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

A vibrating-tube densimeter (DMA 512P, Anton Paar, Austria) was used to inv estigate the densities and volumetric properties of aqueous potassium hydro gen phthalate (KHP) and potassium sodium phthalate (KNaP). Measurements wer e made at molalities m from (0.006 to 0.66) mol . kg(-1), at temperatures f rom 278.15 K to 368.15 K and at the pressure 0.35 MPa. The densimeter was c alibrated through measurements on pure water and on 1.0 mol . kg(-1) NaCl(a q). We also used a twin fixed-cell, power-compensation, differential output , temperature-scanning calorimeter (NanoDSC 6100, Calorimetry Sciences Corp oration, Spanish Fork, UT, U.S.A.) to measure solution heat capacities. Thi s was accomplished by scanning temperature and comparing the heat capacitie s of the unknown solutions to the heat capacity of water. Apparent molar vo lumes V phi and apparent molar heat capacities C-p,C-phi Of the solutions w ere calculated and fit by regression to equations that describe the surface s (V-phi, T, m) and (C-p,C-phi, T, m). Standard state partial molar volumes VzO and heat capacities C-p,2(0). were estimated by extrapolation to the m = 0 plane of the fitted surfaces. Previously determined C (Delta C-r(p,m), T, m) for the proton dissociation for HCl(aq) and NaCl(aq) were used to ob tain reaction of aqueous hydrogen phthalate. This (Delta C-r(p,m), T, m) su rface was created by subtracting C-p,C-phi for KHP(aq) and for NaCl(aq) fro m the sum of C-p,C-phi for KNaP(aq) and for HCl(aq). Surfaces representing (Delta H-r(m), T, m) and (Delta 8a, T, m), where pe, denotes the molality e quilibrium quotient, were created by integration of our (Delta C-r(p,m), T, m) surface using values for (Delta H-r(m), m) and (pK(a), m) at T = 308.15 K from the literature as integration constants. (C) 2001 Academic Press.