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The tetrad effect: The thiocyanate complex stability constants of some trivalent actinides

1972; Pergamon Press; Volume: 34; Issue: 4 Linguagem: Inglês

10.1016/0022-1902(72)80339-7

1878-1225

H.D. Harmon, J.R. Peterson, W.J. McDowell, C.F. Coleman,

Metal Extraction and Bioleaching

A solvent extraction technique, using bis(2-ethylhexyl)phosphoric acid as the extractant, was employed to examine the formation of Am(III), Cm(III), Bk(III), Cf(III), and Es(III) thiocyanate complexes in a NaSCNNaClO4 medium at ionic strength I = 1·0 M and pH = 2·00. Mathematical analysis of the data led to the conclusion that the mono-, di-, and trithiocyanate species exist in the thiocyanate concentration range studied in this work. Values of the overall stability constants β1, β2, and β3 were calculated. As the atomic number of the actinides increased, the β1 values exhibited a gradual increase in stability, which is consistent with the expected effect on complex stability due to the actinide contraction. The β2 values decreased rapidly across the series from americium to einsteinium, with a concomitant increase in β3. In addition, there was evidence of the tetrad effect in the β3 values, and this led to the tentative conclusion that the Act(SCN)3 (where Act = Am, Cm, Bk, Cf, and Es) complexes are of the inner-sphere type. The thermodynamic parameters of the Am(III) thiocyanate complexes were calculated from temperature dependence measurements. The enthalpy and entropy changes were used to aid in the distinction between inner-sphere and outer-sphere complex formation.