Thermodynamics of proton dissociations from aqueous [formula omitted] -tartaric acid at temperatures 278.15⩽⩽⩽ (T / K)⩽⩽⩽ 393.15 and at the pressurep = 0.35 MPa: apparent molar volumes and apparent molar heat capacities of[formula omitted] -tartaric acid and its sodium salts

Abstract

Apparent molar volumes Vφand apparent molar heat capacitiesCp, φ of aqueous solutions ofl-tartaric acid, mono-sodiuml-tartrate, and di-sodiuml-tartrate have been measured at molalities 0.006 ⩽m/(mol · kg−1)⩽ 1.0, at temperatures 278.15⩽T/K⩽ 393.15, and at the pressure p= 0.35 MPa. Apparent molar volumes were obtained from density measurement using a vibrating-tube densimeter (DMA 512, Anton Paar, Austria). Apparent molar heat capacities were obtained from heat capacity measurements using a twin fixed-cell, differential-output, power-compensation, temperature-scanning calorimeter (NanoDSC model 6100, Calorimetric Sciences Corporation, Spanish Fork, UT, U.S.A.). These results were fit by regression to semi-empirical equations to describe the (m, T, Vφ) and (m, T,Cp, φ ) surfaces. These results were also used to estimate the change in volume (m, T, ΔrVm) and the change in heat capacity (m, T, ΔrCp, m) for the two stepwise proton dissociation reactions of aqueousl-tartaric acid. The (m, T,ΔrCp, m ) surfaces for each proton dissociation reaction were integrated numerically to obtain estimates of the change in enthalpy (m, T,ΔrHm ) and proton dissociation molality quotients (m, T, pQa). These integrated results greatly extend the ranges ofm and T over which precise thermodynamic information is available for the two dissociation reactions of aqueousl-tartaric acid.