Dove and Rimstidt (1985) have reported a solubility-product constant for the mineral scorodite that has prompted several questions regarding their investigation. Solubility studies at 25°C can be an excellent source of equilibrium-constant and free-energy data, but only if carefully conceived, planned, and executed. Some of the more important criteria for a successful study include (1) demonstrated reversibility of the reaction (i.e., approaching equilibrium from both undersaturation and supersaturation); (2) variation of one or more critical parameters over a large enough range to test the stoichiometry of the reaction (e.g., vary pH for the solubility of metal hydroxides, or vary CO2 for the solubility of carbonate minerals); (3) identification of the major reactions and minimization of their number by appropriate choice of experimental conditions; (4) proper characterization of the solid phase before and after the experiment to check for purity, including the appearance of secondary phases forming during the experiment; (5) use of solid-liquid separation and analytical procedures appropriate to the investigation; and (6) appropriate choice of models for converting the raw data to thermodynamic functions.

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