Reduction by Lithium Amalgam SIR: The principal method now in use for the separation of europium from fission products takes advantage of the fact that it is the only rare earth element which can be reduced to the divalent state by powdered zinc or zinc amalgam in dilute acid solution (1). Divalent europium shows many properties of the alkaline earth family; therefore, it remains in solution while the other rare earths are precipitated as hydroxides. The principal shortcoming of this method is that the separation is not clean. Generally a series of at least three precipitations each must be made, alternately with europium oxidized and then reduced in the presence of cerous carrier, in order to achieve satisfactory decontamination. I n addition a prior rare earth-fluoride precipitation is necessary to remove most nonrare earth activities. The selective reduction of europium can be made to proceed further. If an alkali metal amalgam is used instead of zinc amalgam, europium is reduced to the free element and enters the mercury phase. McCoy ( 2 ) first demonstrated this extraction by shaking a solution of europium in alkaline potassium citrate with potassium amalgam. The extraction was quite specific for europium. Similar results could also be obtained by direct electrolysis using a mercury cathode and potassium citrate as supporting electrolyte, but this is more timeconsuming. Recently Onstott (4, 5 ) reexamined the reduction, and employed lithium citrate in place of potassium citrate and lithium amalgam in place of potassium amalgam. Potassium sometimes shows the undesirable property of forming insoluble double citrates which lithium does not. Rfarsh (3) and others employed a very dilute acid medium without any complexing agents, but because extraction from this type of solution is not as specific for europium, these conditions were not considered in the present work. Perhaps the reason for the great selectivity in the extraction of europium as compared with other rare earths lies in its ease of forming an intermediate divalent state on reduction. While the electrode potentials for reduction of all trivalent rare earths to the free element are below that of the Li+-Lio couple, citrate complexing could increase those potentials to such an extent as to prevent their reduction by lithium. The 1574
ANALYTICAL CHEMISTRY
reduction of divalent europium, on the other hand, might proceed at a lower potential since complexing by citrate is probably less in this case (assuming that divalent europium acts in a similar manner to alkaline earths). Differences in free energy of amalgamation may also be a factor contributing to more favorable reduction of europium (4). The formation of europium amalgam seems to proceed in alkaline lithium citrate solution at a potential just below that required for lithium reduction (6). Previous studies of the formation of europium amalgam by electrolysis into a mercury cathode or extraction into an amalgam have been aimed a t the purification of europium or its removal from rare earth mixtures. In attempting to apply the extraction to the radiochemical determination of europium in fission products, interference by strontium and cesium must be considered, because these are extracted also. Fortunately, these elements are easily removed prior to extraction by a series of hydroxide precipitations. Of the remaining activities in aged fission product mixtures, only europium is extractable, and because of the high density and insolubility of mercury and the absence of adsorptive effects, the separation is clean. Therefore, in addition to the hydroxide precipitation step, only a single extraction is necessary in most cases to obtain pure europium. EXPERIMENTAL
Apparatus. Lithium amalgam was prepared using the electrolytic cell shown in Figure 1. The cell was a 100-ml. capacity cylindrical separatory funnel, the upper platinum anode and lower platinum lead to the cathode being attached with the aid of graded glass seals. The electrolyte was 1N lithium citrate; lithium hydroxide can also be used. A satisfactory ratio of electrolyte to mercury was 3 : 1 by volume. An applied e.m.f. of 6 volts \\-as required to generate the lithium amalgam. As explained below, sufficient lithium amalgam for a number of determinations can be prepared at one time. Procedure. Pipet 1 ml. of europium carrier (20 mg. Eu/ml.), 1 ml. of strontium carrier (10 mg. Sr/ml.) and a n aliquot of sample into a 40-ml. centrifuge tube. Dilute the solution to about 20 ml., heat, and precipitate europium hydroxide by adding concentrated ammonium hydroxide. Cen-
