Studies on the Dissociation Constants of Phthalic Acid in Aquo-ethanolic Mixtures

United Bank of India, 507, R. B. C. Road, Garifa, North 24-Parganas Department of Chemistry, University of Kalyani, Kalyani-741 235 Department of Chemistry, Vivekananda Mission Mahavidyalaya, Viveknagar, P.O. Chaitanyapur (Haldia)-721 645 Manuscript received 6 April 1993, revised 15 July 1993, accepted 26 August 1993 As in aqueous solutions, potassium hydrogen phthalate solution (0.05 M) may be used as a suitable buffer in mixed solvents also, provided the first and second dissociation constants of phthalic acid in mixed solvents are known. The first dissociation constants have been determined using solubility and conductometric measurements and the second ones have been determined pH-metrically. The quantitative measure of the ion-solvent or solute-solvent interactions have been obtained from the values of Gibbs energy of transfer of phthalic acid, biphthalate ion and phthalate ion by coupling solubility values of H2Ph and the dissociation constant values using appropriate relations . Δ G\(_t^o\)(HPh-) and . Δ G\(_t^o\)(Ph-2) have been found to be increasingly unfavourable in going from water to non-aqueous solvents. The Aº values of H2Ph and KHPh also decrease continuously in going from water to mixed solvents