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LABORATORY EXERCISES IN NUCLEAR CHEMISTRY* ZII. Preparation and Properties of Several Halogen Activities ROBERT H. SCHULER', RUSSELL R. WILLIAMS, JR., and WILLIAM H. HAMILL University of Notre Dame, Notre Dame, Indiana
IT
1s an important and interesting fact, not yet realized by many scientists, that the ~urchasecost of radioisotopes ( I ) is often the smallest item of expense in an experiment utilizing tracer techniques. since the capital investment in permanent equipment for such work may be limited to a few hundred dollars, it is apparent that this very powerful method is well within the means of many industrial and university laboratories. H ~ the ~~ t E~~~~~ ~ ~ ~~ ~ ~i ~ rightly restricts the distribution of radioactive tracers to persons and institutions properly qualified for their use, no doubt leaving many workers with the need desire for such materials, but without the necessary training. This situation is being remedied in a number of ways; by training courses offeredby the Commission (9) and by installationof university courses in the subject. This article is the third in a series (3, 4) describing some student experiments on nuclear chemistry. The preparation and characterization of a radioactive tracer is an important preliminary step in the application of tracer techniques and frequently involves rather simple but specialized chemical procedures. The principles of such operations have been previously discussed (3) and it is the purpose of the present article to describe simple experiments which apply these methods to the preparation of certain halogen activities. The simplest and most generally useful method of preparing radioactive isotopes is through the slow neutron capture reaction. Bromine and iodine yield, with reasonable eEciency, active species as shown in Figure 1. Whiie the specific activity which can he induced in these elements with a 5 mg. Ra-Be neutron source is comparatively small (approximately lo4disintegrations per second per mol) and is practically undetectable without enrichment, the Szilard reaction may he employed to yield a useful specific activity. As previously described (3) this processby virtue of the recoil energy of gamma radiation from the neutron captureinvolves the ejection of the activated atoms from the target molecule into a form which may he chemically distinguishable and separable from * Presented at the 114th Meeting of the American Chemical Society, St. Louis, Missouri, September, 1948. ' Present address, Canisius College, Buffalo, New York.
the large residue of unchanged target molecules. Thus a considerable portion of the activity of lo4dps originating in a mol of target substance may be conce~trated into a millimol or less of extracted halogen. This represents an enrichment of a p ~ r o ~ a t one e l thou~ sandfold and a specific activity of useful magnitude. The experiments described helow apply this method to the production of several halogen activities, yielding specific ~ activities ~, of about ~ loJ disintegrations i per ~ minute ~ Per milligram of halogen. The activities are characterized determination of the decay rate of the active species. Additional experiments in chemical manipulation of these activities are suggested. These
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JOURNAL OF CHEMICAL EDUCATION
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experiments will require only the equipment previously described (4) plus some common chemicals and laboratory glassware. PROCEDURES
The procedures involved in the preparation of the various halogen activities are essentially identical and will therefore be described in general terms, followed by specific comments applying to each case. General Procedure. Approximately 400 ml. of the target substance, a liquid alkyl or aryl halide, is placed in a 500-ml. florence flask. The 5 mg. Ra-Be neutron source is inserted in a centrally located soft glass2test tube arranged to hold the source in a central position as shown in Figure 2 (see also Figure 2 of Reference 3). A few milligrams of free halogen or hydrogen halide should be added to the target substance to act as carrier for the liberated activity. After a bombardment time corresponding to at least one half-life of the active species desired, the neutron sonrce may be removed to its lead shield and the chemical extraction of the active species undertaken. The irradiated alkyl halide is placed in a large separatory funnel and shaken vigorously with about 50 ml. of dilute aqueous sodium hydroxide solution. This serves to hydrolyze and extract the halogen carrier and any activity which has been liberated from the target substance. After separation of the aqueous from the organic layer, it may be mashed several times uith an inert organic solvent to remove all traces of the organic iodide. This step is essential only with organic iodides, which react easily with the silver nitrate to be used in the next step. The halogen activity with carrier is next precipitated Pyrex and other boron-containing glasses are to be avoided PPOSS nert,inn of boron
because of the exceptionally high capture for thermal neutrons.
