Ion Exchange Separation a n d Volumetric Determination of Gallium in Plutonium-Gallium Alloys S I R : For a number of years a solvent extraction-colorimetric procedure utilizing the gallium 8-quinolinol complex, has been used for determining gallium in plutonium-gallium alloys. Frequently, this procedure failed. These failures could be attributed to difficulties inherent in the analytical use of a volatile solvent in a glovebox and to the use of the notoriously unspecific 8-quinolinol as the color-forming reagent. A new analytical procedure has been developed to overcome these problems. Plutonium and gallium are separated by ion exchange rather than by solvent extraction and gallium is determined volumetrically rather than colorimetrically. EXPERIMENTAL
Reagents. A 0.0231 thorium solution was prepared from thorium nitrate and standardized against a standard 0 . 1 X E D T A solution. T h e indicator used was a 0.0270 xylenol orange solution (6OyO water, 40y0 ethanol) screened with a 0.02% aqueous solution of Poirrier's blue (National .hiline Division, Allied Chemical Corp.). Buffer solution was prepared by dissolving 14 grams of NaAc in 1 liter of water containing 114 ml. of glacial HXc. The HC1-ascorbic acid solution was prepared by dissolving 1 gram of ascorbic acid in 100 ml. of SLY HC1. This last solution is unstable, so it must be prepared fresh daily. The ion exchange resin used was Dowex 1 X 4, 50- to 100-mesh, chloride form. Prior to initial use, it was washed several times with ethanol, then water, and finally with 8'37 HC1 until the washings obtained were clear. I t was then washed with 0.5N HC1 and stored moist until needed. Apparatus. Ion exchange columns of conventional design were used. T h e y were approximately 27 cm. long. T h e top third, enlarged to serve as a reservoir, had a n inside diameter of approximately 3.4 cm. The bottom two thirds had an inside diameter of approximately 1.2 cm. A coarse porosity, sintered glass disk supported the resin. To permit more rapid flow rates, these disks were made more porous by carefully passing dilute H F t'hrough them. The columns were filled with resin to a depth of approsimately 7 em. The ion exchange columns were supported in a rack fabricated entirely from ",'*-inch plaat,ic sheet. This all-plastic construction was necessary because of the corrosion of conventional metal equipment in gloveboxes in which HCl is used. Procedures. REGULAR.Weigh approsimately 1-gram alloy sample aliquants to 0.1 mg. and place in 25-ml.
volumetric flasks. Dissolve t h e metal in approximately 5 ml. of the HCIascorbic acid solution. Add this acid solution to the volumetric flask in 0.5-ml. increments to prevent the sample solution from boiling over. Keep the dissolution reaction going, applying heat if necessary. If the reaction stops before the metal is completely dissolved, subsequent dissolution is difficult. Load the sample on a resin column that has just been conditioned by washing with 10 ml. of the HC1-ascorbic acid solution. Elute the plutonium, using approximately 20 ml. of the HC1-ascorbic acid solution, a t the rate of 4 ml. per minute. The plutonium is colored, so completion of elution can be determined visually. After removing the plutonium, elute the gallium from the column into a 250-ml. Erlenmeyer flask with 50 ml. of 0.5M HC1. Add 3 or 4 drops of a 0.1% solution of bromphenol blue to the galliumcontaining effluent, then add 6M KaOH until the solution just begins to turn blue. This will give a p H of 3.5. Add 15 ml. of buffer, then 10 ml. of the standard E D T A solution. The volume of E D T A must be accurately measured. This is conveniently done with an automatic pipet. iifter adding the EDTA, boil the solution for 2 minutes. Boiling hastens the reaction between E D T A and gallium: if it is eliminated, the results will be low. Add 1 ml. of xylenol orange and 2 or 3 drops of Poirrer's blue. Titrate the solution with the standardized thorium solution. I t is not necessary to cool the solution before titrating. Determine combined column and reagent blanks daily by eluting 50 ml. of 0.5M HCI through the column after previously conditioning it with the HClascorbic acid solution. Carry the 0.5M HCI effluent through the rest of the procedure as outlined above. The blank titration will require approximately 17 ml. of the thorium solution. SULFIDE PRECIPITATION. For samples known to contain more than 250 p.p.m. copper, the above procedure is modified to remove copper as follows. To the column effluent containing the gallium, add 1 ml. of a FeC13 solution containing 4 mg. of iron per ml. and 3 ml. of a sulfosalicylic acid solution containing 0.2 gram of the acid per ml. Iron serves as a carrier for impurities and sulfosalicylic acid prevents gallium from hydrolyzing when the solution is made basic. Add 6M NaOH until the color changes from deep purple to yellow-orange. The p H should be about 12. Then add 2 ml. of Na2S solution containing 30 mg. of NazS per ml. and 5 drops of the flocculant solution prepared by dissolving 0.1 gram of Separan, a Dow Chemical Co. flocculant, in 100 ml. of the wash solution described below. Stir, using a magnetic stirring bar, for 1 minute and
