1045
V O L U M E 2 6 , N O . 6, J U N E 1 9 5 4 ACKNOWLEDGMENT
Glaustcne, S.,"Textbook of PhyJical Chemistry," 5th ed., p. 577, Sew York, D. \'an Sostrand Co., 1940. Groves, T. R., J . Chem. Soc., 11, 97 (1859). Hesse, O., Ann., 176, 189 (1875). Kolthoff. J. AI., Biociiem. Z., 162,289 (1925). Levi, L., and Farmilo, Ch.. SAL. CHEM.,25, 909 (1953). Xeubauer, C., 2. anal. Chew&., 2, 79 (1863). Tanret. C., J . pharm. chini., 28, (5th series). 433, 490 (1893). Taurins, .k.,2 . anal. Chem.. 97, 27 (1934). Travell. ,J., ,J. A m . P h a r m . Assoc., Sei. Ed., 23, 689 (1934). Valser. .I.,"Repertoire de Chimie Pure et Appliqu6e." p. 460, 1862: Thesis, Paris, 1562; .I. A m . Pharm. dssoc.. 11, 16s
The authors wish t o thank L. I. Pugsley for valuable advice and discussions. They are indebted t o L6o llarion of the Sational Research Council for use of his polarimeter, and W. H. Barnes of the National Research Council and Charles E. Hublejof the Dei-nee Research Board for the x-ray diffraction patterns and infrared absorption spectra of the compounds. They also thank 11.Butler for drafting the experimental data reported in Figures 2, 3, and 4, and i , and C. A. Kerr of the Photographic Laboratory of the Department of Sational Health and Welfare for preparing prints of the illustrat,ive material.
(1803).
LITERATURE CITED
(1) Francois, RI., and B l a n c , L. G., Bull. (4th series), 1208, 1304 (1922).
soc.
cliim. France, 31,
RECEIVEDfor review Tovember 12, 1953. Accepted A p r i l t i . 1054. Presented before the Division of Analytical Chemistsy at tile 124th 11eeting of the . ~ V E R I C . ~ SCHEXIC.AL SOCIETY, Chicago, Ill.
Separation of Niobium and Tantalum by liquid-liquid Extraction J. Y. ELLENBURG, G. W. LEDDICOTTE,
and
F. L. MOORE
A n a l y t i c a l Chemistry Division, O a k Ridge N a t i o n a l Laboratory, Carbide a n d Carbon Chemical Co., a n d Carbon Corp., O a k Ridge, Tenn.
The limitations of existing conventional methods for the separation of niobium and tantalum make it desirable to consider any other method that offers the possibility of a separation of these two elements. Studies, using inactive niobium and tantalum as well as radioactive tracers of these elements, indicate that the separation of niobium from tantalum b:- liquid-liquid extraction of the hl-drochloric acid solution of the elements with long-chain aliphatic and aromatic amines in organic solvents is quantitative. Further studies showed that niobium and tantalum sulfate and oxalate complexes extract with the solvated amines. However, in the case of tribenzjlamine in methylene chloride, the niobium sulfate complex may be quantitatively separated from the tantalum sulfate complex if the ratio of organic to aqueous volume is maintained at 1.5 to 1. These methods should be valuable for rapid separations of these elements and the amine systems may be applied to industrial separations of niobium and tantalum.
T
H E separation of niobium and tantalum is one of the most difficult of inorganic problems. Previous methods based on repeated fractional separations are both time-consuming and often uncertain. More recently, increased interest in liquidliquid extractions led t o the esploration of this technique as a more efficient method of separation for these elements. The rssential criteria for extraction are a suitable solvent and a favor:tl>le partition coefficient. Thus, one element may be estrac.ted quantit,ntively while the other remains in the original medium. In laboratory scale operations, a favorable distribution coefficient is essential to keep the analytical procedure simple and the apparatus uncomplicated. Solutions of long-chain aliphatic and aromatic amines in organic solvents have been s1ion.n to be efficient acid extractants (4,9). The use of methyldioctylamine in xylene for the separation of niobium and tantalum has been drscribed (3). I n this study, tribenzylamine, TBA, a product of Eastman Kodak Co., Rochester, S . T.,was of particular interest. It is u-ater-insoluble and is known to form the vorreipondin:: amine acid salts which are also waterinsolutilc l)ut, in grneral, s01ul)le i n various organic solvents.