tRa-Be neutron source
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I
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Time after Removal of Neutron Source (Minutes) Figure 3. Decay of I 118
in acid solution by addition of silver nitrate solution. The precipitate is collected on paper in a small Biichner or Hirsch funnel and dried by washing with alcohol and ether. Covered with cellophane and mounted on a card, the sample is placed next to the Geiger tuhe for observation of its activity as a function of time. The entire time elapsed from the end of bombardment until the beginning of the activity measurements should occupy no more than one half-life of the 'ctive species. and preferably less. This requires that the student prepare his reagents and apparatus for the extraction process during the bombardnent, in order to bring the sample to the counter as early as possible.. The target halide should always be saved for, later use, since no appreciable chemical decomposition is produced. Iodine. Ethyl iodide is suggested as a suit,ablr target substance for the preparation of iodine activity. A small crystal of iodine may be dissolvedin the alkyl iodide to serve as carrier for the liberated iodime atoms. Typical decay data taken wit,h a sample of active iodine prepared as described above are shown hFigure 3. Bromine. Ethyl bromide or ethylene dibromide are suggested as suitable target materials for the preparation of bromine activities. A few drops of. a previously prepared solution of bromine should be added to the target substance to furnish carrier bromine for subsequent chemical operations. Bombardments of duration less than 2 ~ k m u t e will s yield principally the 18-minute lower isomeric state of Bra" while bombardments of several hours will yield in addition, the 4.4-hour upper isomeric state BrW*. A small yield of Bras, depending on bombardment time. may also be expected, but this may be neglected if all subsequent beta counting is conducted through a t least 100 mg./cm.z of absorber (glass wall counter plus approximately 70 r n g . / ~ m .Al). ~ This absorber will cut off the weak beta particles of 34hour Bras, while not seriously decreasing the intensity of the hard betas from BrnO. While the radiations from 4.4-hourBflo*weundetectable in the counting arrangement commonly used, its 18-minut,eBra0 daughter is readily detectable and serves to denote its presence. The decay curve of samples subjected t,o several honrs bombardment will show two
DECEMBER, 1949
components, corresponding to the half-lives of 18 minutes and 4.4 hours. This behavior is illustrated in Figure 4. Note that activity determinations must be made at frequent intervals during the first stages of decay, while later measurements may be made as much as an hour apart, since the decay rate is much smaller. The isomeric transition Blbn* +Bra0 isaccompanied by a chemical effect similar to that. induced by neutron capture, which may be demonstrated as follows: After the decay rate of the extracted bromine activity has decreased to a value indicating absence of 18-minute activity (about two hours after the end of bombardment) the organic bromide sample may be treated again to extract bromine activity, using a small amount of carrier as before. This extract will be active, due to the presence of retained 4.4-hour Br'O* in the target substance and due to the molecular rupture resuking from the isomeric transition process. However, the active extract obtained in this second extraction should show a pure 18-minute half-life, and consists of Br" formed by isomeric transition from B P * , rather than directly by neutron capture. All of the 4.4-hour B?O* not extracted in the initial treatment must have reentered an organic form, and can leave that form only as a consequence of the decay reaction which yields 18-minute B P . Exchange Reactions. Some simple variations on the procedures given above may be used to demonstrate the phenomena of isotopic exchange, which are of paramount importance to nuclear chemistry. Iodine activity is suggested for such experiments, since it can be quickly prepared in high specific activity by the procedure outlined above. To demonstrate the rapid isotopic exchange between iodide and iodine, prepare an aqueous extract of iodine activity by neutron bombardment of pure ethyl i.dide omitting the use of I2carrier, and followed by extraction with an aqueous solution containing about 20 mg. of iodide ion. Divide this solution into two equal portions; save one and shake the ot.her vigorously with carbon tetrachloride containing about 10 mg. of elementary iodine. Separate the two phases and extract the i d m e from carbon tetrachloride into an aqueous thiosulfate solution. Acidify, boil, and precipitate silver iodide from this solution, and from the second portion of the original aqueous extract. Dry, mount, and count the two precipitates. Presence of activity in the sample extracted from carbon tetrachloride denotes exchange between iodine and iodide. If measured amounts of iodine and iodide have been used it is possible to predict, from the activity of the control solution, the activity of t,he iodine fraction. Lack of iostopic exchange between iodide and iodate in alkaline solution may be demonstrated as follows: Prepare an aqueous solution of active iodine with iodide carrier as above, make this solution alkaline and add 100 mg. of iodine as iodate. Divide the solution into two portions and from one precipitate barium iodate and count to test for exchange. Acidify the other portion, which will permit oxidation of the iodide by the excess
669
iodate present. The iodine may be extracted into carbon tetrachloride and discarded. The remaining iodate may be precipitated as barium iodate and counted to test for exchange between iodate and iodine in acid solution. Other exchange reactions such as those between iodides or elementary iodine and alkyl iodides or elementary iodine and alkyl iodides may be tested in similar fashion. However, the success of the SzilardChalmers reactions with these substances attests to the lack of exchange tinder the conditions of the experiments. If an ultraviolet lamp is available it will he possible to-demonstrate a photochemical exchange between iodine and an organic iodide, preferably methyl iodide, which absorbs at wave lengths shorter than about 3500 A. The primary photochemical process is probably CHJ
