Column Chromatographic Separation of Niobium, Tantalum, Molybdenum, and Tungsten James S. Fritz and Lionel H. Dahmer Institute f o r Atomic Research and Department of Chemistry Iowa State University, Ames, Iowa Distribution ratios are reported for extraction of niobium(V), tantalum(V), molybdenum(Vl), and tungsten(V1) into methyl isobutyl ketone from various aqueous mixtures of hydrofluoric, hydrochloric, and sulfuric acids. Quantitative separations cannot be obtained by simple batch extractions, so a column chromatographic technique was employed using a Teflon-6 support impregnated with methyl isobutyl ketone. Sequential elution with eluents of different compositions separates niobium(V) and tantalum(V), molybdenum(Vl), and tungsten(VI), and a mixture of all four elements from each other. Using radioactive tracers, as little as 0.1 ppm of one metal was separated from the others.
THE SEPARATION of mixtures of niobium, tantalum, molybdenum, and tungsten by classical gravimetric methods is very lengthy and difficult. This is true partly because of the similarity of their chemical properties and partly because of the ease with which compounds of these metals are hydrolyzed in acidic aqueous solution. In 1952 Burstall and Williams (I) achieved a major breakthrough by separating niobium and tantalum from each other and from many other metal ions through elution from a cellulose column with methyl ethyl ketone-hydrofluoric acid mixtures. However, their method requires large volumes of organic solvent for elution. More recently, separations of niobium, tantalum, molybdenum, and tungsten have been accomplished by anion exchange chromatography using various eluents containing hydrofluoric acid (2-5). Although useful, these separations require exacting conditions and the separation factors are not as favorable as might be desired. Several authors have shown that tantalum(V) may be extracted into organic solvents to a greater extent than niobium from aqueous solutions containing hydrofluoric acid plus a strong mineral acid (6-8). Werning et al. (9) devised a large-scale separation of niobium and tantalum by countercurrent extraction from hydrochloric-hydrofluoric acid aqueous solution into methyl isobutyl ketone (MIBK). The separation methods in this paper are based on partition of tantalum(V), niobium(V), molybdenum(VI), and tungsten(V1) between an aqueous phase containing hydrochloric and hydrofluoric acids and MIBK as the organic (1) F. H. Burstall and A. F. Williams, Analyst, 77,983 (1952). ( 2 ) E. J. Dixon and J. B. Headridge, Zbid., 89, 185 (1964). (3) S. Kallman, H.Oberthin, and R. Liu, ANAL.CHEM., 34, 609 ( 1962). (4) K . A. Kraus and G. E. Moore, J. Am. Chem. SOC.,77, 1383 (1955). (5) J. L. Hague and L. A. Machlan, J. Research NBS, 62,11 (1959). (6) G. W. C. Millner, G. A. Barnett, and A. A. Smales, Analyst, 80, 380 (1955). (7) S . Nishimura, J. Muriyama, and I. Kushina, Trans. Japan Znst. Metals, 5, 32 (1964). (8) P. C. Stevenson and H. G . Hicks, ANAL.CHEM., 25, 1517 (1953). (9) J. R. Werning, K. B. Higbie, J. T. Grace, B. F. Speece, and H.L.Gilbert, Znd. Eng. Chem., 46,644 (1954).