Table I. Radionuclide
A Division o f Union Carbide
Nuclear Data for Radioniobium and Radiotantalum Production
TI/z, Days
Characteristic Radiations
EXPERI MENT.4 L
I n a typical experiment, an aqueous solution containing either niobium-95 radioact'ive tracer or tantalum-182 radioactive tracer was extracted wit,h an equal volume of the amine in an organic solvent. Each phase was checked for niobium-95 y or tantalum-182 -1 radioactivity by use of a y-scintillation counter having a sodium iodide crystal. Esperiments of this type showed that niobium-95-tantalum-182 tracer mistures and nonradioactive niobium-tantalum separations could be effected by this solvent estraction met,hod. The nurlear propert,ies of the two radionuclides used in thew experiments are described in Table I. This information has been tabulated from data by K a y et al. ( I O ) . Radiochemical purity of the radioactive tracer and end products of solvent extraction technique were checked b y means of decay studies and absorption measurements. The radioactive tracers were prepared as follows: Siobium-95. Radioniobium product was obtained from the Radioisotope Sales Department, Oak Ridge National Laboratory. Decay &dies and @-absorption measurements indicated the radiochemical purity to be 99%. Tantalum-182. Tantalum metal was bombarded in the Oak Ridge National Laboratory graphite reactor. After irradiation, the material was processed by dissolving the metal in concentrated nitric acid-hydrofluoric acid mixture followed by a precipitation of tantalum hydrated oxide upon the addition of concentrated ammonium hydroxide. The precipitate was then dissolved in concentrated hydrochloric acid. Aliquots were taken for each experiment. The Tribenzylamine System. The experimental work with methyldioctylamine (3) suggested that the hydrochloric acid , tem would probably yield satisfactory results in the tribenzylamine extractions. In experiments with tribenzylamine, equal volumes of the sample and extracting solvent were shaken for 5 minutes. The layers were allowed t o separate and the activity
1046
ANALYTICAL CHEMISTRY
Table 11. Partition Coefficient with % Tribenzylamine in Chloroform" 70 T B A
a
KX h K T ~ 1.25 0.20 0.003 2.5 0.80 0,004 5.0 9.4 0.004 8.0 74 4 0,002 10.0 72.6 0.038 11M HC1 solution, equal volumes solvent-aqueous phase.
