Carbon Isotope Fractionation in the Thermal Decomposition of Uranyl Formate

Abstract

Thermal decompositions of uranium(VI) chelates have been used in the past for quantitative determinations of uranium. The mechanisms of these pyrolyses have been investigated several times by the method of differential thermal analyses to clear up the inconsistencies in the literature [l-7]. The discrepant results have been reported also in the case of the much simpler thermal decompositions of uranyl formates and discussed also [8-13]. The irresolvable mechanistic problems of reactions in solutions are usually studied by supplementing the classical kinetic observations with kinetic isotope effects (KIEs) determinations. The much more specific solid state reactions are less frequently probed by the heavy atom KIEs methods. We investigated recently the carbon kinetic isotope effects (13C KIEs) in the decarbonylation (and oxidation) of liquid formic acid as such [14,14a] and in the presence of uranium oxide, uranyl formate (UF) and uranium peroxide [15,16]. The experimental 13C KIEs were found to be constituent with 13C KIEs calculated assuming that in the case of decarbonylations taking place in liquid phase they are determined by the rupture of the single carbonoxygen bond. In the case of oxidation of liquid formic acid with uranium peroxide the 13C/12C fractionation was found to be governed by the carbon-hydrogen bond rupture [16]. The differences between solid phase and liquid phase decompositions of formic acid and formates should be more precisely revealed by 13C KIEs determinations. We therefore initiated 13C KIEs studies in the thermal decomposition of uranyl formate in vacuum glass reaction vessel in a 260-390°C temperature interval. We quantitatively isolated carbon dioxide, carbon monoxide, volatile formic acid and water as decomposition products (Eq. 1). The [CO2]/[CO] ratio was increasing with the increase of the decomposition temperature from 0.90±0.02 (at 275-316°C) to 2.06 (at 382°C). At “low” temperatures the [C02]/[CO] ratio was increasing also with the increase of the degree “f” of decomposition of solid formate. The first portion of gaseous products collected at 275 "C contained carbon dioxide and carbon monoxide in a ratio (0.163 1 /0.5723) = 0.285 only.