reaction of fluoride with boron in the 96% silica glass to form compounds that were not hydrolysed completely in the receiving flask to yield fluoride ion. It follows that boron in samples would constitute an interference in this method. The substitution of argon for oxygen as a carrier gas gave results comparable with those obtained with oxygen. Although the effect of the carrier gas on pyrolysis was not studied thoroughly, it is believed that nitrogen, argon, or air could be employed successfully. Hillebrand et al. (6) have emphasized . that some fluosilicates are not decomposed easily by acid in the procedure used for the separation of fluoride by distillation. I n such cases a preliminary fusion with sodium carbonate is necessary. By pyrolysis and fusion pyrolysis (8), fluosilicates and refractory fluorides evidently are decomposed completely without a preliminary fusion. Fluoride probably occurs in ores at times as calcium fluoride, and calcium fluoride is one of the more difficult pyrolyzable substances. To determine the recovery of fluoride from calcium fluoride, samples of fluorspar (NBS No. 79, 94.8% CaF2) were mixed with 1.5 grams of tungstic oxide accelerator and pyrolyzed. As shown in Figure 2, fluoride recovery from fluorspar was complete within 15 minutes a t 1000° C. when the fluorspar was admixed with iron ore, although recovery was only about 90% complete after 15 minutes in the absence of iron ore. Evidently, iron ore aids in accelerating the reaction. These data support the data of Tables I and 11,which indicate complete fluoride recovery for the samples investigated. The rate of fluoride recovery decreased as the temperature was decreased to 950’ C., Figure 2.
100
a
? 75 8 a
Figure 2. Pyrolytic W separation of fluoride 0 so from fluorspar and iron YA ore with tungstic oxide Iw present as accelerator
5
9 25 n
13.6mg.F
4 0
15
30
45
60
T I M E , minulei
As shown, fluoride in fluorspar can be separated in 40 minutes a t 1000° C. in the absence of iron ore; this indicates that fluorspar may be analyzed by this method. It is not possible to assess fully the accuracy of the pyrolytic method because of the limited number of certified fluoride standards. As an estimate of the accuracy, the average absolute difference between the determined and reported fluoride values was 0,022% for samples containing less than 1% fluoride, and 0.12% for samples containing more than 1% fluoride. The relative standard deviation of a single determination was 0.02270 for samples with less than 1% fluoride, and 0.052% for samples with more than 1% fluoride. The precision of the new and of the conventional method cannot be compared because the individual determinations used to obtain the reported values by the conventional methods are not available. However, the new method appears to be as accurate and precise as the conventional procedure.
LITERATURE CITED
(1)Bertolachi, R. J., Barney, J. E., ANAL.CHEM.30,202 (1958). (2) Fine, L., Wynne, E. A., Microchem. J. 3, 515 (1959). (3)Fisher Scientific Co., Pittsburgh, Pa.,
Technical Data Bull. TI)-138. October
1959. ~. . (4)Hensley, A. L., Barney, J. E., ANAL. CHEM.32, 828 (1960). (5) Hillebrand, W. F.,Lundell, G. E. F., Bright, H. A., Hoffman, J. I., “Applied Inoreanic Analvsis.” “ , Wilev. New York. 1953: (6) Knietsche, R., Be?. deut. chem. Ges. 3, 4095 (1901). ( 7 ) Powell, R. H., Menis, O., ANAL. CHEM.30, 1546 (1958). (8).Powell, R. ,H., Menis, O., “Separation of Fluoride from Refractorv Materials b Fusion-Pyrolysis,” U.”S. At. Energy 6omm., ORNL 2512, April 1958. (9) Reilley, C. N., ed., “Advances in ~
“
I
Analytical Chemistry and Instrumentation,” Vol. I, pp. 163, 185, Interscience, New York, 1960. (10) Silverman, H. P., Bowen, F. J., ANAL.CHEM.31, 1960 (1959). (11) Warf, J. C.,Cline, W. D., Tevebaugh, R. D., Zbid., 26, 342 (1954).
RECEIVED for review February 28, 19C1. Accepted May 19,1961.
Separation of Thorium by Anion Exchange JOHANN KORKISCH and FOUAD TERA Analytical Institute, Universify o f Vienna, IX. W:hringerstrasse
b Two methods are described for th’e separation of thorium from a number of elements by a two-cycle anion exchange. The first method utilizes the adsorption of the negatively charged nitrate complex of thorium on the strongly basic anion exchanger Dowex 1 -X 8 (nitrate form) from a sobtion consisting of 90% methanol and 10% 5N nitric acid. All elements except barium, lead, bismuth, lanthanum, and rare earth elements, which are also adsorbed, can be separated from thorium quantitatively. To separate thorium from these elements 1264
ANALYTICAL CHEMISTRY
38, Austria
(except bismuth) a second ion exchange method was employed, based on the fact that thorium at pH 4 forms a negatively charged ascorbinate complex which i s strongly adsorbed on Dowex resin 1-X 8 (ascorbinate form), whereas the accompanying elements pass into the effluent unadsorbed. Thoronol was used as a colorimetric reagent far the spectrophotometric determination of thorium.
