Radiochemically Pure Cerium New Specification of Chemical Purity MURRELL L. SALUTSKY' and
H. W. KIRBY
M o u n d Laboratory, Monsanto Chemical Co., Miamisburg, O h i o
LAURENCE L. QUILL K e d z i e Chemical Laboratory, M i c h i g a n State University, Fast Lansing, M i c h .
Radiochemically pure cerium was prepared as a carrier for tri- and quadrivalent ions in extremely low level radiochemical analyses, such as urinalysis. One requirement for such a reagent is that the alpha activity should not exceed one count per hour per milligram of cerium. Analyses of a series of commercial salts showed that none of the commercial compounds was suitable for such low level radiochemical work. The alpha activity in the commercial salts was due to thorium impurity, which varied from a few hundredths of a per cent to several per cent depending on the salt and its source. Radiochemically pure cerium was prepared by three methods: fractional precipitation of the iodate from homogeneous solution, fractional crystallization of the double magnesium nitrate, and fractional crystallization of ceric ammonium nitrate.
I T H the growth of radiochemistry there is an increasing need for a new specification of chemical purity, t h a t of radiochemical purity. Many reagent grade chemicals may meet all the present specifications and yet be totally unsuitable for low level radiochemical work, because of the presence of small quantities of naturally occurring radioactive elementq. An important need for radiochemically pure conipounds arose in the determination of the amounts of radioactive materials ingested by exposed personnel. For example, in the development of urinalysis procedures cerium salts were needed as carriers for tri- and quadrivalent ions ( 1 ) . One requirement as that the alpha radioactivity should not exceed one count per hour per milligram of cerium. Although a series of commercial compounds was analyzed, none was satisfactory for such lo\? level radiochemical work.
count per hour per milligram of cerium. All commercial cerium compounds showed a definite count above the blank value. ANALYSIS OF COMMERCI4L CERIUM SALTS
Fifteen commercial cerium salts were analyzed. The salts were purchased from the major cerium producers and were the highest quality obtainable. They are listed in Table I and are arranged in order of increasing alpha radioactivity. A wide range of radiochemical purities is represented by alpha activities of from 6 t o 1245 counts per hour per milligram of cerium. In every case, except samples 6 and 7 , the compounds came directly from the manufacturer, and the bottles had not been opened previously. A Kahlbaum sample, sample 12, was a museum specimen which had not been opened since it mas originally packaged bv the German concern in 1912. In general, the cerous salts contained less radioactivity than the ceric salts. This may indicate that the manufacturers are concentrating thorium, the most probable radioactive impurity in cerium salts, in the process of producing the ceric compounds. The amount of radioactivity in the same cerium compound may vary considerably-for example, the alpha activity in the three ceric ammonium nitrate samples, samples 7, 11, and 13, varied from 89 to 202 t o 693 counts per hour per milligram of cerium. Salts from the same source may vary considerably in radiochemical purity. The cerous oxide from company A, sample 1, had a count of only 6, whereas their ceric oxide, sample 14, had a count of 853, or contained over 100 times as much radioactivity.
Table I. Sample
Compound
Source
1
e
3
PROCEDURE
3
The following procedure was used to determine the alpha activity in the cerium compounds. A dilute nitric acid solution of each of the compounds was prepared so that the concentration of cerium was about 100 mg. of cerium per milliliter of solution. In some cases, such as hydrated oxides, oxalates, etc., it was necessary to convert the compounds to the nitrates. Twentyfive microliters of the solution, containing approximatelv 2.5 mg. of cerium, was mounted in 100 to 150 small droplets on a 2-inch stainless steel disk. The droplets \?ere evaporated by placing the disk under an infrared lamp. The disk was ignited gently to convert the dried cerium compound to the oxide. Strong ignition was avoided, as high temperatures caused the ovide to shrink and lose its adherence to the stainless steel disk. The alpha activity on the disk was counted for 16 hours in a Nuclear Measurements Corp., Model PC-1 proportional counter. The cerium solution was standardized gravimetrically by precipitation of cerium oxalate followed by ignition to the oxide. The alpha activity in counts per hour per milligram of cerium was calculated. Except for the 25-pl. micropipet, all net? glassware was used for each sample. B series of blank determinations on the micropipet, forceps, disks, etc., gave an alpha activity of 1.2 f 0.6 counts per hour. Since 2.5 mg. of cerium was mounted on each disk, the limit of detectability was approximately 0.5
8
1 Present address, Inorganic Chemicals Division, Monsanto Chemical Co., Nicholae Road, Dayton, Ohio.
Radiochemical Purity of Cerium Compounds
7 8 9 10 11 12
13 14
15 a Alpha counts per hour per milligram of cerium.
