Determination of Antimony by Radioactivation J. E. HUDGENS, JR., AND P. J. CALI U . S. Atomic Energy Commission, New Brunswick, N . J . Slow neutron activation and radiochemical separations have been used to determine very small concentrations of antimony in samples of pure materials. The generalized technique of radioactivation analysis and the precautions that must be taken for the irradiation are given. Sensitivities of 1 part in lo* are attainable and the determination is capable of excellent accuracy. Methods of calculating the amount of activity formed by a neutron irradiation and of checking the radiochemical purity of the isolated radioelement are discussed.
T
HE activation of the stable elements by a high intensity neutron source as a method of determining small concentrations of various elements is a technique that has come into prominence as a result of the construction of large uranium graphite reactors. Keutron intensities of 4 X 10’2 neutrons per second per square centimeter (4)are available and the limit of detection gram of the element sought per is approximately lo-* or gram of the major constituent. The limit of detection is determined by the intensity of the neutron beam to which the sample is exposed, by the cross section, and by the length of time to which the sample is exposed to the neutron beam. Radioactivation as a method of analysis xas first used by Hevesy and Levi (17),and Seaborg and Livingood (26) used the cyclotron as a source of the activating particles. Kumerous papers describing the technique and setting forth its potentialities have been published ( 2 , 5-7, 12, I S , 28). Saturally occurring antimony is made up of t n o isotopes, one of mass 121 (57.25%) and one of mass 123 (42.7570) (29, 30). When these isotopes are subjected t o a neutron bcmbardment five radioactive isomers are formed. These isomers are produced by the reactions
flux, X is the decay constant
___
of theisotope formed, and
t i s the length of time the sample remained in the neutron flux. A more useful form of the equation can be derived by including in the equation a factor t h a t combines the known constants and the known thermal neutron cross sections of Sb1Z1(6.8 barns) ( 2 7 )and of SbIz3(2.5 barns) ( 2 7 )for the production of the 2.8-day Sb122 and the 60-day SblZ4isotopes, respectively, the accurately known isotopic abundances for the naturally occurring element, and the equation for the decay of the 2.8-day and 60-day isotopes. The equation
AT = W(f)
[“ -
e-hitz)(e-hit?)
+ (1 -
e-Azti)(e-hzti)
191
3
is obtained. I n this equation A r i s the total number of antimony atoms disintegrating per unit time, W is the weight in grams of naturally occurring antimony exposed to the neutron flux, f, XI is the decay constant of the 2.8-day isotope, XZ is the decay constant of the 60-day isotope, t l is the length of time the sample was exposed to the neutron flux, and t 2 is the length of time that elapsed between the cessation of bombardment and the radioactivity determination. The activity of either of the two important antimony isotopes can be made to predominate a t the time the sample is removed from the neutron flux by a proper selection of the irradiation time. In Figure 1 the amount of Sb122 and Sb12*radioactivity present after an irradiation time, tl, is given. These curves also make possible an estimation of the “sensitivity” of the method. It is evident that for the bombardment of gram quantities of the major constituent a few thousandths of a part per million of antimony could be detected. BIONITORING O F NEUTRON FLUX
One of the most important problems of radioactivation analysis is the determination of the intensity of the neutron flux t o which the sample has been exposed. This problem may be solved by a careful calibration of the neutron source, by irradiating a sample of known cross section a t the same time that the unknom-n sample is irradiated, or by irradiating a sample of antimony with the unknown sample. I n the latter case the antimony in the sample may be counted a t the same time that the antimony monitor is counted, and a simple proportion for calculating the concentration of the antimony in the sample may be used. I n samples in which the major constituent has a large absorption cross section, a correction for the decrease of the neutron beam intensity toward the center of the sample must be made. The correction to be applied is calculated from the expression
GO tlnys (10, 18, 29)
After a short time has elapsed, the only nuclides that remain are the 2.8-day and the 60-day antimony isotopes, in addition to the other radioactive isotopes which may be formed by a neutron activation of other impurities or of the major constituent of the sample. The total quantity of antimony 122 or antimony 124 activities formed as a result of a given neutron irradiation may be calculated from the equation (%6)
A =
( u ) ( . f ) ( N )1(
-
,-At)
where A is the activity of the isotope in units of atoms disintegrating per second, u is the thermal neutron cross section of the parent atom in units of sq. cm. (barns), j is the neutron flux in units of neutrons per second per square centimeter, N is the number of atoms of the parent isotope exposed t o the neutron
(66): f = fo(e-g.VzX)
where j is the flux of neutrons (neutrons/cm.-*/second) after
171
ANALYTICAL CHEMISTRY
172
passing through a thickness X of the material having a cross section u , N , is the density of the material in atoms per cc. The importance of this correction factor is decreased as the physical dimensions of the sample are decreased and for very small samples t h e correction factor can usually be neglected entirely. By integrating this equation between the limits o and X and dividing by X the average flux t o which the sample was exposed can be obtained. For a capsule 0.5 cm. in diameter containing antimony metal powder, t h e calculated average flux is 99+% of the flux t o which the sample was exposed.
