Apparent molar volumes and apparent molar heat capacities of aqueous 2-amino-2-hydroxymethyl-propan-1,3-diol (Tris or THAM) and THAM plus equimolal HCl

We used a vibrating tube densimeter (DMA 512P, Anton Paar Austria) to investigate the densities and volumetric properties of aqueous 2-amino-2-hydroxymethyl-propan-1,3-diol (Tris or THAM) and THAM plus equimolal HCl. We made measurements at molalities m from (0.005 to 0.5) mol . kg(-1) and at temperatures from 278.15 K to 393.15 K. We calibrated the densimeter through measurements on pure water and on 1.0 mol . kg(-1) NaCl at the pressure 0.35 MPa. We used a fixed-cell, power-compensation, differential-output, temperature-scanning calorimeter (NanoDSC 6100, Calorimetry Sciences Corporation, Prove, UT, U.S.A.) to measure solution heat capacities at molalities from (0.005 to 0.5) mol . kg(-1) and at temperatures from 278.15 K to 368.15 K. This was accomplished by scanning temperature and comparing the heat capacities of the unknown solutions to the heat capacity of water. We calculated the apparent molar volumes V-phi and apparent molar heat capacities C-p,C-phi of the solutions and fitted them to equations that describe the surfaces (V-phi against T against m) and (C-p,C-phi against T against m). Standard state partial molar volumes V-2(0) and heat capacities C-p,2(0) were estimated by extrapolation to the m = 0 plane of the fitted surfaces. We used previously determined C-p,C-phi for HCl(aq) to obtain Delta(r)C(p,m) for the proton dissociation reaction of THAM.H+(aq). The (Delta(r)C(p,m) against T against m) surface was coated by subtracting C-p,C-phi of THAM(aq) and HCl(aq) from the heat capacity of THAM.HCl(aq). We created surfaces representing Delta(r)H(m) and pQ(a) by integration of our Delta(r)C(p,m) surface over T while using values for Delta(r)H(m) and pQ(a) at T = 298.15 K from the literature as integration constants. (C) 2000 Academic Press.