P
research (2, 8, 13, 16) on ion exchange in mixed solvents has shown that thorium is efficiently reREVIOUS
tained on anion exchangers of the strongly basic type from mixtures of aliphatic alcohols and mineral acids, whereas its adsorption from pure aqueous solutions of these acids is very slight and not suitable for analytical separations (4, 6, 6, 16). Among the aliphatic alcohols methanol proved to be the most suitable solvent, together with nitric acid. This is understandable, as the distribution coefficient of thorium in such media is known to be high (7, 16). To separate those elements which remain adsorbed together with thorium, a second method was
developed, utilizing the fact that thorium a t p H 4 forms a negatively charged ascorbinate complex, which is also strongly adsorbed on the resin form), Dowex 1-X8 (ascorbinate whereas the other elements are not retained. The principle of isolation of elements such as uranium, titanium, zirconium, and vanadium as their ascorbinate complexes has been used in many analytical applications (9-12). REAGENTS
The resin used for the separation experiments and equilibrium studies was Dowex 1-XS (100- to aOO-mesh, chloride form). The nitrate form of the resin was prepared from the chloride form by washing the resin with 5N nitric acid until no chloride ions could be detected in the effluent. The resin was then soaked in a solution of 90 volume % methanol (chemically pure) and 10 volume % 5N nitric acid. For the determination of the distribution coefficients the airdried form of the same resin (nitrate form) was used. A 0.1% aqueous solution of Thoronol [Thorin, Thoron, APANS, disodium salt of l-(o-arsenophenylazo)-2-naphthol-3,6disulfonic acid] (3) was used as a reagent for the spectrophotometric determination of thorium. Standard solutions of thorium and other elements investigated were prepared by dissolving reagent grade nitrates in 5N nitric acid. The methanol-nitric acid solution mentioned above was also employed for washing the columns (wash solution). APPARATUS
The columns were of the same type and dimensions earlier described (9). The absorbance measurements were carried out in l-cm. cells a t a wave length of 545 mp using a blue-sensitive photocell in a Beckman Model B spectrophotometer. PROCEDURE
Separation of Thorium from Uranium, Iron, Titanium, Zirconium, Aluminum, Cobalt, Nickel, Copper, etc. (first column operation). Pretreatment of Resin Bed. The resin was transferred to the ion exchange column, taking care t h a t no air bubbles were introduced into the resin bed, and was treated with 30 to 50 ml. of the wash solution. Sorption Step. The sorption solution consisted of 90 ml. of methanol (pure) and 10 ml. of 5N nitric acid with a known amount of thorium and a definite quantity of the element from which thorium was to be separated. This solution was passed through the column a t a flow rate of less than 50 ml. per hour (rate of flow is not critical because of the extremely high distribution coefficient of thorium). During this operation the thorium is strongly adsorbed on the resin, whereas most of the other elements pass into the effluent unadsorbed.
Table 1. Separation of Thorium from Other Elements in "03-Methanol Medium Thorium, pg. Foreign Ion Remarks Used, Mg. Taken Found Iron imparts a deep yellow Fe(III), 15 10 9.4 color to sorption solution as Fe(III), 15 100 102 well as to effluent. No iron Fe(III), 15 250 250 was detectable in thorium Fe(III), 15 1000 1010 eluate-i.e., it is not held Fe(III), 15 2500 2500 Fe(III), 50 5000 5020 Fe( III), 50 100 101 Co not retained on resin 100 100 Co(II), 50 Ni not retained on resin Ni(II), 50 99 Cu not retained on resin Cu(II), 50 101 Ag not retained on resin 100 Ag(I), 50 Mg not retained on resin 102 Mg(II), 50 Ca not retained on resin Ca(II), 50 100 Sr not retained on resin Sr(II), 50 100 Ba strongly adsorbed on resin Ba(II), 10 98 Zn not retained on resin Zn(II), 50 100 Cd not retained on resin Cd(II), 50 102 Al not retained on resin AI(III), 50 99 Y not retained on resin Y(III), 50 100 La strongly adsorbed on resin La(III), 20 250 Ce strongl adsorbed on resin Ce(IV), 20 150 Nd strong$ adsorbed on resin Nd(III), 20 180 Pr(III), 20 Pr strongl adsorbed on resin 200 Yb(111))20 Yb strong& adsorbed on resin 190 Pb strongly adsorbed on resin Pb(II), 10 101 Ti not retained on resin Ti(IV), 1 100 Ti(IV), 10 104 Zr not retained on resin Zr(IV), 1 100 Zr(IV), 10 103 Hf not retained on resin Hf(IV), 10 102 Bi strongly adsorbed on resin. Bi(III), 10 99 Thorium measurable after addition of KI V not retained on resin 97 v(v)J Cr not retained on resin Cr(III), 50 100 Mo weakly adsorbed but can Mo(IV), 30 98 be completely removed by prolonged washing