Parameter-free activity coefficient equations were tested in addition to those containing one, two, three or four electrolyte-dependent parameters with the experimental activity coefficients obtained from the literature data for aqueous solutions of the following electrolytes at 298.15 K: KCl, NaCl, RbCl, KBr, RbBr, CsBr, KI, RbI, KNO 3 , and KH 2 PO 4 . The experimental activity coefficients of each electrolyte considered can be reproduced within the uncertainty of the measurements up to the molality of the saturated solution by using a three-parameter equation of the extended Hückel type. The best Hückel equations are given for all electrolytes in question. The results from the present studies reveal that the parameterfree equations can be reliably used in thermodynamic studies only for very dilute electrolyte solutions. On the other hand, in most cases, a good agreement with the experimental data is obtained with the one-parameter equations of Bromley [14] and Kusik and Meissner [13], with the two-parameter equation of Bretti et al. [15], and with the three-or four-parameter equation of Hamer and Wu [19] in addition to the three-parameter Pitzer equation [23], with almost all parameter values suggested in the literature. In several cases, these equations seem to apply to much higher molalities than those used in the parameter estimations. Therefore, the best of these equations may have important applications in calculations associated with the dissolution and crystallization processes of these salts.

Magnetite (Fe3O4) is a key economically valuable component in iron ore and is extracted by dissolution processes, but among the Fe (oxyhydr)oxides its solubility behavior is one of the least understood. The objective of this study was to improve understanding of magnetite dissolution mechanisms leading to thermodynamic equilibrium by comparing the dissolution of two solid samples, one synthetic and one industrial, using oxalic, sulfuric, and nitric acids at varying concentrations and temperatures. Of the three solid-liquid systems investigated, only the system consisting of magnetite and oxalic acid reached an equilibrium state within the duration of an individual experiment (6 h). In this system, increasing the acid concentration resulted in a significant increase in the equilibrium concentration of dissolved Fe. When dissolving synthetic and industrial magnetite, increasing the temperature not only increased the rate of reaction but also affected the concentration of dissolved Fe. Significant effects were observed when increasing the temperature from 15 to 35°C, but only slight differences were seen on further increases in temperature. Observations regarding the equilibrium state of the sulfuric and nitric acid systems could not be made because equilibrium was not reached. The most important individual observation regarding the equilibrium state of the nitric- and sulfuric-acid systems seems to be that in future studies a much longer reaction time is necessary, due to slow kinetics of the dissolution mechanism. A proton-based mechanism has been hypothesized as the one governing the dissolution of magnetite by these two acids, but only the dissolution of the industrial sample yielded results that were similar for these two acids and consistent with that hypothesis.

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