trifuge and discard the supernatant to waste. Dissolve the precipitate in 1 ml. of concentrated hydrochloric acid, add 1 ml. of strontium carrier and dilute t o 20 ml. Repeat the hydroxide precipitation using 12M sodium hydroxide as precipitant. Dissolve the second precipitate in hydrochloric acid and reprecipitate with 12M sodium hydroxide as before, but omit the addition of strontium carrier. Dissolve the third hydroxide precipitate in the minimum quantity of hydrochloric or citric acid solution, and transfer the solution to a 125-ml. separatory funnel, using 20 ml. of 1M lithium citrate to aid in the transfer. Adjust the p H of the solution to 8-10 using saturated lithium hydroxide and testing externally with p Hydrion paper. Extract europium from the solution by gently shaking with 3-ml. of lithium amalgam for 3 minutes. Drain the mercury phase from extraction into a clean separatory funnel and wash once by shaking momentarily with 10 ml. of dilute (-0.1M) lithium citrate solution. Drain the mercury phase into a 10-ml. beaker and strip the europium from it by stirring with three 2-ml. portions of 1M nitric acid. Transfer all portions to a 40-ml. centrifuge tube and dilute to 20 ml. with water. Precipitate europium hydroxide with ammonium hydroxide after adding a few drops of saturated bromine water and heating to ensure oxidation of europium to the trivalent state. Centrifuge and discard the supernatant. Dissolve the precipitate in 1 ml. of 8M nitric acid, dilute to 20 ml. with water and heat on a boiling water bath. Add 5 ml. of saturated oxalic acid and digest the resulting precipitate for 15 minutes. Cool, filter the precipitate on a weighed Whatman No. 42 filter circle, dry by vacuum desiccation, and weigh to determine the chemical yield. Mount on a n aluminum plate and count the gamma activity of europium-155 with the counter threshold set for minimum energy cut-off of about 30 k.e.v. When the sample contains more than 1 mg. of plutonium, it is recommended that this element be removed prior to extraction, because of the possible hazard involved in handling this amount in a separatory funnel. This can be accomplished as follows: Dissolve the first hydroxide precipitate, mentioned above, in 1 ml. of concentrated nitric acid, and dilute to 20 ml. with water. Add a few drops of 1M sodium dichromate, heat the solution on a boiling water bath for 10 minutes, then cool. Precipitate europium fluoride by adding 1 ml. of concentrated hydrofluoric acid. Allow to digest at room temperature for
IM LITHIUM CITRATE
.MERCURY
-lw Figure 1. Electrolysis cell used for preparation of lithium amalgam
5 minutes, centrifuge, and discard the supernatant. Wash the precipitate once with 1M nitric acid-lM hydrofluoric acid. Dissolve the precipitate in 1 ml. of concentrated hydrochloric acid and 5 drops of saturated boric acid, add 1 ml. of strontium carrier and perform the second and third hydroxide precipitations as described above. Dissolve the final precipitate and adjust the solution for extraction as described above. RESULTS
Interfering Ions. I n preliminary experiments europium was found to be extracted with high yield from a solution in 1M lithium citrate when the initial p H was 8 to 10. Nitrate ion interfered, perhaps because of preferential reduction by lithium amalgam. This ion is removed from the sample during the hydroxide precipitation steps used to remove strontium and cesium. Ammonium ion also interferes seriously, therefore the last two hydroxide precipitations prior to extraction should be made using 12M sodium hydroxide as precipitant. Effect of Electrolysis Time. Figure 2 shows the results of an experiment made to measure the rate of formation of lithium amalgam. As electrolysis progressed, 3-ml. portions of the mercury phase were withdrawn from the cell. Lithium was stripped from each portion with a n excess of 1 M nitric acid and the excess acid back-titrated with 1 M sodium hydroxide. Assuming that lithium was the only basic substance stripped from the mercury, then the number of milliequivalents in 3 ml. found with time of electrolysis are as shown in the lower curve of the figure. The initial rate was about 0.05 meq. per minute, and this decreased only slightly with time. Another series of similar runs was made, but the lithium in 3-ml. portions
of the mercury phase was used to extract europium from a solution containing 20 mg. of that element in 1M lithium citrate. This amount of europium is generally used as a carrier in its determination. The yields of europium extracted, as found by stripping into 1M nitric acid and precipitating as europium oxalate, are shown in the upper curve of Figure 2. After about 20 minutes electrolysis time sufficient amalgam was formed in 3 ml. of mercury phase to provide a high yield (about goyo) in the extraction of europium. Continued electrolysis merely ensured an excess of lithium in the mercury phase, which amount did not affect the yield of extraction significantly. Upon considering the results for 15 minutes electrolysis, Figure 2 shows that 0.65 meq. of lithium amalgam were produced in 3 ml. of mercury phase. This was sufficient to extract about 90% of 20 mg. or 0.36 meq. of europium. Therefore the efficiency of extraction was a t least &yo of theoretical, which is satisfactory. Stability of Lithium Amalgam. A series of europium extractions was usually made using portions of lithium amalgam taken directly from the electrolysis