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 HVO42-, H2VO4-, and (VO3-), as well as the pKa of H2VO4- (7.62�0.07). Metavanadate is probably a mixture of V3O93- and V4O124- 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-.