Spectrophotometric studies of the metavanadate system. I. Solution equilibria

Abstract

Spectrophotometric absorbance measurements on sodium metavanadate solutions in the region 240-300 nm were made at 10°, 25°, and 40°C on: (1) a series of solutions 2.46 x 10^-5M in vanadium and covering the pH range 7-9; and (2) a series of solutions at constant pH and covering the concentration range from 4x10^-5 to 1.2x10^-2M. All solutions contained 3.0M NaClO₄ and 0.10M Tris buffer. From these data were computed the extinction coefficients of HVO₄^2-, H₂VO₄^-, and (VO₃^-), as well as the pK_a of H₂VO₄^- (7.62±0.07). Metavanadate is probably a mixture of V₃O₉^3- and V₄O₁₂^4- under the conditions of the experiment. Upper limits on the formation constants at 25° are K₃ = 10.8±0.5x10⁶ 1² mol^-2 and K₄ = 73±7x 10⁹1³ mol^-3. The temperature dependence of the data gives for the enthalpy of polymerization, ΔH° = -21±2 kJ mol^-1 H₂VO₄^-.