Radiochemical Determination of Sodium-24 and Sulfur-35 in Seawater DANIEL L. LOVE and DANIEL SAM
U. S.
Naval Radiological Defense laboratory, San Francisco 24, Calif.
b Rapid radiochemical procedures were developed for determining Na*4 and S35 in seawater containing fission product radionuclides. Sodium-24 i s separated from other radionuclides b y scavenging with lanthanum hydroxide; two sodium chloride precipitations with hydrogen chloride gas follow. The disintegration rate i s determined b y measuring the area of the 1.368or 2.750-m.e.v. gamma photopeak. The working time for a single analysis is 0.5 hour; the precision, the chemical yield, about 7070; and the chemical decontamination factor from fission products, >1 06. Sulfur-35 in seawater is separated from other radionuclides by precipitating barium sulfate. The sulfate i s then reduced to hydrogen sulfide with hydrogen iodide. Subsequently, the sulfide i s oxidized to sulfate in an alkaline peroxide solution. The working time required for a single analysis i s less than 1 hour; the precision, &5%; the chemical yield, 70 to 8OY0; and the decontamination factor from fission products, >lo7. Although the samples tested are salt solutions contaminated with fission products (nuclear explosions in the ocean and in salt domes), these methods are applicable to many other types of Na24 and S35 samples containing fission products and induced activities.
j~l7~;
N
in seawater produces a mixture of radionuclides some of nhich are formed through iicutron captiire by those elements in the water. The composition of the miuture of these induced radionuclides depends upon tho time elapsed after the event. Dyrqsen and S j n i n n (6) calculated the relative activities of the slomneutron-induced radioelements in seawater. They shoved that after the first hour, S a z 4 constitutes nearly all the induced actiritv throughout the first weck, after nhich almost all the activity is due to S35. Sodium-24 is formed from a (TI,?) reaction on stable NaZ3, and S35 iq produced by the (n,p) reaction on stable Cl35 and the (n,y) reaction on stable S34. ApproxiUCLEAR DETOP;ATIOT\'
336 *
ANALYTICAL CHEMISTRY
mately 200 times more S35is formed by the (n,p) reaction on C136than by the (n,?) reaction on S34. Since Na24 and S35 are the two most abundant radionuclides induced in seawater following a nuclear detonation, it was desirable to develop simpler and more rapid radiochemical procedures for their analyses. Sodium-24. T h e NaZ4decay scheme consists essentially of a beta particle of maximum energy of 1.39 m.e.v., followed by two gammas in cascade at 1.368 and 2.750 m.e.1'. (11). The half period of NaZ4is 15.06 hours (18). Prestwood (15) reported a radiochemical separation procedure of Sa24 from a mixed fission product solution. After an initial iron scavenge on the sample, sodium is precipitated as the sodium magnesium uranyl acetate. The sodium is finally converted to the chloride for counting. The cheniical yield is about 50 to 60%. While excellent decontamination is obtained, this procedure is tedious and time consuming. Since Na24 has a relatively short half period, a more rapid method of separation, adaptable to the analysis of numerous samples, is necessary. A procedure employing gamma-ray spectrometry in conjunction with limited chemical separations n as deiised. Sulfur-35. Sulfur-35 is a pure beta-emitter having a half period of 87.16 days (16) and a maximum beta energy of 0.1674 m.e.v. ( 3 ) . All the sulfur in seawater is assumed to be present as the sulfate ion and is usually determined by precipitation as barium sulfate. The procedure and errors involved are adequately discussed in standard works (19). TKOprocedures are knonn for radiochemically separating S35 from mixed fission products. Metcalf (13) described a procedure in n-hich bromine is used t o convert sulfide to sulfate, rs hich is then precipitated as benzidine sulfate. Decontamination is effected by repeated extractions of the benzidine sulfate in basic solution with isopropyl ether. This is followed by reprecipitation of benzidine sulfate with benzidine hydrochloride in acid solution. A ferric hydroxide scavenge is made, and a final benzidine sulfate precipitation is