then allow the precipitate to settle for 1 minute. Filter into a 250-ml. suction flask using a medium porosity sintered glass filter funnel. Wash the precipit'ate with a small amount of wash solution made by adding 12 to L6 drops of 6-11 S a O H to 1 liter of water. Adjust the p H of the filtrat,e in the suction flask with 81M HCl until the solution changes from green to pink. The p H will be about 3.5. Add 14 ml. of buffer solution and 5 ml. of st:ondard EDTA$ solution and continue a8 in the regular procedure outlined above. The sample turns cloudy when the E:DTd is added, but' clears when the solution is boiled. DISCUSSION A N D RESULTS
Ion Exchange Separation of Gallium. T h e separation of gallium and plutonium by ion exchange is based on the difference in the types of complex ions formed by the two elements. I n hydrochloric acid or chloride salt solutions, plutonium, as P u ( I I I ) , does not form anionic Cornplexes (3) while gallium does (6). The reverse situation exists in nitric acid or nitrate salt solutions ( 9 ) . The separation of thie two elements was investigated initially in nitric acid solutions. This system was selected because the ion exchange behavior of plutonium in nitric acid has been investigated extensively a,t Rocky Flats, at Hanford (9) and more recently a t Los Alamos ( 7 ) . Preliminary results, obtained using the conventional technique of passing the sample through a column of resin a t room temperature, indicated a n unacceptably early breakthrough of plutonium and a n incomplete recovery of gallium. Investigations of the hydrochloric acid system indicated no comparable problems and a successful quantitative separation procedure was developed. It consists of dissolving a weighed alloy sample in hydrocliloric acid and eluting the sample solution through a n anion exchange resin column. After completely removing the plutonium, the eluate is changed and the gallium is quantitatively removed and determined. This separation procedure is comparable to one that has been developed in England ( I ) . Volumetric Determination of Gallium. A number of methods are available for the determination of gallium - colorimetric, fluorimetric, polarographic, gravimetric, and volumetric. They are summarized in a n article by Milner (8). Of these, a volumetric method based on a n EiDTA titration (2, I O ) appeared best suited to the needs of this work. VOL. 37, NO. 8, JULY 1965
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Table 1. Accuracy and Precision of Ion Exchange-Volumetric Procedure Synthetic samples, 10.00 mg. of Ga
Gallium recovered Std. Mean Pro- No. of Mean. dev. error. cedure samples mg. mg. % Regular 10 10.02 1 0 . 0 3 0.2 Sulfide ppt’n, 18 9.99 1 0 . 0 5 -0.1 Table II.
Precision of Ion ExchangeVolumetric Procedure
Std. dev. calcd. on basis of Sample Prosource cedure Regular Synthetic Laboratory Laboratory Sulfide Synthetic ppt’n. Laboratory
70Ga in alloy
No. of samples 10 21b gC 19 36d
Std. dev.,
7,”
f O iz0 f O f O iz0
03 02 01 05 03
Pooled std. dev. on laboratory samples. Duplicate analyses on 21 samples. c Duplicate or triplicate analyses on 6 samples. d Duplicate or triplicate analyses on 36 samples. a
b
The reaction between gallium and EDTA is relatively slow, so a n EDTAback titration is required. The xylenol orange-thorium system gave the best visual color change a t the end point. Later it was found that by adding Poirrier’s blue as a screening agent the color change a t the end point could be made even sharper. The titration was carried out in a weak acid solution (pH 3 . 5 ) , primarily to prevent hydrolysis of gallium. But in addition, the more acid the solution, the less interference there is in the E D T A titration from cationic impurities. Interferences. A number of metals present as impurities in plutoniumgallium alloys can interfere in the volumetric gallium determination. These include aluminum, nickel, chromium, lead, copper, and iron. Aluminum, nickel, and chromium, if prese n t as chromium(III), are not absorbed by the resin from t h e 5 N HC1 solutions used in separating gallium from the plutonium ( 5 ) . They would be eluted, therefore, along with the plutonium. Lead is only slightly absorbed so unless it is present in concentrations higher than is normally experienced, it does not interfere. The ion exchange absorption behavior of copper is comparable to that of lead. But, because copper has been present at times in relatively high concentrations, the level at which interference would begin was determined. Copper spikes of increasing concentrations were added to aliquants of a standard gallium solution and these samples run through the
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ANALYTICAL CHEMISTRY
