Medium Effects on the Dissociation of Weak Acids in Methanol-Water Solvents

A spectrophotometric method has been used to determine the dissociation constants of o-chloroanilinium ion, m-nitroanilinium ion, and 4-chloro-2,6-dinitrophenol in methanol- water solvents at 25 °C. The ranges of solvent compositions (in weight percent methanol) were as follows: o-chloroanilinium ion, 0 to 99.9; m-nitroanilinium ion, 0 to 93.7; and 4-chloro-2,6-dinitrophenol, 0 to 33.4. The pK of the first two acids falls with addition of methanol and passes through a minimum when the mole fraction of methanol in the solvent mixture is about 0.7. The pK of the substituted phenol, however, rises as the dielectric constant of the solvent is decreased by addition of methanol. It is suggested that the total medium effect for both types of acid can be explained by the superposition of an electrostatic effect and a nonelectrostatic effect. The latter is a constant for each particular solvent composition and probably characterizes the acid-base property of the medium itself.