performed for chemical yield determination and counting. Handley and Reynolds (7) improved Metcalf's procedure by performing three ferric hydroxide scavenges, four benzidine sulfate precipitations, and three isopropyl ether extractions. A decontamination of greater than 107 and a chemical yield of 80% were obtained from a mixed fission product sample. Approximately 3 hours are required t o perform four determinations. These procedures are rather slow and unsatisfactory for analyzing numerous S35 samples. A more rapid radiochemical procedure was therefore developed. Seawater Samples. I n the development of the two radiochemical procedures, the seawater used was obtained 40 miles off the coast of California. Since seawater contains large concentrations of sodium ion and sulfate ion, relatively constant a t 8.650 and 2.372 gram per kg. of qeawater. these carriers were not added. S o suspended matter (inorganic nor organic particles) was visible in the sean-ater. The concentration of particulate matter (8) in the coastal rmters off Massachusetts (depth 70 meters) is 0.4 to 2 mg. per liter, while that in the ocean Lvater. 100 miles hcyond the continental shelf is 0.2 nig. per liter. Also, dissolved organic substances in seanater from the Sorth Atlantic hare been estimated to be about 5 to 6 mg. pt.r liter. Yaz4 and S3j w e not concentrated by marine organisms (6). Thus, the presence of sodium and sulfur agglomerated in naturally occurring particulate or organic matter was considered nepligiblc. Particulate matter formed from nuclear weapon debris also mould contain negligihle amoiirits of S a p 4and S35. SODIUM-24 PROCEDURE
The KaZ4procedure is as follons: To a hot solution of 25 ml. of seawater and 1 ml. of lanthanum carrier (10 mg. of La per ml.) in a 50-ml. centrifuge tube, add ammonium hydroxide until lanthanum hydroxide precipitates. Centrifuge the solution and filter it through a S o . 42 Whatman filter paper into a clean centrifuge tube. Cool the solution in an ice bath. Pass anhydrous hydroAnalysis.
gen chloride into the solution by means of the anhydrous hydrogen chloride saturation apparatus (description below). Saturate the solution with the gas under pressure (5 p.s.i.g.) for 3 minutes after sodium chloride precipitates. Centrifuge the solution for 30 seconds and discard the supernatant solution. Wash the sodium chloride precipitate with 10 ml. of hydrochloric acid wash solution (concentrated hydrochloric acid saturated with hydrogen chloride gas) and discard the wash. Dissolve the sodium
t
I
-
0 - i
-
r :* !.. .
0
I
IU
chloride in 10 ml. of distilled water. Again precipitate sodium chloride with anhydrous hydrogen chloride. Wash the precipitate once with 5 ml. of the hydrochloric acid wash solution, then filter through a medium-pore sinteredglass filter. Wash i t tn-ice with 5-ml. aliquots of ethyl alcohol saturated with hydrogen chloride gas, and twice with 5-ml. aliquots of diethyl ether saturated with hydrogen chloride gas. Spread out the precipitate a t the bottom of a preweighed weighing bottle (standard
,.
-
'
FISSION PRODUCT M I X T U R E . 3 x 1012 TOTAL F I S S I O N S FISSION PRODUCT M I X T U R E , 3 x I O t o T O T A L FISSIONS ~ 0 2 S 4 OURCE, e x 105 D I S I N T E G R A T I O N S PER M I N . FISSION PRODUCT M I X T U R E . 3 x I O 8 TOTA- F I S S I O N S N a 2 4 SOURCE, 8 x I O 3 DISINTEGRATIONS PER MIN Y o Z 4 S O J R C E , 8 x1O'DlSINTEGRATlONS PER MIN F I S S I O N PRODLCT M I X T U R E - 3 ~ 1 0 ~
-
-
-
-
I
/U
I I368
I
2 750
