Hydrolysis constants of tetravalent neptunium by using solvent extraction method

Abstract

Abstract The hydrolysis constants of tetravalent neptunium (Np(IV)) were determined by solvent extraction method using thenoyltrifluoroacetone(TTA). In order to avoid colloid formation, a stock solution of carrier-free 239Np(V) was from 243Am milked. The valence of Np in the solution was then reduced to Np(IV) by using zinc amalgam. The hydrolysis constants (β m ) of the reactions, Np4++mOH−=Np(OH) m (4−m)+ was evaluated by using distribution ratios at ionic strengths (I)=0.1, 0.5 and 1.0. All experiments were performed in oxygen- free 0.5% H2−N2 atmosphere (below 1.0 ppm of O2) in a glovebox at room temperature (23±2 °C) to avoid oxidation of Np(IV). The β m values were extrapolated to the standard state (I=0) by using the specific ion interaction theory (SIT), and the formation constants at I=0 were determined to be logβ 1°=13.91±0.23, logβ 2°=27.13±0.15, logβ 3°=37.70±0.30 and logβ 4°=46.16±0.30. The ion interaction coefficients were also evaluated to be ε(NpOH3+, ClO4 −)=0.49±0.15, ε(Np(OH)2 2+, ClO4 −)=0.35±0.11, and ε(Np(OH)3 +, ClO4 −)=0.29±0.15.