Spectrophotometric measurement of uranium(VI)-tributylphosphate complex in supercritical carbon dioxide

UV-visible absorption spectra of uranium(VI)-tributylphosphate (U(VI)-TBP) complex dissolved in supercritical CO2 at 40-60 degrees C and 100-250 kg cm(-2) were recorded. Wavelengths and molar extinction coefficients for the absorption peaks of U(VI)-TBP were determined and confirmed to be in good agreement with those of UO2(NO3)(2)(TBP)(2) complex dissolved in organic solvents such as n-hexane. The absorbance at a given wavelength was proportional to the concentration of U(VI) species in supercritical CO2, indicating a feasibility of in-situ determination of U(VI) concentration in CO2 phase. A lower detection limit of U(VI)-TBP complex was estimated to be ca. 1 x 10(-3) M. The molar extinction coefficient of U(VI)-TBP in supercritical CO2 decreased slightly with an increase of the density of CO2 medium, suggesting that the solute-solvent interaction of U(VI)-TBP complex with CO2 was affected by the density. On the basis of the spectra obtained, phase behavior and solubility of UO2(NO3)(2)(TBP)(2) + H(NO3)(TBP) + TBP in supercritical CO2 were elucidated. (C) 1998 Elsevier Science B.V. All rights reserved.