| Summary: | The stoichiometric solubility constant of eitelite (NaMg 0.5 CO 3 +2H + ⇄ Na ++0.5Mg 2+ +CO 2 (g)+H 2 O, log *K pso I =14.67±0.03 was determined at I=3 m (mol kg -1) (NaClO 4 ) and 25°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 (-1033.60±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 (Mg 2++2CO 3 2- ⇄ Mg(CO 3) 2 2-, log β z = 3.90 ± 0.08). A set of ion-interaction parameters was obtained from solubility and dissociation constants for carbonic acid in 1 to 3.5 m NaClO 4 media {Mathematical expression} 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 NaClO 4 {Mathematical expression}. The model and Gibbs functions of solid phases derived here reproduce original solubility data (-log [H +], [Mg 2+ ] tot ) measured in perchlorate medium within experimental uncertainty.
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