Determination of Thorium in Low-Grade Ores Using a Cation Exchange Separation-EDTA Titration Procedure F. W. E. STRELOW National Chemical Research laboratory, South African Council for Scientific and Industrial Research, Pretoria, South Africa
b The selective cation exchange separation of thorium on A G 5OW-X12 resin i s combined with a sensitive EDTA titration method using Xylenol Orange as indicator to determine thorium in low-grade ores. The end point is sharp when a back-titration with thorium solution is used. Small amounts of fluoride influence both the sharpness of the end point and the titration volume. Procedures for the determination of thorium in carbonatites and in complex tantaloniobates are described. Interference from traces of ferric iron which blocks the indicator can be suppressed b y reduction to the ferrous state with hydroxylamine hydrochloride.
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HE accurate analytical determination of thorium in lon--grade ores is very often a complicated procedure comprised of a number of separation steps before the final determination can be attempted. It was therefore felt that the very selective, cation exchange separation of thoiiuiii (9), combincd n ith a sensitive and accurate ED'T.1 titration, should offer an accurate and dq)endable method for the detcwiiination of this elenicnt in complel low-grade ores. This method qerniz to offer in many caws advantages o w r othrr mcthod.. Phbil and Iiotos ( 8 ) developed a complesometric microdetermination of thorium using 0.00131 E D T A as titration agent and Xylenol Orange as indicator. This seemed to be the most promising method for the titration, but other indicators were investigated to select the most favorable one for the EDTA titration of thorium at the lowest possiblc concciitration. EXPERIMENTAL
Selection of Indicator. The sharpness of t h e visual end point of several indicators n-hich are advocated in the literature for the E D T h titration of thorium was tested. The indicators, 1-(2-pyridylaz0)-2-naphthol (PAS) (14). 1648
I ,8-dihydrouy-2-(p-sulfophcnT1ANALYTICAL CHEMISTRY
azo) - naphthalene - 3,3-disulfonic acid (SPADXS) ( I ) , Alizarin Red S ( 3 ) , azoarsenic acid ( 4 ) , Eriochrome Cyanine R C ( I S ) , dinitrosochromotropic acid ( 8 ) ) Fe(I1)-cacotheline ( 7 ) , Catechol Violet ( I I ) , and Xylenol Orange (5, 8 ) were investigated. Most of these gave usable end points when a 100-ml. solution containing 20 mg. of Tho2 was titrated with 0.0131EDTA. K h e n a 100-ml. solution containing 2 mg. of T h o 2 was titrated n.ith O.OOlJ1 ZDTA, only Xylenol Orange gave a sufficiently sharp color change at the end point to warrant further investigation, Xylcnol Orange also needs less rigorous p H control than most of the other indicators. The p H of the solution may be between 1.5 and 3.5. Thus, one initial adjustment to a p H of 2.5 i 0.3 is satisfactory in most cases. Reagents and Apparatus. Reagents of analytical reagent quality were used whenever possible. The AG 5OWX12 and iiG 5OW-X8 resins were supplied by the Bio-Rad Laboratories, Berkeley, Calif., and the E D T A
Table I.
Direct Titration of Standard Solutions Ratio >I1 111 Th All. Tli Solution EDTA Solution AI1 EDTA 0.751 X 0991 X 10-3ni lO-3W Solution 5.00 3 68 1 359 10 00 7 42 1 347 20 00 15 00 1 333 30 00 22 61 1 327 40.00 30 45 1.320
Table II.