U not retained on resin 98 UO*(II), 5 100 Mn(II), 50 Mn not retained on resin Alkali metals, 50 e ach Li, Na, K, Cs not retained on 98 (Li, Na, K, Cs) resin 97 Phosphate ion does not interHzPO4-, 50 fere SO,,' 50 100 Sulfate ion does not interfere 101 C1-, 50 Chloride ion does not interfere Nitrate ion does not interfere NOa-, 50 100 Washing and Elution. After complete sorption, the resin was washed portionwise with a total of 300 ml. of the washing solution. In most cases, complete removal of the elements to be separated from thorium was achieved with less than 100 ml. of the washing solution. This demonstrates that most of the elements, in comparison with thorium, have a negligible tendency to be absorbed as nitrate complexes. The thorium was eluted by passing 100 ml. of 1N nitric acid through the column. Determination of Thorium in Eluate. The eluate was evaporated to dryness on a water bath, the residue was dissolved in a few milliliters of concentrated hydrochloric acid, and a few drops of 30% hydrogen peroxide were added to destroy organic matter. After 3 to 4 hours the solution was heated, to eliminate exceas hydrogen peroxide, and evaporated on the water bath. This operation should be repeated once more. Thereafter the destruction of organic matter is complete. Instead of using this oxidizing mixture perchloric acid could also be applied, thus
shortening the procedure to some extent. The residue was taken up portionwise in 40 ml. of 0.1N hydrochloric acid, and simultaneously transferred to a 50-ml. measuring flask. Then 5 ml. of the 0.1% aqueous Thoronol solution were added and the volume was made up to 50 ml. with 0.1N hydrochloric acid. The absorbance of this solution was measured against a reagent blank at 545 m p and converted to concentration of thorium by a calibration curve, obtained by spectrophotometric measurements of solutions with known thorium If less than 100 pg. of contents. thorium were used for the column operation, the dilution was carried out in a 10-ml. measuring flask with a proportionally decreased volume of the reagent solution. When more than 100 pg. of thorium were used, the determination was performed in an aliquot portion of the eluate. Results and Discussion. The results of a series of separation experiments are recorded in Table I. Plutonium is very likely to behave like VOL 33, NO. 9, AUGUST 1961
1265
thorium, since it is k n o w to form negatidely charged nitrate complexes in aqueous nitric acid solutions, which are strongly adsorbed on strong base anion exchange resins (1, 14). Of the coadsorbed elements, barium and lead do not interfere with the determination of thorium. I n the case of bismuth a few drops of a potassium iodide solution must be added to the 0.1N hydrochloric acid used to facilitate the dissolution of the evaporation residue. It is possible to elute thorium without removal of bismuth from the column, by employing 6 N hydrochloric acid for the elution, inasmuch as the chloride complexes of bismuth remain adsorbed on the resin. As is evident from Table I, lanthanum and the rare earth elements tested interfere strongly with the thorium determination, so that another separation method had to be developed to eliminate this interference (see below). The distribution coefficients of some of the elements used in the separation experiments are shown in Table 11. The distribution coefficient, Kd, is given by the following equation: pg. of element/gram of resin Kd = pg. of element/ml. of solution This coefficient was determined as follows: 22.5 ml. of methanol plus 2.5 ml. of 5N nitric acid containing the element in question plus 1 gram of resin (nitrate form) were transferred to a \conical flask, which was stoppered and agitated on a shaking machine for 24
Table II. Distribution Coefficients of Some Elements in HN03-Methanol Medium on Dowex 1-X 8
Element Th(1V) Fe(II1)
UOr(I1) Cu(I1) Ni(I1) Co(I1) TiUW ZriIvj
Distribution Coefficient 12 ,475 0.0 33
0.0 0.0 0.0 0.0
1.0
Table 111. Separation of Thorium in Ascorbinate Medium
Thorium, Taken Found 100 1000 100 1000
1266
100 98 970 100 1010
Foreign Ion Used, Mg. Ce(III), 1 .O Ce(III), 1 0 . 0 Ce(III), 10.0 La(III), 10.0 La(III), 10.0
ANALYllCAL CHEMISTRY