A B
i:
B B
B
C .1
R
B
D .1
6
Radiosctivitya 6 16 16 16 26 30 89 129 143 186
202 364 693 833 124.5
The radioactive impurity can be separated by fractional crystallization. This is shown by samples 4 and 5 (specially prepared by Company B) which are the first and second crop of crystal?, respectively, from a series of cerous nitrate crystallizations. If all the alpha radioactivity in the cerium compounds were due to thorium impurity, the percentage of thorium would vary from a few hundredths of 1%for sample 1 t o several per cent for sample 15. The calculated alpha counts per hour per milligram of cerium for varying amounts of thorium impurity are shown in Table 11. Thorium was detected in three of the cerium compounds, samples 2, 10, and 11, by isolation and identification of its ra-
1960
1961
V O L U M E 2 7 , NO. 1 2 , D E C E M B E R 1 9 5 5
lower spectrum is due to the radioactive impurity in ceric ammonium nitrate. The spectra are nearly identical. Therefore, the major radioactive impurity in commercial cerium salts is thorium as proved by the separation and identification of radium-224, by x-ray fluorescence spectroscopy, and by gamma pulse height analysis.
RAOIUM SERIES
P
I
U
M SERIES
PREPARATION OF RADIOCHEMICALLY P U R E CERIU11
DAYS
Figure 1. Decay of radium isotopes separated from ceric ammonium nitrate
4
Because none of the commercial cerium salts was suitable for use in urinalysis, radiochemically pure cerium was prepared in the laboratory by three methods: fractional precipitation of the iodate from homogeneous solution (6); fractional crystallization of the double magnesium nitrate ( 2 ) ; and fractional crystallization of ceric ammonium nitrate (3). Fractional precipitation of ceric iodate from homogeneous solution was accomplished by a met'hod similar to that reported by Willard and Yu (6). Iodic acid and potassium chlorate were added to a solution of cerous nitrate in approximately 5-Vnitric arid. The solution was boiled; and as the cerium was slowly oxidized, it precipitated as ceric iodate. Several successive iodate fractions were precipitated. The cerium remaining in solution after the fifth iodate precipitation v-as recovered as the hydroxide. After filtration, the iodate fractions TTere converted t o nitrates by evaporating with hydrochloric acid and then with nitric acid. The hydroxide residue was filtered, washed, and dissolved in nitric acid. After 3 to 4 weeks equilibrium n-as reestablished and samples of the nitric acid solutions were mounted as described. The alpha activity on each disk \vas determined. The results of these anal>-separe shown in Table IT.
L
a
Table 11. Radioactivity in Cerium Compounds Due to Thorium Impurity a Activitya, Counts/Hr./Alg. Ce 4.50 46 4 6 0.5 Calculated; assuming radioactive equilibrium.
Thorium, yo 1. 0 0.1 0.01 0.001
a
I 0
50
v
:.
00
Figure 2.
I50 KW
200
2%
3M
Gamma spectra
Table 111. Analyses of Commercial Cerium Compounds Sample Compound Thorium, % 2 10
11
Ce(S0s)s .6H20
Ce(I0s)a (liHd2Ce(NOs)e
0 03 0 4
0.4
dium-224 daughter. Ten milligrams of barium carrier was added to a solution containing 1 gram of the cerium salt, and the barium was precipitated as the sulfate. The barium sulfate was centrifuged, washed, and mounted on a stainless steel disk. The disk was counted daily for alpha activity. The thorium content was calculated from the amounts of the radium daughter detected. The results of these calculations are shown in Table 111. The data for Sample 11 are plotted in Figure 1. The straight lines represent the decay a t equilibrium of the alpha-emitting radium isotopes in each of the three natural decay series, the radium, actinium, and thorium series, respectively. It is obvious that the radium isotopes separated from the ceric ammonium nitrate decays a t a rate equivalent to that in the thorium seriesi. e., radium-224. The presence of thorium in the ceric ammonium nitrate, Sample 11, was confirmed by x-ray fluorescence spectroscopy (4). (Arc spectrographic analysis is less sensitive than the x-ray method, because the thorium spectrum is mmked by that of cerium.) The thorium was also detected by gamma pulse height analysis (Figure 2). The upper spectrum is that of thorium nitrate, whereas the
Table IV.
Analyses of Iodate Fractions
1 2 3 4
9 19 39 22
..
