ANALYTICAL CHEMISTRY
176 I n general, the strict precautions observed in working with carbon monoxide should be practiced in working with the cobalt carbonyls. Dicobalt octacarbonyl has a low vapor pressure and there is little danger of inhaling its vapor. Cobalt hydrocarbonyl has a high vapor pressure and may be very toxic, but, in the absence of an atmosphere of carbon monoxide, it starts to decompose well below room temperature into hydrogen and dicobalt octacarbonyl. The apparatus used in the analytical determinations was placed in a large well ventilated room with an exit tube leading to the hood for venting carbon monoxide. The hydrocarbonyl was prepared in a hood. ACKNOWLEDGMENT
The authors wish to thank Jack Sharkey and R. A. Friedel for the gas analyses and Sol Metlin for assistance in preparation of dicobalt octacarbonyl.
LITERATURE CITED
Adkins, H., and Krsek, G., J. Am. Chem. Soc., 70, 383 (1948). Blanchard, A.A., and Gilmont, P., Ibid., 62, 1192 (1940). Hieber, W.,Muhlbauer, F., and Ehmann, E. A., Ber., 65, 1090 (1934).
Hieber, W., and Schulten, H., 2.anorg. u. allgem. Chem., 232, 17 (1937). Ibid., 243,145 (1939). Hieber, W., and Teller, U., Ibid., 249, 43 (1942).
Koller, C.R.,and Barusch, M.R., TND. ENG.CHEM.,ANAL.ED., 14,907 (1942).
Orchin, M., and Wender, I.,A N ~ LC. H E Y . ,21, 875 (1949). This apparatus is now available from H. Tarmy Co., 1035 Watson St., Pittsburgh 19, Pa. Wender, I., and Orchin, hl., Bur. Mines, Rept. Invest. 4270 (1948). RECEIVED June 5 , 1951.
Nonorthophosphate Contaminant of NeutronIrradiated Rock Phosphates Procedures f o r Its Removal A . J. MAcKENZIE AND J. W. BORLAND Bureau of Plant I n d u s t r y , Soils, a n d Agricultural Engineering, U . S . D e p a r t m e n t of Agriculture, Beltsville, Md. Orthophosphate compounds, upon irradiation with slow (thermal) neutrons, yield a radioactive product, the P32content of which is only partly in the orthophosphate condition. The compounds are therefore not suited for tracer experiments without treatment to convert all the P32to the orthophosphate form. Calcium phosphates tested showed a considerable amount of nonorthophosphate P32,depending partially upon conditions of irradiation. A method is presented for determining nonorthophosphate P32in
S
LOW (thermal) neutron irradiation of orthophosphate com-
pounds, no matter how pure they may be, yields a radioactive product, the P3*content of which is only partly in the orthophosphate condition. The nonorthophosphate, presumably produced as a consequence of the recoil of the P31 nucleus from gamma-ray emission as it passes to the ground state, has been identified as phosphite in irradiated alkali phosphates ( 4 , 6 , 8 )and as hypophosphate in irradiated calcium phosphates ( 3 ) . The possible presence of P32 in more than one form in irradiated samples necessitates a careful examination of any irradiated phosphate that is to be used as a source of radioactive phosphorus in tracer experiments. The basic requirement for a proper label is that all the P32in the labeling compound be in, or readily convertible to, the same form as the phosphorus compound to be labeled. Recent work in this laboratory ( 9 ) points to pyrophosphate as the dominant form of nonorthophosphate in the irradiated potassium dihydrogen phosphate distributed by the Oak Ridge S a tional Laboratory-the Pa2 source used extensively in late years by this bureau for labeling phosphate fertilizers for greenhouse and field experiments (6). Moreover, it was found that the labeling processes provide conditions favorable to the reconversion of the nonorthophosphate, so that the labeled fertilizers carry an insignificant amount of it. The same end could probably be attained in any labeling process involving synthesis of a compound. On the other hand, the introduction of Pa2 into a natural material, such a8 phosphate rock, which cannot be synthesized in the laboratory, must be accomplished by direct irradiation of the
calcium phosphates. A treatment is shown that will result in an irradiated rock phosphate suitable for plant investigations. This work should impress users of neutron-irradiated compounds with the frequency of occurrence of this phenomenon. The presence of more than one radioactive isotope, if not noted and eliminated by conversion or compensation, invalidates tracer experiments. A n important fertilization material, rock phosphate, can now be used in tracer experiments.