1000
[ -
100
o W
S e w Brunswick Laboratory, used the chromous chloride precipitation method as a substitute for the sulfide precipitation and the electrolysis steps given in the procedure be!ow with excellent results. Antimony has been separated as stibine ( I , 9] 14,15,23,24). However, interference by arsenic, selenium, and several other metals is serious. Maxn-ell et al. (20) separated carrier-free antimony from a tin cyclotron target by using a combination of sulfide precipitations and a distillation with hydrobromic acid. The distillation of antimony as the chloride and bromide and electrolytic precipitations are classical methods of separation (SI). Edwards and L'oigt studied the isopropyl ether extraction of antimony(V) as the chloride (11). The number of different types of chemical separations which must be performed t o separate antimony from other radioelements formed in the irradiation is determined by the concentration] cross section, and decay characteristics of the other elements as well as the specificity of the chemical separations used. Because the zirconium oside samples used to test the procedure (Table I ) contained appreciable quantities of elements whose chemistry is similar t o that of antimony, a more complicated separation procedure than is usually necessary was used. Distillation of antimony trichloride followed by a precipitation ( b y electrolysis or chromous chloride) of antimony metal is usually sufficient to prepare the sample for counting.
ln
\ ln
CHEIIICAL PROCEDURE
-
Wash the sealed quartz capsule containing the zirconium oxide (-0.5 gram) with a 10% hydrochloric acid solution containing a few drops of hydrofluoric acid, and after rinsing with distilled water break the capsule under 50 ml. of concentrated sulfuric acid. Add t o the sulfuric acid-zirconium oxide mixture an aliquot of antimony trichloride solution containing the equivalent of 100 mg. of antimony ion. Allow the mixture to stand until the solution of the zirconium oxide is complete. Transfer the solution t o a 100-ml. volumetric flask and dilute to the mark with sulfuric acid. Rinse into the distillation apparatus (31) 1 gram of hydroxylamine hydrochloride and 20 mg. each of arsenic and tin chlorides, with a I+ 4 minimum amount of hot distilled water. Transfer 10 ml. of the zirconium oxide sample soluL f tion to the distillation flask, and rinse the walls I+++ I-++ of the dropping funnel with a 10 m1.-volume of BODY concentrated sulfuric acid. During the addition of the sample and 6 - 3 2 SET S C R E W rinse solutions, cool the flask with water. Add 125 ml. of distilled water slowlv, heat the contents of the distilling flask to 150" c., and start carbon dioxide bubbling through the solution. Add 20 ml. of CAP concentrated hydrochloFigure 2. Aluminum Irradiric acid through the funnel and collect the antiation Capsule mony trichloride distilDimensions in inches late over the temperature range 155' t o 165' C. After the completion of the distillation the distillate should be approximately 7 M in hydrochloric acid. Oxidize the antimony to the pentavalent form by adding dropwise a saturated solution of potassium permanganate. E uilibrate 50 ml. of peroxidefree isopropyl ether with an e q u a volume of 7 hl hydrochloric acid solution, discard the aqueous layer and, using two 25-m1. aliquots of the ether solution, extract the pentavalent antimony into the isopropyl ether. The addition of catechol (50 mg. per liter of ether) to the stock solution of isopropyl ether will prevent oxidation ( d b ) and subsequent loss of antimony caused by this effect. Combine the organic phases and place in a large separatory funnel, Add 35 ml. of distilled water and 2 drops of 12 M hydro-
-0
IO'
I
t > I-
o
a
I(
L
'
12
"
"
24 IRRADIATION
'
36
.
"
40
'
60
72
TIME (DAYS)
Figure 1. Amount of Antimony Radioactivity Present after Irradiation of tl Days Disintegration rates shown were calculated for irradiation of 1 of antimony of natural isotopic composition in a flux of 5 X 1011 neutrons per second per sq. cm.