Back-Titration of Standard Solutions Ratio 111. 311. T h 111. T h EDTA Solution Solution 0.991 X 311. EDTA 0.751 X 10-3~i lo-3.W Solution 5 30 -1 00 1 325 8 00 1 322 10 58 21 11 16 00 1 319 34 2 6 26 00 1 318 12 21 3.2 00 1 320
by Merck, Darmstadt. €3. Siegfried, Zofingen, supplied the Xylenol Orange. The ion exchange columns have been described (IO). Titration of Standard Solutions. A standard solution containing 198.3 wg. of Thon per ml. was prepared by dilution from a solution containing 19.83 mg. of T h o z per ml. The concentrated solution was standardized by the A gravimetric oxalate procedure. known amount of the dilute solution was added to about 50 mi. of distilled water, about 1 to 10 ammonium hydroxide or 1 to 10 hydrochloric acid was used to control the pH, and 6 drops of a n aqueous 0.1% Xylenol Orange solution were added as an indicator. The solution was then titrated a-ith an EDTA solution, about 0.001J1,until the indicator turned from rcd to straw yellon-. The results of the titratioiis are givcn in Table I. For these direct titrations, the end points wcre not very sharp. The slight - but distinct decrease in thp value ml. T h solution>with inof the ratio ml. EDTA solution creasing titration volume as shown in Table I suggests a small systematic error in the procedure. This error is probably due to a small undertitration caused by an indistinct end point. A back-titration procedure mas tried next. The experimental details w r e similar; but the thorium solution n a s titrated with 0.0OlM EDTA until the indicator turned yellon-. Another 2 to 3 ml. of EDTA solution was added. and the solution then back-titrated with 0.001Xthorium solution until the first orange-red color appeared. This was taken as the end point. 4 further 2 drops of 0.00131 thorium solution turned the indicator distinctly red. The results of tlirie titration$ are giwn in Table 11. With the back-titration method, the color changc (yellow to red) was much sharper than that for the direct titration. Furthprmorc, the titration ratio was reasonably constant, as is shov-n in Table 11. A back-titration proctdure employing about 0 001.11 EDTh and 0.001 1I thorium solution w m c ~ t lto be
the most suitable method for the volumetric determination of small Table 111. Influence of Fluoride amounts of thorium. Further experiM1. EDTA M1. Th Solution pg. NaF ments showed that, with photometric 0.991 x 10-aM 0.751 X 10-3M Added End Point end point detection and reducing the 20 * 00 26.41 zt 0.03 Nil Sharp total volume of the solution to 10 ml., 20.00 26.50 0 . 1 25 Less sharp 50 Indistinct 20.00 -26.8 zt 0 . 2 as little as 20 to 40 pg. of thorium can be 20.00 -27.2 i 0 . 5 100 More indistinct determined with an error of ~ 1 % . 20.00 -28.0 i 1 . 0 200 Very indistinct For these low amounts, about 0.0001M solutions of EDTA and thorium and 2 drops of the 0.17, indicator solutions Table IV. Results of Analyses of Thorium Ores were employed. The visual end point Sample Thoz Found % Thoz by Other Methods was satisfactory too, but gave less f 0.002 0.59, 0.59b Carbonatite An 0.582 accurate titration results for small 0.225 f 0.002 0 . 2 2 , 0.22 Carbonatite Ba titration volumes. 0.407 f 0.001 0.42, 0.43 Carbonatite C a 1.057 =t0.002 Phalaborwa l a Influence of Fluoride. Two methPhalaborwa 20 2.293 f 0.004 ods can be employed to remove the 30 0.003 f 0.001 Phalaborwa thorium from the resin column after Phalaborwa 4~ 0.004 f 0.001 the other elements have been eluted. 0.0527 + 0.0004d Euxenitec 0 0524 f 0.0003 0.707 i 0.0003 Yttrotantalitee 0 706 f 0.002 Either thorium can be eluted with a 0.348 i 0.0007 Samarskitee 0 348 i 0.001 suitable eluting agent such as 6LV 0.426 f 0.0007 Daviditee 0.426 0.001 sulfuric acid, or the resin can be ashed All analyses done in triplicate. and the residue dissolved in nitric acid 2-g. samples used for analysis. containing a trace of fluoride. The Determined by ion exchange x-ray fluorescence method (12). simpler ignition procedure was pre10-g. samples used for analysis. ferred. A trace of fluoride must be Determined by ion exchange separation with gravimetric finish using 10-g. samples added to the nitric acid to assist in the e 10-g. samples used for analysis and aliquot representing 2 g. taken for titration. dissolution of the ignited thorium oxide residue. Since thorium forms an insoluble fluoride, the influence of separated by filtration and the filter of oven-dry AG 50W-Xl2 cation exfluoride on the titration had to be invespaper was ashed. About 0.5 t o 1 change resin (200 to 400 mesh), tigated. Even 50 pg. of fluoride in the hydrogen form, a t a flow rate ml. of concentrated sulfuric acid and influenced both the sharpness of the of 0.8 to 1.0 ml. per minute. The 2 t o 5 ml. of hydrofluoric acid were end point and the titration volume. preparation of the columns has been added and the sample was evaporated With 25 pg. the interference was less described (10). Zirconium, rare earths, almost to dryness. I n many cases, serious but still observable. Some this treatment effected complete disand other cations were eluted with 500 results of titrations of standard solutions solution of the sample. If any inml. of 4N hydrochloric acid a t a