a rlctivity, Counts/Hr./AIg. Ce 16 93 0 0
Residue
3 3
24
Fraction Original
Total Cerium Precipitated, 5
0 0
One gram of cerium as the nitrate was used as the original material. The reagent grade cerous nitrate hexahydrate had an alpha activity of 16 counts per hour per milligram of cerium, The increase in activity in the first iodate fraction t o 95 counts per hour per milligram of cerium is undoubtedly due t o thorium which should concentrate in this fraction. The final residue should contain any actinium and radium isotopes along with their daughter activities, and for this reason showed a count of 24. The center fractions composing nearly 90% of the original cerium sample were radiochemically pure. -4nalyses were made of samples from two fractionation series,
1962
ANALYTICAL CHEMISTRY
NCe and CE, in which cerium had been recrystallized from concentrated nitric acid as the double magnesium nitrate [3Mg (NOa)2 2 C e ( N 0 3 ) ~ . 2 4 H ~ O ]It. was estimated that 100 to 150 recrystallizations had been made in each series. Samples from each fraction in the two series were analyzed for alpha activity by the method described. If the radioactive impurity were thorium, it would be expected to concentrate in the more soluble end-i.e., larger numbered fractions-of a double magnesium nitrate series. The results of analyses listed in Table V show that the radioactive impuritv did concentrate in the more soluble end of the series. The cerium in about one half of each series was radiochemically pure. The alpha-emitting impurity increased by an order of magnitude in each of the last four fractions of each series. Nine bottles of cerium samples taken from various steps in a cerium purification process, similar to a commercial process, were supplied by G. Frederick Smith. Bottle 1 contained a sample of technical grade ceric oxide. Several pounds of this material was leached with hot nitric acid, and a sample of the nitric acid insoluble residue, containing silica, some phosphate, etc., was packaged in bottle 2. T o the nitric acid soluble portion was added excess ammonium nitrate to precipitate ceric ammonium nitrate, a sample of which was contained in bottle 3. -4series of fractional crystallizations of the ceric ammonium nitrate yielded a second, third, fourth, and fifth crop of crystals, samples of which were in bottles 4, 5, 6, and 8, respectively. The filtrates from the first, second, and third crop of crystals were apparently discarded as no samples of these materials were available for analysis. However, samples of the filtrates from the fourth and fifth crops of crystals were supplied, bottles 7 and 9, respectively. The radiochemical analyses of the nine samples are shown in Figure 3. The alpha activity in the cerium samples shows the distribution of thorium in the purification process. The fifth crop of crystals, bottle 8, is very pure, since it contains a maximum of 0.001% thorium. Therefore, radiochemically pure cerium can be prepared by the method of Smith, Sullivan, and Frank ( S ) , provided a sufficient number of crystallizations are made. The yield in this experiment rras rather low as only 3 ounces out of more than 4.7’5 pounds was recovered in a radiochemically pure form. Radiochemical analyses indicate that the original cerium, bottle I, contained about 0.75% thorium and that the first crop of crystals, bottle 3, contained only about 0.25%. The thorium appears to concentrate in the filtrates as is shown by the analyses of the samples in bottles 7 and 9. In these cases the filtrates contain 10 times more alpha activity than the crystals, bottles 6 and 8, respectively. The nitric acid leach of the technical grade ceric oxide seems to have had little effect on the separation of thorium as the alpha activity of the cerium in bottle 2 is only slightly greater than that of the original, bottle 1. CONCLUSION s
Radiochemically pure cerium with an alpha activity of less than one count per hour per milligram of cerium can be prepared by anv of the three described methods. Fractional iodate precipitation seems suitable only as a laboratory method for rapidly
Table V. Analyses of Double iMagnesium Nitrate Fractions Fraction NCe
a Activity Counts/Hr./Mi. Ce
Fraction, CE
a Activity Counts/Hr./h&
Ce
{ORIGINAL C#02,) ACT.341
BOTTLE 2 H N 0 3 SOLU0LE
ALPHA ACTIVITY EXPRESSED AS COUNTS PER HOW PER MILLIGRAM OF CERIUM BOTTLE 8
‘j
CROP 5 CRYSTALS
Figure 3.
BOTTLE 9 CROP 5 FILTRATE ACT.5 0
Ceric ammonium nitrate fractions
preparing small quantities of radiochemically pure cerium E l e n though a high yield may be obtained, the high cost of chemicals prohibits the large scale use of the method. Either fractional crystallization of the double magnesium nitrate or ceric ammonium nitrate is satisfactory for producing large quantities of radiochemically pure cerium on a routine scale. The number of crystallizations required to obtain a high radiochemical purity depends on the purity of the technical grade ceric oxide used a8 the source material. The detection of radioactivity in cerium salts is only one example of radioactivity in reagent grade chemicals. The authors have detected radioactivity in several common chemicals and for this reason are suggesting a new chemical purity epecification, radiochemical purity. ACKNOWLEDGMENT
The authors thank C. R. Hudgens, George Charles, and George Pish for the x-ray and spectrographic analyses. They also thank G. Frederick Smith for preparing the ceric ammonium nitrate series of samples. LITERATURE CITED
Kirby, H. W., and Brodbeck, R. &I., U. S. Atomic Energy Commission MLM-1003, Aug. 20,1954. (2) Pearce, D. W., “Inorganic Syntheses,” vol. 2, pp. 52-8, MoGrswHill, New York, 1946. (3) Smith, G. F., Sullivan, V. R., and Frank, G., IND.ENQ.CHEM., (1)
A N A L . E D . , S(1936). ,~~~ (4) Von Hevqsy, G., “Chemical Analysis by X-Rays and Its Applications, McGraw-Hill, New York, 1932. (5) Willard, H. H., and Yu, S.T., ANAL.CHEM., 25, 1754 (1953). R ~ C E I V Efor D review July 18, 1955. Accepted September 2, 1955. Division of Analytical Chemistry, 127th Meeting, ACS, Cincinnati, Ohio. April 1955. Abstracted from E. S. Atomic Energy Commission MLM-1002, “Radiochemically Pure Cerium,” Mound Laboratory, Monsanto Chemical Co., Miamisburg. Ohio, August 20, 1954. Mound Laboratory is operated b y Monsanto Chemical Co., under A.E.C. Contract AT-33-1-GEN-53.
