2009
V O L U M E 2 7 , NO. 1 2 , D E C E M B E R 1 9 5 5 Hydrogen peroxide, 100 volume 11.& B. “for analysis,” diluted as required and determined daily by titration with standard 0.1N potassium permanganate. Ethyl acetate, Baker’s Analyzed. PROCEDURE
The solutions were measured out. from microburets into 50-ml. stoppered flasks. For each 5 ml. of the total aqueous solution, 6 nil. of ethyl acetate were added. The flasks were cooled to about 10” C., then hydrogen peroxide was introduced last, and the solutions were well mixed for 30 seconds. The liquid layers were allowed to separate for 2 minutes, whereupon an aliquot of the blue perchromate solution was transferred by pipetting into the spectrophotometer tube, and its absorbance was compared with that of the pure solvent. One milliliter of the same sample solution was simultaneously taken for the diphenylcarbazide determination, decomposed by potassium hydroxide, extracted with water, and further treated as in ( 2 ) . Lastly, the direct method was tested by two analyses of alloy steel from Bureau of Analyzed Samples, Ltd. The samples n-ere certified to have the percentage composition:
C Si S P Mn Ni 17 hlo 0.34 0 . 2 5 0.036 0.014 0.64 2.59 0 43 0.80 .. .. . . 1:k7 4:11
I
..
I1
..
W 5:66
Cr 0.75
4.40
The percentage of chromium found by the photometric estimation of the blue perchromic acid was: I
I1 4.24 =t0.04%
0 . 7 1 2 f 0.008%
Identical results were obtained whether or not the iron v a s precipitated by ammonia before extraction. LITERATURE CITED
(1) Brookshier, R. K., and Freund, H., ASAL. CHEW, 23, 1110 (1951). (2) Sandell, K. B., “Colorimetric Determination of Traces of Metals,” pp. 191-5, Interscience, New York, 1944. RECEIVED for review March 22, 1955. Accepted August 5 , 1955.
Radiochemical Separation of Actinium and Its Daughters by Means of lead Sulfate N. E. ROGERS and R. M. WATROUS M o u n d Laboratory, Monsanto Chemical Co., Miamisburg, O h i o
A radiochemical separation of actinium-227 and its daughter elements from rat urine was required as an analytical method in the determination of these radioactive elements. A method is described in which actinium and its decay products are separated from urine salts on precipitated lead sulfate. Approximately 90% of the radioactivity in rat urine can be recovered by coprecipitation on lead sulfate in hot solutions. The lead is removed by precipitation with hydrogen sulfide, leaving the activity in solution. The actinium fraction is free of interfering salts except for a small quantity of urine salts coprecipitated with the lead sulfate. The method requires about 8 hours.
on a controlled diet, the chemical composition of the urine remained relatively stable during the course of this work. Composition of a sample of typical rat urine is presented in Table I.
Table I.
a
I
N T H E study of the effect of actinium-227 and its daughter elements on rats, a reliable radiochemical separation of these elements from rat urine was needed. Data obtained from such an analytical procedure lvould be of material assistance in ascertaining the ratio of radioactivity administered into a rat to that excreted daily through the kidneys. Lead sulfate has been used as a carrier for radium in solution. Ames and others ( I ) described a method for the determination of small amounts of radium in uranium ore8 by the separation of the radioactivity on precipitated lead sulfate. This method was later adopted, with slight modifications, for the separation of radium from large volumes of human urine ( 7 ) . The technique of using lead sulfate as a carrier for actinium-227 was reported by McLane and Peterson (6). They stated that over 98% of a carrier-free actinium tracer is precipitated -with 1 gram of lead per liter of 6 M sulfuric acid. Lead sulfate has been used as a carrier for actinium-227 onlv in relativelv salt-free solutions. The presence of concentrated rat urine salts tends to contaminate the lead sulfate precipitate and makes it difficult to obtain quantitative separations. The major components found in digested rat urine are sodium, potassium, and magnesium chlorides. As the rats are maintained
Composition of Digested Rat Urine Salts % 38.0 7.5
Element Potassiuma Chlorine Sodiuma
27.5 1.5 0.5 0.1 6.5 1.6
Magnesium Iron Calcium Phosphates Sulfates Flame photometer.