taper joint No. 29/12), for counting in a previously calibrated gamma-ray spectrometer. After the counting, dry the sodium chloride a t 105' C. in an oven, and cool in a desiccator. Weigh to determine its chemical yield. If necessary, detPrmine the initial amount of sodium in each sample by flame photometry. However, in almost all cases, the amount of sodium in the seawater is known accurately enough. Spectrometer Calibration. Place a known aliquot of a S a z 4solution on a gold-plated VYNS (polyvinylchloride acetate copolymer) film used for 4-pi beta counting. Dry it under an infrared lamp and count in a 4-pi gas-flow beta proportional counter to determine the KaZ4absolute disintegration rate. Pate and Jaffe (14) give a detailed description of the 4-pi beta counting method. Then calibrate the gammaray spectrometer with an aliquot of this Ka24 solution. To obtain a precise measurement of NaZ4activity, evaluate the data obtained from the 1.368 or 2.750 m.e.v. total absorption peak by the method of Covell ( 4 ) . I n this method a given fraction of the area of the total absorption peak is determined by analyzing the digital data defining that area of the peak. This measured total absorption peak is directly proportional to the disintegration rate of the Naz4 source. The Covell method was used because it is particularly insensitive to overlapping photopeaks (where the peaks are distinguishable) compared to other methods known to the author and because it is rapid. The choice of photopeak area to be used is influenced by photopeaks of impurities close in energy, and how well defined these photopeaks are. 256-Channel Gamma Analyzer. The gamma-ray spectrometer used is a modified version of the RCL 256channel pulse-height analyzer, Mark 20, Model 2603 (Radiation Counter Laboratory, Inc., Nucleonic Park, Skokie, Ill.). The detector is a 3 X 3 inch NaI(T1) crystal coupled to a 3-inch Dumont, Model 6363 multiplier phototube, output from which is coupled t o a linear amplifier through a cathode-follower preamplifier. The data print out in digital form. Saturation Apparatus. A 50-ml. centrifuge tube, having a flared top, is fitted with a two-holed rubber stopper. A clamp holds the rubber stopper tightly in the centrifuge tube. One hole holds a gas dispersion tube which is connected to a tank of anhydrous hydrogen chloride. This tube dips below the surface of the solution to be saturated. The other hole holds a glass tube fitted with a glass stopcock. This is connected to a funnel with Tygon tubing. The funnel is inverted and dips 1 mm. into water to trap escaping hydrogen chloride gas.
I
4 118
G A M M A R A Y ENERGY L M E V )
Figure 1 . Gamma-ray spectra of fission product mixture (38 hours old) and radiochemically pure N a 2 4sources
RESULTS AND DISCUSSION
Sodium-24 Procedure. The present procedure for Na24is based on a limited radiochemical separation followed by VOL. 34, NO. 3, MARCH 1962
337
,
ooico
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_
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z
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v
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SCaCC;
0
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;
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_1
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1011
-
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SUCTION
7
51
Figure 2. Efficiency factor for counting the 1.368m.e.v. photopeak of N a Z 4in the 256-channel gamma analyzer
G
Figure 3. A.
a n assay of a high energy Na24 gammaray photopeak. This technique gives a very large effective decontamination from other radionuclides. The precision and accuracy of the procedure mere determined by the assay of the 1.368-n1.e.v. NaZ4 photopeak. The 1.368-, 2.750-, or the 4.118-m.e.v. addition peak may be used for assaying NaZ4. Figure 1 shows the relative advantages of using any one of the photopeaks under various experimental conditions. The analyst often knows how many fissions have taken place in his sample. Dividing the total fissions in the sample by the chemical decontamination factor of 3 X lo5 nil1 give the appropriate pulse height distribution of the fission products in Figure 1. Since age and types of fission products plus induced activities will vary so greatly for different samples, it is assumed here as R rough approximation that the pulse height distribution of the fission products before and after decontamination is the same. The relative areas of the 1.368-, 2.750-, and 4.118-m.e.v. photopeaks of K a Z 4 are 700 to 200 to 1, respectively With this information, the Na24 photopeak to use would be the 1.368-m.e.v. peak if the amount of KaZ4activity was low
Table I.
