Anal. Chem. 1986, 58, 2408-2412
2408
arise from calibration and detection procedures. If appropriate calibration standards and detector response factors are employed, however, to overcome the latter, the precision in values determined by SEC approaches that of measurements by VPO and agreement between the two methods is good (Table V). SEC also affords, of course, a value of Mw and allows the complete MM distribution to be determined.
ACKNOWLEDGMENT We thank A. A. Herod for the mass spectra and W. R. Ladner and staff of the National Coal Board Coal Research Establishment for making available samples with analytical data. Special thanks are due to C. E. Snape for numerous helpful discussions.
LITERATURE CITED (1) Lee, M. L.; Yang, F. J.; Bartle. K. D. Open Tubular Column Gas Chromatography: Theory and Practice; Wiley: New York, 1984. (2) Bartle, K. D.; Zander, M. Erdoel Kohle, Erdgas Petrochem. 1983. 38, 15-21. (3) Peaden, P. A.; Lee, M. L. J . Liq. Chromatogr. 1982, 5 (Supp. 2) 179-221. (4) Bartle, K. D.; Collin, G.; Stadelhofer, J. W.; Zander, M. J . Chem. Technol. Blotechnol. 1979, 29. 531-551. (5) Wilson. M. A.; Rottendorf, M.; Colin, P. G.; Vassallo, A. M.; Barron, P. F. Fuel 1982, 61, 321-328. (6) Mulligan. M. J.; Bartle, K. D.; Taylor, N.; Gibson, C.; Mills, D. G. Fuel 1983, 62, 1181-1185. (7) Bartle, K. D.; Mulligan, M. J.; Taylor, N.; Martin, T. G.; Snape, C. E. Fuel 1984, 63,1556-1560. (8) Ladner. W. R.; Snape, C. E. Proceedings International Conference on Coal Science, Sydney, 1985; Pergamon Press: Sydney, 1985; pp 949-952. (9) Davies, G. 0. Chem. Ind. (London) 1978, 560-566. (IO) Bartle, K. D.; Ekinci, E.; Frere, B.; Sarac, S.; Snape, C. E. Chem. Geol. 1981, 34, 151-164. (1 1) Bartle, K. D.; Ladner. W. R.; Martin, T. G.; Snape, C. E.; Williams, D. F. Fuel 1979, 58. 413-422. (12) Martin, T. G.; Smith, C. A,; Snape, C. E.; Starkie, M. c. ,%e/ 1981, 60, 365-366.
(13) Snape, C. E.; Bartle, K. D. Fuel 1979, 58, 898-900. (14) Drushel, H. V.; Schulz, W. W. Paper presented at the Symposium on Techniques for Characterization of Residual Fuels, ACS Meeting, Las Vegas, NV, August 1980. (15) Greinke, R . A.; O'Connor, L. H. Anal. Chem. 1980, 52, 1877-1881. (16) Philip, C. V.; Anthony, R. G. Fuel 1982, 61, 357-363. (17) Thomas, K. M.; Mulligan, M. J. Fuel 1986, 65,694-703. (18) Yau, W. W.; Kirkland, J. J.; Bly, D. D. Modern Size-Exclusion Liquid Chromatography; Wiley: New York, 1979; pp 222-224. (19) Holstein, W. A.; Hellmann, G.; Severin, D. ErdoelKohle, Erdgas Petrochem. 1984. 37, 175. (20) Coleman, W. M.; Wooton, D. L.; Dorn, M. C.; Taylor, L. T. J . Chromatogr. 1976, 123,419-428. (2 1) Richards, D. G.; Snape, C. E.; Bartle, K. D.; Gibson, C.; Mulligan, M. J.; Taylor, N. Fuel 1983, 62, 724-731. (22) Brown, R. S.; Hausler, D. W.; Taylor, L. T. Anal. Chem. 1980, 52, 1511-1515. (23) Brown, R. S.;Hausler. D. W.; Taylor, L. T.; Carter, R. C. Anal. Chem. 1981, 53, 197-201. (24) Nosyrev, I.E.; Kuzaev, A. I.; Kochkanyan, R. 0.; Baranov. S. N. Khim. Tverd. Top/. (Leningrad) 1981, 15,69-75. (25) Baltisberger, R. J.; Jones, M. B.; Schwan, J. F. Prep.-Am. Chem. Soc., Div. FuelChem. 1985, 30,257. (26) Bakisberger, R. J.; Jones, M. 8.;Schwan, J. F. Fuel Process, Technol. 1985. 11, 213. (27) Bly. D. D.; Stoklosa, H. J.; Kirkland, J. J.; Yau, W. W. Anal. Chem. 1975, 47, 1810-1813. (28) Schulz, W. W. J . Liq. Chromatogr. 1980, 3 ,941-952. (29) Mori, S.; Suzuki, T. Anal. Chem. 1980, 52, 1625-1629. (30) Mori, S. J . Appl. Polym. Sci. 1977, 21, 1921-1926. (31) Tchir. W. J.; Rudin. A,; Fyfe, C. A. J . Polym. Sci. Po/ym. Phys. 1982, 20, 1443-1451. (32) Baike, S. T. Quantitative Column Liquid Chromatography; Elsevier: Amsterdam, 1984: pp 170-172. (33) Yau, W. W.; Kirkland, J. J.; Bly, D. D. Modern Size-Exclusion Liquid Chromatography; Wiley: New York, 1979; pp 108-113. (34) Schwager, I.; Lee, W. C.; Yen, T. F. Anal. Chem. 1977, 49, 2363-2365.
