The aim of this work was to study the formation and chemical equilibrium of sodium-jarosite in synthetic Fe(III)-Na-HSO4-H2O solutions at 70 degrees C. Solutions were prepared adding sodium as NaOH and ferric iron as FeH(SO4)(2), Sulphuric acid solutions of different acidities and containing different initial concentrations of ferric iron were prepared, introduced in sealed flasks and left for 45 days in a rotating oven until equilibrium was reached. Final equilibrium solutions, after separation of formed precipitates by filtration, were analyzed by free acidity, total acidity, pH, Eh, and concentration of sodium, iron and sulfate. Formed precipitates were characterized by chemical analysis, DRX, Raman and FT-IR spectroscopy. Equilibrium solution data was analyzed using a thermodynamic model which described ionic interactions in the system. Experimental studies on the formation of sodiurn-jarosite at 70 degrees C showed that at pH 1.72 sodium-jarosite is obtained forming mixtures with goethite. A pure crystalline natro-jarosite phase is obtained only in solutions with pH <= 1.42. Chemical analysis of this crystalline sodium-jarosite phase, however, indicates that is formed by a series of hydronium:natro-jarosites with iron deficiency and excess water, in which hydronium partially replaces sodium in the structure. A good fitting of the experimental data for sodium-jarosite solubility was obtained using a thermodynamic model which described ionic interactions in terms of Davis model for activity coefficients. The standard equilibrium constant obtained for sodium-jarosite formation at 70 degrees C was calculated assuming for the solid the ideal stoichiometry Na[Fe-3(SO4)(2)(OH)(6)], giving Log K-0 = 16.5 +/- 0.3. Speciation calculations conducted with this model showed that dissolved iron is highly associated being iron-sulphate-complexes FeH(SO4)(2(aq)) and Fe(SO4)(2)(-) the main species present. (c) 2006 Elsevier B.V All rights reserved.16FONDEF3FONDEF92