material. As the irradiated rock must be utilized directly as the tracer source, a nonorthophosphate content is a serious drawback (4).Phosphate rock is used extensively (more than 800,000 short tons in 1949) for direct application to the soil, and thus considerable interest attaches to the measurement of its effectiveness as a source of phosphorus in the growing of vegetation with the aid of the radioactive tracer technique. Looking to this end, a study was made of the amount and distribution of nonorthophosphate in irradiated phosphate rock, and of treatments that might be used to destroy the nonorthophosphate without seriously altering the properties of the rock. Results of this study are reported here. PHOSPHORUS SPECIFIC ACTlVlTY AND NONORTHOPHOSPHATE CONTENT OF SOME IRRADlATED CALCIUM PHOSPHATES
In the spring of 1950 eight calcium phosphates and a sample of potassium acid phosphate m-ere irradiated in the neutron pile a t Oak Ridge under various conditions of neutron flux and temperature. Samples of each of the nine materials Tere exposed to a high neutron flux with an intermediate (water-cooled) temperature, a low neutron flux with a low temperature, or a high neutron flux with a high temperature. These samples, after irradiation, were returned to the laboratory and phosphorus specific activity and nonorthophosphate content were determined (Table I). It can be seen from Table I that a great part of the phosphorua activity in neutron-irradiated calcium phosphates may exist as a nonorthophosphate contaminant, The nonorthophosphate content in each case was higher in the calcium phosphates than in
V O L U M E 2 4 , NO. 1, J A N U A R Y 1 9 5 2 Table I.
177
Nonorthophosphate Content of Phosphates Irradiated under Selected Conditions
Table 111. Konorthophosphate Content and Phosphorus Specific Activity of Phosphate Rock
spz:k1l&oCAoNonorthophosphate
Phosphate KHd'On Florida land pebble Curacao rock Hydroxylapatite Morocco rock Steamed bone Fluorapatite Tricalcium phosphate
Irradiation tiaityb, Relative % of Treatment a Units total Pa1 B 0.364 18.31 B 0.561 26.69 C 1.650 0.57 A 0.362 48.28 B 0.526 48.86 C 1.549 31.01 A 0,340 46.33 B 0.518 52.51 C 1,504 30.33 4 0 349 37.85 B 0.511 40.46 C 1.534 37.46 A 0.378 29.52 B 0.565 41,27 C 1,570 18.83 A 0.394 31.51 B 0.567 36.42 C 1.502 31.04 A 0.365 34.59 B 0.515 37.73 C 1.420 26.72 A
B C
Phosphate glass
A
B C
0.416 0.564 I. 567 0.387 0.533 1.357
51.78 48.45 42.79 37.97 33.49 31.33
Days after irradiation 39 22 22 40 23 23 41 24 24 104 87 87 105 88 88 146 129 129 146 129 129 146 129 129 146 129 129
Treatment A. High neutron flux, intermediate temperature (watercooled). Irradiation period, April 24 to N a y 5, 1950. Treatment B. Low neutron flux, low temperature. Irradiation period, 4 weeks beginning April 24, 1950. Treatment C. High flux, high temperature. Irradiation period, 4 weeks beginning April 24, 1950. b Results corrected to activity of J u n e 13. 1950. a
Table
11. Observed
Phosphate Iirr2p04 Florida land pebble Curacao rock Hydroxylapatite Morocco rock Steamed hone Fluorapatite Tricalcium phosphate Phosphate glass
Half-life of hctivity
Nonorthophosphate
Observed Half-life, Days 13.5 14.6 14.2 14.0 13 6 13.9 13.6 13.0 14.3
t h e potassium acid phosphate. The phosphorus specific activity
of all materials for each treatment u-as not significantly different Treatment differences in specific activity were due to the short time of irradiation in Treatment A and to the differences in neutron flux between Treatment B and Treatment C. Half-life determinations on the activity given by the nonorthophosphate fractions of the irradiated phosphates indicated that the activity was due to P32 (Table 11). The exact forms of the nonorthophosphate have not yet been determined. However, analysis using manganese chloride a t p H 4.2 as a precipitant ( 2 ) showed 16 to 307, of the nonorthophosphate in irradiated phosphate rock to be pyrophosphate. Investigating further the nonorthophosphate content and phosphorus specific activity of irradiated calcium phosphates, four samples of phosphate rock (Florida land pebble) Ivere sealed in quartz ampoules under vacuum, and under atmospheres of argon, air, and oxygen, respectively. These samples then lvere irradiated and later analyzed for nonorthosphosphate content and phosphorus specific activity. The results of the determinations on the four samples are given in Table 111. There appeared to be no significant differences in the phosphorus specific activities of the four samples, although there was some variation in the nonorthophosphate content. Apparently, under the above conditions, there v a s no large effect on the nonorthophosphate content due to the differences in atmosphere during irradiation.