RECEIVED for review February 27, 1986. Accepted May 13, 1986. This work was supported by the science and ~ ~ neering Research Council through a grant and a studentship (M.M.) and by the National Coal Board.
Determination of Traces of Titanium down to Sub-Part-per-Million Levels in Molybdenum Metal and Compounds by Ion Exchange Chromatography/Spectrophotometry F. W. E. Strelow National Chemical Research Laboratory, CSIR, P.O. Box 395, Pretoria 0001, Republic of South Africa
Traces and up to 10 mg of titanium can be determined in large samples of molybdenum or molybdenum compounds (up to 10 g) by cation exchange chromatography with columns containing only 2 or 4 g of AG50W-X4 resin followed by determination uslng spectrophotometry of the Tlron complex in the presence of compiexing agents. Separations are very satisfactory, leaving only between 5 and 30 pg of moiybdenum in the titanium fraction when 10 g of MOO, is present originally. Recoveries of titanium from synthetic mixtures are quantitative and 5 pg of titanium (0.5 ppm) in 10 g of MOO, can be determined with a standard deviation of 0.2 pg (0.02 ppm). Relevant elution curves and results for the analysis of synthetic mixtures and some actual samples are presented and the influences of nitric and phosphoric acid concentrations on the separation are discussed. I n addition a method is presented to avold excessive catalytic decomposition of H202,which can interfere with the column separation.
Table I. Results for the Analysis of Synthetic Mixturesa found taken
g of
@gof
MOO,
Ti
MOO,
10.00
5.0 50.0 1000 10000
C
10.00 10.00
10.00
pg o f Ti (corrected for blanksIb
5.0 f 0.2
49.9 f 0.2 1001 f 2 9997 3 8
pg of Mo in Ti fractions
11-28 8-31 5-24
7-21
.Results of 5-f analyses w i t h calculated standard deviations. Results f o r triplicate b l a n k r u n s were typically 0.4 f 0.1 pg of Ti. N o t determined.
For the determination of trace amounts of titanium in molybdenum metal or its compounds, it is advantageous, and for very small traces even necessary, to separate the titanium
0003-2700/86/0358-2408$01.50/0 0 1986 American Chemical Society
~
i
ANALYTICAL CHEMISTRY, VOL. 58, NO. 12, OCTOBER 1986
from the matrix elements when accurate results are required. Bandi et al. (I)and Shakashiro e t al. (2) have described anion exchange procedures using oxalic-citric-hydrochloric acid mixtures with peroxide and oxalic-sulfuric acid mixtures, respectively. Since, in the procedures described, molybdenum is more strongly retained by the anion exchange resins than titanium, this approach is not very suitable for separating traces of titanium from gram amounts of molybdenum because inconveniently large columns would be required. Donaldson (3)has developed a solvent extraction procedure t o separate titanium from high-purity molybdenum before spectrophotometric determination. Between 5 and lo00 ppm of titanium could be determined. Smaller concentrations caused problems because of sample size (up t o 0.5 g) and reagent blank (approximately 4 pg) ratios. In addition, the method is rather time-consuming, especially when a larger number of samples have to be analyzed. When traces of titanium have to be separated from gram amounts of molybdenum, one would like to retain the titanium on the column and let the matrix element pass through, preferably without adsorption. Alimarin e t al. ( 4 ) , using a weakly acid Russian resin, and Fritz e t al. ( 5 ) , using the strongly acid Dowex W-X8 resin, have shown that titanium can be retained from dilute sulfuric acid of p H 1 or 0.01 M, respectively, containing hydrogen peroxide, while molybdenum passes through the column. The author, using the AG50W-X8 strongly acid cation exchanger, has separated titanium from 17 other elements (6). Mo(VI), V(V), and Nb(V) were eluted with 0.25 M sulfuric acid containing 1%hydrogen peroxide while titanium was retained and could be eluted selectively with 0.50 M sulfuric acid containing 1%hydrogen peroxide leaving many other elements on the column. The higher acid concentration was employed t o avoid the hydrolysis of titanium when larger amounts were present (up to about 500 mg). From the above Spano et al. (7) have developed a method for the determination of seven trace elements in molybdenum metal and molybdenum trioxide by combining an ion exchange separation method with a determination by X-ray fluorescence spectrography. The authors say in their discussion that their method was unsuitable for the determination of titanium because this element was only partially separated from molybdenum in the ion exchange part of the procedure. An investigation of relevant distribution coefficients (8) and ion exchange column behavior reveals that the reasons for titanium partially passing through the column are 2-fold. Firstly, the concentration of nitric acid in the solution from which absorption took place apparently was considerably too high, even when one assumes that a part of the 12 mL concentrated nitric acid added originally is used u p in the oxidation of the molybdenum metal. Secondly, the presence of appreciable amounts of phosphoric acid also tends t o mobilize titanium(IV) on the column, as will be shown. This paper describes a modified sample dissolution and ion exchange separation procedure that results in a complete separation and recovery of titanium from as much as 10 g of molybdenum. At the low concentration of strong acid, the low cross-linked AG50W-X4 cation exchange resin has been used instead of the high cross-linked DoweX-X12, because t h e low cross-linked resin provides much better exchange kinetics for the retained element and allows fast elution even at relatively low acid concentration (2 M nitric acid). The separation is combined with a spectrophotometric procedure measuring the yellow color of the titanium(1V)-Tiron complex a t 420 nm ( 9 ) .
