Potentiometric pK(a) determination of standard substances in binary solvent systems

A new method has been developed for the determination in binary solvent sys tems of acid dissociation constants of compounds (molecular acids or phenol s) used as pK(a) standards in potentiometric measurements. In the first ste p, a rough pK(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 bi nary solvent mixture and in one of the single solvents of the binary system . The pK(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 det ermination of equilibrium constants in solutions. By this novel method, pK( a) values have been determined in binary systems of acetonitrile-dimethyl s ulfoxide, acetonitrile-acetone and acetonitrile-methanol for compounds freq uently used as standards in potentiometric measurements (picric acid and 2, 6-dinitrophenol). The solvent mixtures consisted of both polar aprotic solv ents (the protophobic acetonitrile and acetone, as well as the protophilic dimethyl sulfoxide) and the amphiprotic methanol. In each of the binary sol vent systems studied, similar as in individual solvents, the relationship b etween e.m.f. and the logarithm of the concentration ratio of components of the standardising system (tetra-n-butylammonium picrate-picric acid or tet ra-n-butylammonium 2,6-dinitrophenolate-2,6-dinitrophenol) has been found t o 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 relation ship between pK(a) of the standards and molar fraction was non-linear in al l the solvent systems studied, thus revealing the absence of additivity of the pK(a) values in the solvent mixtures. (C) 2000 Elsevier Science B.V. Al l rights reserved.