Holmium nitrate complexation with tri-n-butyl phosphate in supercritical carbon dioxide

Holmium nitrate pentahydrate was reacted with tri-n-butyl phosphate in supercritical carbon dioxide at 308 K. The products of the complexation reaction were measured under supercritical fluid conditions using UV-vis spectroscopy. The solubility of the metal complexes in the supercritical fluid phase was measured. The mole-ratio titration method was used to determine the stoichiometry of the soluble complexes. Conditional extraction coefficients were calculated from spectral data using least-squares regression and hard-equilibria models. Data indicate that the holmium nitrate-tributyl phosphate system forms 1:2 and 1:4 holmium-tributyl phosphate complexes. Conditional extraction coefficients were calculated as log Kex2 = 5.8 ± 0.1 and log Kex4 = 3.1 ± 0.1 giving a log Kext value of 8.9 ± 0.1. The conditional formation constant for the reaction ML2(scf)+2L⇌ML4(scf)+2H2O in the supercritical fluid phase was calculated as log β204(scf) = 2.4 ± 0.1.