hours. The resin was filtered off and the element determined in the filtrate using a suitable analytical procedure. From Table I1 it can 'be seen that thorium has a very high distribution coefficient, whereas the other elements show only very small Kd values under identical conditions, illustrating the ease with which they can be separated from thorium. Separation of Thorium from Rare Earth Elements Lanthanum, Barium, and Lead (second column operation). After transformation of the resin bed (nitrate form) into the chloride form by thorough washing with 1N hydrochloric acid, the resin was mashed with distilled water until theeffluent showed the p H talue of distilled water. Then 50 t o 100 ml. of a solution prepared from 100 ml. of 0.1N hydrochloric acid, 800 ml. of water, and 20 grams of ascorbie acid (pure, Wiener Heilmittelwerke) buffered to p H 4 by dropwise addition of concentrated ammonia solution were passed through the resin, which was thus transformed into the ascorbinate form. The sorption solution was prepared in the following way: The residue obtained after evaporation of the eluate (first column operation), and containing thorium and one or more of the coadsorbed elements (with the exception of bismuth), was completely dissolved in 10 ml. of 0.1N hydrochloric acid and diluted with 80 ml. of distilled water. After addition of 2 grams of ascorbic acid, the solution was buffered to p H 4 by ammonia solution. Close adjustment to this p H value is imFortant, as the rare earth elements, if present in large amounts, start to hydrolyze a t p H 4.5. Bismuth hydrolyzes at p H 4 and the simultaneous separation of this element is thus not possible. The sorption solution was passed through the column at a flow rate of about 50 ml. per hour. The thorium was adsorbed on the resin as the ascorbinate complex, whereas all of the other elements except bismuth passed into the effluent. The column was then washed at the same flow rate, with 200 mi. of the buffer solution used for the pretreatment of the resin. Finally, the resin was washed portionwise with 30 to 60 ml. of distilled water and the thorium was eluted with 100 ml. of IN hydrochloric acid. The eluate was evaporated to dryness on the water bath after addition of 20 ml. of concentrated nitric acid to destroy the bulk of ascorbic acid present. 'The dry residue, which still contained some organic matter, was then transferred to a platinum dish with approximately 20 ml. of 5N nitric acid, and again evaporated to dryness. The organic matter was removed by careful ignition of the residue (wet oxidation of the organic matter with perchloric acid is in this case more time-consuming because the organic compounds left are very resista n t even to the strong oxidizing action of this acid), which was subsequently
dissolved by evaporating once with a few milliliters of concentrated nitric and hydrofluoric acids on the water bath, and four times with the same amount of concentrated nitric acid. The residue was then taken up in O.1N hydrochloric acid and the thorium determined spectrophotometrically as described above. RESULTS
The results of a series of experiments, carried out as described above, are shown in Table 111. The separation of thorium from the element in question is complete and thorium is recovered quantitatively in the eluate. The distribution coefficient of thorium as a n ascorbinate complex under the conditions applied for the separation is extremely high, with a value of 49,800. PRECISION
The following values for precision were obtained : Standard deviation of nitrate system: =t1.64.
Standard deviation of ascorbinate system: 3=1.75.
For these calculations all the experimental values obtained by taking 100 pg. of thorium (see Tables I and 111) were used, except for separation experiments during which coadsorption of other elements occurred (nitrate system). LITERATURE CITED
(1) Allison, G. M., Atomic Energy of Canada, Chalk River, Ontario, PAB-57 (1952). (2) Antal, P., Korkisch, J., Hecht, F. J.Inwg. & Nuclear Chem. 14,251 (1960). (3) Banks, C. V., Byrd, C. H., ANAL. CHEM.25,416 (1953). (4) C a n w U, D. J., J. Znorg. & N u c k a r Chem. 3,384 (1957). (5) Danon, J., Zbid., 5,237 (1958). ( 6 ) Hyde, E. K., Proceedings of Inter-
national Conference on Peaceful Uses of Atomic Energy, Geneva, 1955, Vol. 7, p. 281, United Nations, 1956. (7) Korkisch, J., Progress Report to International iltomic Energy Agency and U. S. Atomic Energy Commission under Contract 67/US (February 1961). (8) Korkisch, J., Antal, P., 2. anal. Chem.
171,22 (1959). (9) Korkisch, J., Farag, A., Mikrochim. A d a (Wien) 1958.416. (10) Ibld., p.'646. ' (11) Ibid., p. 659. (12) Korkisch, J., Farag, A., 2. anal. Chem. 166, 181 (1959). (13) Korkisch, J., Tera, F., J . Znorg. & Nuclear Chem. 15, 177 (1960). (14) Ryan, J. L., Wheelwright, E. J., Znd. Eng. Chem. 51,60 (1959). (15) Saito, N., Sekine, T., Bull. Chem. Soc. J a p a n 30 ( 5 ) ,561 (1957). (16) Tera, F., Korkisch, J., Hecht, F., J . Inora. & Nuclear Chem. 16, 345 (1961). "
RECEIVED for review December 19, 1960. Accepted April 17, 1961. Research sponsored by the International Atomic Energy Agency and the U. S. Atomic Energy Commission under Contract 67/US.
