Enthalpy of solution of californium oxychloride; calculation of the standard enthalpy of formation of CfOCl

Abstract

The enthalpy of solution of CfOCl in 1.022 mol dm −3 HCl was measured on the 0.4–1.5 mg scale, and the standard enthalpy of formation at 298 K for this compound was calculated. The CfOCl was prepared by heating in air at 280–320°C the solid hydrated trichloride, obtained by slow evaporation of Cf(III) in HCl solution. X-ray powder diffraction analysis confirmed the material to be the expected oxychloride, which exhibits the PbFCl-type tetragonal structure. Dissolutions of the oxychloride were made at 25.0°C in an isoperibol solution microcalorimeter. Enthalpy of solution values were calculated according to the equation: CfOCl (cr)+2HCl (sln)→CfCl 3(sln)+H 2O (sln). The Δ f H 0 (CfOCl (cr)) was determined to be −920±7 kJ mol −1. Interpolation within Δ f H 0 (AnOCl (cr)), where An=U, Pu, Am, Cm, and Cf, as a function of An(III) ionic radius yielded Δ f H 0 (BkOCl (cr))=−944±10 kJ mol −1.