Potentiometric Studies on the Protonation Constants and Protonation Energies of Some Diamines in Methanol + Water Mixtures

Abstract

The protonation constants of diamines such as ethylenediamine, 1,2-diaminopropane, 1,3-diaminopropane, o-phenylenediamine, m-phenylene-diamine, p-phenylenediamine were determined on the basis of Bjerrum and Calvin method in methanol-water mixtures. A pH metric method was used for calculation of protonation constants. The effects of solvents on protonation constant have been determined at ionic strength 0.2 M dm^-3 (NaClO₄) and temperature 30±0.1^oC under nitrogen atmosphere. FORTRAN (IV) programs were used for calculation of protonation constants and distribution of species like H₂L, HL, L in equilibrium state. The logarithm of the protonation constants decrease in aliphatic diamines and increase in aromatic diamines with increase in methanol content in mixed equilibria. The verification of constants are explained on the basis of solute-solvent interaction, solvation, proton transfer processes and dielectric constant of equilibria. Protonation energies have been calculated theoretically using computational methods and these protonation energies for aromatic diamines are higher than aliphatic diamines.