Apparent molar volumes and apparent molar heat capacities of aqueous potassium hydrogen phthalate (KHP) and potassium sodium phthalate (KNaP) at temperatures fromT = 278.15 K toT = 393.15 K at the pressure 0.35 MPa

Abstract

A vibrating-tube densimeter (DMA 512P, Anton Paar, Austria) was used to investigate the densities and volumetric properties of aqueous potassium hydrogen phthalate (KHP) and potassium sodium phthalate (KNaP). Measurements were made at molalities m from (0.006 to 0.66)mol · kg−1, at temperatures from 278.15 K to 368.15 K and at the pressure 0.35 MPa. The densimeter was calibrated through measurements on pure water and on 1.0 mol · kg−1NaCl(aq). We also used a twin fixed-cell, power-compensation, differential-output, temperature-scanning calorimeter (NanoDSC 6100, Calorimetry Sciences Corporation, Spanish Fork, UT, U.S.A.) to measure solution heat capacities. This was accomplished by scanning temperature and comparing the heat capacities of the unknown solutions to the heat capacity of water. Apparent molar volumes Vφand apparent molar heat capacities Cp, φof the solutions were calculated and fit by regression to equations that describe the surfaces (Vφ, T, m) and (Cp, φ, T, m). Standard state partial molar volumesV2o and heat capacities Cp,2owere estimated by extrapolation to the m= 0 plane of the fitted surfaces. Previously determinedCp, φ for HCl(aq) and NaCl(aq) were used to obtain (ΔrCp, m, T, m) for the proton dissociation reaction of aqueous hydrogen phthalate. This (ΔrCp,m, T, m) surface was created by subtracting Cp,φfor KHP(aq) and for NaCl(aq) from the sum of Cp,φfor KNaP(aq) and for HCl(aq). Surfaces representing (ΔrHm, T, m) and (pQa, T, m), where pQadenotes the molality equilibrium quotient, were created by integration of our (ΔrCp,m, T, m) surface using values for (ΔrHm, m) and (pKa, m) at T= 308.15 K from the literature as integration constants.