Silicon and boron, in moderate amounts, are also present in digested rat urine. These ti?-o elements are probably leached from the glassrare during the digestion period. PROCEDURE
To a 3-day sample of rat urine (approximately 120 ml.) in a 250-ml. boiling flask, add 60 ml. of concentrated nitric acid. Enclose the flask in an electric heating mantle and evaporate the contents nearly to dryness. Add small portions of nitric acid, with repeated evaporations, until the solution is clear and the residual urine salts are white. Finally, evaporate the contents of the flask to dryness. Dissolve the dried urine salts in the flaqk with 10 to 15 ml. of dilute nitric acid and transfer the contents quantitatively to a clean 150-ml. beaker. Evaporate the salts to dryness and take up the residue in 100 ml. of O . 1 N nitric acid. Slowly add 3 ml. of concentrated sulfuric acid to the solution. Heat the solution to 75” C. on a hot plate with mechanical stirring, and add dropwise 100 mg. of leadnitrate solution (0.5 ml. of a solution prepared by dissolving 32 grams of lead nitrate in 100 ml. of distilled water). Stir the lead sulfate precipitate for 30 minutes with the temperature maintained a t 75’ C. Rinse the stirring rod and thermometer with sufficient mater to bring the volume of the
2010
ANALYTICAL CHEMISTRY
solution t o 100 ml. Cover the beaker with a watchglass and let the precipitate digest a t 60" to 65" C. for a t least 4 hours, or preferably, overnight. Stir occasionally. Cool the beaker and contents to room temperature, transfer to a centrifuge tube, and centrifuge. Rinse the beaker thoroughly with portions of the supernatant liquid, add to the centrifuge tube, and centrifuge until the supernatant liquid is clear. If there are floating particles on top of the solution add a drop of 5% aerosol (5% aqueous solution of Aerosol-OT, dioctyl sulfosuccinate) and centrifuge again. Discard the supernatant liquid which contains the inactive urine salts. The major portion of the radioactivity has been carried with the lead sulfate. Set aside the original 150-ml. beaker for use in the next step. Add 15 ml. of concentrated hydrochloric acid t o the lead sulfate precipitate. Heat on a steam bath with occasional stirring until the precipitate is dissolved and then evaporate to dryness. Add dilute hydrochloric acid slowly to the residue until it dissolves and transfer to the original 150-ml. beaker with distilled water. The total volume of the solution in the beaker should be a proximately 50 ml. Adjust the hydrogen ion concentration of t i e solution with dilute ammonium hydroxide so that it is weakly acid (pH 3.0 t o 3.5) to alkacid paper. Heat almost to boiling and bubble hydrogen sulfide gas into the solution until it becomes saturated. Add 50 ml. of cold distilled water and continue to pass in hydrogen sulfide until the solution is clear and the recipitate settles to the bottom of the beaker. Centrifuge a n z t h e n wash the precipitate twice with 5-ml. portions of hydrogen sulfide water. Dissolve the lead sulfide precipitate in a little hot concentrated hydrochloric acid, evaporate the resultant solution t o dryness, redissolve in a small quantity of dilute hydrochloric acid, adjust the pH, and reprecipitate as before. After the reprecipitated lead sulfide has been washed with hydrogen sulfide water, discard the precipitate. Combine the two supernatant solutions, heat almost to boiling, and test for completeness of precipitation by saturating the solution with hydrogen sulfide gas. Evaporate the combined solution to dryness. Rinse the sides of the beaker containing the residue with about 10 ml. of concentrated nitric acid. Reduce the volume of the solution to 2 to 3 ml. by evaporation. Cool, and rinse the sides of the beaker again with nitric acid, followed by a second rinsing with distilled water. Again reduce the volume by evaporation to 2 t o 3 ml. Transfer the solution quantitatively to a clean 10-ml. volumetric flask, using small portions of distilled water to wash the contents of the beaker into the flask. Fill the volumetric flask to the mark and mount an appropriate aliquot on a stainless steel disk, the mount being spread over as large an area as possible to reduce self-absorption, For best results, the volume of the mounted aliquot should be selected so as to produce an alpha count within the range of 1000 t o 50,000 counts per minute. When the mount is nearly dry, lower the heat lamp and heat strongly until all excess sulfuric acid has been volatilked. Carefully ignite the mounted sample in the flame of a Bunsen burner, Set the ignited sample aside for 3 to 4 hours to permit the short-lived daughters to attain equilibrium and then submit the sample for alpha counting.