Sample 1 2
(Irradiated in vacuum. argon, oxygen, and air) Total Phosphorus Konorthophnsphate, Specific Activity, % of Total P31 Relative Cnits Treatment 29.26 1.370 Sealed in vacuum Sealed in atmosphere of oxygen 31 66 1.439
3
Sealed in atmosphere of argon
34 36
1.413
4
Samule ienited and sealed inatmosphere of air
38 53
1.400
ELIlIINATION O F NONORTHOPHOSPHlTE CONT.AMIK.AST O F 3EUTRON-1RRADIATED ROCK PHOSPHATES
It has been shown that neutron-irradiated calcium phosphates possess a radioactive phosphorus contaminant which is not of the ortho form. T o make these materials suitable for radioisotope studies it is necessary to convert the nonorthophosphate P32 to the orthophosphate form. Experiments on irradiated phosphate rock have been performed v ith this goal in mind. The irradiated phosphate rock was treated in various manners in an effort to reduce the nonorthophosphate content to a negligible amount without changing the physical or chemical properties of the rock under various treatments involving heat, pressure, and moisture. After each decontamination treatment] nonorthophosphate content and specific activity were determined, either by dissolving the entire sample and determining the average nonorthophosphate content and specific activity, or by performing a stepwise dissolution with dilute acid, followed by determination of the nonorthophosphate content and specific activity in each extract. Any changes in the distribution of the nonorthophosphate content or specific activity could he observed by the use of the latter procedure. The various treatments and the results of decontamination treatments on a sample of irradiated phosphate rock (Florida land pebble, sample C, Table I ) are given in Table IV. Of the procedures listed in Table IV, only Treatment 9, where the irradiated phosphate rock was heated in air a t 500" for 91 hours, was successful in reducing the nonorthophosphate content to an insignificant amount without altering the material. Heating a t lower temperatures for even longer periods failed to decontaminate the phosphate rock as successfully. Bomb treatments substantially reduced the nonorthophosphate content, but alteration of the material was observed a t the 300" and 350" C. heat treatments. Alteration was indicated by the formation of a
Table 17'. Treatment KO.
Description
1 2
Check Heated in water a t 90' C. for 3 hours Heated in water a t 90' C. for 6 hours Heated a t 200' C. for 91 hours, sealed in bomb with water present Heated a t 300' C. for 91 hours, sealed in bomb with water presentb Heated a t 350' C. for 91 hours, sealed in bomb n i t h water presentb Heated a t 200' C. for 91 hours, dry a t atmospheric pressure
3 4 D
6 7 8 9 a b
Effect of Decontamination Treatments on Irradiated Phosphate Rock" Sonorthophosphate, % of Total P31 27.5 18 2 24.5
.
Heated a t 200' C. for 241 hours. dry a t atmospheric pressure Heated a t 500' C. for 91 hours, dry a t atmospheric pressure
Florida land pebble phosphate rock. sample C , Table I. Pho3phate rock altered by treatment.
16.2
7.0 4 3 22 3 17.5 1.6
ANALYTICAL CHEMISTRY
178 Table V. Distribution of Nonorthophosphate Contarninant and Phosphorus Specific Activity in NeutronIrradiated Phosphate Rocka }:\tract
(As shown by stepwise dissolution analysis) Dissolved Pa', Specific Activity, Nonorthophosphate. No. % Total P31 Relative Units 0'9 of Total P32
sample 2, Table 111). The treatments involved heating of the phosphate rock in the presence of steam a t atmospheric pressure for 48 and 168 hours, respectively. The results of the treatments are shown in Table VI. Almost complete decontamination was accomplished by the 450" C. heat treatment for 168 hours. Chemical and petrographic examination of the treated samples showed that no great chemical or physical alterations had occurred in the material due to the treatment. This product would be suitable for phosphate investigations with plants. Chemical and petrographic analysis of the phosphate rock samples before and after various decontamination treatments are given in Table VII. DETER.MIVATION OF NOYORTHOPHOSPHATE CONTEIIT
a
I'hospliate rock sample, Treatment 8. Table I V .
