SOLID-SOLUTE PHASE-EQUILIBRIA IN AQUEOUS-SOLUTION .6. SOLUBILITIES, COMPLEX-FORMATION, AND ION-INTERACTION PARAMETERS FOR THE SYSTEM NA+ - MG2+ - CLO4(-) - CO2 - H2O AT 25-DEGREES-C

The stoichiometric solubility constant of eitelite (NaMg0.5CO3 + 2H+ reversible Na+ + 0.5Mg2+ + CO2(g) + H2O, log *K(ps0)I = 14.67+/-0.03 was determined at I = 3 m (mol kg-1) (NaClO4) and 25-degrees-C. The stability of magnesium (hydrogen-)carbonato complexes in this ionic medium was explicitely taken into account. Consequently, trace activity coefficients of free ionic species, calculated from the Pitzer model with ion-interaction parameters from the literature, were sufficient for an evaluation of the thermodynamic solubility constants and Gibbs energies of formation for eitelite (-1039.88+/-0.60), magnesite 33.60+/-0.40), hydromagnesite (-1174.30+/-0.50), nesquehonite (-1724.67+/-0.40), and brucite (-835.90+/-0.80 kJ-mol-1). The increasing solubilities of nesquehonite and eitelite at higher sodium carbonate molalities were explained by invoking a magnesium dicarbonato complex (Mg2+ + 2CO32- reversible Mg(CO3)22-, log beta2 = 3.90+/-0.08). A set of ion-interaction parameters was obtained from solubility and dissociation constants for carbonic acid in 1 to 3.5m NaClO4 media (theta(HCO3-,ClO4-) = 0.081, theta(CO32-,ClO4-) = 0.071, psi(Na+, HCO3-, ClO4-) = -0.019, psi(Na+, CO32-, ClO4-) = -0.006, lambda(ClO4-, CO2) = -0.076) which reproduce these constants to 0.02 units in log K. The following Pitzer parameters are consistent with the previously studied formation of magnesium (hydrogen-)carbonato complexes in 3m NaClO4 (psi(Mg2+, HCO3-, ClO4-) = -0.36, lambda(ClO4-, MgCO3) = 0.081). The model and Gibbs functions of solid phases derived here reproduce original solubility data (-log [H+], [Mg2+]tot) measured in perchlorate medium within experimental uncertainty.