Thermodynamics of proton dissociations from aqueous l-methionine at temperatures from (278.15 to 393.15) K, molalities from (0.0125 to 1.0) mol · kg−1, and at the pressure 0.35 MPa: Apparent molar heat capacities and apparent molar volumes of l-methionine

We have measured the densities of aqueous solutions of l-methionine, l-methionine plus equimolal HCl, and l-methionine plus equimolal NaOH at temperatures 278.15 ⩽ T/K ⩽ 368.15, at molalities 0.0125 ⩽ m/mol · kg−1 ⩽ 1.0 as solubilities allowed, and at p = 0.35 MPa using a vibrating tube densimeter. We have also measured the heat capacities of these solutions at 278.15 ⩽ T/K ⩽ 393.15 and at the same m and p using a twin fixed-cell differential temperature-scanning calorimeter. We used the densities to calculate apparent molar volumes Vϕ and the heat capacities to calculate apparent molar heat capacities Cp,ϕ for these solutions. We used our results and values from the literature for Vϕ(T, m) and Cp,ϕ(T, m) for HCl(aq), NaOH(aq), and NaCl(aq) and the molar heat capacity change ΔrCp,m(T, m) for ionization of water to calculate parameters for ΔrCp,m(T, m) for the two proton dissociations from protonated aqueous cationic l-methionine. We integrated these results in an iterative algorithm using Young’s Rule to account for the effects of speciation and chemical relaxation on Vϕ(T, m) and Cp,ϕ(T, m). This procedure yielded parameters for Vϕ(T, m) and Cp,ϕ(T, m) for methioninium chloride {H2Met+Cl−(aq)} and for sodium methioninate {Na+Met−(aq)} which successfully modeled our observed results. Values are given for ΔrCp,m, ΔrHm, pQa, ΔrSm, and ΔrVm for the first and second proton dissociations from protonated aqueous l-methionine as functions of T and m.