Hindawi Journal of Chemistry Volume 2019, Article ID 7091781, 6 pages https://doi.org/10.1155/2019/7091781 Research Article Thermodynamic Stability Areas of Polyvanadates of Alkaline Earth Metals Igor Povar ,1,2 Inga Zinicovscaia,1,2,3 Oxana Spinu,1 and Boris Pintilie1 1Institute of Chemistry, 3 Academiei Str., MD 2028, Chisinau, Moldova 2Joint Institute for Nuclear Research, Joliot-Curie Str., 6, 141980 Dubna, Russia 3Horia Hulubei National Institute for R&D in Physics and Nuclear Engineering, 30 Reactorului, Str. MG-6, Bucharest-Magurele, Romania Correspondence should be addressed to Igor Povar; [email protected] Received 28 September 2018; Revised 15 November 2018; Accepted 3 December 2018; Published 2 January 2019 Academic Editor: Ana Moldes Copyright © 2019 Igor Povar et al. *is is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. A thermodynamic method of the global Gibbs energy variation calculation for describing heterogeneous equilibria of trans- formations 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. *e existing experimental data confirm the results on the thermodynamic stability of polyvanadates obtained in this paper. 1. Introduction aluminum-vanadium alloys for alloying structural materials based on titanium, applied in space technology. Inorganic Polyvanadates obtained by solid-phase synthesis and sub- materials containing polyvanadates are used for production sequent study of their chemical properties in water-salt of optical quantum generators, ionic conductors, ferro- systems have a huge practical significance, because re- electrics, dielectrics, ceramics, etc. [1]. *e demand for gardless of the type of source of raw materials, their pro- polyvanadates is projected to grow by seven percent a year cessing includes stages of the synthesis of final products with till 2025 [2], thanks to the usage of vanadium-containing desired properties (high or low solubility, etc.), with the steels in both traditional areas and the implementation of transfer of vanadium in the solution and its subsequent new technologies for the manufacture of batteries [2]. *e precipitation. *e abovementioned processes are associated concentrations of vanadium in surface waters caused by with extremely complex chemical equilibria, the study of industrial wastes are small and, in general, are within the which would allow the development of technological basis natural content (up to 65 μg/L) [3]. Literature sources of a number of combined productions: obtaining techni- provide information on vanadium concentrations in surface cal and pure vanadium pentoxide, vanadium catalysts, dyes, waters of industrial wastes up to 2 mg/L [3]. *e chemistry of etc. [1, 2]. Up to 85–87% of the total amount of poly- aqueous solutions of vanadium is complicated by the ex- vanadates is used in ferrous metallurgy as highly efficient istence in solutions of numerous ions and polyions, the and habitually an indispensable alloying additive in the transition of which into an equilibrium state depends on the production of diverse steels. About 10–12% of polyvanadates pH value and oxidation-reduction potentials in the systems, is used in nonferrous metallurgy, mainly in the form of as well as its concentration and the content of other 2 Journal of Chemistry elements. *us, the composition and stability of solid phases Table 1: Standard Gibbs energy of formation of the soluble and formed in the MeO–V2O5–H2O system, where Me is the insoluble species at 295.15 K [4, 5]. alkaline earth element, exhibit complex functions of the 0 0 Species ΔGf (i) Species ΔGf (i) chemical composition of solution, pH, etc. Experimentally, it 2+ Ca −552.75 Ca3V10O28 −9376.52 has been demonstrated [4–6] that depending on the con- 2+ Sr −563.90 Sr3V10O28 −9428.23 ditions in the MeO–V2O5–H2O system, different poly- 2+ Ba −546.83 Ba3V10O28 −9403.50 vanadates MeV12O31, MeV6O16, Me3V10O28, Me(VO4)2, + VO2 −588.30 CaV6O16 −5031.30 Me2V2O7, and Me3(VO4)2 can precipitate. For the com- H2O −237.24 SrV6O16 −5043.05 2+ 2+ 2+ − pounds of Ca , Sr , and Ba the same authors determined VO3 −785.70 BaV6O16 −5029.60 3− the solubility products [4, 5], taking into account the mutual VO4 −897.60 CaV12O31 −9295.90 2− transformation reactions of the vanadium (V) ionic species. HVO4 −979 SrV12O31 −9307.60 − *e coexistence areas of these polyvanadates were also cal- H2VO4 −1021.12 BaV12O31 −9293.97 culated in the cited papers