Solvent Effect on Deprotonation Equilibria of Acids of Various Charge Types in Non-aqueous Isodielectric Mixtures of Protic Ethylene Glycol and Dipolar Aprotic N,N-Dimethylformamide at 298.15 K

Deprotonation constants of phthalic (H2A) and biphthalic (HA−) acids and of mono-protonated (BH+) and di-protonated (BH22+) piperazine acids have been determined at 25 °C by measuring the Emf of galvanic cells comprising H+-sensitive glass GE(H+) and Ag,AgCl electrodes in non-aqueous isodielectric mixtures of protic ethylene glycol (EG) and dipolar aprotic N,N-dimethylformamide (DMF). Solvent effects on deprotonation of the acids: ∂(ΔGdisso)=2.303RT[p(sKa)−p(RKa)], have been dissected into transfer Gibbs energies, ΔGto, of the species involved by evaluating ΔGto of the uncharged phthalic acid and base piperazine (B) from the measured solubilities of the acid and base, respectively, and using ΔGto of H+ based on the TATB reference electrolyte assumptions, as evaluated earlier. The contributions of the different species involved in the protolytic equilibria i.e., H+,H2A,HA−,BH22+ and BH+ and their respective conjugate bases HA−,A2−,BH+ and B have been discussed in terms of their solvation behavior as guided by the ‘acid-base’, dispersion, structural and electronic characteristics of the acid-base species and of the co-solvent molecules and binary mixtures, ignoring the Born-type electrostatic interactions on the ionic species as the solvent system is quasi isodielectric.