of the radioactive tracers, niobium-95 and tantalum-182, was determined. The partition coefficient for niobium varied with the hydrochloric acid concentration and per cent tribenzylamine present in the solvent are shown in Tables I1 and 111. Quantitative separation of niobium from tantalum was effected by extracting an 11144 hydrochloric acid solution of the sample with an 8% solution of tribenzylamine in chloroform or methylene chloride. Chloroform and methylene chloride were the only satisfactory solvents for tribenzylamine among those tested. A white solid precipitated upon the addition of concentrated hydrochloric acid to solutions of tribenzylamine in benzene, carbon tetrachloride, toluene, and xylene. Material balance studies showed the precipitate was amine hydrochloride. I n Tables 11, 111, and IV
as shown in Table IV. St higher concentrations of sulfuric acid, the methylene chloride extracted the tantalum from the aqueous phase. Increasing the concentration of tribenzylamine beyond 8% was unfavorable as the tantalum sulfate complex began to extract into the solvent. The alternative lay in a change in the volume ratio of the phases. Bush and Densen ( 1 ) showed that for a mixture of two substances having partition coefficients, K , and Kb, the greatest fractional separation for a given extraction was attained if the ratio of the volumes of the two solvents, 1 X and Y , was maintained at: V,/V, = Theoreti-
.\i&
cally, when this relationship was applied to Rxt, = 0.58 and K T = ~ 0.001, the ratio of solvent phase to aqueous phase was calculated as 41 to 1. However, a solvent ratio of 15 to 1 gave Kxb equal to 99.2 while K T remained ~ a t 0.001; thus, a quantitative separation had been made. hlacro experiments with niobium concentrations of 10 mg. per ml. and tantalum concentrations of 20 mg. per ml. showed 9970 recovery of both elements in their separate phases. DISCUSSION
In the hydrochloric acid system, niobium is thought to extract into the solvent phase by the formation of an oxychloride complex of the type H(Nb0CL) or H2(SbOCl5) (11). On the other hand, neither chlorotantalates (6, 8) nor mg.A/ml. of solvent phase - counts/min./ml. of solvent phase KA = oxychloride (12) complexes of tantalum are presumed mg.A/ml. of aqueous phase counts/min./ml. of aqueous phase to exist in aqueous solution. This may be a possible explanation for the lack of extraction of the tantalum into the I n macro experiments using both inactive niobium, tantalum, solvent phase. Because complex ions are in equilibrium with niobium-95, and tantalum-182 radioactive tracers, solutions their simple components, increased hydrochloric acid concentracontaining 25 mg. per ml. of tantalum were quantitatively sepation would shift the equilibrium in favor of the complex and conrated from 6 mg. per ml. of niobium. I n order t o identify the sequently increase the extractability of the niobium. radioactivity in each phase, radioactive decay studies were made Iiiobium and tantalum pentoxides are both soluble in sulfuric on aqueous and chloroform phases. The half life of the aqueous acid. The exact configuration of these complexes are in doubt phase was about 110 days which corresponded t o radio-tantalum. ( 8 ) . However, the complexes of both niobium and tantalum, The chloroform phase had a half life of about 31 days which was whatever their structure, are extractable to a degree by the chloroindicative of radioniobium. form-tribenzylamine solvent. For example, with a ratio of 15 parts of organic phase-i.e., of methylene chloride-8% tribenzylamine-to 1 part aqueous phase by volume, a quantiTable 111. Effect of Hydrochloric Acid Concentration on tative separation of niobium from tantalum apparently can be Extraction of Niobium from Tantalum by 8% Tribenzylamine made a t 4.5M sulfuric acid. Although niobium can be separated HCI, M , Solvent K Nb K T ~ from tantalum, the concentration of sulfuric acid is very critical 0.003 Chloroform and should be very carefully controlled a t this molarity. 