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.