McLane and Peterson ( 5 )reported that barium sulfate precipitates adsorbed 96% of an actinium tracer a t 85" C. Because of the similarity of chemical properties of barium and lead sulfates it seemed very probable that lead would exhibit higher adsorptive qualities if precipitated in hot solutions. Accordingly, a number of experiments were conducted in n-hich the lead sulfate was precipitated a t 75' C. and digested for at least 4 hours at 60" to 65" C. The actinium and its daughter elements were carried quantitatively on precipitated lead sulfate from urine salts in hot solution. The lead sulfate precipitate was set aside for further work and aliquot samples of the filtrate were counted to determine the amount of radioactivity lost through removal of urine salts (Table 111).
Table 111. Loss of Activity in Filtrate Following Lead Sulfate Precipitation" Sample 1 2 3
9 10
Av. a
Table 11. rldsorption of Lanthanum-140 on Precipitated Lead Sulfate" Activity Found, % Sample Precipitate b Filtrate 1 24 76 2 29 71 3 45 55 4 32 68 a. Precipitated a t 25' C., digested a t 4 O C.; 0.5 mg. Pb/ml. of solution. 6 Calculated.
1.20 0.77 C., 1 mg. Pb/ml. of solution.
The effect of varying the concentration of lead is shown in Table IV. The optimum quantity of lead for adsorption appears to be 1 mg. per ml. of solution. Increasing the amount of lead to 2 mg. per ml. does not appreciably alter the adsorption of the precipitated lead sulfate toward actinium. Decreasing it to 0.5 mg. per ml. does, however, correspondingly decrease the quantity of radioactivity carried. The data presented in Table IS' record the loss of activitv to the filtrate.
Table IV.
EXPERIMENTAL DATA
In preliminary work, lanthanum-140 (40-hour half life) was used as a stand-in for actinium to determine whether lead sulfate could be used effectively to carry actinium and its daughter products quantitatively from the bulk of rat urine salts. In these experiments, the lead sulfate was precipitated a t room temperature and digested in an ice bath as recommended by Russell, Lesko, and Scbubert ( 7 ) in their work on the determination of radium in exposed humans. The results of these experiments, (Table 11)a6 determined by beta counting, were low and inconsistent.
Precipitated a t 75' C., digested a t 60-65'
0.52 2.30 0.18 0.31 0.21 0.60 0.15 2.00 0.38
a
Effect of Varying Concentration of Leada
Concn. Lead, Mg./Ml, 0.5 0.5 1.0 1.0 1.0 2.0 2.0 2.0 Precipitated in hot solution.
Loss of Activity t o Filtrate, % 12.10 9.75 0.52 0.15 0.38 0.18 0.31 0.21
The lead may be separated from the radioactivity by electrolysis ( 4 ) . In this method, lead is plated a~ the dioxide on the anode from a dilute nitric acid solution. The lead sulfide method is recommended, however, because in the authors' experience a more quantitative separation is obtained and the weight of the dissolved solids remaining in the final filtrate, which contains the activity, is considerably less. The weight of the salt content of the filtrate, after evaporation to dryness, was only 25 to 30 mg. in an average sample. By comparison, the total weight may run as high as 50 to 100 mg. per sample when the lead is removed electrolytic all^. In either case, the lead is almost completely removed and carries less than 2% of the radioactivity (Table V ) . Table VI shows the total activity recovered from rat urine with a single lead sulfate precipitation. Varying amounts of carrier-free actinium-in-equilibrium were added to the urine to produce between 5700 and 4,800,000 alpha counts per minute. The actinium was added directly to the raw rat urine sample prior to digestion with concentrated nitric acid.
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V O L U M E 27, N O . 12, D E C E M B E R 1 9 5 5 Loss of Activity by Removal of Lead Salts
Table V.
Activity on Lead, %
Sample Lead Sulfide Pptn.
1.96 2.30 0.41 1.88 1.65 2.10 1.92
1 2 7 4 5 6 7
Av.
1.75
DISCUSSION
Electrolysis 0.34 0.75 2.10 0.51 2.30 0.71
8 9 10 11 12 13
Av.
