ACQUISITION AND EVALUATION OF THERMODYNAMIC DATA FOR THE REACTION CDSO4·8/3H2O = CDSO4·H2O + 5/3 H2O AT 0.1 MPA

Metal sulfates play important roles in the short-term sequestration of metals in mine-drainage settings. Hydrated sulfate salts of ferrous iron, copper, and zinc are relatively common, but cadmium-sulfate salts are exceedingly rare; only an unnamed monohydrate, found on burning coal-mine wastes, and an unnamed quadrihydrate have been reported. Nevertheless, sulfate salts are likely to be important to the cycling of cadmium through solid-solution effects in more common sulfate salts. Thus, thermodynamic data for end-member salts are essential for understanding more complex natural systems including solid solutions and for modeling speciation in high ionic strength solutions. This study extends the database to the Cd end-member system, which is applicable in the well-known Weston voltage standard cell.

Published estimates for the equilibrium relative humidity (RH) for the reaction: (CdSO₄•8/3H₂O)_solid=(CdSO₄•H₂O)_solid + 5/3 (H₂O)_gas, at 25^οC 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 43^οC 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 43.6^οC (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 43.6^οC for this invariant point; the predicted RH is in excellent agreement with those reported previously.