The antimony (metal) used t o monitor the neutron beam is held a t a known distance from the unknonn sample by making use of a simple irradiation capsule which has been devised for this purpose or by careful spacing of the sealed quartz capsules in the irradiation can. The aluminum capsules are fabricated from pure aluminum (Aluminum Corp. of America designation 2-S) in order t o minimize t h e attenuation of the neutron flux by the sample capsule. The capsules are shonm in Figure 2. Irradiation capsules may be easily fabricated from quartz tubing by sealing the ends after the sample has been introduced. The cross section of silica compares favorably with t h a t of aluminum and less time is required t o fabricate the capsule. The irradiation of several samples in the same irradiation can must be carried out after a careful placement of the capsules, as the flux distribution in the can may be heterogeneous and thus the probability that one sample might receive a higher intensity neutron bombardment than the other samples is increased. The antimony must usually be chemically separated from other sample constituents before a radioactivity determination is made. Boldridge and Hume (3) separated antimony from other sample constituents by precipitation of the metal with chromous chloride, precipitation of antimony pyrogallate, and precipitation of antimony(II1) with cesium chloride, presumably as Cs3SbZClP. Each of these compounds was found t o be a suitable weighing form, although the greatest accuracy was obtained with the pyrogallol and chromous chloride methods. L. C. Nelson, of the
+
'
ct*;4
V O L U M E 2 4 , N O . 1, J A N U A R Y 1 9 5 2 Table I.
Sample
90. 2692 2693 2694 2695 2696
Sb
Added 10 5
2
1.0 0
173
Analyses of Zirconium Oxide Samples (Parts per million) Bi, Co, Cr. Cu, .Il, Fe, h l g , Xi, Sb Zn Pb, Sn Found Added Added 10.7 133 10 5.6 67 5 2.5 26 2 1.5 1 13 0.54
0
0
As, Mn, P, Si, V, T i , M o Added 100 50 20 10 0
.4g, B , Be, I n
Added 1.0 0.5
0.2 0.1 0
chloric acid. Completely evaporate the isopropyl ether by attaching a water aspirator to the top of the funnel and opening the stopcock until a steady stream of air is drawn through the solution. Make the solution 2 111 in hydrochloric acid, heat t o 90" to 100"C., and bubble hydrogen sulfide gas through the solution for 10 to 15 minutes. Collect the recipitate on a 42 Whatman filter paper or a sintered-glass filter z n n e l . Wash the precipitate with a 25- to 30-ml. portion of hydrogen sulfide-saturated distilled water and with a like volume of distilled water.
T O P O S I T I V E LEAD-
t
P
T
AGITATOR
5.0
c Y.
Figure 3.
Cell for Electrodeposition of Antimony
Transfer the antimony to the electrolysis cell (Figure 3) b wsshing the precipitate from the funnel into the cell with 20 m? of sodium sulfide solution containing 140 mg. of sodium sulfide per ml. Add 15 ml. of distilled water and 0.75 gram of potassium cyanide and deposit the antimony by passing a current of 50 ma. (1.3 to 1.5 volts) through the cell for 2 hours. Wash the deposited antimony with distilled water and with ethyl alcohol, dry the deposit for 10 minutes a t 80" C. (constant weight), and determine the weight of the antimony metal. DETERMINATIOK OF ANTIMONY RADIOACTIVITY
The antimony radioactivity was determined by using a belltype thin end-window Geiger-lldller counter tube attached to a commercial scaler (Nuclear Instrument and Chemical Corp. Model 163). As all samples were compared to antimony metal irradiated under identical irradiation conditions, the problems attendant to determinations of absolute disintegration rates were eliminated.
