6th Annual Summer Symposium-Less Familiar Elements
Analytical Chemistry of Uranium CLELMENTJ. RODDEN C'. S. Atomic Energy Commission, New Brccnswick, N . J .
Advances in the analytical chemistry of uranium from 1949 to the present are reviewed. In addition to papers on gravimetric, volumetric, colorimetric, fluorimetric, and radiochemical methods, x-ray absorption and coulometric procedures have been described for the first time.
T
H E present interest in uranium as a fissionable material has evolved considerable investigation of the determination of this element. The interest is such that the range runs the entire gamut from amounts as small as 10-l1 gram per gram to practically 100% pure uranium. The methods used up to 1949 have been reviewed (60, 61). The progress since this time is discussed here. I n addition to papers on gravimetric, volumetric, colorimetric, fluorimetric, and radiochemical methods, x-ray absorption and coulometric procedures have been described for the first time. Methods have been reviewed by Hecht ( S 7 ) , and methods for use in mineral products have been published by Burstall and Ryan ( 8 ) , Burstall and Williams ( I O ) , and Rabbitts (67). SEPARATIONS
Practically all recent advances in methods of separation have been in the use of organic solvents. Some of the work done during the war on the effects of salting agents on the ether extraction of uranyl nitrate ( 3 0 ) as well as the extraction of uranous cupferrate with ether (28) has been published by the Atomic Energy Commission. Scott (66)has studied the effects of salting agents on the extraction of uranyl nitrate with ether. A solution 1 M in ferric nitrate and 3 N nitric acid has been found most suitable for his work. Kaufman and Galvanek (42)have studied the extraction of uranyl nitrate with ether to show how elements which interfere in the alkaline-peroxide colorimetric method can be removed. They devised a simple piece of apparatus in which the sample is roasted, leached, and extracted. The greatest advance in solvent-extraction work has come from England, where studies on the use of paper strip and packed cellulose columns have resulted in time-saving procedures. Initially paper strips were used (1, 7 , 4 7 ) , whereby uranyl nitrate is dissolved and moves with the solvent front as diffusion in the absorbent paper proceeds. Most other metals remain stationary or move only slowly in comparison with the uranium. The uranium in the paper section could then be measured with the polarograph, colorimeter, or fluorimeter. Work on somewhat similar lines with different solvents has been reported by Lacourt, Sommereyns, and Soet6 ( 4 6 ) In an excellent series of articles (9, 44,54) 63, 71, 7 5 ) an extensive study of variables employing the cellulose column was reported. I t was found that activated alumina retained arsenic and molybdenum] which were extracted to some extent when cellulose rolumns alone were used. Ferric nitrate may be used to complex phosphate so it will not interfere. The procedure is simple (63). After the walls of the vessel have been coated with a silicone (DOW DC-200), 5 or 6 grams of cellulose pulp (Whatman) are mixed with ether containing 5 % nitric acid and poured into the glass tube. After agitation and gentle pressing down to form a column, 15 grams of activated alumina (200 mesh) are added on the top of the cellulose. The sample is dissolved in nitric acid
and evaporated to dryness, then taken up in 4 ml. of 25% nitric acid. The solution is absorbed with cellulose, which is then transferred to the top of the column. ilfter extraction with about 200 ml. of ether containing 5% nitric acid, the uranium is entirely in the ether solution and can be determined in any manner desired. Results obtained are comparable with other accepted methods. The method may also be used in the presence of thorium ( 7 6 ) . In this case after phosphate has been complexed with ferric nitrate, the uranium is extracted with ether containing 1% nitric acid, and then the thorium nitrate is extracted with ether containing 15% nitric acid. The method has been applied (46) to the separation of microgram quantities of uranium. As little as 5 X 10-7 of uranium has been separated. I n another application of solvent extraction, tantalum and niobium (50)are separated from uranium in an alloy by extracting the fluorides of the earth acids with methyl ethyl ketone; the aqueous solution contains the uranium. The effectiveness of different solvents for extracting uranyl nitrate was shown by Spence and Streeton (7Z),who also describe a micro-form rotary extractor. GRAVIMETRIC METHODS
Gravimetric methods are occasionally used for the determination of uranium. Duval (62) has used his thermal balance for studying uranium compounds. Isatin p-oxime (38) and 8quinolinol (14) have been re-examined as precipitating agents. If an oxine wash solution instead of water is used, correct results are obtained. Ammonia has been used for the analysis of cobalturanium alloys (49). VOLUMETRIC METHODS
