Dielectric Constant Measurements on the Binary Solvents: Acetic Acid-Acetic Anhydride and Acetic Acid-Nitromethane

Abstract

composed of aromantics and acetonitrile, since interfering reactions involving salt solvation and multiple ion association may occur. However, the availability of reliable formation and dissociation constants for many acids, bases, and salts in glacial acetic acid suggested to us that a binary solvent containing acetic acid as one component would be a suitable system in which to study the relationship of dielectric constant of the medium to spectrophotometrically determined constants. Hoover and Hutchison (8) observed parallel changes in both the electrical conductance and dielectric constant with changes in composition of dilute solutions of water and acetic anhydride in acetic acid. An increase in the apparent base strength of weak bases in acetic acid (with respect to perchloric acid, HCIO4, as the reference acid) occurs upon the addition of a second miscible solvent having a dielectric constant greater than 6.2. Fritz and Fulda (5) attributed the large change in e.m.f. at the equivalence point for the titration of colorless bases in the binary solvents, acetic acid-acetic anhydride and acetic anhydride-nitromethane, to the complete removal of water from the solvent and to HOAc+C10- being a stronger acid than H3O+CIO--. Other potentiometric measurements by Streuli (18) showed that acetic acid-acetic anhydride is a nonleveling solvent toward neutral and anionic bases. Spectrophotometrically determined indicator constants (halfneutralization numbers) exhibited a continuous nonlinear increase in magnitude with increasing mole fraction of acetic anhydride in the same binary solvent system (13).