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

A spectrophotometric method has been used to determ in e the dissociat ion constants of o-chloroanilinium ion, m-n itroanilini um ion , and 4-chloro-2,6-din itrophenol in methanolwater solvents at 25 °C. The ranges of solvent composition s (in weigh t percent metha nol) were as follows: o-ehloroan ilinium ion , 0 to 99.9; ?n-nitroanilinium ion, 0 to 93.7; and 4-ehloro-2,6-dinitrophenol, 0 to 33.4. The 1)K of t h e first two acids fa lls with addition of methanol and passes through a minimum when the mole fract ion of methanol in the solvent mixture is abo ut 0.7 . The pI( of the substituted phenol, however, rises as the dielectric constant of t he solvent is decreased by addition of methanol. It is suggested that the total m edium effect for both t y pes of acid can be explained by the superposit ion of an electrostatic e ffect and a non electrostatic effect. The latter is a constant for each particu lar solvent composition and probably characterize's the acid-base proper t.v of the m edium itself.