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The species and equilibria present in the synergistic extraction system for strontium, composed of didodecylnaphthalene sulfonic acid (HDDNS) and dicyclohexano-18-crown-6 (DC18C6) in CCI4, have been studied by IR and proton NMR spectroscopies, vapor pressure osmometry (VPO), and Karl Fischer (KF) titrations. With the aid of least-squares computer methods to fit the IR and VPO results, a series of proposed equilibria and associated equilibrium constants have been shown to adequately account for all of the observed behavior. The sulfonic acid and the salt of the acid alone in CCI4solution form the micellar species (HDDNS·xH2O) 11(× is in the range 7-8) and (Sr(DDNS)2· 11 H2O) 6respectively. The sulfonic acid alone, under conditions of intermediate loadings with Sr2+, yields the predominant species (Sr(DDNS) 2)3(HDDNS) 5·×H2O. When alone in solution, DC18C6 is a monomer, but it forms adducts with the free sulfonic acid and its strontium salt, yielding the respective product species (HDDNS) 7·DC18C6· ×H2O and Sr(DC18C6)(DDNS)2· 2H2O. An excess of HDDNS or Sr(DDNS)2in the presence of Sr(DC18C6)(DDNS) 22H2O yields predominantly Sr(DC18C6)(DDNS)2(HDDNS)2· ×H2O and (Sr(DC18C6)(DDNS)2) 2(Sr(DDNS)2) 33' · ×H2O respectively.

The quantitative description of synergistic solvent extraction systems is usually a difficult problem in equilibrium analysis because a relatively large number of species can be involved. Both sophisticated methods of interpretation and extensive experimental data are required. The graphical methods of "slope analysis"and "continuous variation"provide useful guidance only when the minimum number of species is present and especially when there are no competing reactions between the two synergistic extractants. A general least squares computer program (SXLSQ)is described to deal with the general case in which many possible species must be considered. It will accept six different kinds of equilibrium data for a given system in a single data set and can be used interactively at a terminal. Analysis of data on the extraction of Mn(II) by an organophilic sulfonic acid in combination with each of two crown ethers showed that a unique identification of the species formed cannot be made without additional information about the degree of association of the sulfonic acid in the organic diluent. But narrow limits could be placed on ratios of the integers defining the principal species formed.

The Pitzer treatment is employed to estimate activity and osmotic coefficients for these aqueous mixtures from literature data. A convenient formalism is presented for the inclusion of electrically neutral species such as undissociated HNO3 in the treatment. The water vapor pressure and the solubility of uranyl nitrate hexahydrate in HNO3 could be fitted reasonably well with parameters derived from solutions of the separate components. Isopiestic data for NaNO3 - UO2 (NO3) 2 mixtures required one mixture parameter. Stoichiometric activity coefficients and water activities are generated for UO2 (NO3)2 - HNO3 mixtures from the derived parameters.

The nature of the hydration and aggregation of di(2-ethylhexyl) sulfoxide (DEHSO) dissolved in wet dodecane at 25 °C has been characterized using Fourier transform infrared (FTIR) spectrophotometry, Karl Fischer titrations, and tritium-tracer equilibrium measurements. Least-squares computer modeling of the water content of 0.01-to-0.4 M DEHSO solutions equilibrated with water (optionally containing nonextractable CaCl2 to adjust the water activity ay in the range 0.28 to 1.0) indicates that the expected 1:1 species DEHSO H20 predominates at low DEHSO concentration (K11 = {DEHSO H20} / ({DEHS0]aw.) = 0.12 ± 0.01) with additional formation of hyarated dimeric species at the higher DEHSO concentrations. Analysis of the water vibrational bands in the FTIR spectra of 0.01-to-0.4 M DEHSO solutions in equilibrium with pure water supports the formation of both the 1:1 species and a dimer, identified by its spectral characteristics as the 2:1 dimer monohydrate. By reference to the available literature on the hydration of weak bases, the results suggest that at least part of the 'nom'deaiityrsquo; of solutions of weak-base extractants in wet nonpolar diluents may be explained by formation of the discrete 1:1 and 2:1 monohydrate species.