Thermodynamics for ionization of water at temperatures from 278.15 K to 393.15 K and at the pressure 0.35 MPa: apparent molar volumes of aqueous KCl, KOH, and NaOH and apparent molar heat capacities of aqueous HCl, KCl, KOH, and NaOH

Abstract

Apparent molar volumes Vφof aqueous KCl, KOH, and NaOH and apparent molar heat capacities Cp, φof aqueous HCl, KCl, KOH, and NaOH have been determined at the pressure p= 0.35 MPa, and at molalities 0.015 ⩽m/mol · kg−1⩽ 0.5. Densities were measured using a vibrating-tube densimeter (DMA 512, Anton Paar, Austria) at temperatures 278.15 ⩽T/K⩽ 368.15. These values were used to calculate the apparent molar volumes. A fixed-cell, differential-output, power-compensating, temperature-scanning calorimeter (NanoDSC model 6100, Calorimetry Sciences Corporation, Spanish Fork, UT, U.S.A.) was used to measure the heat capacities of the same solutions at temperatures 278.15 ⩽T/K⩽ 393.15. Results were fitted by using equations that describe the surfaces (m, T, Vφ) and (m, T, Cp, φ). Using these equations, we have calculated the surfaces (m, T, ΔrVm), (m, T, ΔrCp, m), (m, T, ΔrHm), (m,T , p Qa), and (m, T,ΔrSm ) for the ionization of water in the presence of combinations of the above electrolytes. The last three surfaces were calculated by integration using our (m,T , ΔrCp, m) surface and literature values for the molality dependence of ΔrHmand pQa at T= 298.15 K.