Medium effects on deprotonation of phthalic and biphthalic acids in aqueous binary mixtures of some protic, aprotic, and dipolar aprotic cosolvents

Abstract

Deprotonation constants of phthalic acid (H₂A), (K_a)_HzA, and biphthalic acid (HA^–), (K_a)_HA^–, have been determined at 25 °C by measuring the e.m.f.s of galvanic cells comprising glass and Ag–AgCl electrodes in aqueous mixtures of organic cosolvents of different chemical nature, viz. protic glycerol (GL), aprotic dioxane (D), protophobic dipolar aprotic acetonitrile (ACN), and protophilic dipolar aprotic dimethyl sulphoxide (DMSO). Medium effects on deprotonation of the acids: δ(ΔG°_diss,)=2.303 RT[p(_sK_a)– p(_wK_a)] have been dissected into transfer free energies ΔG°_t of the species involved by evaluating ΔG°_t of the uncharged acid (H₂A) from the measured solubilities of the acid and using ΔG°_t of H⁺ based on the widely used tetraphenylarsonium tetraphenylboride (TATB) reference electrolyte assumption, as reported earlier for the solvents. The contributions of the different species involved in the protolytic equilibria, viz. H⁺, acids (H₂A or HA^–), and their respective conjugate bases (HA^– or A^2–) are discussed in terms of their solvation behaviour as guided by the ‘acid-base’, dispersion, structural, and electronic characteristics of the acid–base species and of the cosolvent molecules and their aqueous mixtures, besides the Born-type electrostatic interactions on the ionic acid–base species.