(21% K 42 Sc46
Mn56
FeSg
c
D. Flask containing 1N N a O H for absorbing HzS, and for reforming HzS with HCl E. Flask containing NHdOH and H202 for absorbing reformed H S (other parts of reduction apparatus are described in text)
(which is usually the case) and the fission product mixture contained less than 1013 fissions; the 2 i.50-m.e.v. peak if the amount of fission products was greater than loi3 fissions or the amount of Na24 n'as sufficiently large to obtain a well defined pulse height distribution vvithin a reasonable counting time; or the 4.118-m.e.~. peak without chemical separation if there are over lo6d.p.m. of Ka24 and less than lo1* total fission-. The stability of the gamma-ray spectrometer, the variation of photopeak counts of Na24 Kith specific activity, and the effect of other gamma-emitting radionuclides were determined and are described below. The precision with uhich Sa24 is measured depends greatly on the stability of the gamma-ray spectrometer. The spectrometer is regularly subjected to quality control procedures to maintain a high degree of stability (4). Experience with the 256-channel analyzer has shown no measurable change in the photoptak counting efficiencies of
Relative Activity of Most Abundant High Gamma Energy Radionuclides Induced b y Thermal Neutron Irradiation of Seawater
Induced Radionuclide Na24
060
cue4
Nie6 Zn66 As75 Bra2 AgllOm
Ti12~ 15 06 h 37 29 m 12 44 h 85 d 2 576 h 45 1 d 527 y
12.8 h 2 564 h 250 d 26 1 h
35 87 h d
270
AS?,*
x 105 x 105 x 104 1 . 5 x 104 1 8
7 1.5
Gamma Emission Rate a t 15 Hr after Irradiation< (Photons 'Pec.) 5 4 x 102 3 x 10-4
1.0 x 107
4 8 5 X lo2 4 x 105 2.1 104
x 53 2 4 x 105 6 9 X lo4 2 x 102
h = hours; m = minutes; d = days; y = years. *0 Normalized From reference ( 1 7 )activity . to the induced by neutrons from 1.32 X occurring in seawater.
0 8 2 5 2 1
5 9 5 6 0 6
s
x 10-7 x 10-4 x 10-3 x 10-9 x 10-4 x 10-10 x 10-2 x 10-4 x 10-9
a
338
ANALYTICAL CHEMISTRY
HI reduction apparatus
HI reduction flask
fission events
C060 for over a year. Also. repeated measurements with the same sample have shown that the variation in counting rate is that predicted by counting statistics. The absolute disintegration rate of Na24 is known through calibration of the spectrometer. The efficiency factor, N'/d.p.m., 2's. the weight of sodium chloride is presented in Figure 2. N' is the amount of activity from a Eaz4 source counted for 100 seconds which is part of the arbitrarily chosen portion of the 1.368-m.e.v. total absorption peak extending 10 channels on either aide of the maximum activity channel. Several radionuclides having gamma energies above 1 m.e.v. could interfere with the measurement of the 1.368or 2.i.50-m.e.v. gamma rays of NaZ4. To establish these interferences, an estimate is made of the relative activities of the pertinent fission products and induced radionuclides occurring in seawater a t 15 hours after irradiation. Table I lists the relative activities of the possible interfering induced activities. Column 3 of this table was taken from an article by Senftle and Champion ( 1 7 ) . Here is defined as the specific initial activity in 1 gram of the target material for 1 second of irradiation with a flux of 10I2 neutrons per sq. cm.-second. Column 4 takes into consideration the relative concentration of the target material in seawater, the relative abundance of the gamma rays having energies above 1 m.e.v., the decay of each nuclide for
a period of 15 hours after fission, and the neutron cross sections for competing reactions. The number of neutrons emitted per fission from UZ3j undergoing neutron fission is approximately 2.5 (2). One neutron is required for the propagation of one fission reaction. Of the 1.5 neutrons left, two nuclides (H1 and (5) absorb approximately 99%. Thus, only a fraction, approximately 6 X lo+, of all
the neutrons released per fission is available for capture by the other components in seawater. Of this, 4.4 X 10-3 is taken by sodium and 2.6 X 10-4 by sulfur, The values in column 4 of Table I were normalized to 1.32 X 1012 fission events to allow direct comparison with calculated fission product activities, given in Table 11. Table I1 lists the fission products having gamma energies above 1 m.e.v. and having relatively high fission yields. Column 3 shows the gamma emission rote of each fission product nuclide at 15 hours post-fission, and takes into consideration the respective mass chain yields ( I O ) . Each value corresponds to the activity from 1.32 X 10I2 fission events. This number of fissions corresponds to the fission products obtained by the irradiation, for 1 second, of 1 gram of enriched U235of 93.2y0 isotopic abundance, at a thermal neutron