20
ANALYTICAL CHEMISTRY
phase. Because quantitative separations cannot be obtained by simple batch extraction, a column chromatographic technique is employed. The column is packed with a special Teflon-6 support onto which is sorbed MIBK; aqueous solutions of various compositions are used to elute the metal ions selectively. By this scheme it is possible to separate niobium and tantalum, molybdenum and tungsten, and a mixture of all four elements from each other. EXPERIMENTAL
Apparatus and Solutions. Columns consisted of 1.2-cm i.d. polyethylene tubing held straight by tightly fitting glass tubing on the outside. A Teflon straight-union reducer attachment was connected to the bottom of the tubing; a Nalgene stopcock was fitted to the bottom of the reducer. The resin was supported by a small plug of Dyne1 wool. A 125-ml Nalgene separatory funnel at the top of the column served as a reservoir for the eluent. Irradiation of molybdenum and tungsten samples was performed at the Ames Laboratory Research Reactor. Counting was done using a 4- X 4-inch well-type NaI-(TI) crystal in conjunction with a single channel gamma scintillation spectrometer. The window of the analyzer was set to measure the following gamma ray energies: 35 day g5Nb, 0.76 MeV; 115 day 'UT'a, 1.2 MeV; 24 hour lU7W,0.48 MeV, 0.68 MeV; 66 hour Q9Mo,0.141 MeV. Tee Six, an inert form of Teflon-6, was obtained from Analytical Engineering Laboratories, Hamden, Conn. The presieved resin was available as 70180 mesh or 1601170 mesh. Before use the Tee Six powder was stirred with a mixture of hydrochloric acid, hydrofluoric acid, and methyl isobutyl ketone. Washing in a Buchner funnel with acetone and ethyl ether, followed by air-drying, made the support ready for use. Methyl isobutyl ketone (MIBK) obtained from Eastman was used without purification. Niobium(V), tantalum(V), molybdenum(VI), and tungsten (VI) stock solutions were prepared by dissolution of the metal in a hydrofluoric-nitric acid mixture. The solution was evaporated to dryness, followed by addition of appropriate amounts of hydrofluoric acid and hydrochloric acid. The resulting solution was then transferred to Nalgene bottles for storage. Solutions of radioactive Q5Nband 1*Ta were prepared by transferring a portion of the solutions, obtained from Oak Ridge, Tenn., to a Nalgene bottle containing carrier in 7M hydrochloric acid, 2M hydrofluoric acid. The resulting solutions were stirred and allowed to stand a day to ensure complete exchange. Solutions of radioactive 9 Q Mand ~ IS7Wwere prepared by irradiation of 1 mg of the metal sealed inside a 2-mm i.d. fused quartz tube, After irradiation of the samples, the metal ion solutions were prepared in the usual way by dissolution of the metal in a hydrofluoric-nitric acid mixture. Except for the metal ion solutions listed, all other metal ion solutions were 0.02M to 0.05M solutions of the nitrate or perchlorate salt in dilute nitric or perchloric acid. Chromium(II1) was a 0.05M solution of chromium chloride in dilute nitric acid.
Zirconium(1V) was a 0.05M solution of zirconyl chloride in 0.3M hydrochloric acid. The zirconium(IV) salt was dissolved in concentrated hydrochloric acid and diluted to volume. Titanium(1V) was a 0.025M solution of titanium(lV) chloride in 5M hydrochloric acid. Aqueous eluents were equilibrated with MIBK before use by shaking for about 5 minutes in a separatory funnel. Eluents were used on the same day that they were prepared. Analytical Procedures. Molybdenum(V1) was precipitated with 8-hydroxyquinoline according to the method of Balanescu (10). Niobium(V), tantalum(V), and tungsten(V1) were analyzed according to the homogeneous precipitation technique of Dams and Hoste (11). The details for handling column effluents prior to precipitation have been described (12).
Titration of most other metal ions has done with 0.05M EDTA using standard methods. Chromium was determined by oxidation-reduction titration with iron(I1). Molybdenum(V1) and tungsten(V1) were determined spectrophotometrically by the thiocyanate method (13). Niobium(V) was determined by a spectrophotometric hydrogen peroxide (14) or pyridyl azo resorcinol (PAR) (15) method. Tantalum(V) was determined by a spectrophotometric method employing Arsenazo I(10). Trace amounts of the title elements were determined by gamma scintillation counting. Radioactive column effluents were collected in plastic cylinders and transferred to platinum dishes. Oxalic, citric, or tartaric acid was added to keep the metal ions in solution. The solutions were evaporated to about 4 ml and then transferred to 18- X 150-mm glass tubes for counting. A standard consisted of radioactive tracer solution, which was evaporated and handled in the same manner as column effluents. Except for molybdenum (VI), column effluents were analyzed immediately after column separations were achieved. Molybdenum(V1) solutions were counted after 15 hours to allow 9 9 M and ~ 99Tc to reach transient equilibrium. Column Separation Procedure. Tee Six powder was stirred with enough pre-equilibrated MIBK to form a free flowing slurry. This mixture was allowed to stand for about 10 minutes before transferring it to the column. Additional MIBK was used to complete the transfer. The level of the MIBK was allowed to settle to the top of the resin, and then pre-equilibrated aqueous phase was passed through. A graduated cylinder was placed at the bottom of the column to collect the effluent. When the MIBK phase had been completely displaced by aqueous phase, the volume of MIBK was measured and noted as VM,the volume of mobile phase (interstitial volume). After allowing the level of the aqueous phase to settle to the top of the resin, a sample was carefully introduced by means of a separatory funnel. This sorption step was done at a flow rate of 0.25 ml per minute, about half as fast as for elution of metal ions. Sequential elution of metal ions in the sample mixture was then carried out using pre-equilibrated acid solutions. Fractions were collected and analyzed according to procedures described previously. The total volume ( V T )of MIBK associated with the resin column was obtained by pipetting MIBK into a graduated cylinder containing the resin. The volume of excess MIBK above the top of the resin was then measured. This value
(10) G . Balanescu, 2. Anal. Clzem., 83, 470 (1931). (11) R. Dams and J. Hoste, Tulunfu,8,664 (1961). 37,1272 (1965). (12) J. S. Fritz and L. H. Dahrner, ANAL.CHEM., (13) C. E. Crouthamel and C. E. Johnson, Zbid.,26,1284 (1954). (14) F. C. Palilla, N. Adler, and C. F. Hiskey, Zbid., 25,926 (1953). (15) R. Belcher, T. V. Ramakvishna, and T. S. West, Talanfa,10, 1013 (1963).
