Thermodynamics of proton dissociations from aqueous citric acid: apparent molar volumes and apparent molar heat capacities of citric acid and its sodium salts at the pressure 0.35 MPa and at temperatures from 278.15 K to 393.15 K

• Published in: The Journal of chemical thermodynamics 2001-12, Vol.33 (12), p.1735-1764
• Main Authors: Patterson, B.A., Woolley, E.M.
• Format: Article
• Language: English
• Subjects: apparent molar heat capacity; apparent molar volume; Chemical thermodynamics; Chemistry; citric acid; Exact sciences and technology; General and physical chemistry; General. Theory; proton dissociation; sodium citrate
• ISSN: 0021-9614; 1096-3626
• DOI: 10.1006/jcht.2001.0880

Apparent molar volumes Vφand apparent molar heat capacitiesCp, φ have been determined for aqueous citric acid and its mono-, di-, and tri-sodium salts at the pressure p= 0.35 MPa and at temperatures 278.15⩽T/K⩽ 393.15. Densities were measured using a vibrating-tube densimeter (DMA 512, Anton Paar, Austria) at the pressure p= 0.35 MPa and at temperatures 278.15⩽T/K⩽ 368.15. These results were used to calculate the apparent molar volumes of each solute over the concentration range 0.01 ⩽m/(mol · kg−1) ⩽ 1.0. Heat capacities of these same solutions were measured using a twin fixed-cell, differential-output, power-compensation, temperature-scanning calorimeter (NanoDSC model 6100, Calorimetry Sciences Corporation, Spanish Fork, UT, U.S.A.) at the pressure p= 0.35 MPa and at temperatures 278.15 ⩽T/K⩽ 393. Equations to describe the ( m,T , Vφ) and ( m, T, Cp,φ) surfaces of each solute were obtained by regression. Using these equations we calculated the surfaces ( m, T, ΔrVm), ( m,T , ΔrCp, m), ( m, T,ΔrHm ), ( m, T,pQa ), and ( m, T,ΔrSm ) for each of the three-proton dissociation reactions of aqueous citric acid. The ΔrHmandpQa surfaces were calculated for each proton dissociation reaction by integration using our (m , T, ΔrCp, m) results with the molality dependence of ΔrHm, and pQafor each reaction at T= 298.15 K obtained from the literature. Corrections for the partial dissociation and/or hydrolysis of each citrate species were applied in these calculations.