The main advances in volumetric methods have been in the choice of methods of reduction. Cooke, Hazel, and Mch'abb (17) have used a lead reductor in hydrochloric acid solution to reduce uranium(V1) to uranium(1V). I n this fashion the air-oxidation step is avoided. High concentrations of sulfate can be tolerated, provided a sufficient concentration of hydrochloric acid is present. Sill and Peterson (68) also used a lead reductor for milligram amounts of uranium. The size of the lead particles and temperature of the reductor were shown to be important. Furman, Bricker, and Dilts (29) used a cadmium amalgam reductor to reduce uranium(V1) to uranium(1V) in a sulfuric acid medium, For solutions containing phosphates Schreyer and Baes ( 6 4 ) have used a Jones reductor with 4.5 M sulfuric acid to avoid phosphate precipitate. The reduction of uranium(V1) to uranium(1V) was shown to be quantitative in phosphate solutions. Rowe (62) showed that arsenate ion does not interfere in the procedure using the Jones reductor. Chromous salts have been used to reduce uranyl solutions. Hahn and Kelley ( 5 5 ) reduced uranyl solutions with chromous
1598
V O L U M E 2 5 , NO. 11, N O V E M B E R 1 9 5 3 sulfate followed by a potentiometric titration of uranium(1V) with ceric sulfate. The method can be used in the presence of iron by complexing the iron( 11) formed with 1,lO-phenanthroline. Cooke, Hazel, and McKabb (18) used chromous chloride for reduction of milligram quantities of uranium(V1) to uranium( IV) destroying the excess chromous ion by air oxidation. This is followed by the usual ceric sulfate titration. Sill and Peterson (68) have made a study of the usual volumetric procedure ( 6 0 ) using the Jones reductor. The effect of small amounts of impurities, time, temperature, and acids on the air oxidation of uranium(1V) as well as the effect of phosphoric acid was investigated. Procedures are given for milligram quantities of uranium. Bricker and Sweetser (6) used a cadmium amalgam reductor, followed by a spectrophotometric method for determining the end point !Then titrating with ceric sulfate.
1599 Gerhold and Hecht (31) have used potassium thiocyanate in strong hydrochloric acid. With 30 ml. of 3 N potassium thiocyanate and 15 ml. of hydrochloric acid in 100 ml. reproducible results are obtained. The solutions appear to be stable for a sufficient time. Interfering elements are removed by an ether extraction of the nitrate.
COLORIMETRIC 3lETHODS
Rasin-Streden (58)has given a photometric method using hydrogen peroxide and sodium carbonate which has been used for from 0.2 up to 85% U;Os. Scott ( 6 6 ) showed that sodium bicarbonate caused errors in the carbonate-peroxide method. Kaufman and Galvanek ( 4 2 ) studied the effects of variables employing an ether extraction of uranyl nitrate followed by the alkaline peroxide method. Brackenbury ( 6 ) has given a plant control procedure using alkaline peroxide.
'1
El 20
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I
,350
,300
Y 0
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By measuring the uranium alkaline peroxide solution against a known amount of uranium treated in the same manner in the blank cell, i t is possible to extend the range normally employed and also increase the precision of the analysis. Figure 1 shows such a curve. Using a wave length of 390 mp and IO-cm. cells it is also possible to extend the peroxide method down to 100 y . Figure 2 shows the range when 10-cm. cells are used. Davenport and Thompson ( 2 1 ) have used ammonium thioglycolate in a solution of p H 7 to 11 in the range of 0.1 to 1.6 mg. per 25 ml. Beer's law is obeyed. The interfering elements are described. I n general, an ether separation prior to the analysis would remove interfering elements.
Crouthamel and Johnson ( 1 6 ) have employed the thiocyanate reaction in an acetone medium. The stability of the color is improved, the effect of acid concentratimp i p minimized, and many elements which interfere in aqueous mcdium do not interfere in the acetone medium. -4convenient range is 1 mg. of uranium per 25 ml. at 375 mp -4differential application is also described. The precision is increased and standard deviation not greater than 320.25% was obtained. The thiocyanate should be recrystallized from methanol and air-dried before storing (15). Silverman (69) has used a chloroform extraction of uianium 8-quinolate and is able to measure 0.05 mg. of uranium even in the preEence of iron. COULOMETRIC
;\IETHODS
Two articles have recently appeared (12, 29) on the coulornrtric determination of uranium One by Carson ( 1 2 ) is used on a routine basis a t the General Electric Co., Richland, and ako a t the Argonne Sational Laboratory. I n this procedure the uranium(V1) is reduced to uranium(1V) using a lead reductor. Electrolytically generated bromine in the presence of iron is used to oxidize the uranium( IS-) to uranium(V1). Iron bromide (0.02 M iron) in 3 M hydrobromic acid is used to rinse out the uranium from the reductor. Lead phosphate and sulfate are soluble in this medium. The reduced solution is heated to 95 "C. and the electrolysis started using the derivative polarographic indicator system (59). With care, as little as 2 micrograms of uranium can be determined. The method lends itself to the automatic titration procedure of Carson (11 ). The other method described by Furman, Bricker, and Dilts (29) uses a cadmium amalgam reductor to reduce uranium(V1) to uranium(1V). The uranium( IV) is oxidized by ferric sulfate,