only a solution of the monitor in hydrochloric acid, addition of a known amount of antimony carrier solution, precipitation of antimony sulfide, and electrolysis of the antimony as given in the procedure above. As a correction for chemical yield is made for the monitor in the same manner as that for the unknown sample, the small errors caused by the electrodeposition cancel. [Antimony analyses have been reported by Henz (16) to yield results consistently high by 1.5 to 2% of the total antimony.] If the antimony concentration in the carrier solution is determined by using the electrodeposition procedure, the errors in the chemical yield determination may be cancelled by those cases in which an antimony monitor is not used. The use of an antimony monitor to serve as a check on the neutron intensity of the neutron source also serves the purpose of compensating for long-term variations in counter efficiency, of obviating the necessity of determining accurately the correction factors for the geometry of the counting arrangement, the backscattering of beta-rays from the material on which the sample is mounted, and the self-scattering and/or self-absorption by the sample precipitate, since the weight of antimony precipitate on the counting disk is nearly the same for the antimony monitor as for the unknown sample. RESULTS
The synthetic samples used to test the method were prepared by adding known amounts of antimony to a pure zirconium oxide powder. AE the synthetic samples were prepared for the use of the New Brunswick Laboratory spectrographic group, a large number of other elements were also added in known quantities (Table I). The presence of these elements served as a test of the specificity of the method. e The synthetic samples shown in Table I received a 2-week irradiation in the Oak Ridge Sational Laboratory pile. This irradiation was sufficient to yield a specific antimony 124 radioactivity of approximately 500 disintegrations per second per microgram or a total antimony radioactivity of approximately 10,000 disintegrations per second per microgram. As the antimony was isolated from a small part (50 mg.) of the total irradiated sample and most of the 2.8-day antimony 122 had decayed by the time the chemical separations were performed, the counting rates observed were from 1 to 30 counts per second for the series of samples. The observed counting rates could have been increased to from 20 to 600 counts per second by separating the antimony from other sample constituents immediately after the samples were removed from the pile. For samples containing a smaller concentration of antimony the aliquot size could also be increased. The results given in Table I indicate that the original zirconium oxide contained 0.5 p.p.m. of antimony. I t is unlikely that results consistently high by this constant amount could be caused by errors in the counting rate determination or in the chemical yield determination, or by contamination from external sources, for these errors would be observed either as constant multipliers or as erratic additions. The recovery of added antimony carrier does not usually influence the accuracy of the method, as a correction for losses is applied and the losses are usually less than 20y0 of the inactive antimony added. The energy of the radiations from each of the isolated antimony metal precipitates TTas determined by absorption curve methods as an additional check of the efficiency of the separations. The beta and gamma-ray energies found by feather analysis or half-thickness determinations checked the values given by Way et al. (29).
TREATMENT OF THE KEUTRON FLUX MONITOR
The antimony activity in the monitor is determined in a manner analogous to the sample radioactivity determination. In most cases the separation of antimony activity from gross sample impurities is unnecessary and the chemical steps include
ACKNOWLEDGMENT
The authors wish t o thank H . J. Dabagian for his help in testing the antimony deposition procedure, and .411an R. Eberle and members of the S e w Brunswick Laboratory spectrographic group
ANALYTICAL CHEMISTRY
174 for the preparation of the synthetic samples of zirconium oxide used to test the procedure. LITERATURE CITED
(1) Abelson, P. H., Phys. Rev., 56, 1 (1939). (2) Arrol, W. J., Research, 2 , 253 (1949). (3) Boldridge, W.F., and Hume, 0. N., U. S. Atomic Energy Commission, Rept. AECD-2552-E (1949). (4) Borst, L. B., Physics Today, 4 , 6 (1950). (6) Boyd, G. E., ANAL. CHEM.,21, 335 (1949). (6) Brown, H., and Goldberg, E., Science, 109, 347 (1949). (7) Cohn, W. E., ANAL.CHEM.,2 0 , 4 9 8 (1948). (8) Cook, C. S., and Langer, L. M., Phys. Rev., 73, 1149 (1948). (9) Cook, G. B., U. S.Atomic Energy Commission, Rept. A E R E C/R-424 (1949). (10) der Mateosian, E., Goldhaber, M., Muehlhause, C. O., and McKeown, M., Phys. Rev., 72, 1271 (1947). (11) Edwards. F. C.. and Voiat. A.. ANAL.CHEM.,21, 1204 (1949). (12) Goldberg, E. D‘., and Brown, H., Ibid., 22, 308 (1950). (13) Gordon, C. L., Ibid., 21, 96 (1949). (14) Grummitt, W., and Wilkinson, G., Canadian Atomic Energy Commission, Repl. MC-167 (1945). (15) Hahn, O., and Strassmann, F., Natt.rwissenschaffen, 31, 499 (1943). (16) Hens, F., Z . Anorg. Chem., 37, 31 (1903).
