Application of Ion Exchange Chromatography to Accurate Determilnation of Lead, Uranium, and Thorium in Tantalo-Niobates F. W. E. Strelow National Chemical Research Laboratory, Pretoria, South Africa
TANTALO-NIOBATES often contain small amounts of uranium, thorium, and radiogenic lead. Because they are resistant t o weathering, these minerals are suited for the determination of geological ages by the lead isotope method. Concentrations of the elements to be determined are in the range from 0.001 t o 0.2 %, and conventional methods of separation and determination often d o not produce results of satisfactory accuracy. All three elements can be determined by the isotope dilution method on the mass spectrometer ( I ) , but this also requires elaborate separations and places a heavy burden on the apparatus. A method consisting of a combination of ion exchange chromatographic procedures for separation, and complexometric and gravimetric procedures for the determination of the above elements in tantaloniobates therefore has been developed and applied in this laboratory. Provided large amounts of sample are available, highly reproducible and accurate results can be obtained even at low concentrations. Spectrophotometric procedures can be employed with somewhat lower reproducibility when only small amounts of sample are available. The procedure for large samples will be described in detail including a n improved method for the dissolution of tantalo-niobates which introduces only a minimum amount of nonradiogenic lead. EXPERIMENTAL
Reagents and Apparatus. Analytical grade reagents were used throughout. “Superpure” nitric, hydrochloric, hydrobromic, and hydrofluoric acids containing less than 0.01 ppm lead were obtained from Merck A.G., Darmstadt, Germany. Laboratory distilled water was purified by triple distillation in a special quartz still. All glass and Teflon ware was boiled in 1 :1 nitric acid before use. Borosilicate glass tubes of 20mm diameter fitted with a glass sinter of No. 2 porosity, a tap at the bottom, and a B19 ground-glass joint a t the top were used as exchange columns. The resins, AG50-Xl2, a sulfonated ‘polystyrene, and AG1-X8, a poly-styrene with quaternary ammonium groups, were supplied by the BIOR A D Laboratories of Richmond, Calif. Dissolution of Tantalo-niobates. The dissolution presents a problem because the amount of lead introduced with the reagents must be kept at a minimum. Potassium hydrogen sulfate (2-5), potassium hydroxide (3, 4, sodium peroxide (3, 4 ) , and sodium hydroxide-peroxide mixture (6) cannot be (1) M. G. Inghram, Ann. R e t . Nucl. Sci., 4, 81, (1954). (2) R. Dams and J. Hoste, Talanta, 11, 1605 (1964). (3) W. Fresenius and G. Jander, “Handbuch der Analytischen
Chemie,” third part, Vol. Vb, Springer Verlag, Berlin, 1957, pp. 311-14. (4) W. E. Hillebrand and G. E. F. Lundell, “Applied Inorganic Analysis’’ 2nd ed., Wiley, New York, 1953, pp. 591-5. (5) W. R. Schoeller and A. R. Powell, “The Analysis of Minerals and Ores of the Rarer Elements”, 3rd ed., Griffin and Co., London, 1955, pp. 222-4. (6) Y. Oka and M. Miyamoto, W . Electrockem. SOC.Japari, 17, 63 (1949); C.A., 48,7857 (1954). 1454
ANALYTICAL CHEMISTRY
employed as fusion reagents because purification from small traces of lead is difficult. Potassium carbonate (3, 4 ) and borax (7) can be purified more easily, but a 1O:l weight ratio of reagent to sample is required. Dissolution by heating with HF (8, 9) which is an excellent mode of attack for many other tantalum and niobium-containing minerals, is ineffective in the case of tantalo-niobates (3, 4). The tantaloniobate particles develop a coat of dark brown material, which protects against further dissolution. According to Ito, tantalo-niobates dissolve at 250” C under pressure in hydrofluoric-sulfuric acid mixtures in Teflon-lined vessels (IO). Our tests revealed that the brown material easily could be dissolved by using HCl-HF mixtures in Teflon beakers o n the waterbath. Procedure of Analysis. About 50-gram amounts of tantalo-niobate were weighed out accurately and dissolved in a 500-ml Teflon beaker by heating on the waterbath with about 400 ml of