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

Apparent molar volumes Vφand apparent molar heat capacitiesCp, φwere determined for individual solutions of aqueousl-histidine, of aqueousl-histidine with equimolal HCl, and of aqueousl-histidine with equimolal NaOH at molalities m=(0.015 to 0.66 )mol · kg−1, at temperatures T=(278.15 to 393.15 ) K, and at the pressure p=0.35 MPa. Apparent molar volumes were generated from density measurements obtained with a vibrating-tube densimeter. Apparent molar heat capacities were generated from heat capacity measurements with a twin fixed-cell, power-compensation, differential-output, temperature-scanning calorimeter. These results were then fitted by regression to empirical equations to describe the (Vφ,m, T) and (Cp, φ, m, T) surfaces for each of the three systems. These regression equations were then used to calculate the changes in partial molar volumeΔrVmand partial molar heat capacityΔrCp, mas functions of mand Tfor both the first and second proton dissociation reactions for protonated aqueousl-histidine. The changes in enthalpy ΔrHmand entropyΔrSmand the acid dissociation molality quotientQawere then obtained as functions of mand Tfor each proton dissociation reaction by integration, using our (ΔrCp, m, m,T) results and literature values for ΔrHmandQa. Our results illustrate the unique thermodynamic properties of the cationic, neutral zwitterionic, and anionic forms ofl-histidine in aqueous solution.