silireous crust, both inside and outside the glass sample container, which M hen analyzed was found to contain 1.58% fluorine. A petrographic examination of the treated samples also 'revealed an increase in refractive index of the phosphate (examination made by J . G. Cady). The t-ypical data of Table V were obtained by analyzing a sample of neutron-irradiated phosphate rock by the step\+ise dissolution technique. I n treatments for the elimination of nonorthophosphate contaminant, stepwise dissolution analysis showed that it n a s reduced to its lowest amount in the initial extracts and either remained the same or gradually increased in the succeeding extracts. Phosphorus specific activity showed that the distribution of P 3 2 in the irradiated phosphate rock was uniform, except that there waa a slightly higher concentration in the initial extracts-a condition that points to a concentration of the P 3 2 in the surface layers of the particles Halt-life determinations and aluminum absorption measurements on the radiations from these initial extracts showed that the radiations uere not different from the radiations of the succeeding extracts and that the radiations were due to Pat. Additional decontamination experiments were attempted on another sample of irradiated phosphate rock (Florida land pebble,
Reagents. Hydrochloric Acid, 3 Y. Dilute 250 ml. of concentrated hydrochloric acid (37.69'') to 1 liter. Ammonium Molybdate. Add 111.6 grams of Moo3 to.500 ml. of water. Slowly add 110 ml. of concentrated ammonium hydroxide (specific gravity 0.90) with constant stirring. Filter and dilute to 1 liter. .Immonium Kitrate, 5% solution. Dissolve 50 grams of ammonium nitrate in 500 ml. of water. Dilute to 1 liter. Sodium Dihydrogen Phosphate, 2 mg. of phosphorus per ml. Dissolve 8.91 grams of sodium dihydrogen phosphate monohydrate in 500 ml. of water. Dilute to 1 liter. Hydrogen Ion Exchange Resin. Make a slurry of a cation exchange resin (Amberlite IR-100) with 3 iV hydrochloric acid and allow to stand for several hours with occasional mixing. Decant the liquid and add fresh 3 *V hydrochloric acid, repeating this process several times. Finally decant the supernatant liquid and wash the resin with water until free of hydrochloric acid. Add enough water to cover the resin and store for future use. Nitric Acid, reagent grade, 70y0 (specific gravity 1.42). Ammonium Nitrate, reagent grade crystals. Preparation of Hydrogen Ion Exchange Resin Column. Irradiated phosphate rock contains radioactive cations, calcium and iron, TI hich are conveniently separated from the radioactive phosphorus by passing the solution of the sample through a cation exchange column.
Place a wad of glass wool in the bottom of a 50-ml. buret and add 30 ml. of the hydrogen ion-exchange resin. Wash the column with water until the wash water is neutral. T o regenerate the column after use, back-n ash the column with water and then regenerate the resin by passing 500 ml. of 3 S hydrochloric acid through the column a t a rate of 10 ml. per minute. R a s h with water until the wash water is neutral. The column is then ready for use again. Collect all solutions coming from the Table VI, Decontamination of Irradiated Phosphate column during regeneration and washing and handle as radioactive Rocka by Heating at 450" C. in %earn at Atmospheric li uid wastes of calcium and iron. Pressure Brocedure. For each sample of irradiated rock phosphate dissolve approximately 25 mg. of the sample in 30 ml. of cold 0.1 1%' Treatment Sonorthophosphate. nitric acid. Pass 25 ml. of this solution through the hydrogen NO. Description % of Total Pa2 ion-exchange resin column a t a rate of 10 ml. per minute. Collect 10 Check 31.66 the effluent liquid in a 100-ml. volumetric flask and wash the 11 48 hours 7.20 column with water until the total effluent volume is 100 ml. Desig168 hours 1.93 12 nate this solution, which is free of all radioactive cations, as assay solution I. (' Florida land pebble phosphate rock, sample 2, Table 111. Take a 75-ml. aliquot of assay solution I and transfer to a Table VII. Composition of Phosphate Rock after Nonorthophosphate 250-ml. beaker. Add 5 ml. of Decontamination Treatments the sodium dihydrogen phosCitratephate solution and then disInsoluble Cor. F, solve I5 grams of ammonium Treatment. S o . Treatment Pros, % % ' % Petrographic Examinationn nitrate in the solution. Add 10 1 Check 32.67 1.50 3 . 