Table I. Variation of Distribution Ratio with Hydrochloric and Hydrofluoric Acid Concentrations Mfn = 0.025M
HC1
HF
DTS
7 4 2
2 8 1
392
...
106
Dm 64 33 0.2
Dw 0.1
Dmo 6.2 0.4
was subtracted from the amount added by pipet, giving the value of VT. The difference between VT and VM gives VS, the volume of MIBK sorbed to the resin (volume of stationary phase). RESULTS AND DISCUSSION
Distribution Ratios. Distribution ratios were determined for extraction of niobium(V), tantalum@), molyl;denum(VI), and tungsten(V1) from 1 M hydrofluoric acid-1M nitric acid into methyl isobutyl ketone. This system was investigated briefly because a successful paper chromatographic separation of niobium(V) from tantalum(V) was reported using the same aqueous phase and diethyl ketone (16). However, the distribution ratios did not suggest a separation scheme for all four metals. Werning et al. (9) found tantalum(V) to be 99 extracted into methyl isobutyl ketone from 3M hydrochloric acid-1.1 M hydrofluoric acid while niobium(V) is only 1 . 6 z extracted under the same conditions. However, their data show that increasing both the hydrochloric and hydrofluoric acid concentrations increases the extraction of niobium to greater than 90%. Because a separation scheme including molybdenum(VI) and tungsten(VI) was desired, the distribution ratios of these metal ions were studied. Earlier investigators found that molybdenum(V1) is extracted appreciably into MIBK from 3M to 7 M hydrochloric acid (17) but that addition of hydrofluoric acid greatly decreases the extraction and addition of sulfuric acid increases it somewhat. We found that addition of low concentrations of sulfuric acid increase the extraction of tantalum, niobium, and tungsten slightly, but the increase in extraction of molybdenum is more pronounced. The effect of varying hydrochloric and hydrofluoric acid concentrations on extraction of the four metal salts studied is shown in Table I. These distribution ratios indicate very favorable separation factors for separation of tungsten from the other metal ions using 7 M hydrochloric acid-2M hydrofluoric acid, separation of molybdenum from tantalum and niobium with 4M hydrochloric-8M hydrofluoric, and of niobium from tantalum with 2M hydrochloric-1 M hydrofluoric acid. The distribution ratios of several other metal ions were determined for partition between 7 M hydrochloric-2M hydrofluoric acids and methyl isobutyl ketone. Aluminum (111), chromium(III), cobalt(II), copper(II), manganese(II), nickel(II), titanium(IV), and zirconium(1V) have distribution ratios of 0.1 or less; vanadium(1V) has a distribution ratio of 0.19. Iron(II1) is strongly extracted by the MIBK phase, having a distribution coefficient of 59.
z
(16) I. Martin and R. I. Magee, Talunfu,10, 1119 (1963). (17) G . Latwesen, M. S. Thesis, Iowa State University, 1966. VOL 40, NO. 1, JANUARY 1968
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2.
