dissociation constants of compounds (molecular acids or phenols) used as p K a standards in potentiometric measurements. In the first step, a rough p K a value of a standard is estimated from the difference in e.m.f. of equimolar solutions of acidic and basic buffer components in the binary solvent mixture and in one of the single solvents of the binary system. The p K a value of the standard substance estimated in this way provides subsequently the basis for the determination of the accurate value by using a procedure based on the ‘general’ Kostrowicki and Liwo method for the determination of equilibrium constants in solutions. By this novel method, p K a values have been determined in binary systems of acetonitrile–dimethyl sulfoxide, acetonitrile–acetone and acetonitrile–methanol for compounds frequently used as standards in potentiometric measurements (picric acid and 2,6-dinitrophenol). The solvent mixtures consisted of both polar aprotic solvents (the protophobic acetonitrile and acetone, as well as the protophilic dimethyl sulfoxide) and the amphiprotic methanol. In each of the binary solvent systems studied, similar as in individual solvents, the relationship between e.m.f. and the logarithm of the concentration ratio of components of the standardising system (tetra- n-butylammonium picrate–picric acid or tetra- n-butylammonium 2,6-dinitrophenolate–2,6-dinitrophenol) has been found to be linear, and the slope of the glass electrode characteristics was close to the theoretical Nernst value of 59.14 mV. On the other hand, a relationship between p K a of the standards and molar fraction was non-linear in all the solvent systems studied, thus revealing the absence of additivity of the p K a values in the solvent mixtures.