2 0.004 Chloroform 3 The use of tribenzylamine as the amine solute for the oxalic 0.002 Chloroform 4 0.003 Chloroform 6 acid system results in some interesting anomalies. Both ni0.24 Chloroform 8 5.05 Chloroform obium and tantalum oxalato complexes (6, 7 ) are extracted by the 9 13.34 Chloroform 10 chloroform and methylene chloride solvents. However, when 74.38 Chloroform 11 Methylene 80.50 11 the only change in experimental conditions is the replacement of chloride the solvent with benzene, carbon tetrachloride, toluene, or xylene, neither niobium nor tantalum was extracted from the aqueous phase. The behavior of the niobium and tantalum oxalate complexes Material balance studies with oxalic acid show that the acid was similar. They were both extracted by tribenzylamine in is extracted by the tribenzylamine solvated in chloroform or chloroform or methylene chloride. The partition coefficients remained a t 3 for niobium and 1 for tantalum with varying concentrations of tribenzylamine or added oxalic acid. There was Table IV. Partition Coefficient with Sulfuric Acid no measurable extraction of niobium or tantalum when the solvent Concentration was benzene, carbon tetrachloride, toluene, or xylene. HzSO4, M KN b K T ~ The niobium sulfate complex in 0.45 t o 9M sulfuric acid had 0.02 0.45O 0.004 a distribution coefficient of less than 0.15 when chloroform or 0.42 4.0 0.003 0.58 4.5 0.003 benzene and its related compounds were employed as the solvent. 0.002 0.56 4.7 K T was ~ less than 0.01. However, the behavior of the sulfate 0.53 4.8 0.004 1.2 8.3 4.9 complexes with methylene chloride was radically different. 2.4 5.0 20.1 2.7 17.0 5.5 The difference in coefficients a t 4.511.1 sulfuric acid appeared sig6.4 12.1 6.0 nificant enough to encourage further work on the sulfate system. a 8% TBA in methylene chloride. Experimental data. obtained by With 8% tribenzylamine in methylene chloride a t 4.5M sulfuric extracting equal volumes solvent/aqueous phase. acid, niobium had a K of 0.58, whereas tantalum had a K of 0.0030,
-
1047
V O L U M E 2 6 , NO. 6, J U N E 1 9 5 4 methylene chloride but not in tribenzylamine solvated in carbon tetrachloride or benzene and its homologs. These methods for the separation of niobium from tantalum should prove valuable t o those interested in a rapid separation of the two metals. Of the systems reported, the separation of the two metals in the hydrochloric acid system is much more desirable for tantalum does not readily extract from any hydrochloric acid concentration. Technologically, these amine systems may be applied successfully t o the industrial separation of niobium from tantalum with cheap chemicals and replace the present tedious fractionation methods. ACKNOWLEDGMENT
The authors wish to thank P. Z. TTesterdahl and L. hl. Frakes for performing many of the experiments in obtaining the data.
LITERATURE CITED
Bush, 11. T., and Densen, P. &I., ANAL.CHEY.,20, 121 (1948). Kiehl, S.J., Fox, R. L., and Hardt, H. B., J . Am. Chem. Soc., 59, 2395 (1937). Leddicotte, G. W., and Moore, F. L., Ibid., 74, 1618 (1952). hIoore, F. L., U. S. Atomic Energy Commission, “USAEC Secret Report,” ORNL 1314 (July 1952). Rosenheim, A., and Roehrick, E., 2. anorg. Chem., 204, 342 (1932). Ruff. 0.. , and ~ - Schiller. E.. Ihid..72. 329 11911). , , Russ, R., Ihid., 31,~42 (1902). Sidgmick, N. V., “The Chemical Elements and Their Compounds,” Oxford, Clarendon Press, 1950. Smith. E. L.. and Page. J. E., J . Soc. Chem. I n d . , 67, 48 (1948). Way, K., et al., Natl-Bur. Standards, CUT. 499 (1950). Weinland, R. F., and Storz, L., Ber., 39, 3056 (1906). Weinland, R. F., and Storz, L., 2. anorg. Chem., 54, 223 (1907). ~
~~
,
I
RECEIVED for review April 22, 1953. Accepted April
9, 1954.
Fluoride Determination by Electron Transfer Catalysis W.
D. ARMSTRONG
and
LEON SINGER
Department of Physiological Chemistry, M e d i c a l School, University
The marked catalytic effect of fluoride on the cerousceric electron exchange reaction suggested that this reaction could be used as a microanalytical method for fluoride determination without the necessity of removing ions which interfere in present methods. Cations in general were found to exert negligible catalytic effects. The catalytic effect of fluoride, was about 900 times that of sulfate and about 370 times that of phosphate. These circumstances limit the application of the method to those samples which do not contain these ions or in which the effect of the interfering ions can be made constant. The fluoride content of bone ash has been determined by making constant the effect of interfering ions. The results of electron transfer catalysis by various negative ions are considered to be in qualitative agreement with Libby’s description of the mechanism of electron transfer catalysis.