1
1
1
1
1
1
1
1
1
1
1
l
l
1
1
l
l
i
Nb
1.. I
l
Z
l
-6
l -4
l
l -2
W
r
l
l
0
2
4
LOG [METAL ION] aq,-
Figure 1. Variation of distribution ratio with initial metal ion concentration It is useful to know whether the distribution ratio of a metal ion remains constant or changes over a range of metal ion concentrations. In Figure 1 the logarithm of distribution ratio is plotted against the logarithm of the aqueous metal ion concentration before extraction. Except for tungsten, which has a low distribution ratio over the range studied, the
5
22
distribution ratios remain constant in dilute solution but decrease sharply above 0.01 M tantalum(V), 0.05M niobium (V), and above 0.25M molybdenum(V1). An important implication of this information in column chromatographic separations is that symmetric elution curves should be obtained with fairly low concentrations of the metal ions, but tailing might occur at higher concentrations owing to the rapid increase in distribution ratio with decreasing metal ion concentration. Column Separations. Quantitative separations of the metal ions studied may be obtained with a column filled with MIBK-impregnated Teflon-6 using eluents of the composition shown in Table I. Niobium(V) and tantalum(V), in equal and variable molar ratios, were separated on a 16- X 1.2-cm column. Niobium is eluted by approximately 50 ml of 3 M hydrofluoric-lM hydrofluoric acid at a flow rate of 0.5 ml per minute. Tantalum(V) is then stripped from the column with 100 ml of 15% hydrogen peroxide at the maximum flow rate (about 2 ml per minute). Data for quantitative separations are given in Table 11. Favorable separation factors for molybdenum(VI) and tungsten(VI) are exhibited in 7 M hydrochloric acid-2M hydrofluoric acid or in 6 M hydrochloric3M hydrofluoric1M sulfuric acid, Enough hydrofluoric acid must be present to complex tungsten sufficiently to prevent hydrolysis, but excessive concentrations of hydrofluoric acid lessen the extraction of molybdenum into MIBK. Results of a quantitative separation of molybdenum and tungsten on a 26- X 1.2-cm column are shown in Table 11. Spot tests and spectrographic analyses indicated that tungsten was completely eluted with 15 to 20 ml of the 6-3-1M eluent. The 3-10-1M eluent used to remove molybdenum from the column does not cause niobium to be eluted. Therefore, a quantitative separation of tungsten, molybdenum, and niobium was also successful. Tantalum(V) is highly extracted into MIBK from all of the solutions that were used to elute tungsten, molybdenum, and niobium in the preceding separations. Thus, all four metal ions may be separated on asingle column using the same
Table 11. Quantitative Column Separations Columns are 16 X 1.2 cm filled with 70-mesh Teflon-6 for Nb-Ta separations; 26 X 1.2 cm filled with 170-mesh Teflon-6 for other separations Flow rate -0.6 ml per minute except for tantalum (-2.0 ml/min) Eluent Oxide, mg Taken Found Composition Metal Vol, ml 67.0 66.6a Nb 50 3M HCl, 1.1M HF Ta 100 15% HzOz 110.7 111.2a 12.1 11.7 Nb 50 3M HCl, 1.1M HF 221 .o 221.9 Ta 100 15% HzOz 59.1 58.7 W 20 6M HCl, 3M HF, 1M 3M HC1, 10M HF, 1M HzSO4 38.3 38.5 Mo 50 6M HC1, 3M HF, 1M HSOd 59.1 59.6 W 20 3M HCl, 10M HF, 1M HzSOa 38.3 38.2 Mo 50 29.0 28.7 Nb 50 3M HC1,lM HF 6M HCI, 1M HzSO4, 3M HF 59.1 59.4 W 20 3M HCI, 1M HzS04,lOM HF 38.3 38.2 Mo 50 29.0 28.6 Nb 50 3M HCl, 1M HF 54.2 53.4 Ta 50 MIBK 57.8 57.1 W 20 7M HCl, 2M HF 38.5 38.4 Mo 50 6M HC1,6M HF 33.1 32.0 Nb 50 3M HCI, 1M HF 56.6 56.3 Ta 50 MIBK Average of 7 column separations and analyses.
ANALYTICAL
CHEMISTRY
Difference -0.3 $0.5 -0.4 +0.9 -0.4 +0.2 +0.5 -0.1 -0.3
$0.3 -0.1 -0.4 -0.8 -0.7 -0.1
-1.1 -0.3
I -1
ELUTION CURVE NIOBIUM ( V I
14w0t 5000b
HETP:O.OS CY. THEORETICAL VR' 216 ml. EXPERIMENTAL V, = 223 ml.
