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 M L 2 ( scf ) + 2 L ⇌ M L 4 ( scf ) + 2 H 2 O in the supercritical fluid phase was calculated as log β204(scf) = 2.4 ± 0.1.