H
ORNIG and Libby (1) discovered a marked catalytic effect
of fluoride ions on the rate of electron transfer between cerous and ceric ions in nitric acid solution and showed that the increase in electron transfer rate was proportional t o the fluoride concentration. A proposed mechanism (1, 2 ) of electron transfer catalysis describes the placing of a small negative ion (in this case fluoride) between the positive exchanging ions, so as to form a symmetrical transition complex in which the hydration atmospheres of the positive ions are shared. This model of electron exchange catalysis suggested the possibility that the effect of fluoride on this type of electron transfer might be so specific as to permit the determination of fluoride in natural materials, without the necessity of separation of fluoride from other ions in the sample. This report presents a comparison of the catalytic effect of several ions commonly present in natural materials with that of fluoride, and indicates the limitations of fluoride analysis based upon this principle. EXPERIMENTAL
The stock solution of radioactive cerium-144 was treated with hydrogen peroxide in order t o convert the ion t o the cerous state. The excess hydrogen peroxide was removed by heating the solution on a water bath. Concentrations which would give approximately 75,000 counts per minute of the radioisotope, under conditions of radioactive counting used in this method, were employed in each reaction mixture.
of
Minnesota, Minneapolis, M i n n .
The reactions were carried out in 50-ml. centrifuge tubes suspended in a large, stirred bath of ethylene glycol, which was placed in a household-type deep-freeze cabinet. The temperature of the bath was regulated a t -14.2’ C. and rose no more than 0.4’ over an 8-hour working period. i2t room temperature 3 ml. of 0.003M cerous nitrate hexahydrate solution in 12M nitric acid, labeled with cerium-144, were transferred t o the reaction tubes and mixed with 3 ml. of an aqueous solution of the ion t o be tested. After a temperature equilibrium had been established, usually by standing overnight, the reaction was started by the addition of 3 ml. of nonradioactive cerium(1V) solution (0.0003M ceric ammonium nitrate in 6 M nitric acid) a t - 14.2’ C. The reaction was stopped after 160 minutes and the cerium(ITr) separated from the reaction mixture by rapid and forceful injection of 20 ml. of cold ether into the reaction mixture with a syringe. Ten minutes later, 5 ml. of the ether layer containing the cerium( IV) were removed with a cold pipet and transferred to a 25-ml. volumetric flask. The ether was removed by aeration and the flask diluted t o volume with a solution containing 2 mg. per liter of nonradioactive cerium(1V). This solution acted as a carrier and diluent to impede possible adsorption of the radioactive ceric ions on container walls. The flask was allowed t o stand a t least 175 minutes (usually overnight) in order t o permit the praseodymium-144 daughter t o grow t o a concentration in equilibrium with that of cerium-144. The contents of the flask were transferred to paraffined drinking cups and the radioactivity was measured ( 3 ) . It is presumed that under these conditions only the energetic beta radiation from praseodymium-144 decay was measured. * A41dehydesand peroxides in untreated ether would reduce ceric ion during its extraction from the reaction mixture. The ether was satisfactorily purified in volumes sufficient for 5 days’ use by washing with ceric ammonium nitrate in approximately 8M pitric acid until the color of the ceric ions remained in the washings. The ether was then washed eight times with water, or until the washings were neutral, and stored in a dark bottle in the deep-freeze cabinet. In agreement with Hornig and Libby ( I ) , the dependable and rapid separations of the ether and aqueous phases, in the extraction of cerium(1V) from nitric acid solutions, were obtained with difficulty. The conditions specified were found by trial-anderror experiments, in which the relative volumes of ether and acid solutions were varied, to be optimal. Phase separation could not be obtained a t the low temperature employed when 15 ml. of ether and 9 ml. of acid were used, or when the acid concentration was increased t o 8.7M. RESULTS AND DISCUSSION
The results given in the figures are shown as the per cent of tho exchange that has not yet occurred, plotted against time or con