3 3
i
Table III. Separation and Radiochemical Determination of Niobium(V), Tantalum(V), Molybdenum(W, and Tungsten(VI) 26 X 1.2 cm. Tee Six column (170-mesh), flow rate 0.5 ml/minute; 20 ml of 7 M HCI, 2M HF elutes tungsten; 50 ml of 6 M HCI, 6M HF elutes molybdenum; 50 ml of 3M HCI, 1M HF elutes niobium; 50 ml of MIBK elutes tantalum Active ion Mixture Counts/ recovery, metal ion ratio Fraction analyzed minute 114,837 Mo, W, Ta:Nb Nb standard 106:l Nb 114,140 99.4 Mo, W,Ta:Nb Nb standard 11,232 Nb 11,109 99.0 107:1 7,036 Mo, W,Nb:Ta Ta standard Ta 7,018 99.7 107 :1 Mo, Nb,Ta:W W standard 1,077 W 1,076 99.9 107:1 W, Nb, Ta:Mo Mo standard 2,104 106:l M O 2,059 98.9
z
I 160
180
I 200
I
I 220
I
I
I
240
MILLILITERS OF 7HHCI
I 260
280
300
320
, 2MHF-
Figure 2. Elution of 1 X 10-8 mmole radioactive niobium(V) from a 11.3- X 1.2-42111 column of 170-mesh Tee Six eluents as in the separation of tungsten, molybdenum, and niobium. Tantalum is last to be eluted using MIBK at the maximum flow rate. Results are given in Table 11. Another separation of all four metal ions was performed in which the system was simplified by omitting sulfuric acid from the eluents. Distribution ratios for the metal ions in the 7 M hydrochloric-2M hydrofluoric acid system with MIBK are given in Table I. Spectrographic analyses of each fraction for the presence of other metals showed that the column separations were complete. For 29 individual analyses of tungsten, molybdenum, niobium, and tantalum after column separations the average recovery was 99.3% with a standard deviation of +0.8%. The low bias in the results is believed to be caused by slight difficulties in analysis of the separated fractions and not by incomplete separation or partial retention by the column. Spectrographic analyses of various fractions showed no contamination by other metal ions; also spectrographic analysis of column material in several instances showed no evidence of metals retained by the column after elution. Extremely small amounts of one element were separated from macro amounts (-25 to 50 mg) of each of three other elements using radioactive tracers to follow elution of the minor sample constituent. Results are shown in Table 111. As little as 0.1 ppm (on a mole basis) of one metal was separated from the other metals with excellent recovery. The gamma count of the other metal fractions seldom exceeded the background count, giving additional evidence as to the excellence of the separations obtained at the trace level. The only exception was in separations involving radioactive niobium where a small percentage of the gamma counts were consistently found in the tungsten fraction. A gamma ray spectrum obtained on a multichannel analyzer showed no other radioactive metal ion impurity that might have been introduced with the niobium tracer. The presence of a nonextractable niobium radio colloid, therefore, seemed likely. This was confirmed by extraction of radio niobium stock solution into MIBK from 7 M hydrochloric-2M hydrofluoric acid, followed by back-extraction with 2 M hydrochloric-lM hydrofluoric acid. This solution was adjusted to the proper acid composition, added to the chromatographic column, and eluted with 7 M hydrochloric-2M hydrofluoric acid as before. This time no activity appeared in the tungsten fraction. Column Parameters. Separations based on partition chromatography have a definite advantage over batch type
solvent extraction methods in that only a significant difference in the extractability of the sample components is needed for a chromatographic separation. If a chromatographic separation involves only a partitioning effect-Le., no adsorption or ion exchange interaction of the solute with the solid support-then the elution of solutes from a column should be quantitatively predictable from liquid-liquid distribution ratios using the relationship,
V,
= V,
+ DV,
(1)
where V , = volume of eluent required to elute the solute to its maximum concentration in the effluent, V, = volume of mobile phase (interstitial volume), D = batch distribution ratio for the extraction of solute from aqueous into organic phase, and V, = volume of stationary liquid phase. In order to test this equation, 10-8 mmole of radioactive niobium(V) was introduced to a 11.3- X 1.2-cm Teflon-6 column impregnated with methyl isobutyl ketone. Elution was carried out with a 7 M hydrochloric-2M hydrofluoric acid solution, and 5-ml fractions were collected for counting. An elution curve is shown in Figure 2. In this experiment the flow rate was 0.3 ml per minute, V, was 5.0 ml, Vswas 3.3 ml, and D was 64. The theoretical V , calculated from the equation above was 216 ml as compared to the experimental V, of 223 ml. Agreement between experiment and theory is often not nearly this good owing largely to the difficulty in measuring V, accurately. Nevertheless, this experiment supports the idea that the column process is one of pure partition uncomplicated by any adsorption interaction with the solid support. The height equivalent of a theoretical plate is a recognized measure of column efficiency. The number of theoretical plates in the column used in the preceeding experiment was determined from Figure 2 using the formula
N=8(;)' where N is the number of theoretical plates and /3 is the width of the elution curve at a solute concentration c = cWJe = 0 . 3 6 8 ~ ~ N~ ~was . calculated to be 124, corresponding to a theoretical plate height of 0.09 cm. This is considered to be quite good for a liquid chromatographic column. RECEIVED for review January 12, 1967. Accepted July 19, 1967. Work performed in Ames Laboratory of the U. S. Atomic Energy Commission. VOL 40, NO. 1 , JANUARY 1968
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