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Apparent molar volumes V φ and apparent molar heat capacities C p,φ were determined for individual solutions of aqueous L-histidine, of aqueous L-histidine with equimolal HCl, and of aqueous L-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 (V φ ,m,T ) and (C p,φ , m, T ) surfaces for each of the three systems. These regression equations were then used to calculate the changes in partial molar volume r V m and partial molar heat capacity r C p,m as functions of m and T for both the first and second proton dissociation reactions for protonated aqueous L-histidine. The changes in enthalpy r H m and entropy r S m and the acid dissociation molality quotient Q a were then obtained as functions of m and T for each proton dissociation reaction by integration, using our ( r C p,m , m, T ) results and literature values for r H m and Q a . Our results illustrate the unique thermodynamic properties of the cationic, neutral zwitterionic, and anionic forms of L-histidine in aqueous solution.

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