Equilibria in the vanadium(III)-fluoride-hydroxide system. 1. A potentiometric study of the binary V(III)-F- complexes.

The complex formation between V3+ and F- ions has been studied in a 3 M NaC lO4 medium, at 25 degrees C, by measuring the potentials of a hydrogen and of a fluoride ion electrode, which provided the equilibrium concentrations of H+ and F-. The analytical concentrations, B, of vanadium(III) and, A, of fluoride have been varied in the ranges 0.003 M less than or equal to B le ss than or equal to 0.01 M and 0 less than or equal to A less than or equal to 0.06 M, respectively, in the course of experiments carried out at const ant [H+] levels, h. The potentiometric data at -logh less than or equal to 2 have been explained by assuming the formation of the binary species VFn(3 -n+), with n = 1 to 4, according to reaction (1), V3+ + n HF = VFn(3-n+) + n H+ (1) with the following overall equilibrium constants: log beta(1) = 1.99 +/- 0. 04, log beta(2) = 2.96 +/- 0.06, log beta(3) = 2.87 +/- 0.10 and log beta(4 ) = 2.70 +/- 0.08. Data collected at 2 less than or equal to -logh < 3 indi cate the formation of hydrolytic species as well. A tentative interpretatio n of the measurements at -logh = 2.3 has been made by considering reaction (2) in addition to (1), V3+ + H2O = VOH2+ + H+ (2) with the equilibrium constant log chi(-1,1) = -2.3 +/- 0.2. However a more complete interpretation of the data, including ascertainment of possible fo rmation of mixed complexes, requires a precise knowledge of the hydrolysis of V3+ in 3 M NaClO4.