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

Author： Ford T.D.   Call T.G.   Origlia M.L.   Stark M.A.   Woolley E.M.  

Publisher： Academic Press

ISSN： 0021-9614

Source： Journal of Chemical Thermodynamics, Vol.33, Iss.3, 2001-03, pp. : 287-304

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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 volumesV2oand heat capacities Cp,2owere estimated by extrapolation to the m = 0 plane of the fitted surfaces. Previously determinedCp, φfor HCl(aq) and NaCl(aq) were used to obtain (∆rCp, m, T, m) for the proton dissociation reaction of aqueous hydrogen phthalate. This (∆rCp,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 (∆rHm, T, m) and (pQa, T, m), where pQadenotes the molality equilibrium quotient, were created by integration of our (∆rCp,m, T, m) surface using values for (∆rHm, m) and (pKa, m) at T = 308.15 K from the literature as integration constants.

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