7 5 ml. of concentrated nitric acid, 4 Heated at 200' C. for 91 hours, sealed then 25 ml. of the ammonium in bomb with water present . . . . . . . . . . . . KO alteration noticeable molybdate reagent with stirHeated a t 300' C. for 91 hours, sealed 5 ring. Allow the solution to in bomb with water present . . . . . . , . 3 . 5 0 Alteration indicated b y formastand for 30 minutes. Stir oction of siliceous crust and incasionally during that time, and crease in refraction index decant the supernatant liquid 6 Heated a t 350' C. for 91 hours, sealed in bomb with water present .... . . . . . . . . .%Iteration indicated b y formathrough a Whatman KO.42 tion of siliceous crust and infilter into a 200-ml. volumetcrease in refraction index ric flask. Wash the precipi9 Heated a t 500' C. for 91 hours, d r y a t tate several times by decantaatmospheric pressure 31.73 1 . 5 4 3 . 5 6 KO alteration noticeable Heated a t 450 C. for 168 hours in tion with 5% ammonium niateam a t atmospheric pressure 32.85 1.55 3 . 4 3 No alteration noticeable trate solution. Petrographic examinations made b y J. G. Cady. To the combined filtrate and washings in the 200-ml. volu-
..
.
V O L U M E 24, NO. 1, J A N U A R Y 1 9 5 2 metric flask, add concentrated ammonium hydroxide until the solution is alkaline to phenolphthalein. Allow to cool to room temperature and dilute to volume with water. Designate this solution, which contains the nonorthophosphate contaminant, as assay solution JI. Dissolve the ammonium phosphomolybdate precipitate left in the beaker and filter in 1 S ammonium hydroxide and discard as P3* n-aste Take a 5-nd. aliquot of the cation-free assay solution I and make up to a volume of 200 ml. with 0.1 N nitric acid. Designate this solution as assay solution 111. This solution contains the activity of the irradiated phosphate. total Count assay solutions I J and I11 to 10,000 total counts by the use of a solution counter. Calculate the nonorthophosphate content as a percentage of the total P 3 2 activity from the following equation: Sonorthophosphate yo of total P32 = counts per second on solution I1 x 100 counts per second on solution I11 DETERRIINATION O F T O T 4 L PHOSPHORUS
('olorimetric P 3 I determinations were made on assay solution I b j the ammonium metavanadate procedure proposed by Mission ( 7 ) and also used by Barton ( 1 ) in the analysis of the phosphate rock. Sonorthophosphate is not measured by thisprocedure, but its conversion to orthophosphate may be effected by boiling in an oxidizing acid solution. This is not necessary, however, because the substance is carrier-free and the weight of nonorthophosphate, like that of the total P32 complement, is negligibly small in comparison with the total phosphoms. Reagents. Nitric ,kcid, T.5 S. .imnionium hletavanadate. Thssoive 2.345 grams of ammonium metavanadate in 500 ml. of hot water, add 10 ml. of nitric acid (specific gravity 1.42), and dilute the solution to 1liter. .kmmonium Molybdate. Dissolve 90 grams of ammonium molybdate tetrahydrate in 500 ml. of warm water (50" C.) containing 10 ml. of ammonium hydroxide (specific gravity 0.90). Filter and dilute to 1 liter. Procedure. Pipet an aliqout of assay solution I into a 50-ml. volumetric flask. This aliquot should contain between 50 and 300 micrograms of phosphorus and should not be more than 25 ml. If the aliquot is less than 25 ml., dilute to 25 ml. with water. Add 10 ml. of 7.5 N nitric acid, mix well, and add 5 ml. of ammonium metavanadate reagent. Mix well and add 5 ml. of the ammonium molybdate reagent. Dilute to volume with water and mix n ell. Allow the color to develop for 0.5 hour and then read on a suitable colorimeter using a 420 mM filter. Convert to micrograms of P31, using a calibration curve prepared by the above procedure on knon n amounts of phosphorus. Calculate micrograms of P31 per milliliter of assay solution I11 by the use of the dilution factor DETERRIIRATIOh OF T O T 4 L 1WOSl'HORL S SPECIFIC ACTIbITY