cell, the first extractions being carried out after electrolysis had progressed for a t least 20 minutes. Electrolysis was continued until all extractions in the series of runs were completed. Use of the freshly prepared lithium amalgam in this way eliminated any problems due to its relative instability. Recently it was found that storage stability is quite good under a layer of mineral oil, the concentration of amalgam decreasing only slightly during one month. Therefore, an alternative way of handling the amalgam is to prepare a large quantity by electrolysis and to store it in a separatory funnel under mineral oil. Extractions can be made using portions withdrawn from the funnel. Optimum Amalgam Quantity and Extraction Time. The yields of europium extracted were quite dependent on the amount of amalgam used. Thus, while 2 ml. gave a yield of about 55%, 3 ml. provided a n 80% yield; in other runs this latter varied from 70 to 90%. Greater amounts than 3 ml. were not considered necessary to increase the yield further. Extraction of europium into lithium amalgam was quite rapid. Thus, in one series of runs a half-minute shaking resulted in 85% of the europium being extracted. The yield improved slightly for longer periods of extraction, and a 3minute extraction time is recommended. Shaking should be gentle so as not to cause the mercury to "break" into fine droplets. Evidently, when the mercury breaks, reaction of lithium amalgam
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Figure 2. Amounts of lithium amalgam formed in electrolysis and yields of europium extracted, with time of electrolysis with water is speeded up and this causes low yields in the extraction of europium. Reproducibility of Analysis. Table I shows results obtained for the determination of europium-155 in a sample of plutonium reactor fuel which had aged for more than a year after removal from the reactor. The relative standard deviation for a single determination was 1%. The overall chemical yields for each run are also shown in the table. This particular sample showed a high gamma activity due to cobalt-60, arising from the fact that the original fuel was a plutonium-cerium-cobalt alloy. The cobalt-60 activity was extracted into the mercury phase, however no detectable amount was stripped into the 1X nitric acid with europium. Decontamination from Other Fission Products. Decontamination factors were measured in duplicate runs for strontium-90, americium-241, cesium-137, zirconium-95, ruthenium103, cerium-141, -144, and samarium153. The values found were all
Table 1. Gamma Activities and Yields Found for Europium-155 in Fission Product Sample Gamma-act., c.p.rn." Yield, yo 15214 62.7 15130 71.8 15165 64.5 74.9 15187 15295 53.5 50.3 15622 15188 15419
15306 15132 15351
2 =
0
15421 15286
66.3
77.1 73.4 75.8 68.0
80.8 68.3
Rel. std. dev. l . O ~ o Corrected for yield.
VOL. 37, NO. 12, NOVEMBER 1965
1575
above lo4 except for samarium, which value was 800. This latter is to be expected since samarium is the fission product rare-earth element which most cIosely resembles europium in its behavior toward reduction. The decontamination factor of BOO, along with the low efficiency of counting the gamma activity of the samarium-151 isotope present in long-aged fission products under the conditions employed, combine to minimize interference due to this isotope in the determination of europium. Decontamination factorsfor most fission products would be expected to improve when the fluoride-precipitation step is inserted into the procedure. Preliminary results indicate that d e contamination from short-lived fission products is satisfactory. Two runs were
made, using the unmodified procedure, on 1-mg. samples of uranium-235, 3 days after each had undergone 2 X 1012 fissions in a reactor. There were obtained 1791 and 1820 gamma c.p.m. of activity. This activity was made up of two components, decaying with halflives of about 19 hours and 16.7 days, respectively. These values agree closely with that expected for the short-lived europium isotopes produced in fission, 15.4-hour europium-157 and 15.4-day europium-156. The gamma spectra seemed to be free of foreign activities. Time of Analysis. With the unmodified procedure 10 determinations can be made in 5 hours. Inclusion of the fluoride-precipitation step increases the time of analysis by about 10 minutes per determination.
LITERATURE CITED
( I ) Keller, R. N., “Collected Radiochemical Procedures” Los A l a m s Scienti$c Lab. Rept. LA-1721 (2nd ed.), J. Kleinberg, ed., 1958. (2) McCoy, H. N., J . Am. Chem. SOC. 63, 1622 (1941). (3) Marsh, J. K., J . Chem. SOC. 1942, 398. (4) Onsott, E. I., J . Am. Chem. SOC.77, ~~~
2129 (1955). (5) Ibid., 78, 2070 (1956). (6) Rulfs, C. L., University of Michigan, Ann Arbor. Mich.. unmblished communication,’September i964. JOSEPH BUBERNAK MARIONS. LEW GEORGE M. MATLACK University of California Los Alamos Scientific Laboratory Los Alamos, N. M. WORKdone under the auspices of the U. S. Atomic Energy Commission.
