<p><span lang="EN-US">The forms of occurrence of vanadium metal are determined by the major chemical reactions in the aquatic environment such as hydrolysis, oxidation, reduction, and precipitation. Depending on pH, potential and total concentration of inorganic ions and organic ligands, vanadium compounds may undergo various transformations to produce a whole range of chemical forms in aqueous systems. In this paper, a novel approach has been applied for calculating potential</span><span lang="EN-US">-</span><span lang="EN-US">pH (Pourbaix) diagrams, based on the developed thermodynamic analysis of chemical equilibria in the V–H₂O system. On the basis of currently revised thermodynamic data for V(III), V(IV) and V(V) hydrolysis and original thermodynamic and graphical approach used, the repartition of their soluble and insoluble chemical species has been investigated. By means of ΔG–pH diagrams, the areas of thermodynamic stability of V(IV) and V(V) hydroxides have been established for a number of analytical concentrations of vanadium in heterogeneous mixtures. The obtained results, based on the thermodynamic analysis and graphic design of calculated data, are in good agreement with available experimental data.</span></p>

A thermodynamic method of the global Gibbs energy variation calculation for describing heterogeneous equilibria of transformations of calcium, strontium, and barium polyvanadates, that occur in systems MeO-V2O5-H2O, where Me is the alkaline earth element, has been developed and used. Its quintessence consists in the thermodynamic analysis of the real conditions of various processes on the basis of their total thermodynamic characteristics. On the basis of the selected thermodynamic data for involved species, the thermodynamic stability areas of solid polyvanadates towards the solution pH and vanadium and alkaline earth metal ion concentrations in heterogeneous mixtures have been established, taking into account the complex formation reactions in multicomponent heterogeneous systems. The existing experimental data confirm the results on the thermodynamic stability of polyvanadates obtained in this paper.

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