as a function of the solution pH H3VO4 −1043.95 Ca(VO3)2 −2165.56 4− V2O7 −1723.94 Sr(VO3)2 −2190.70 and alkaline earth metal concentrations. At the same time, the 3− HV2O7 −1767.73 Ba(VO3)2 −2181.97 reciprocal relationship between the solution composition and 3− V3O9 −2400.70 Ca2V2O7 −2870.75 solid phases was established in the form of diagrams log CV � 4− + + V4O12 −3202.23 Sr2V2O7 −2910.45 f(log[H ]) and log CM � f(log[H ]). *is procedure is not 6− V10O�28 −7688.78 Ba2V2O7 −2891.35 always efficient. In particular, the solubility diagrams provide 5− HV10O�28 −7721.88 Ca3(VO4)2 −3553.23 valuable information only if the solubility of solid phase is 4− H2V10O�28 −7742.43 Sr3(VO4)2 −3604.49 low. Ba3(VO4)2 −3574.97 In this paper, based on calculations of the global Gibbs energy variation of the dissolution-precipitation process of the polyvanadates of alkaline earth metals as a function of pH C 12C and total concentrations of vanadium and alkaline earth G ( ) � G0( ) + RT V Ca: (3) Δ S 1 Δ S 1 ln + 14 metals, their thermodynamic stability areas are determined. It []H has been proved that the value of global Gibbs energy vari- Note that the Gibbs energy variation in Reaction (1) ation under real conditions constitutes a more objective strongly depends on the pH value. At the same time, criterion for estimation of the stability areas of polyvanadates Equation (3) cannot also serve as a characteristic of the of alkaline earth metals than the solubility diagrams. thermodynamic stability of calcium dodecavanadate, be- cause the vanadyl ion, in function of pH and CV, is subjected 2. Theoretical Part to complex chemical transformations in the solution (Table 2). In a series of papers [6–8], it has been shown that as a strict Obviously, in calculating ΔGS, all these equilibria must criterion of solid-phase stability serves the value of the Gibbs be taken into account. A rigorous thermodynamic analysis, energy variation of the solid-phase formation-dissolution developed in a series of papers [7, 8], shows the global Gibbs process. *is criterion will be applied for the determination energy variation of Reaction (1) under real conditions, of the thermodynamic stability areas of the alkaline earth considering the equilibria (Table 2), as described by the metal polyvanadates. following equation: *e essence of the calculation method will be explained C 12C ΔG (1) ��ΔG0(1) � −�RT ln α12 � + RT ln V Ca!; by a concrete example of equilibrium of calcium dodeca- S S V []H+ 14 vanadate with the saturated aqueous solution: ( ) + + 2+ 4 CaV12O31(S) + 14H � 12VO2 + Ca + 7H2O: (1) where αV is the coefficient that takes into account the *e Gibbs energy variation under standard conditions is contribution of the equilibria (Table 2), which is determined equal: in the following way: G0( ) � G0 � + G0�2+ � + G0+ � + −2 + −4 + −3 + −2 Δ S 1 7Δ f H2O Δ f Ca 12Δ f VO2 αV � 1 + k1�H � + k2�H � + k3�H � + k4�H � G0� �; + −1 + + −6 + + −5 − Δ f CaV12O31(S) + k5�H � + 2� VO2 �k6�H � + 2� VO2 �k7�H � (2) + 2 + −6 + 3 + −8 + 3� VO2 � k8�H � + 4� VO2 � k9�H � 0 + 9 + −16 + 9 + −15 where ΔGf (i) is the standard Gibbs energy of formation of 0 + 10� VO2 � k10�H � + 10� VO2 � k11�H � the i species. *e ΔGf (i) values used (kJ/mol) are recal- + 9 + −14 culated from the selected equilibrium constants [4, 5] and + 10� VO2 � k12�H � ; G0( ) shown in Table 1. *e Δ S 1 value cannot, however, serve (5) as a characteristic of the thermodynamic stability of calcium + dodecavanadate under real conditions. In the latter case, the where [VO2 ] is the equilibrium concentration of vanadyl + equilibrium is described by the equation of reaction iso- ion VO2 , computed for certain values of pH and CV from therm, which for Reaction (1) takes the following form: the mass balance conditions: Journal of Chemistry 3 Table 11 2: Complex chemical transformations of the vanadyl ion in 1 the solution at 295 ± 1 K [4, 5]. 1.0 Nr Equation of reaction log K + − + 0.8 2 1 VO2 + H2O � VO3 + 2H log K1 � −6.98 + 3− + 3 2 VO2 + 2H2O � VO4 + 4H log K2 � −29.94 + + � 2− + + � 3 VO2 2H2O HVO4 3H log K3 −14.68 0.6 4 + − + i 4 VO2 + 2H2O � H2VO4 + 2H log K4 � −7.30 f + + 5 VO2 + 2H2O � H3VO4 + H log K5 � −3.30 + 4− + 0.4 6 VO2 + 3H2O � V2O7 + 6H log K6 � −28.80 5 + 3− + 7 2VO2 + 3H2O � HV2O7 + 5H log K7 � −21.13 + 3− + 13 8 3VO2 + 3H2O � V3O9 + 6H log K8 � −13.30 0.2 + 4− + 7 9 4VO2 + 4H2O � V4O12 + 8H log K9 � −17.51 + 6− + 10