Thermodynamics of proton dissociations from aqueous l-proline: apparent molar volumes and apparent molar heat capacities of the protonated cationic, zwitterionic, and deprotonated anionic forms at temperatures from 278.15 K to 393.15 K and at the pressure 0.35 MPa

Abstract

Apparent molar volumes V φ and apparent molar heat capacities C p, φ were determined for aqueous solutions of l-proline, l-proline with equimolal HCl, and l-proline with equimolal NaOH at the pressure p=0.35 MPa. Density measurements obtained with a vibrating-tube densimeter at temperatures (278.15⩽ T/K⩽368.15) were used to calculate V φ values, and heat capacity measurements obtained with a twin fixed-cell, differential-output, power-compensation, temperature-scanning calorimeter at temperatures (278.15⩽ T/K⩽393.15) were used to calculate C p, φ values. Speciation arising from equilibrium was accounted for using Young’s Rule, and semi-empirical equations describing ( V φ , m, T) and ( C p, φ , m, T) for each aqueous equilibrium species were fitted by regression to the experimental results. From these equations, the volume change Δ r V m and heat capacity change Δ r C p,m for the protonation and deprotonation reactions were calculated. Additionally, the Δ r C p,m expression was integrated symbolically to yield values of the reaction enthalpy change Δ r H m, reaction entropy change Δ r S m, and equilibrium molality reaction quotient Q for both reactions. The results provide a much-improved thermodynamic characterization of aqueous l-proline and of its protonation and deprotonation equilibria.