Radiochemical studies of some transplutonium elements

Radiochemical studies undertaken by the groups of radioelectrochemistry and gas thermochromatography of the laboratory of radiochemistry are reviewed. Radiocoulometric and radiopolarographic methods suitable for studies of elements at the tracer scale concentration level are briefly described. These two techniques have been recently used for studying the chemistry of mendelevium in aqueous solution. It was concluded that mendelevium is reduced at - 1.50 V/NHE in non complexing medium; effect of citrate, acetate and chloride ions indicates that its chemistry is very similar to this of divalent fermium. It cannot be considered as a cesium like or silver like ion in the experimental conditions. Preliminary experiments are described on diffusion coefficient measurements of some lanthanide and actinide ions in aqueous solution. Trivalent americium and europium ions have similar Stokes radii, as expected, but first experiments on californium and einsteinium seem to indicate differences with lanthanide homologues. The gas thermochromatographic method has been applied to the study of the tetravalent fluorides of Pu, As, Cm, Bk, Cf and Es. Deposition temperature Tsub(c) has been measured for all the considered elements. They are compared to this of penta and hexafluorides. The value obtained for einsteinium, Tsub(c) = 6950C is consistent with the existence of the gaseous EsF4 compound. This is the first experimental indication of the existence of tetravalent einsteinium, which would be thermally less stable than that of the lighter actinides