flux of 10l2 neutrons per sq. cm.becond. Column 3 of Table I1 is directly comparable to column 4 of Table I. Decontamination tests mere made \T ith various radionuclides in the absence of NaZ4. The chemical decontamination factors of the more abundant radionuclides, given in Table 111, are sufficient for satisfactory removal of the interfering radionuclides. The actual decontamination factors are up to several orders of magnitude higher because of the noninterference of the possible contaminating gamma photopeaks on the area measured for the SaZ4 photopeak when using the technique of Cove11 (4). Decontamination of YQ2aiid Ag112 w r e not tested; however, it has heen obqerved that yttrium radiokotopcs are effectively removed by a lanthanum hydroxide scavenge, and that silver radioisotopes are separated by formation of the silver chloride comp k x during the precipitation of sodium chloride iii the presence of a high hydrochloric apid concentration. Excellmt recorery by this procedure was obtained from a solution containing 4.06 X l o 4 d.p.m. of NaZ4,fission productb of 2.1 X 10'0 fissions, and 25 ml. of seawater. The rmilts for four aliquots are prrsented in Table IV. Chemical yields of 70g1 were obtained. The final sodium chloride precipitate, after separation of the fission products, showed no visible contamination of the NaZ4gamma-ray photoprak. SULFUR-35 PROCEDURE
Analysis. To a 10-ml. aliquot of seawater, add 0.5 ml. of concentrated ammonium hydroxide and 5 ml. of hydrogen peroxide. Boil. Add distilled water to make up a total volume of 500 nil. Add slowly 1 ml. of concentrated HC1 and 5 ml. of 1 M barium chloride solution to precipitate barium sulfate. Filter precipitate through a filter tower containing a filter paper
Table II. Relative Activity of Most Abundant High Gamma Energy Fission Products in a Solution Containing 1.32 X 1 0l2 Fissions
Gamma
Emission Rate at 15 Hours after Irradiation T I / * , Hours (Photons/Sec.) 9.7 2 x 104 3.6 4.9 x 104 3.2 1.1 x 102 30 1 . 6 x 103 2.4 7.3 x 102 22.4 9 . 2 x 103 6.7 2 x 106 40 3 . 7 x 103 3.7 1 . 2 x 104
Fission Product Nuclide Sr91 Figure 4. Efficiency factor for counting S36in low background anticoincidence beta counter
YS2
Ag112 Te131m I132 I133 I136
La140
circle, and wash with hot distilled water. If it is necessary to know the amount of sulfate in the seawater sample, a t this point filter the barium sulfate quantitatively, wash with water, alcohol, and acetone, dry in an oven, and weigh. P u t the barium sulfate precipitatefilter paper, and 50 ml. of concentrated hydriodic acid into a 200-ml. 2-necked flask. See Figure 3 for the hydriodic acid reduction apparatus. Reflux this solution until all the barium sulfate dissolves. During this time, apply suction through a GO-ml. separatory funnel (H).The hydrogen sulfide gas is absorbed in 7 5 ml. of 1 N sodium hydroxide in a 150-ml. flask (D)after passing through a gas dispersion tube (Cj. $fter the BaSOl reduction is complete, apply suction through a 150ml. flask ( E ) . Expel H2S gas by adding 20 ml. of concentrated hydrochloric acid to the sodium hydroxide solution in Flask D. Collect, with suction, hydrogen sulfide gas through a gas dispersion tube ( F ) into flask E which contains a solution of 25 ml. of concentrated ammonium hydroxide and 60 ml. of 307, hydrogen peroxide. An ice bath, G, is necessary to cool the reaction vessel and prevent foaming. Use rubber stoppers and Tygon tubing. i h e n the oxidation and collection of hydrogen sulfide are completed, transfer the solution from Flask E to a 1-liter flask containing approximately 200 ml. of distilled water. Boil to drive off excess ammonia and hydrogen peroxide. (Care is required as this solution boils vigorously.) -4dd about 0.5 ml. of concentrated hydrochloric acid and 5 ml. of Llf barium chloride to precipitate the barium sulfate. After digestion, filter, n-ith the aid of a filter tower, the barium sulfate onto a pren-eighed KO. 42 Whatman filter paper circle. Wash the precipitate twice with hot distilled water, ethyl alcohol, and acetone. Then dry in an oven a t l 0 5 O C., cool in a desiccator, and weigh. Mount the precipitate on a brass planchet assembly and cover with a thin film of rubber hydrochloride. A rubber O-ring fitted around the brass planchet keeps the precipitate in place. Count the barium sulfate source in a low background beta counter. Calibration. To facilitate determining the absolute disintegrations per
La141
Table 111. Chemical Decontamination Factors of Some Interfering Isotopic Radionuclides in Na24 Procedure
Radionuclides Srg Ba133
Te fission-product isotopes in equilibrium with respective I daughters 1131
Fission products (38 hours old) Table IV.
