Composition and formation kinetics of sodium polyvanadates in vanadium(IV, V) solutions

We study how VO2+ ions affect the composition and formation kinetics of polyvanadate precipitates in solutions with 1 <= pH <= 3 at 80-90 degrees C. The compounds have the general formula Na-2.1 - xHxVy4+V12-y5+O31-delta center dot nH(2)O ( 0 <= x <= 1.1, 0.2 <= y <= 2.3, 0.1 <= delta <= 1.4). The maximal vanadium(IV) concentration in the precipitates ( y = 2.2 and 2.3) is achieved for the V4+/V5+ ratio in the solution equal to 0.5 and 0.3 and pH of 1.7 and 3.0, respectively. The polyvanadate precipitation at pH 1.7 has a long induction period, which is not observed when V4+/V5+ > 0.02. In the solutions with pH 3.0, the precipitation occurs only when VO2+ are added. The processes are controlled by second-order reactions on the polyvanadate surface.