oxine added appears to be based on the partition of the neutral metal oxinate molecule between the two phases, rather than on the partition of a metal ion or ion pairs, where the activity of the metal ion is simply reduced by the presence of the complexing agent in the melt. The improvement factor may depend chiefly on the relative size of the species that must enter the crystal lattice of the nearly pure solid. Thus, provided solubility in the melt is not a problem, larger chelating agents such as the substances tested by Sloan (7) should give even larger a-values for these metals. An incidental advantage of adding a chelating agent that forms a n intensely colored product such as the black Fe(Ox)z is that the removal of the metal can be followed visually down t o at least the ppm level. In studies on the system Fe(II1) oxine, in urethane, it was noted that black Fe(Ox)a had a marked tendency to segregate and rise to the top in normal freezing, making the preparation of a homogeneous charge difficult. In fact, growths of concentrated iron oxinate were observed to ooze slowly out of the
+
impure melt on slow cooling, in a fashion simulating a geological process. The contraction of the solid on freezing apparently produced enough internal pressure t o squeeze out veins of the still molten, impure material. The fact that each system of solute plus solvent has its own unique properties and crystallization habits, such as the above observations on Fe(II1) oxine in urethane, makes it difficult to generalize in the field of zone refining.
+
ACKNOWLEDGMENT The authors acknowledge many helpful discussions with G. E. Janauer.
RECEIVED for review August 12, 1968. Accepted October 7, 1968. Presented at the Division of Analytical Chemistry, 156th National Meeting, ACS, Atlantic City, N. J., September 1968. Based on the M.A. thesis of Louis N. Jones, State University of New York at Binghamton, Binghamton, N. Y., 1968.
Solvent Extraction Separation of Tantalum and Niobium FIuorides with N-Benzoyl-N-PhenyIhyd roxylamine J. S. Erskinel, M. L. Sink, and L. P. Varga2 Department of Chemistry, Oklahoma State University, Stillwater, Okla. 74074 The solvent extraction behavior of trace and millimolar concentrations of aqueous tantalum and niobium fluorocomplexes with N-benzoyl-N-phenylhydroxylamine (BPHA) in chloroform was studied as a function of fluoride and perchloric acid concentrations using radioactive tracer techniques. Least squares curvefitting calculations on the distribution data indicated that the principal extracting species were TaF, ZBPA and H,NbFs . ZBPA with extraction equilibrium constants 2.0 x l O * O and 3.0 X lo7, respectively. Single stage decontamination factors up to 70 were achieved from 1.0 x 10-3 molar excess fluoride ion concentrations in the absence of perchlorate on the extraction of millimolar concentrations of both metals into 2.5 X 10-2 molar BPHA in chloroform. e
N-BENZOYL-N-PHENYLHYDROXYLAMINE, BPHA, has the desirable property of forming stable complexes with metal ions of high charge/radius ratio in strong acid solutions where hydrolysis of the metal ion is minimal. Solvent extraction of the chelate formed has been found to be sufficiently selective so that useful separations of this group of high charge/radius ratio ions from other groups have been accomplished (1-5). When separations within the easily hydrolyzed group, niobium, tantalum, and protactinium are considered, the extraction usually is carried out in the presence of a complexing 1 2
Present address, Dow Chemical Co., Freeport, Texas. To whom reprint requests should be made.
(1) G. H. Morrison and H. Freiser, “Solvent Extraction in Analytical Chemistry,” John Wiley and Sons, Inc., New York, N. Y. and London, 1962. (2) R. W. Moshier, “Analytical Chemistry of Niobium and Tantalum,” The Macmillan Co., New York, N. Y., 1964. (3) J. Stary, “The Solvent Extraction of Metal Chelates,” The Macmillan Co., New York, N. Y., 1964. (4) D. Dryssen, Acra Chem. Scartd., 10, 353 (1956). (5) H. Freiser, ANAL.CHEM., 40, 522R (1968). 70
ANALYTICAL CHEMISTRY
ligand, such as fluoride ion used to bring the original material into solution. Thus the chelating agent must compete with the solubilizing ligand t o form an extractable chelate, or the extracting species must contain both complexing ligands. Lyle and Shendrikar (6) reported the separation of protactinium, tantalum, and niobium from fluoride solutions using BPHA, and curve-fitting studies by Varga and co-workers (7) indicated the most probable nature of the extracting tantalum-BPHA adduct over a given fluoride concentration range, but quantitative comparisons between the several metal ion systems have not been made. The present comparative studies on the tantalum and niobium systems at various salt, acid, metal, fluoride, and BPHA concentrations were designed to define the extraction model over a wider fluoride concentration range and to determine optimum separation conditions. THE EXTRACTION MODEL Tantalum and niobium tend t o form hydrolytic polymeric species at lower acidities and higher metal concentrations, Zirconium behaves similarly and in 2 M perchloric acid begins to polymerize at zirconium concentrations of about 2 X 10-aM(8). In 1Mperchloric acid polymerization occurs at about 2 X 10T4M. Paramonova and Kolychev (9) reported that niobium in nitric acid solutions was not in the ionic state but was as niobium oxide particles (
