This research work has two sections. In first section, the equilibrium constant for protonation processes of DL-phenylalanine (K1, and K2) were determined in binary mixed solvents of water–methanol, containing 0, 10, 20, 30, 40, 50, 60, 70, and 80 % (v/v) methanol, at T = 298.15 K and constant ionic strength (0.1 mol.dm-3NaCl). The obtained data (K1, and K2) were analyzed using Kamlet, Abboud, and Taft parameters. In second section, for DL-phenylalanine in aqueous solution, the values K1, and K2 were determined at T = (298.15, 303.15, 308.15, 313.15, and 318.15) K and constant ionic strength (0.1 mol.dm-3NaCl). Using these K1, and K2, the thermodynamic properties (changes of enthalpy, ΔH, changes of entropy, ΔS, and changes of Gibbs free energy, ΔG) were calculated for DL-phenylalanine in aqueous solution. In both sections, the values of K1, and K2 were determined using the spectrophotometric and potentiometric methods.