flow with added amounts of fluoride are rate of 0.8 to 1.0 ml. per minute. The soluble material remained, it was shown in Table 111. Because of this separated by filtration, and, after the resin was removed from the column filter paper had been ashed, the residue interference, it is necessary to use only quantitatively with distilled water, was fused with a small amount of potasseparated from the water by filtration, a minimum amount of fluoride for the sium bisulfate. The fusion product was and ashed a t 600" to 800" C. in a platidissolution of the ignited thorium dissolved in the combined filtrates, from num crucible. The residue was disresidue, and again remove most of it which most of the hydrochloric acid had solved by boiling with 2 to 5 ml. of by evaporation. About 10 to 20 pg. been removed by boiling. The solution concentrated nitric acid containing of sodium fluoride per milliliter of was diluted to contain not more than about 50 pg, of sodium fluoride. The nitric acid used was satisfactory when 0.5N free hydrochloric acid. sample was evaporated to dryness on a the thorium oxide had not been heated The tantaloniobates were decomposed hot plate to remove most of the fluoride, higher than about 800" C. by prolonged heating with hydrofluoric taken up in 5 ml. of 1 to 5 nitric acid, The apparent decrease in the titration acid in a platinum dish on the water and transferred into a suitable titration bath. A hydrofluoric-hydrochloric acid vessel. in milliequivalents caused by the mixture was used in the case of daviIn the case of the tantaloniobates presence of fluoride is considerably dites. When decomposition had been where the zirconium had already been higher than can be explained by the effected, the samples were evaporated separated by a fluoride precipitation of formation of insoluble thorium tetra1 to 5 to dryness and taken up in about the thorium, a column of 10 grams of fluoride. This seems to indicbte that hydrofluoric acid. The uranium was oven-dry AG 50W-X8, 100- to 200thorium fluoride compounds with less reduced to the quadrivalent state by mesh resin in the hydrogen form was than four fluoride atoms per thorium the addition of solid stannous chloride used. Five hundred milliliters of 3N atom are present in the solution. to act as a n additional carrier for the hydrochloric acid were employed as thorium, and the sample was kept eluent and the flow rate was increased overnight in a dark place. The into 2.5 f 0.5 ml. per minute. DETERMINATION OF THORIUM IN LOW-GRADE soluble fluorides containing the thorium Titration of Thorium. The volume ORES were separated by filtration and disof the solution was adjusted to about As a result of the described work, a solved by digestion with nitric and 30 ml. and the p H brought t o 2.5 -f perchloric acids. Finally, the solution method for the determination of thorium 0.5 with 1 t o 10 ammonium hydroxide was taken to fumes of perchloric acid, was developed and applied to the using a narrow-range p H paper as and, after cooling, diluted with distilled indicator. Four to 6 drops of a 0.1% analysis of three carbonatites, four water to contain about 0.5N free peraqueous solution of Xylenol Orange other thorium-containing ores from chloric acid. were added, followed by a measured Phalaborwa (Transvaal), and four thoamount of 0.0005 or 0.001M EDTA rium-containing tantaloniobates from Ion Exchange Separation of which had previously been standardized Thorium. T h e thorium solution conRhodesia and Swaziland. against a standard thorium solution. taining about 0.5N free hydrochloric The solution was back-titrated with Dissolution of Samples. The carboor perchloric acid was passed through natites were dissolved in hydrochloric standard 0.0005 or 0.001M thorium a column of about 21-cm. length and acid. Any insoluble material was 1.15-cm. diameter containing 10 grams solution until the first permanent
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VOL. 33, NO. 12, NOVEMBER 1961
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appearance of orange red. As a check, the addition of a further 2 drops of thorium solution turned the color of the indicator distinctly red. The results of the analyses are presented in Table IV. DISCUSSION
Table IV indicates that the described method combines a good sensitivity with a high rcproducibility and accuracy. I t is more time-consuming than the ion exchange x-ray-fluorescence method (12) but more accurate and flexible. All foreign cations are eliminated by the ion exchange chromatographic separation, so that a pur? thorium solution is available for the final titration. This ensures accurate and dependable results and makes the method very suitable for reference and standard analyses. For highest accuracy, the total amount of thorium should be about 2 to 5 mg., when about 0.0005M solution is used for the titration. Khen much smaller amounts of thorium are determined and more dilute solutions are used for titration, the interference from ferric iron becomes serious. Traces of iron can be picked u p from all kinds of sources. Small amounts of iron may even be enclosed in the resin during the production step. This iron can be inaccessible for elution because i t is not bound by ion exchange adsorption. Since ferric iron forms a very stable violet compound with Xylenol Orange and a very stable EDTA complex, i t is partly titrated by E D T A and can partly block the indicator. Tests revealed that as little as 20 pg. of ferric iron will obscure the end point