Separation of Condensed Phosphates on Thin Layer of Starch SIR: Intensive application of the salts of phosphorus acid in the form of condensed phosphates (particularly in the food industry) justifies the effort to achieve a quick method for qualitative investigation of commercial mixtures of condensed phosphates. For this investigation paper chromatography was applied. In 1951 Ebel et al. ( 2 ) used ascending paper chromatography. Later this method was developed and modified by many investigators (3-5, 7-14,16). Recently Clesceri and Lee (1) succeeded in separating monophosphate and diphosphate by thin layer chromatography, (cellulose powder was used for the thin layer), but they could not obtain a suitable reaction by Hanes and Isherwood (6) spray reagent. Therefore, they used phosphorus-32-tagged compounds. The positions of monoand diphosphate on the chromatogram was obtained by autoradiography on film exposed 24 to 48 hours. This paper reports the separation of the mixture of condensed phosphates on a thin layer of corn starch. In addition to the usual thin layer chromatographic technique, circular thin layer chromatography was applied, and better separation was obtained. The Hanes and Isherwood spray reagent was used to locate the phosphate spots, and ultraviolet light was used to reduce the heteropoly compounds. EXPERIMENTAL
Commercial corn starch (“Servo RIihalj,” Zrenjanin, Yugoslavia) was washed three times in distilled water to which a few drops of chloroform were added, filtered through the Buchner funnel, and dried first in air and then 1576
ANALYTICAL CHEMISTRY
in an oven a t 105” C. The dried starch was milled in Waring Blendor and sieved through 150 mesh (ASTM). A 30-gram sample of starch was dispersed in 40 ml. of distilled water, and the suspension was applied to glass plates 200 x 200 mm. a t a thickness of 250 microns, according to the method of Stahl (16). This amount is sufficient for five plates. The plates for circular chromatography were prepared in the following way: 10 grams of starch were dispersed in 20 ml. of water and the suspension was sprayed a t a distance of 20 to 30 cm. onto a glass plate 250 mm. in diameter. This technique gave a uniform coating 200 to 300 microns in thickness. The plates were dried a t room temperature. A 2-pl. sample of 1% aqueous solutions of commercial mixtures of condensed phosphates was applied as spots on the plates by means of a 10-J. Hamilton microsyringe. At the same time the 1% aqueous solutions of disodiumphosphate (Merck) and sodiumdiphosphate (Merck) were chromatographed. For the circular technique the spots were applied in a circle 4 cm. in diameter. For separations the following solvent systems were used: (1) 5 grams of trichloroacetic acid, 80 ml. of isopropanol, 39 ml. of distilIed water, 1 ml. of 0.1M EDTA, and 0.3 ml. of 25% ammonia (modified acidic Gassner’s solvent) (4); (2) 30 ml. of isobutanol, 30 ml. of absolute ethanol, 38 ml. of distilled water, 1 ml. of 0.1M EDTA, and 1 ml. of 25% ammonia [basic Ebel’s solvent (S)] . Development of chromatograms by the ascending technique was carried out in the usual way. Circular chromatography was done between two lids of a desiccator, The chromatograms were developed a t room temperature. The development b y the ascending technique was stopped when the solvent
front was about 15 cm. from the bottom of the plate, and was stopped for the circular technique when the solvent front came to the edge of the plate. The time for development of a chromatogram was approximately 6 to 8 hours. Chromatograms were dried in thf air and sprayed with Hanes and Ishei wood reagent (6). After spraying, the plates were dried in a stream of warm air. It is very important for the plates to be well dried, for wet plates under ultraviolet light (used for the reduction of hydrolyzed phosphates to blue colored heteropolymolybdophosphoric acid) are stained in blue over the whole surface. After 2 to 3 minutes of exposure under ultraviolet light, blue spots of phosphate on the white background of starch appear’on the dry plate. The destruction of starch was not observed under influence of the reagents. The yellow color sometimes observed on the white background disappears after a few hours. The chromatograms are stable for several weeks. RESULTS
Ascending Technique. Figure 1 shows the separations of mono-, di-, tri-, and tetraphosphate; the phosphates with a condensation degree of more than four could not be separated. The separation of tri- and tetraphosphate (Preparation No. 3) is not enough, while with other preparations containing high condensated phosphates the tetraphosphate cannot be observed. These separations are obtained by solvent I. By solvent 11, the cyclic and chain phosphates were separated. The cyclotetra- and cyclotriphosphate on the chromatogram may be observed. Circular Technique. Good separations of mono-, di-, tri-, tetra-,