EXPERIMENTAL SECTION Reagents. Reagents were of A.R. grade purity, and distilled water was further purified by passing through an Elgastat deionizer. A standard solution containing 500 ppm of titanium
2409
was prepared by dissolving 3.697 g of titanyl potassium oxalate of A.R. reagent quality in 300 mL of 0.05 M oxalic acid and removing the potassium by cation exchange. The oxalate in the eluate was then destroyed by wet digestion with sulfuric and nitric acids and the solution finally made up to 1000 mL volume containing about 0.5 M nitric acid, 0.05 M sulfuric acid, and 0.3% hydrogen peroxide. Solutions containing lower concentrations of titanium were prepared by dilution when required. Titanium-free solution of molybdenum for quantitative separations of synthetic mixtures were obtained by dissolving the required amount of molybdenum trioxide in 5 mL of 2 M nitric acid and about 60 mL of 3070 hydrogen peroxide as described later. After removal of the remaining traces of undissolved material by filtration and heating on the water bath to remove excess hydrogen peroxide, the cold solutions were passed through columns containing 12 mL of AG50W-X8 cation exchange resin [ i o mm length, 15 mm diameter]. This removed not only the last traces of titanium but also other elements such as iron, which catalyze the decomposition of hydrogen peroxide. The resin was the AG50W-X4 low-cross-linked sulfonated polystyrene cation exchanger of 100-200 mesh particle size marketed by Bio-Rad Laboratories of Richmond, CA. An AG50W-X8resin was used for prior removal of titanium and other impurities from molybdenum solutions used for separations of synthetic mixtures. Columns and Apparatus. Borosilicate glass tubes of 12.5 mm i.d. and 150 mm length, fitted with a no. 1 porosity glass sinter plate and a buret tap at the bottom were used as columns (2 g of resin). At the upper part the tubes were joined to a wider part of 20 mm i.d. and 120 mm length, fitted with a B19 female ground-glass joint at the top to receive a separatory funnel as reservoir. In some cases (4-g resin columns) glass tubes of 15 mm i.d. and 300 mm length, fitted with a B14 female ground-glass joint at the top were used. These columns had no wider part at the top. The columns were filled with a slurry of AG50W-X4 resin until the settled resin reached a 8.7-mL mark (2 g of resin) or a 17.3-mL mark (4 g of resin) in the wider columns. The resulting resin columns were 73 and 100 mm long, respectively (in H,O). The columns were purified by passing through about 100 mL of 5 M nitric acid, followed by 30 mL of deionized water. Spectrophotometric measurements (titanium) were carried out with a Zeiss PMQ I1 UV-visible spectrophotometer. A VarianTechtron AA-5 instrument was used for atomic absorption measurements (molybdenum). Elution Curves. a. Mo(ZV) and Ti(ZV) in Absence of Phosphoric Acid. A 5.0-g portion of Moos was dissolved in a high-walled 250-mL covered beaker by heating with 10 mL of 1 M nitric acid and 30 mL of 30% hydrogen peroxide on the water bath and swirling the contents repeatedly. Further 10-mL amounts of hydrogen peroxide were added a t intervals when necessary to replace decomposed reagent. A small amount of insoluble residue was separated by filtration and excess hydrogen peroxide removed from the filtrate by further heating on the water bath until the color of the solution started turning reddish brown. Then about 20 mL of the titanium standard solution (10 mg of titanium) was added and the solution (about 100 mL in volume) cooled to about 5 O C in an ice bath and passed through a column containing 2 g of AG50W-X4 resin as described above. The column had been equilibrated by passing through about 20 mL of cold (-5 "C) 0.01 M nitric acid containing 0.1570 hydrogen peroxide. The solution was washed onto the resin with a few small portions of 0.01 M nitric acid containing 0.15% hydrogen peroxide and the column rotated a few times in a vertical position between two flat hands to loosen and remove any gas bubbles that had formed a t the top of the resin. Then molybdenum was eluted with 0.01 M nitric acid containing 0.15% hydrogen peroxide using a flow rate of 3.0 f 0.5 mL/min. Though elution of molybdenum was complete after 60 mL (
