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The interpretation of chemical processes in aqueous systems requires the use of modern electronic computers, particularly in the calculation of multicomponent, multiphase equilibria. Commonly, the first concern of solution chemists and aqueous geochemists is to calculate the distribution and activities of species on the assumption that equilibrium exists throughout the aqueous phase. Species distribution can then be used in several areas of analytical and applied chemistry, e.g. to examine the availability of free and reactive ions, to test solubility hypotheses, and to determine the potential bioavailability of nutrients or toxic substances. Species distribution also forms the basis for more complex computations involving solutions which change composition by reaction with other solutions and with gases and solids. Equilibrium calculations of this type are particularly helpful in solving interpretive problems encountered in such fields as chemical and environmental engineering, geochemistry, biochemistry and aquatic ecology.This symposium demonstrates quite clearly that we depend heavily on chemical models, especially computerized models, to interpret aqueous chemical processes. Several computer programs which solve problems of simultaneous chemical equilibria are being used by a rapidly increasing number of investigators and it is necessary to review the inherent assumptions and limitations of these aqueous models. There is a temptation to use these models as ready-made interpretations 1

Speciation calculations for aluminum, in water samples taken from a drainage basin containing acid mine waters, demonstrate a distinct transition from conservative behavior for pH below 4.6 to nonconservative behavior for pH above 4.9. This transition corresponds to the pK for the first hydrolysis constant of the aqueous aluminum ion and appears to be a consistent phenomenon independent of field location, ionic strength, and sulfate concentration. Nonconservative behavior is closely correlated with the equilibrium solubility of a microcrystalline gibbsite or amorphous aluminum hydroxide.

This review critically evaluates the reported thermodynamic data on chromium metal, oxides, hydroxides, free aqueous ions, and hydrolysis species. Several discrepancies and inconsistencies have been uncovered and resolved to improve equilibrium calculations for chemical modeling and related engineering purposes. A revised set of data is derived from evaluation of electrochemical measurements, silver chromate solubility measurements, and auxiliary post-1980 data, reevaluation of earlier data, and reconsideration of the path for the thermodynamic network. The recommended thermodynamic values for Cr(cr), C , C , Cr , Cr2 , Cr2O3(cr), CrO3(cr), FeCr2O4(cr), CrCl2(cr), CrCl3(cr), and KFe3(CrO4)2(OH)6(cr) at 25 °C, 1 bar, and infinite dilution are given.

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