ACQUISITION AND EVALUATION OF THERMODYNAMIC DATA FOR THE REACTION CDSO4·8/3H2O = CDSO4·H2O + 5/3 H2O AT 0.1 MPA
Published estimates for the equilibrium relative humidity (RH) for the reaction: (CdSO4•8/3H2O)solid=(CdSO4•H2O)solid + 5/3 (H2O)gas, at 25οC range from 71.5 to 74.9%. To evaluate these data, the humidity-buffer technique (Chou et al., 2002, Amer. Mineral., 87, 108-114) was used to determine equilibrium constants for this reaction between 25 and 43οC at 0.1 MPa. Reversals along four humidity-buffer curves yield ln K=28.61 - 10395.1/T, where K is the equilibrium constant, and T is temperature in K. The derived standard Gibbs free energy of reaction is 15.51 kJ/mol, which agrees within ±0.05 kJ/mol with values based on the vapor pressure measurements of Carpenter and Jette (1923, J. Am. Chem. Soc., 45, 578-590), and Ishikawa and Murooka (1933, The Science Report of Tohoku Univ., Ser. 1, 22, 138-155). It also agrees within ±0.10 kJ/mol with values calculated from the data compiled by Wagman et al. (1982, J. Phys. Chem. Ref. Data, v. 11, Sup. No. 2) and DeKock (1986, Bur. Mines Inf. Cir., 9081). Previous studies indicate that the temperature of the invariant point for the assemblage (CdSO4•8/3H2O + CdSO4•H2O + aqueous solution + vapor) is near 43.6οC (Linke and Seidell, 1958, Amer. Chem. Soc.); the %RH values predicted from Carpenter and Jette (ibid.) and Ishikawa and Murooka (ibid.) at this temperature are 88.5 and 89.8, respectively. Our extrapolated data predict 89.4 %RH at 43.6οC for this invariant point; the predicted RH is in excellent agreement with those reported previously.