1. 12
Table 1'1. Total -4ctivity Recovered from Rat Urine with Lead Sulfate" Total Activity, Counts per Minute -4dded Recovered 4 800 000 4 '500 '000
4'500:OOO 4 50O,000 900,000 900,000 900,000 900,000 225,000 66,000 66,000 21,000 21,000 5,700 5 , 700 5,700
4,358,000 3,812,000 3,932,000 3,954,000 784,000 775,000 763,000 792,000 206,800 59, GOO 56,100 19,640 20,400 5,020 5,825 5,120
Activity Lostb,
Activity Recovered,
%
%
3.80 2.82 0.59 2.19 1.69 2.31 2.56 0.49 2.76 0.89 2.40 3.40 1.,28
91 85 87
E
c
A study was made to determine which of the three major activities of actinium-in-equilibrium was being lost through chemical separations. Samples of both urine salts and separated lead carrier were counted a t regular intervals for 30 or more days. A plot showing the growth and decay curves of the activity in both the urine salts and lead carrier tends to follow very closely pure thorium-227. This indicates that nearly all of the activity lost through chemical separations is thorium-227.
88
87 86
85
88 92 90 86 94 97 88 102 90
-4ctinium-227 is a beta emitter n-ith a 22-year half life. Its principal decay products are thorium-227 (18.6-day half life), radium-223 (11.2-day half life), and a number of short-lived products including actinon or radon, bismuth, polonium, and lead isotopes. Since the beta rays are extremely w a k , (0.02 ni.e.v.), the alpha radiation is utilized for counting purposes although it comprises only 1.20% (6) of the total iadiation. Both tho1ium-227 and radium-223 are alpha emitters. Purified actinium grom rapidly for a number of weeks, owing to the formation of its daughter products, and then comes to equilibrium a t the end of 185 da3.s. At the end of this period, actinium decays according to its 22-year half life. The use of specific carriers employed \vith precipitation methods for the determination of actinium-227 and its daughters pave incomplete separations due to the presence of urine salts. The removal of total activity from the bulk of these salts v..as found to be a practical method.
h
loo
Precipitated in hot solution. b Combined losses of urine and lead salts. C Represents alpha counts below accurate range of counting instruments used. a
Approximately 90% of the actinium-in-equilibrium originally added to the rat urine v a s recovered in the final filtrate (Table VI). The combined losses of sctivity due to the separation of the urine and lead salts amounted to a little more than 27,. The discrepancy b e t m e n the starting alpha counts and total alpha counts found (activity in the final filtrate plus the combined losses in the urine and lead salts) is due to sorption on glaEsxare, inherent Crrors in the counting instruments, arid szlf-absorption. Self-absorption appearcd to be of little significttnce except in the residual or discarded fractions. I n the case of separnt,ed urine salts, tbe solutions contained very small amounts of radioactivity and a high concentration of solids (approxiniately 3 grams per sample), making it necessary t,o prepare niounts of appreciahle thiclmess. Tile nzight of a typical mounted sample varied from 3 to G mg. per sq. cm., depending on the volume of the fraction required to obtain a mount of sufficient sclivity for accurate counting. Reference to Figure 1 ( 2 ) shon-s that 54 to 75% of the alpha radiation in these deposits would be counted by a proportional counter, the remainder being absorbed within the sample itself. While this may appear to be a significant error, actually it is of lit,tle importa.nce if it is taken into consideration that only slightly more than 1% of the tot,al activity is represented. The same is true of the separated lead fraction, only to a lesser degree, as in this case a mounted sample would weigh 0.5 mg. per sq. cm. or less. I n contrast, sample mounts of the final radioactive filtrate n-eigh less than 0.1 mg. per sq. cm., constituting a loss of not more than 2 to 3% due directly to self-absorption (Figure 1).
2
4
6
8
10
12
14
16
18
Thickness of Deposit (mg/cm2)
Figure 1. Alpha absorption of radioactive (actinium227 in equilibrium) rat urine salts (dried at 700' C. on slide)
Individual activities may be determined by application of the data obtained from alpha counting. The authors found the technique of differential decay (3)to he an excellent method of determining individual activities using data obtained from counting procedures. Briefly, it consists of substituting known data in simultaneous equations and solving for the one possible combination of activities and the proportion of each present. In the work on actinium, the final filtrate was essentially in equilibrium with its daughters and it was found expedient to break the chain so that the counting period could be shortened. This was accomplished by the removal of thorium-227, the first important daughter, by homogeneous precipitation with a suitable carrier, as the iodate (8). ACKNOWLEDGMENT
Special acknowledgment is made to J. J. Daub>-,Mound Laboratory, who carried out the absorption studies reported in thie
2012
ANALYTICAL CHEMISTRY
paper. The authors also m
