The effect of sodium chloride on thermodynamic paraemeters of ion association for cysteine and methionine with L-ascorbic acid in aqueous solution at different temperature

Abstract

Molar conductivity for hydrogen bonds ion association of ascorbic acid (AA) with Cysteine (Cys) and Methionine (Met) at concentration (2–5×10−3, 2×10−3 and 2×10−3 M respectively) in water and in (2×10−3 M) sodium chloride (NaCl) solutions was examined at temperatures range (298 K to 313 K). The conductance equation of Shedlovsky was used to calculate the association constants (KA) and the limiting molar conductance (Λo). The association heat, the Gibbs free energy, the change of entropy and the activation energy (ΔHo, ΔGo, ΔSo, and ΔES respectively) also calculated. All results are computed. The data showed increasing of Λo, with increases in temperature and the same trend in decreasing for KA. The data showed negative values for ΔHo and ΔGo parameters. This refers to the exothermic association processes, and values increased with increasing in AA concentration, while ΔSo and ΔES parameters show a positive value related to the higher mobility of the ions and decreasing of ion association solvation in water and NaCl solutions. The ΔEs and ΔHo of a different solution in present sodium chloride show decreasing in values as compared with water solution at different concentration of AA, while the opposite trend values appear for ΔGo and ΔSo with some exception.