ANALYTICAL CHEMISTRY
1600 and ceric ions are generated electrolytically to oxidize the ferrous ion produced. The analysis is run a t room temperature, and with the cadmium amalgam reductor it is not necessary to wash with iron bromide as above. The procedure is applicable to amounts of uranium as low as 5 y. POLAROGRAPHIC R.I ETHODS
The polarographic method is used to a considerable extent, usually hydroxylamine hydrochloride and sodium tartrate or ammonium citrate being employed. Phosphoric acid interference may be removed by fuming with sulfuric acid ( 4 8 ) or by using ferric ion (4). X-RAY iMETHODS
A determination of uranium using x-ray absorption may be of interest. Bartlett ( 2 ) showed that elements of low atomic weight did not interfere, and that with cells 19 mm. long the lower limit, of detection was 0.1 gram per liter. Chemical separation was necessary where appreciable quantities of other elements were present. Peed and Dunn (55) have done preliminary work on using the absorption method for 150 to 30,000 p.p.m. of uranium in water. Another interesting application of x-rays is Birks and Brooks' (3) determination of 0.05 gram per liter of uranium by x-ray fluorescence. Heavy elements such as lead interfere but light elements have no effect. Solids are used by evaporation of liquids into spherical cups. About 10 minutes for each determination is all that is necessary. SPECTROGRAPHIC METHODS
Kaufman found that the spectrographic determination of uranium in ores was not satisfactory ( 4 3 ) ; the sensitivity was only 0.05% UaOs and it was subject to many interferences. Chemical separation was usually necessary, after which a colorimetric or fluorimetric analysis is more convenient. Strasheim ( 7 3 ) has ( a )used direct evaporation of the sample in a direct current arc, ( b ) extracted uranium and then following as in ( a ) , and ( c ) used an enhancing agent in the direct current arc. An accuracy of 11% is claimed by an extraction procedure recommended where calcium is high. RADIOCHEMICAL METHODS
The determination of uranium in ores by a counting method is used chiefly as a means of obtaining the range. Generally this is followed by a colorimetric or fluorimetric analysis. Hardy ( 3 6 ) has described a rapid method using a scintillation detector and ratemeter counter. By simultaneous measurement of /3 and y activities it is possible to determine uranium and thorium in ores (23). Curie and Faraggi (19) have determined uranium and thorium in minerals by irradiating with thermal neutrons. A nuclear emulsion is used to observe the fission tracks; a-particle autoradiography permits determination of thorium by difference. The method has been used on autunite, uranite, andeuxenitewith a precision of about 6%. For determining uranium in rocks with as little as 0.0003% uranium, Smales (70) has used neutron irradiation in the Harwell pile. After irradiation, the barium fission product is separated and counted. The results show good agreement with chemical methods of analysis. The method is suggested where only small amounts are available. The chemical manipulation appears to be more than in the usual fluorescence analysis. The method has been extended (67) for use in determining U*36in naturally occurring uranium isotopes. FLUORIMETRIC METHODS
The fluorimetric method is by far the most sensitive for the determination of uranium and is used by a large number of work-
ers. Generally the uranium sample is fused with sodium fluoride or a mixture of sodium fluoride and sodium-potassium carbonate, and the resultant disk is meamred in a fluorimeter. The excellent work of Price, Ferretti, and Schwartz (.56) which was done during the war has finally been published. This is a thorough investigation of the fluorimetric procedure and describes a fluorimeter which is so sensitive that quenching impuritiep can be decreased by dilution and which still has sufficient sensitivity for the uranium. The technique described has been used by a large number of workers. Unfortunately, many workers have found that contamination is a very serious problem with the dilution technique, and a solvent purification had to be employed prior to the determination. The question of fluorimeters is a rather complicated one. It almost appears that every woi ker designs his own fluorimeter. Numerous fluorimeters are used ( I S , 20, 64, 26, 39, 41),practically all of which employ a source of ultraviolet light with a filter. This light strikes the sample, and the fluorescent light given off after passing through a filter to remove ultraviolet light then impinges on a photocell, generally