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Thermodynamics of proton dissociations from aqueous serine at temperatures from (278.15 to 393.15) K, molalities from (0.01 up to 1.0) mol AE kg À1 , and at the pressure 0.35 MPa: Apparent molar heat capacities and apparent molar volumes of serine, serinium chloride, and sodium serinate AbstractWe have measured the densities of aqueous solutions of serine, serine plus equimolal HCl, and serine plus equimolal NaOH at temperatures 278.15 6 T/K 6 368.15, molalities 0.01 6 m/mol AE kg À1 6 1.0, and at the pressure p = 0.35 MPa, using a vibrating tube densimeter. We have also measured the heat capacities of these solutions at 278.15 6 T/K 6 393.15 and at the same m and p using a fixed-cell differential temperature-scanning calorimeter. We used the densities to calculate apparent molar volumes V / and the heat capacities to calculate apparent molar heat capacities C p,/ for these solutions. We used our results and values from the literature for V / (T,m) and C p,/ (T,m) for HCl(aq), NaOH(aq), and NaCl(aq) and the molar heat capacity change D r C p,m (T,m) for ionization of water to calculate D r C p,m (T,m) for proton dissociations from protonated aqueous cationic serine and from the zwitterionic form. We integrated these results in an iterative algorithm using YoungÕs rule to account for the effects of speciation and chemical relaxation on the observed V / (T,m) and C p,/ (T,m) of the solutions. This procedure yielded parameters for V / (T,m) and C p,/ (T,m) for serinium chloride {H 2 Ser + Cl À (aq)} and for sodium serinate {Na + Gly À (aq)} which successfully modeled our observed results. We have then calculated D r C p,m , D r H m , D r V m and pQ a for the first and second proton dissociations from protonated aqueous serine as functions of T and m.

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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.