when 2 drops of O.lcoXylenol Orange solution are used as indicator, nhile 100 pg. have a detrimental effect when 6 to 8 drops of indicator solution are used. The interference of small amounts of iron can be suppressed as follolvs : After the ignition residue has been dissolved, the nitric acid is removed by evaporation with 10 to 15 drops of sulfuric acid. The sample is taken up in dilute hydrochloric acid, and about 100 mg. of hydroxylamine hydrochloride are added to the cold solution. The solution is left standing for about 10 minutes, diluted with water, and the titration conducted without delay. A platinum crucible which has been used for silicate analysis for some time can easily be the source of one or even several hundred micrograms of iron. Other elements t h a t form very stable complexes with Xylenol Orange are bismuth and zirconium, but these are less common elements than iron, and it is fairly improbable that they will be picked up after the ion exchange separation in sufficient amounts to interfere with the titration. The described method can be applied to the determination of larger amounts of thoiium as well. About 0.01M or 0.0561 standard solutions of E D T A and thorium are used for the titration, but, when more than 50 mg. of T h o ? are present, the direct weighing of the ignition product from the resin column with the application of a correction for the resin ash is the shortest method and at least as accurate as the volumetric procedure (10). While this paper u as being prepared for publication, a two-cycle anion CY-
change separation of thorium using the nitrate and ascorbinate complexes was published by Korkisch and Tera (6). The method was adapted to the separation and determination of thorium in the microgram range. The method described in this paper seem6 to be simpler, and, for larger amounts, more accurate than that by Korkisch and Tera, but it is probably less sensitive. LITERATURE CITED
(1) Brtnerjee, G., 2. anal. Chem. 148, 349 (1955) (2) . . Datta, S. K., Anal. Chirn. S c t a 16, 115 (1957). ' (3) Flits, J. J., Ford, J. J., ANAL. CHEM. 25, 1640 (1953). ~. (4) Fritz:, J. S., Oliver, It. T., Pietrzyk, D. J., Ibid., 30, 1111 (1958). (5) Kort11. J., PEibil., R.,, Chemist Analvst 45,10:2 (1956). (6) Korlcisch. J.. Tera. F.. ANAL.CHEM. 33,12164 ( i Q 6 i j . ( 7 ) NarE.tyana Rao, V., Gopala Rao, G., 2. anal. Chem. 155,; I34 (1957). (8) . , Pfibil. R.. Koros. E.. Maoval- Ke'm. Folydirhl 64, 55 (1958). (9) Smit, J. van R., Peisach, If., Strelow, I
\ - - - - I
,
I
I
II
F. W. E., 2nd International Conference of Peaceful Uses of Atomic Energy,
Paper 1119, 1958. (10) Strelow, F. IT. E., h s a ~ .CHEW 31, 1201 (1959). (11) Suk, V., Maltit, If., Ryba, O., Collection Czechoslov. Chem. Communs.
19, 679 (1954). (12) Niekerk, J. N. van, Strelow, F. W.
E., Wybenga, F. T., J . -4ppl. Spectroscopy, in press. (13) Willard, H. H., hfosen, A. W., Gardner, R. D., ANAL.CHEM.30, 1614 ( 1958). (14) Wilkins, D. H., Anal. Chim. Scta 19,440 (1958).
RECEIVED for review April 14, 1961. Accepted Julp 11, 1961.
Separation of Iridium from Rhodium by Extraction with Tributyl Phosphate RAYMOND B. WILSON and WILLIAM D. JACOBS Department of Chemistry, University of Georgia, Athens, Ga.
b A procedure based upon the extraction of Ir(1V) from hydrochloric acid solutions into tributyl phosphate has been studied as a method for separating Ir(1V) from Rh(lll) on a microgram scale. Ir(lV) is extracted into the organic phase, while Rh(lll) remains in the aqueous phase. The rhodium content in the aqueous phase is determined spectrophotometrically with N,N' bis(3 dimethylaminopropy1)dithio-oxamide in the range 60 to 200 pg. and with p-nitrosodimethylaniline in the range 1.5 to 25 pg. of rhodium. N o attempts were made
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ANALYTICAL CHEMISTRY
to determine Ir(lV) in the nonaqueous phase, since the procedure is designed specifically for the separation and determination of rhodium.
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HE interference of iridium in the determination of trace quantities of rhodium has been a problem for many years. Milligram quantities of rhodium and iridium can be separated by both cation (6) and anion exchange (3). Microgram quantities are reportedly separated by precipitation (8), paper chromatographic methods (6), and
anion exchange ( 7 ) . The precipitation method requires 10 hours of ashing in a muffle furnace. The drawback in the paper chromatographic method is the necessity of obtaining the sample in a very small volume, which is applied to the paper strip. In the anion exchange method reported by Marks and Beamish large quantities of indium relative t o rhodium were not mentioned. Solvent extraction has not been widely used as a means of resolving mixtures of the platinum metals. Distribution coefficients determined by Berg and Senn ( 1 ) indicate that rhodium and