resin column. The effluents were titrated and the bias introduced by the copper was calculated. Interference began at 250 p.p.m. of copper-Le., 250 pg. of copper per gram of plutonium. Therefore, if copper is known or suspected of being present in quantities in excess of 250 p.p.m., it will have to be separated. The sulfide precipitation method discussed in the procedure was developed to effect this separation. Iron is the only remaining purity of any consequence which would interfere in the EDTA titration of gallium. It can be removed in the ion exchange step, along with plutonium, if it is present as Fe(I1) because Fe(I1) is not retained by a n anion exchange resin if the HCl concentration is less than 6-44 ( 4 ) . Iron can be kept in the ferrous state by adding a reductant (ascorbic acid) to the acid used to dissolve the alloy sample and by conditioning the resin column with a n HC1-ascorbic acid solution just prior to the addition of the sample. The efficiency of this method for removing iron was demonstrated by spiking standard gallium solution with increasing concentrations of iron, adding HC1-ascorbic acid solution, and following the regular procedure given above. There was no interference from iron in concentrations as high as that equivalent to 6000 pg. of iron per gram of plutonium. Because this represents more iron than is experienced in usual alloy samples, no higher concentrations were studied. PRECISION A N D ACCURACY
The accuracy of the ion exchangevolumetric procedure for determining gallium was evaluated using synthetic samples. The precision was evaluated using both synthetic and laboratory samples. I n addition, results obtained by this new procedure on laboratory samples were compared with results obtained on these same laboratory samples by the previously used solvent extraction-colorimetric procedure. Synthetic samples were prepared by dissolving 1-gram aliquants of pure plutoriium metal in hydrochloric acid, then adding aliquants of solutions containing 10 mg. of gallium and the desired impurities (4 mg. of iron, 2 mg. of nickel, 1 mg. each of lead, copper, and aluminum). These samples were then analyzed by either the regular or the sulfide precipitation procedure. The results, shown in Table I, indicate acceptable precision and accuracy. The precision of the regular and sulfide precipitation procedures obtained on both synthetic and laboratory samples are reported in Table 11. The results on the laboratory samples seem somewhat more precise (although not significantly so, statistically, a t the
95y0 confidence level) than results on synthetic samples. This is caused, perhaps, by the relatively high total concentration of impurities in the synthetic samples. Although this made the synthetic samples more impure than the laboratory samples analyzed concurrently, it did test the ion exchangevolumetric procedure under difficult impurity conditions. For a period of time, laboratory samples were analyzed by both the solvent extraction-colorimetric procedure and either the regular or the sulfide precipitation ion exchangevolumetric procedure. On 21 samples using the regular ion exchange-volumetric procedure, the mean difference between the results obtained by the solvent extraction-colorimetric procedure and the ion exchange-volumetric procedure was zero. Likewise, a zero mean difference was obtained on 33 samples analyzed by the sulfide precipitation ion exchange-volumetric procedure and the solvent extraction-colorimetric procedure. These data show that the new ion exchange-volumetric procedure gives results that are in good agreement with results obtained by the solvent extraction-colorimetric procedure. I n addition, the new method has acceptable precision and accuracy and it does not suffer from the frequent failures associated with the solvent extractioncolorimetric procedure. LITERATURE CITED
(1) Donaldson, J. M., CKAEA, AWRE,
Aldermaston, private communication to J. T. Byrne, The Dow Chemical Co., 1959. ( 2 ) J. T. Baker Chemical Co., Phillipsburg, X . J., “The EDTA Titration,” p. 28 (1957). 13) Katz. J. J.. Seabore. G. T.. “Chemistry of the Actinide Elements:” p. 264, Wiley, Sew York, 1957. (4) Kraus, K. A., Moore, G. E., J . Am. Chem. SOC.75, 1460 (1953). ( 5 ) Kraus, K. A., Kelson, F., Proe. Intern. C m f . Peaceful Uses of Atomzc Energy, 7, 113 (1955). (6) Kraus, K. A,, Selson, F., Smith, G. W., J . Phys. Chem. 58, 11 (1954). ( 7 ) Kressin, I. K., Waterbury, G. R., ANAL.CHEM.34, 1598 (1962). (8) Milner, G. W. C., Analyst 81, 632
J
~
( 1 9.56). \ - - - - ,
(9) Ryan, J. L., Wheelwright, E. J., Ind. Eng. Chem. 51, 601$1959). (10) Welcher, F. J., The Analytical Uses of Ethylenediamine tetraacetic Acid,” p. 176, Van Nostrand, Princeton, 1958.
The Dow Chemical Co. Rocky Flats Division P. 0. Box 888 Golden, Colo.
F. J. MINER R . P. DEGRAZIO
INFORMATION contained in this paper was developed diiring the course of work done under Contract AT(29-1)-1106 for the C . S. Atomic Energy Commission.