Sam-
Decontaniination Factor 1 x 104 1 x 102 6 X lo3 6 X lo2
1 3
x 104 x 105
Recovery of Sodium-24
Sodium-24, D.P.R.I. Taken Found 4.06 x 104 4.15 x 104 4.06 x 104 4.02 x 104 4.06 x 104 4.12 x 104 4.06 x 104 4.10 x 104 AV. 4. io x 104
No. 'le 1 2
3 4
=t1.2cj,
minute of S35, the counter must be calibrated for geometry, self-absorption, and self-scattering. An efficiency factor, the ratio of coupts per minute to disintegrations per minute as a function of the source weight of barium sulfate, can be constructed as follows: P u t a n equal aliquot of S3j of known disintegration rate into each of six 250-ml. Erlenmeyer flasks. (The S35 used was calibrated a t the Sational Bureau of Standards and was reported to have been determined with a precision of =k3% by gas counting as sulfur dioxide.) The flasks contain various known amounts of a standardized sulfuric acid carrier. After mixing thoroughly, precipitate barium sulfate, weigh, mount on brass planchet assemblies, and count. Plot the ratio of the net count rate to the absolute disintegration rate corrected for chemical yield against the weight of barium sulfate precipitate (Figure 4) I
VOL. 34, NO. 3, MARCH 1962
339
Table V.
Recovery of Sulfur-35 from Seawater
SamPle
Sulfur-35, D.P.M. Found KO. Taken 1 8 . 2 X 10’ 8 . 3 X 1 0 ’ 5 5.170 2 8 . 1 x 103 7 . 9 x 103 i 5.470 3 1.62 X l o 4 1.47 X l o 4 i 3.1y0
Table VI. Decontamination Factors of Various Radionuclides in Y5Procedure
Radionuclides
Decontamination Factor > 107 > 107 2107 107 >I06 >io7
1106 > 106
105
Mixed Fission Products (18 days old)
> 106 >107
RESULTS AND DISCUSSION
Sulfur-35 Procedure. T h e initial treatment of the sample with hydrogen peroxide oxidizes a n y hydrogen sulfide t o sulfate ion and ensures complete exchange between 5 3 5 and its sulfate carrier. Bush ( I ) reported that with concentrated sulfuric acid and excess concentrated hydrogen iodide the reaction
product is mainly hydrogen sulfide according to the equation 2H’
+ SOL-* + 8HI
-+
Hi3
+ 412 + 4HzO
However, it is posbible to obtain sulfur dioxide if the HI is slightly diluted. The results of measurements of recovery of known amounts of S35 introduced into seaiyater are presented in Table V. The usual corrections n-ere applied for chemical yield and counting efficiency corresponding to the n-eight of barium sulfate precipitate recovered. Each value represents the average of triplicate determinations, and the precision of such determinations was about = ! ~ 5 7 ~Errors ,. are due mainly to difficulties in obtaining barium sulfate precipitate mounts which were sufficiently reproducible for precise counting of the weak beta-rays of S35. This problem can be partially eliminated by adopting the procedure for assaying S36through gas counting (12) or liquid scintillation counting (9) techniques. The procedure gives decontamination factors of >IO7 from mixed fission products (Table VI), chemical yields of 70 to 80%, and a precision of about &5%. This procedure is also adaptable to the simultaneous determination of a large number of samples. LITERATURE CITED
(1) Bush, F., J. Phys. Chem. 33, 613 (1929). (2) Charpie, R. A., Dunworth, J . V., ed., “International Series of Monographs on Nuclear Enerev: Phvsics of Nuclear Fission,,, p. 75, &gam& Press, 1958. (3) Conner, R. D., Fairweather, I. L.,
