Liquid-liquid equilibrium analysis in perspective. II. Complete model of water, nitric acid, and uranyl nitrate extraction by di-2-ethylhexyl sulfoxide in dodecane

New and previously reported data on the extraction of water (W), HNO3 (HX), and UO2(NO3)(2) (MX2) from aqueous mixtures by DEHSO (B) in dodecane are c ombined to develop a comprehensive model of U(VI) extraction from nitric ac id by DEHSO. The computer program SXFIT used for this modeling estimates so lute activity coefficients and solvent activities in the aqueous phase by t he Pitzer treatment and in the nonaqueous phase by the Hildebrand-Scott tre atment. Included is a critical evaluation of the Pitzer parameters needed a nd an estimate of the smaller effect of the solubility parameters assigned to the nonaqueous solutes. The resulting model indicates that: (1) The smal l amount of water extracted by dodecane alone is monomolecular (W). (2) The additional water extracted by B requires three hydrated species (BW, B2W a nd an aggregate assigned as B2W5). (3) The extraction of HX by B requires f our species (BHX, B(HX)(2)W-5, and two species of B2HX, probably B2HX, and B2BXW2). (4) Finally, the extraction Of MX2 yields two additional species ( principally MB2X2 and a hydrate of MB3X2). To account adequately for all th e data, no less than 11 nonaqueous solute species are required as well as t he adjustment of two Pitzer parameters, psi [UO22+-H+-NO3-] and psi [Na+-H-NO3-]. The treatment of non-ideality, the low interdependence of the adjus ted parameters, the diversity of the data (fitted in stages), and the consi stency of the resulting model with the behavior of related neutral, oxygen- donor extractants strongly suggest that the complexity of this system is re al, rather than an artifact of over interpretation.