a mixture approximately 1 2 M in HF and 2 M in HCI. Normally a heating time of 2 to 3 hours is sufficient. When dissolution had been effected, the cover was removed and the solution was evaporated to a moist sludge. About 400 ml of 1.OM HF were added and soluble salts were dissolved by warming and stirring. Small amounts of insoluble material, made up from insoluble fluorides and mineral impurities, were ignored. After about 5 grams of solid tin(I1) chloride had been added, the solution was stirred and left standing for at least 24 hours. Insoluble fluorides then were separated by filtration using a polyethylene funnel and washing carefully with 0.5M HF. The precipitate was rinsed back into the original beaker with water and, after the filter paper had been ashed and added, the precipitate was dissolved by prolonged heating with 1 :1 nitric acid. Finally about 0.5 gram of boric acid was added, and, after heating for another hour and diluting with 3 times the volume of water, the remaining insoluble material was separated by filtration. The filter paper was ashed and the residue fused with a small amount of sodium peroxide (about 5 times its weight) in a small nickel crucible. The melt was dissolved in 1 :10 nitric acid and added to the main solution. Oxygen or air was bubbled through the filtrate from the fluoride precipitation for about 15 to 20 minutes to convert Sn(1I) to Sn(1V). Then traces of lead were recovered by co-precipitation with about 50 mg of Cu(I1) as the sulfide a t p H 4 i 0.5, using ammonia to raise the p H to the desired value. The sulfides were separated by filtration, dissolved in nitric acid, and the solution was added to the main solution. The combined nitric acid solution was evaporated almost to dryness and the residual salts were dissolved in 50 ml of 2 M HBr. Determination of Lead. A column of 10 grams (23 ml) of AGl-X8 resin of 100- to 200-mesh particle size in the bromide form was prepared and equilibrated with 0.1M (7) P. G. Jeffrey, Analyst, 82, 66 (1957). (8) W. Fresenius and G. Jander, “Handbuch der Analytischen
Chemie,” part 111, Quantitative Analysis, Vol. Vb, Springer Verlag, Berlin, 1957, pp. 363-5. (9) R. C. Wells,J. A m . Chem. SOC.,50, 1017 (1928). (10) J. Ito, Bull. Chem. SOC.Japari, 35, 225 (1962).
HBr. The elements in hydrobromic acid solution were passed through and washed onto the column with 0.1M HBr. Rare earths, U(VI), and Th(1V) were eluted with 250 ml of 0.10M HBr. The lead then was eluted selectively with 400 ml of 0.30M H N O B containing 0.025M HBr. A flow rate of 3.0 i- 0.2 ml per minute (linear flow rate 1.0 f 0.1 cm per minute) was maintained throughout. After the eluate had been taken to dryness, about 10 ml of HNOI was added and the solution taken to dryness again to remove bromide. The salts were dissolved in about 10 ml of water containing a few drops of nitric acid, and lead was titrated with 0.002M EDTA at pH 5.4 to 5.8 using xylenol orange as indicator and hexamethylene tetramine as buffer. After titration the complexing agent was destroyed and the lead recovered for the determination of isotope ratios. Determination of Thorium. The first eluate from the lead separation was passed through a column containing 10 grams (23 ml) of AG50-Xl2 resin of 200- to 400-mesh particle size in the hydrogen form and washed onto the column with 0.1M HCI. When the total amount of Th(1V) plus rare earths present is larger than about 500 mg, a 20-gram (46 ml) column should be used with correspondingly larger elution volumes. Remaining traces of Ta(V) and Nb(V) were eluted with 200 ml of 0.50M HCl. Then uranium, Cu(II), and traces of some other elements were eluted with 250 ml of 2.00M HCI, while Zr(IV), Hf(IV), and Th(IV) were retained by the column quantitatively, together with the largest part of the rare earths. The rare earths, Zr(IV), and Hf(IV) were eluted with 400 ml of 4.00M HCl while thorium was retained quantitatively. A flow rate of 3.0 i 0.2 ml per minute (linear flow rate 1.0 i 0.1 mi per minute) was maintained throughout. After having been removed from the column quantitatively, the resin was ashed in a platinum dish a t a low temperature (