[’roc. Phys. SOC.(London) 70A, 769 (1957). ( 4 ) Covell, D. F., ASAL. CHEM.31, 1785 (1959); U. S. Naval Radiological Defense Laboratory Technical Rept., USNRDL-TR-288, December 1958. (5) Dyrssen, D., Nyman, P. O., Ada Radiol. 43, 421 (1955). (6) “Effects of Atomic Radiation on Oceanography and Fisheries,” Publication KO.55, National Academy of Sciences, National Research Council, Washington, D. C., 1957. (7) Handley, T. H., Reynolds, S. *4., Oak Ridge National Laboratory, CF-56-7118, July 23, 1956. ( 8 ) Harvey, H. W.,“The Chemistry and Fertility of Seawater,” p. 9, Cambridge University Press, Cambridge, 1960. (9) Jeffay, H., Olubajo, F. O., Jewel], W.R., AXAL.CHEW32, 306 (1960). (10) Katcoff,S., A‘ucleonics 18,201 (1958). (11) Knowles, J. IT., Can. J . Phys. 37, 203 (1959). (12) Merritt, W. F., Hawkings, R. C., ANAL.CHEK 32, 308 (1960). (13) Metcalf, R. P., “Radiochemics;! Studies, The Fission Products, Book I, C. D. Coryell, N. Sugarman, eds., National Nuclear Energy Series, Paper No. 47, p. 478, McGraw-Hill, 1951. (14) Pate, B. D., Yaffe, L., Can. J . Chenz. 33, 610 (1955). (15) Prestwood, R. J., “Collected Radiochemical Procedures,” LA-1721, 2nd ed., J. Rleinberg, ed., Los Alamos Scientific Laboratory, Los Alamos, August 1958. (16) Seliger, H. H., Mann, W.B., Cavallo, L. M., J . Research AJatl. Bur. Standards 60, 447 (1958). (17) Senftle, F. E., Champion, W. R., Nuovo Cimento, Suppl. 12, 549 (1954). (18) Sreb, J. H., Phys. Rev.81,469 (1951). (19) Willard, H. H., Furman, N. H “Elementary Quantitative Analysis,;’ pp. 363 ff., Van Nostrand, New York, 1940.
RECE~VED for review September 11, 1961. Accepted January 2, 1962.
Radioassay of Metha nethio I-S u Ifur- 35 a nd Methanethio I-Ca rbon- 14 ROLFE H. HERBER Ralph G. Wright Laboratory o f Chemistry and the Nuclear Science Center, Rutgers, The State University, New Brunswick,
b A method for the radioassay of sulfur-35-labeled methanethiol is based on the precipitation of the mercuric salt of the methyl sulfide anion when the labeled mercaptan is transferred by high vacuum gas handling techniques into an aqueous solution of mercuric cyanide. The precipitates are filtered into M-grade sinteredglass crucibles, washed with distilled water and 95% alcohol, and then dried to constant weight a t 85 =t 5 ” C. Radioassay of the precipitates which have been prepared in layers of uniform thickness is carried out 340
0
ANALYT!CAL CHEMISTRY
using a thin-window gas flow counter, Correction factors for the self absorption of the soft beta radiation in the samples have been determined and good reproducibility of specific activity has been obtained. This method is equally applicable to the radioassay of methanethiol-carbon-14.
I
an investigation (9, 11) of the chemical consequences of the Cl35 ( ~ , P ) Sreaction, ~~ it has been found that when thermal neutron irradiated hexachloroethane is scavenged under N
N. 1.
high vacuum conditions with methanethiol, sulfur-3&labeled CH3SH can be obtained. The quantitative study of such hot atom reactions requires the exploitation of a reproducible radioassay technique for the sulfur radioactivity which is produced. Due to the low energy (Emax, = 0.167 m.e.v.) (4) of the S35radiation, gas phase radioassay methods ( I , 10, l 4 , 1 6 ) which have been developed for the quantitative determination of (2186, Br*Q,1’28, P3*, and other “hard” beta emitters, cannot be employed in the present case. Alternatively, a number
