Hydrolysis constants of tetravalent neptunium by using solvent extraction method

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 239 Np(V) was from 243 Am 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, Np 4+ + m OH - = Np(ΟH) 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% H 2 -N 2 atmosphere (below 1.0 ppm of O 2 ) in a glove-box 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 log β ο 1 = 13.91±0.23, log β ο 2 =27.13± 0.15, log β ο 3 =37.70±0.30 and log β ο 4 = 46.16±0.30. The ion interaction coefficients were also evaluated to be e(NpOH 3+ , ClO 4 - ) =0.49±0.15, e(Np(OH) 2 2+ , ClO 4 - ) = 0.35±0.11, and e(Np(OH) 3 + , ClO 4 - ) = 0.29±0.15.