Aquatic Chemistry of Pentavalent Plutonium: Determination of the First Hydrolysis Constant

Abstract

Up to now, no recommendations have been made by the Organisation for Economic Co-operation and Development Nuclear Energy Agency (OECD-NEA) for the first two hydrolysis constants of pentavalent plutonium. We have determined them, as well as those of Np(V), by means of capillary electrophoresis coupled with inductively coupled plasma mass spectrometry (CE-ICP-MS) in 0.1 M NaCl at 25 °C. The hydrolysis constants of Pu(V) were compared to those of Np(V), for which consensual values have been proposed by the OECD-NEA. The first hydrolysis stability constant for Pu(V) extrapolated at zero ionic strength $(\log {}^{*}{\beta }_1^0=-(11.50\pm 0.12))$ is, as expected, close to that of Np(V) $(\log {}^{*}{\beta }_1^0=-(11.36\pm 0.13))$. We have obtained an excellent agreement with the OECD-NEA value for Np(V) [$\log {}^{*}{\beta }_1^0=-(11.3\pm 0.7)$]. Based on eight independent values including our own, we propose a new, robust value for the first hydrolysis of Np(V): $\log {}^{*}{\beta }_1^0=-(11.22\pm 0.20)$. The determination of the second hydrolysis constant for Np(V) by CE-ICP-MS $[\log {}^{*}{\beta }_2^0=-(24.40\pm 0.33)]$ deviates from the value adopted by the OECD-NEA $[\log {}^{*}{\beta }_2^0=-(23.6\pm 0.5)]$. This discrepancy might be explained by the association of a sodium counter cation with the species ${\left[{NpO}_2{\left(OH\right)}_2\right]}^{-}$. A stability constant is proposed at zero ionic strength and 25 °C for this association: $\log K_{Na\left[{NpO}_2{\left(OH\right)}_2\right]}^0=(1.6\pm 0.5)$. As a result, it is considered that the second hydrolysis constant for Pu(V), $[\log {}^{*}{\beta }_2^0=-(24.24\pm 0.26)]$, is also a mixed constant, taking into account both the hydrolysis and the association with a sodium cation.