C. F. Baes

THE INTERPRETATION OF EQUILIBRIUM DATA FROM SYNERGISTIC SOLVENT EXTRACTION SYSTEMS

ABSTRACT 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...

SXLSQA, A Computer Program for Including Both Complex Formation and Activity Effects in the Interpretation of Solvent Extraction Data

Abstract A previously reported computer program for the interpretation of solvent extraction data in systems that can include two extractants, one acidic and one neutral, has been extended to treat the effects of: (1) variation of activity coefficients of solute species and of water activity in the aqueous phase, calculated by the Pitzer treatment; (2) variation of activity coefficients of solute species in the organic phase, calculated by the Hildebrand-Scott treatment; and (3) product species formed in the aqueous phase. The interaction parameters of the Pitzer treatment and the solubility parameters of the Hildebrand-Scott treatment can be refined along with the formation constants of various assumed product species to fit the data. Like its predecessor, the program is capable of fitting simultaneously a variety of data, including the distribution of an extractable cation, anion, or water, as well as spectra, vapor pressure, or heats of mixing for the organic phase. Use of the program is illustrated by...

MODELING SOLVENT EXTRACTION SYSTEMS WITH SXFIT

SXFIT is the latest in a series of programs designed to model solvent extraction systems of increasing complexity. Unlike its predecessors, SXFIT permits the user complete freedom to define the constituents from which the composition of each phase is to be specified in the data and from which all species of a model for a system are to be formed. This paper describes the manner in which the physical chemistry of the nonaqueous and aqueous solutions involved has come to be treated in this modelling effort. Included are: (1) solute activity coefficients and solvent activity in the nonaqueous phase, estimated by the Scatchard-Hildebrand-Scott treatment; (2) the effect of ionization on these quantities; (3) solute activity coefficients and water activity in the aqueous phase, estimated by the Pitzer treatment; (4) the effect of non ideal behavior on the total concentration of nonaqueous solute species as rendered by a vapor pressure osmometer; (5) a similar treatment of heats of mixing of nonaqueous solutions; (6) the estimation of apparent molar volumes of aqueous species and, from these, the solution density, needed for the conversion of concentrations from the molarity to the molality scale; (7) the conversion of equilibrium constants from one concentration scale to another.

Equilibrium Analysis of Aggregation Behavior in the Solvent Extraction of Cu(II) from Sulfuric Acid by Didodecylnaphthalene Sulfonic Acid

Abstract By use of the principles of equilibrium analysis, the liquid-liquid cation exchange of Cu(II) from aqueous sulfuric acid at 25°C by didodecylnaphthalenesulfonic acid (HDDNS) in toluene may be understood in terms of small hydrated aggregated species in the organic phase. Extraction data have been measured as a function of organic-phase HDDNS molarity (1.0 × 10−4 to 1.0 × 10−1), aqueous copper(II) sulfate molarity (1.2 × 10−8 to 1.3 × 10−2), and aqueous sulfuric acid molarity (0.03 to 6.0). Graphical analysis of linear regions of the data support a model in which organic-phase aggregates of HDDNS function by cation exchange to incorporate Cu(II) ions with no apparent change in aggregation number at low loading. Supporting FTIR spectra and water-content measurements of HDDNS solutions in toluene show that the HDDNS aggregates are highly hydrated. By use of the computer program SXLSQA, a comprehensive equilibrium model was developed with inclusion of activity effects. Aqueous-phase activity coefficie...

SPECTRAL STUDIES AND EQUILIBRIUM ANALYSIS OF THE DIDODECYLNAPHTHALENE SULFONIC ACID, DICYCL0HEXAN0-18-CR0WN-6, Sr2+EXTRACTION SYSTEM

ABSTRACT 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·...

LIQUID-LIQUID EQUILIBRIUM ANALYSIS IN PERSPECTIVE.PART 1. SLOPE ANALYSIS OF THE EXTRACTION OF URANYL NITRATE FROM NITRIC ACID BY DI-2-ETHYLHEXYLSULFOXIDE

ABSTRACT Slope analysis is critically examined as a stand-alone method for analyzing the representative extraction system uranyl nitrate, nitric acid, sodium nitrate, water, di-2-ethylhexylsulfoxide (DEHSO). n-dodecane at 25 °C. The use of detailed computer calculations of free extractant concentrations and activity coefficients has permitted assessment of commonly employed assumptions of slope analysis in the data treatment. On the basis of slope analysis alone, the extraction of nitric acid in the absence of U(VI) can be partly described by the formation of the 1:1 adduct (DEHSO)(HNO3), but complications due to nonideality effects and the formation of at least two other species hinder further reliable analysis. Under the limiting conditions of low DEHSO molarity, low aqueous nitric acid molarity, and low U(VI) loading, a slope-analysis treatment of U(V1) extraction data shows unambiguously that the predominant extracted species of U(VI) is UO2(N03)2(DEHSO)2(H20)w (w not determined). However, it is diffi...

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 combined to develop a comprehensive model of U(VI) extraction from nitric acid by DEHSO. The computer program SXFIT used for this modeling estimates solute activity coefficients and solvent activities in the aqueous phase by the Pitzer treatment and in the nonaqueous phase by the Hildebrand-Scott treatment. Included is a critical evaluation of the Pitzer parameters needed and an estimate of the smaller effect of the solubility parameters assigned to the nonaqueous solutes. The resulting model indicates that: (1) The small amount of water extracted by dodecane alone is monomolecular (W). (2) The additional water extracted by B requires three hydrated species (BW, B2W, and an aggregate assigned as B2W5). (3) The extraction of HX by B requires four species (BHX, B(HX)2W5, and two species of B2HX, probably B2HX, and B2HXW2). (4) Finally, the extraction of MX2 yields two additional species (principally MB2X2 and a hydrate of MB3X2). To account adequately for all the data, no less than 11 nonaqueous solute species are required as well as the adjustment of two Pitzer parameters, ψ[UO2 2+ −H+ −NO3 −] and ψ[Na+ −H+ − NO3 −]. The treatment of non-ideality, the low interdependence of the adjusted parameters, the diversity of the data (fitted in stages), and the consistency of the resulting model with the behavior of related neutral, oxygen-donor extractants strongly suggest that the complexity of this system is real, rather than an artifact of over interpretation.

ESTIMATING ACTIVITY AND OSMOTIC COEFFICIENTS IN UO2 (NO3)2 - HNO3 - NaNO3 MIXTURES

ABSTRACT 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.

Comprehensive equilibrium analysis of the complexation of Cu(II) by tetrathia-14-crown-4 in a synergistic extraction system employing didodecylnaphthalene sulfonic acid

Abstract Liquid-liquid extraction experiments, UV/vis spectra, and equilibrium modeling by use of the program SXLSQA have been employed to reveal the origin of synergism in the extraction of Cu(II) from sulfuric acid by the tetradentate macrocycle tetrathia-14-crown-4 (TT14C4) combined with the cation exchanger didodecylnaphthalene sulfonic acid (HDDNS) in toluene. The key feature of the system is the 1:1 complexation of Cu(II) by TT14C4. HDDNS functions both as a source of exchangeable protons and as a solvating agent that promotes the formation of small, hydrated aggregates. In the absence of the cation-exchange vehicle provided by HDDNS, the macrocycle does not extract Cu(II) detectably. Combined with HDDNS, however, TT14C4 significantly enhances the extraction of the metal by HDDNS. Comparative UV/vis spectrophotometry indicates the formation of the deep-blue chromophore attributed to the complex ion Cu(TT14C4)2+, wherein the planar set of four endo sulfur donor atoms of TT14C4 circumscribes the metal...