of the photomultiplier type. Fletcher, May, and Anderson ( 2 7 ) have employed a trarihmission fluorimeter whereby the ultraviolet light passes through the fused disk. Several studies have been made of the variables encountered in fluorescence analysis (66, 39, 40, 56, 74). Jacobs (39) has shown that with a 5% sodium fluoride-95% sodium-potassium carbonate fusion mixture variations in fluorescence were noted when the concentrations of calcium, magnesium, tellurium, antimony, vanadium, beryllium, aluminum, and molybdenum increased. His conclusions were that the dilution techniques of Price, Ferretti, and Schwartz ( 5 6 ) ~houldbe used with raution. The quenching action of iron on uranium fluorescence (74) was shown to be serious and would substantiate the above conclusions. Other variables have been studied by Fletcher ( W ) , who showed that when a 9% sodium fluoride-91 % sodium-potassium carbonate fusion mixture was used the temperature of the fusion R a' important. High temperatures tended to dissolve platinum, which gave low results. She recommended a furnace with controlled temperatures a t 650°C. Kaufman ( 4 0 ) has found that furnace fusions are dependent on the atmosphere in the furnace and the temperature. In general, it appears the Fletcher burner icl more satisfactory. Jacobs ( 5 9 ) obtained satisfactory results a i t h both a furnace and an open burner. Because of the higher sensitivity and lower blank that can be obtained with pure sodium fluoridr, this flux was preferred b y Kaufman (40)to the so-called miwd flux. However, pure sodium fluoride does present a problem bwaube of the high temperatures necessary for fusion (1050" C . ) . Platinum dishes must be used, but a t these temperatures there is some tendency for platinum to be attacked by the flux under oxidi7ing conditions, and the platinum oxide forms a brown film on the surface of the melt and decreases uranium fluorescence. By proper contrQ1 of the fusion conditions brown melts can be prevented. Such control is possible in well-equipped laboratories but is somewhat difficult in the mining areas where laboratory facilities frequently are rudimentary. Therefore, for field use, the ethyl acetate extraction procedure given below was adapted to be used in conjunction with the mixed flux and gold dishes The following procedure has been teclted and proved by Kaufman (40). To a 40-ml. screw-cap vial add 1 nil. of a sample containing between 5 and 50 y of u308, 15 ml. of nitrate solution (750 grams of magnesium nitrate monohydratr, 50 ml. of concentrated nitric acid, and 100 ml. of saturated calcium nitrate solution per liter), and 10 ml. of ethyl acetate; shake for 1 minute. Place pellets of sodium fluoride (approximately 375 mg.) in the platinum dishes. Allow the ethyl acetate and water layers to separate and pipet out 0.1-ml. aliquots of ethyl acetate to absorb on the sodium fluoride pellets.
V O L U M E 25, NO. 11, N O V E M B E R 1 9 5 3
1601
Evapoiate the ethyl acetate under infrared heaters. Transfer the platinum dishes to a modified Fletcher gas-sir burner and heat the dishes for 3 minutes. Allow melts to cool on burner, transfer to fluorimeter, measure the fluorescence, and compare with standards.
Hahn, R. B., and Kelley. M. T., Atomic Energy Commission, AECD-3311, CF-51-12-183 (December 1951). Hardy, H. R., Dept. of Mines and Technical Surveys, Mines Branch, Ottawa, Canada, NP-4232,TR-103-52 (October 1952). Hecht, F., Milcrochemie ver. Mikrochim. Acta, 36-37, 1083
The original sample can be in nitric or dilute sulfuric acid. If phosphate is present, it is complesed by the addition of ferric nitrate. If the sample contains so little uranium that 5 y cannot be obtained in 1 ml., the sample is concentrated by evaporation. If the sample is too concentrated. it is diluted with 5% nitric acid before the 1-ml. aliquot is taken. The fluorescence method has been used for uranium in sea water (61-63), low-grade ores (39, 78, 79), phosphoric acid (77), sulfuric acid ( 7 6 ) , and shales, lignites, and monazites (34). h field method has been describrd ( 3 3 ) in which shales, high-silica rocks, and phosphate rorks are fused directly with a standard flux and the fluorescence is measured directly.
Hovorka, V., and Hoozbecher, Z., Collection Crechslou. Chem.
(1951).
Communs., 14,40-58 (1949).
Jacobs, S., Chem. Research Laboratory, Teddington, England, CRL-AE-54 (April 1950). Kaufman, D., A4merican Cyanamid Co., private communication. Kaufman, D., Cawtillo, M.,and Koskela, U., -4tomic Energy Commission, MITG-A-70 (July 1950). Kaufman, D., and Galvanek, P., Jr., Atomic Energy Commission, AECD-3137, MITG-A67 (July 1950). Kaufman, D., and Perkins, C. W., Jr., Atomic Energy Commission, AECD-2834 (August 1948). Kember, N. F., Analyst, 77,78 (1952). Kennedy, R. H., Atomic Energy Commission, AECD-3187, MITG-A84 (June 1950). Lacourt, A., Sommereyns, Gh., and SoetB, J., Milzrochemie ver.
LITERATURE CITED
Arden, T. V., Burstall, I?. H., and Linstead, R. P., J. Chem. Soc. (London), Suppl. 2,311-3 (1949). Bartlett, T.W., AN.
