Thermodynamics Research Center / ThermoML | Journal of Chemical Thermodynamics

ABSTRACT

We 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?1 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 Cp,/ for these solutions. We used our results and values from the literature for V/(T,m) and Cp,/(T,m) for HCl(aq), NaOH(aq), and NaCl(aq) and the molar heat capacity change DrCp,m(T,m) for ionization of water to calculate DrCp,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 Young?s rule to account for the effects of speciation and chemical relaxation on the observed V/(T,m) and Cp,/(T,m) of the solutions. This procedure yielded parameters for V/(T,m) and Cp,/(T,m) for serinium chloride {H2Ser+Cl?(aq)} and for sodium serinate {Na+Gly?(aq)} which successfully modeled our observed results. We have then calculated DrCp,m, DrHm, DrVm and pQa for the first and second proton dissociations from protonated aqueous serine as functions of T and m.