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 indic ate differences with 1 anthanide homologue s. The gas thermochromatographic method has been applied to the study of the tetravalent fluorides of Pu, Am, Cm, Bk, Cf and Es. Deposition temperature Tc has been measured for all the considered elements. They are compared to this of penta and hexafluorides. The value obtained for einsteinium, T = 695° C is consistent with the existence of the gaseous EsF4 compound. This i the first experimental indication of the existence of tetravalent einsteinium, which would be thermaly less stable than that of the lighter actinides. Finally some thermodynamical consequences of these data are drawn. 3/0 redox potentials are estimated and stability of oxidation states (2 to 6) are compared in the actinide series. Enthalpy of hydration is also calculated with a semi empirical expression. INTRODUCTION In this report, I present an overview of recent radiochemical studies of heavy actinides, undertaken by the radiochemistry groups of the Institut de Physique Nucleaire. It is well known that many of the chemical and thermodynamical data .are lacking for the second half of the actinide series. Therefore one must develop radiochemical methods applicable to the tracer scale amount of studied element and even applicable to the level of hundreds of atoms. Electrochemical data obtained by radioelectrochemistry as well as data on fluoride compounds of some actinides obtained by gas thermochromatography will be presented. Moreover, I will describe new experiments we have just started for transport parameter measurement of ions in aqueous solutions. These experiments will able us to get information on the charge, radius and structure of the hydrated ions. Preliminary, results on the diffusion coefficient determination of some 4 f and 5 f ions are also reported. Finally, I will draw some thermodynamical consequences of these studies. RADIOELECTROCHEMISTRY Radioelectrochemical investigations mainly on the heaviest actinide elements are of special interest. Further to their big advantage of being applicable to the tracer scale amount, they allow good reliability and comparable accuracy to that of classical electrochemistry. How-