Modeling of equilibria in acidified solutions of sodium orthotungstate in the presence of calcium(II) ions

Abstract

The interactions of the Ca2+–WO42––H+–H2O system initially acidified to the molar ratio (acidity) Z=(H+)/(WO42–)=1.00 were studied by the methods of pH-potentiometry, mathematical modeling and conductometry in the interval Z=0.60–2.00 at 2980.1 K with NaNO3 (I=0.05–0.30 moll–1) as the supporting electrolyte. The logarithms of the equilibrium concentration constants were calculated by the Newton's method, and previously unknown logarithms of the thermodynamic constants and Gibbs energy of the reactions of the formation of ion associates Ca2+,[W4O14(OH)2]4–; Ca2+,[W6O20(OH)2]6–; Ca2+,[W12O40(OH)2]10–; Ca2+,H[W12O40(OH)2]9–; Ca2+,H2[W12O40(OH)2]8–; Ca2+,H3[W12O40(OH)2]7–; [W12O38(OH)2]6–; and H[W12O38(OH)2]5– were calculated using the Pitzer method. The formation of particles with a ratio of Сa2+:[W12O40(OH)2]10–=1:1 and Сa2+:[W6O20(OH)2]6–=1:1 in the solutions was shown by the conductometric titration method. The crystals of calcium paratungstate B Ca5[W12O40(OH)2]30H2O were synthesized from Na2WO4 solution acidified to acidity Z = 1.00. By using ATR-FTIR and Raman spectroscopies, it was shown that the isopolyanion in the composition of the salt belongs to the structural type of paratungstate B. The structure of Ca5[W12O40(OH)2]30H2O was determined by single crystal X-ray diffraction: monoclinic, P21/n, a=15.3619(4) Å, b=11.7537(3) Å, c=18.1471(5) Å, =109.2950(10)0, V=3092.58(14) Å3, R1=0.0298, wR(F2)=0.1387.