(17) Hevesy, G., and Levi, H., Kg1. Danske Videnskab. Selskab, Math.-Fys. Medd., 14, 5, (1936); 15, 11 (1938). (18) Kern, B. D., Zaffarano, D. J., and Mitchell, A. C. G., Phy.9. Rev., 73, 1142 (1948). (19) Livingood, J. J., and Seaborg, G. T., Ibid., 55, 414 (1939). (20) . . Maxwell. R. D.. Raymond. H. R., and Garrison. W-.M.. J. Chem. Phys., 17, 1340 (1949). (21) Miller. L. C.. and Curtiss. L. F.. Phws. Rev.. 70. 983 (1946). (22j Myers; R. J.; Metzler, D.’ E., and Swift, E.’H.,’J. Am. Chem. SOC.,72,3767 (1950). (23) Newton, A. S.,Phys. Rev., 75, 17 (1949). (24) Reid, A. F.. and Weil. A. S..U. S.Atomic Energy -. Commission, Rept. AECD-2324 (1948). (25) Seaborg, G. T., and Livingood, J. J., J . Am. Chem. SOC.,60, 1784 (1938). (26) Seaborg, G. T., Wilson, S.G.. Wilson, V. C., and Coryell, C. D., U. S. Atomic Energy Commission, Rept. MDDC-763 (1946). (27) Seren, L., Friedlander, H. N., and Turkel, S.H., Phys. Rev., 72, 888 (1947). (28) Tobias, C. A., and Dunn, R. W., Science, 109, 109 (1949). (29) Way, K., Fano, L., Scott, M. R., and Thew, K., Natl. Bur. Standards, C ~ T C499 . (1950). (30) White, J. R., and Cameron, A. E., Phys. Rev., 74, 991 (1948). (31) Willard, H. H., and Diehl, H., “Advanced Quantitative Analysis,” pp. 348-9, New York, D. Van Nostrand & Co., 1943.
RECEIVED May 14, 1951.
Analysis of Mixtures of Dicobalt Octacarbonyl and Cobalt Carbonyl Anion e
HEINZ W. STERNBERG, IRVING WENDER, AND MILTON ORCHIN Bureau of Mines, Bruceton, Pa.
A study of the hydroformyIation (oxo) process in this laboratory required the analysis of mixtures containing both dicobalt octacarbonyl, [Co(CO),],, and cobalt carbonyl anion, [Co(CO)4]-, the latter being present either as cobalt hydrocarbonyl, HCo(CO)4, or as a cobalt salt. Because no method was available for this purpose, the authors devised the present procedure. The two-step method depends on the decomposition of all carbonyls, by treatment with iodine, to yield carbon monoxide; and the selective precipitation of the anion by nickel o-phenanthroline chloride, followed by decomposition of the precipitate with liberation of carbon monoxide. As the determination is based on a gasometric procedure, it is suited for the analysis of mixtures, such as those from the hydroformylation reaction, which may contain cobalt salts and metallic cobalt.
M
OST of the mechanisms that have been proposed (1, 9)
for the hydroformylation (oxo) reaction involve the postulation that the homogeneous catalyst is either dicobalt octacarbonyl or cobalt hydrocarbonyl, but no method for the determination of these carbonyls in the reaction products has yet been devised. The dark solution of aldehydes, ketones, and alcohols that is removed from the reaction vessel often contains cobalt not only as dicobalt octacarbonyl, cobalt hydrocarbonyl, cobalt salts of the hydrocarbonyl, and cobalt salts of organic acids, but also as finely divided metal (attracted by a magnet). I n the course of an investigation in this laboratory, it became desirable to devise a simple and rapid method for the accurate determination of dicobalt octacarbonyl, [ C O ( C O ) ~and ] ~ , cobalt hydrocarbonyl, HCo(CO)d, in the mixtures usually obtained in the hydroforniylation reaction. I n a study of these carbonyls ( 3 ,4 ) i t was shown that dicobalt octacarbonyl is quantitatively decomposed by bromine in glacial acetic acid or iodine in benzene according to Equation 1 (8).
[Co(CO),]z
+ 212
--f
2C012
+ 8Cb
(1)
Although the decomposition of the free cobalt hydrocarbonyl by halogens is not described in the literature, Hieber and Teller (6) mention that excess iodine in benzene decomposes cadmium cobalt carbonyl, Cd [ C O ( C O ) ~ ]quantitatively ~, into the corresponding metal halides and carbon monoxide. An aqueous solution of nickel o-phenanthroline chloride has been used by Hieber and coworkers for the quantitative determination of cobalt hydrocarbonyl in aqueous ( 5 )and ammoniacal solution (4). It should be emphasized that this reagent is not specific for the acid, H C O ( C O ) ~but , rather for the cobalt carbonyl anion, [Co(CO),]-. OUTLINE OF METHOD
The present method is based on the following facts: Dicobalt octacarbonyl, cobalt carbonyl anion, and cobalt hydrocarbonyl are quantitatively decomposed by excess iodine according to Equations 1 , 2 , and 3.
+ 312 +2CoI2 4-8 3 0 + 212HCo(CO)r + 312 2CoL + 8 C 0 + 2HI 2[ Co(Co)4]-
--f
(2) (3)