The activity of assay solution I11 per unit of P31present is a measurenient of the phosphorus specific activity of the irradiated sample. A unit of P31 may be expressed as microgram5 of P31 per milliliter of assay solution 111. Procedure. Count assay solution I11 to 10,000 counts with the use of a solution counter. Calculate the specific activity by dividing by the micrograms of P3: per milliliter of assay soiution 111. COMM~IEATS ON PROCEDURE. The degree of accuracy attainable in the determination of specific activity by the above procedure was investigated. Four separate samples of an irradiated Sample \$ere taken and specific activity measurements made. The counting technique was invcstigated by making tr5-o 10,000 total counts on cach solution and also by repeating the counting on three separate days. From the results (Table VIII) it appears that a variation of as much as &2.5% may be expected in specific activity measurements determined by this procedure. STEPWISE DISSOLUTION MEASUREMENTS
I n studying the radioactive phosphorus distribution within neutron-irradiated calcium phosphates, a stepnise dissolution was
179 performed on the materials by successive extractions with dilute acid. Analysis of each extract for nonorthophosphate content and phosphorus specific activity then gave information on the distribution of the nonorthophosphate contaminant and the amount of radioactive phosphorus in each of the extractable layers of the irradiated phosphates. Reagents. Nitric acid extracting solutions, 0.001 N , 0.005 S, 0.01 S,and 0.1 AT. Procedure. Transfer a sample (50 to 200 mg.) of the irradiated calcium phosphate to a 15-ml. conical-shaped centrifuge tube, and extract it successively n-ith 10-ml. portions of dilute acid. Upon each addition of the dilute acid, stir the mixture with a glass rod for 2 minutes and then centrifuge for 10 minutes. Decant the clear supernatant liquid after each extraction into a prepared hydrogen ion-exchange resin column. Pass each extract through a column a t a rate of 10 ml. per minute collecting the effluent liquid in a 100-ml. volumetric flask. Wash the column with water until the total effluent volume is 100 ml. Analyze each extract for nonorthophosphate content and specific activity a8 described above.
COMMESTS O N PROCEDURE. The practice of making eleven or twelve extractions on the irradiated material with dilute acid, extracting about 2 to 370 of the total phosphorus in each extract and then dissolving the remainder of the sample in 0.1 N acid, is usually followed. The sum of the phosphorus in all solutions is taken as the total phosphorus in the sample.
Table VIII.
Reproducibility Attainable in Determination of Specific iictivity
S o . of Rep-
licates
Date Counted
4 4 4
11/20 11/22 11/24
Average Deviation f r o m Mean, % Specific Activity PSI activity 0.68 0.90 1.32 1 08 0.41
0.90 0.90
2.00 2.50
The percentage of the total phosphorus dissolved by each extraction depends upon the size of the sample and the amount of acid present. The time of each extraction was standardized at 2 minutes. In the course of these investigations it was necessary to vary the size of sample and acid concentration so as to obtain extracts that had moderate amounts of P32activity present. As the specific activity of an irradiated sample decreased through decaj-, the use of larger samples was necessary. Sample weights varied from 50 to 200 mg. Concentrations of the acid extracting solutions varied from 0.001 h ' to 0.01 IY, higher acid concentrations being necessary with the larger samples of material in order to extract the desired 2 to 370 of the total phosphorus. ACKNOW L E D G M E h T
The authors wish to express their appreciation to IF'. L. Hill and L. A. Dean of this bureau for their invaluable help and constructive criticisms during these investigations and to J. H. Gillette and associates of the Oak Ridge Sational Laboratory for their cooperation and suggestions. Preparation of the samples and arrangements for their irradiation weie carried out by Xaurice Fried of this bureau LlTERATURE CITED
Barton, C. J., h s . 4 ~CHEM., . 20, 1068 (1948). Borland, J. W., unpublished data. Fiskell, J. G. A., Science, 113, 244 (1951). Fried, hf., and MacKensie, -4.J., Ibid., 111, 492 (1950). Hill, R. L., Fox, E. J., and Mullins, J. F., I n d . Ew. Ckem., 41, 1328 (1949). (6) Libby, F'. W., J . Bm. Chem. SOC.,62, 1930 (1940). (7) Mission, G., Chem. Ztg., 32, 633 (1908). (8) T h o m a s , E. D. E., and Nicholas, D. J. D., Nature, 163, 719 (1949). (1) (2) (3) (4) (5)
RECEIVED M a y 31, 1951. Investigation supported i n part by the U. S. Atomic Energy Coinmission.