+ hu = CHa + I
It can be shown (6) by using iodine 128, that about 99 per cent of the methyl radicals are removed hy the following process, CHn
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Bombard ethyl iodide containing 2 mg. of carrier iodine and extract with water containing 10 mg. of NapSzOa. Add HNOa to t,he aqueous extract to oxidize I- to d and shake with 25 ml. of MeI. Illuminate 10 ml. of the Me1 solution of iodine in a quartz tube with t,he full light of a ultraviolet lamp for 10 min. Meauwhile, extract another 10 ml. of the same solution with dilute NaOH and convert the iodine to silver iodide. Measure the activity of t,his control sample. The irradiated portion of the Me1 solution should be treated similarly. Since some of the activity will now be present as MeI, the difference in activity between the two portidns of silver iodide will he a measure of t,he extent of exchange. Carriw-free Activity. It will be of some interest to observe the behavior of halogen act,ivity in the absence of carrier. Bromobenzene will provide an appr~priat~e target because of its chemical stability. It should he shaken with aqneons thiosulfat,e, water, and dried
JOURNAL OF CHEMICAL EDUCATION
670
with cdcium chloride and bombarded with neutrons for 15 minutes. Pass this liquid several times through a filter paper on a small Buchner filter, with suction. Rinse the paper with a little carbon tetrachloride, dry and count. Some of the activity will have been retained on the filter paper. If the empty flask, in which neutron bombardment was performed, is shaken with dilute NaOH, containing 20 mgs. of NaBr carrier, it will be found that appreciable activity has been absorbed on the glass. Finally, if a drop of bromine is added to the bromobenzene and extracted with dilute aqueous NaOH, more activity will be obtained. The total yield from all sources will, however, be less than had carrier been added before bombardment. TREATMENT AND DISCUSSION OF RESULTS The decay curves obtained from iodine and chlorine activation should be of the simple exponential form. Construction of the best straight line through the points on a semilogarithmic graph (after appropriate background corrections) will permit estimation of the halflives. The yield of activity obtained by the extraction process will vary considerably from one experiment to another and in no case will it be 100 per cent of the active atoms produced. A portion of the activity will always be "retained" by the target material, and a large part as the target molecular form itself. This phenomenon has been discussed previously (3). The decay curve obtained from bromine will show two components, as indicated in Figure 4. The linear portion obtained after a few hours decay may be extrapolated back to zero time to estimate its contribution in early observations. Subtracting these values from the total activity in early measurements, an exponential line corresponding to the shorter bromine half-life will be obtained. hours The observed decay curve indicatm that after the end of neutron bombardment the activity is exclusively due to 4.4-hour BrsQ(through the betaactivity of its daughter 18-minute Br8a). This applies to the bromine activity retained in the ornanic target material as well as to -the inorganic extraci. Further extractions then yield only 18rminute activity, which is rendered extractable by the isomeric transition process. Iodine was chosen for the examples of exchange reactions because of the simplicity and convenience of
its nuclear properties. Similar experiments could be performed with the other halogens. Szilard-Chalmers enrichment of elements other than the halogens is quite feasible. With a 5 mg. Ra-Be neutron source and limited time for laboratory experiments, the choice is limited to those which have neutron capture cross sections approaching 1 X om.' and half-lives of a few minutes to a few hours. In addition, the chemical properties of these elements must be such as to permit choice of an appropriate target form and chemical separation of the active recoil atoms. Slow neutron capture by M n 6 6 forms a 2.6-hour betaemitter with an activatibn cross section of 13 X lo-=' em.= Libby (6) has studied the Szilard-Chalmers reaction in this case and has shown that bombardment of aqueous permanganate solutions of pH less than 10 yields active manganese dioxide with high efficiency. Even the small amount of manganese dioxide formed by passing permanganate solution through filter paper is sufficient to act as carrier for the active manganese and retain it on the filter paper. The short half-life and high cross section for formation of this element recommend it for student experiments. Additional exchange experiments on various valence forms may be devised. Antimony forms two active isotopes by slow neutron capture. The shorter-lived. Sbl" has a half-life of 2.8 days and an activation cross section (natural element) om.% Successful enrichment of this of 3.8 X activity has been accomplished through the bombardment of triphenyl stib'me (7), followed by a two-liquid extraction process. The organic target material may be dissolved in ether and a large fraction of the active antimony can be extracted into aaueous solutions.
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LITERATURE CITED (1) Catalogue, Isotopes Division, U. S. Atomic Energy Commission, Oak Ridge, Tennessee. (2) Department of Special Training, Oak Ridge Institute of Nuclear Studies, P.O. BOX117, OakRidge, Tennessee. (3) W I L L ~ A M R.~R., , JR., W. H. HAMILL, AND R. H. SCHUI.E% THISJOURNAL, 26,210 (1949). (4) HAMILL, W. H., R. R. WILLIAMS, JR., AND R. H. SCHULER, 2
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April 1, 1949. (6) LIBBY,W. F.,J. Am. Chem. Soe., 62,1930 (1940). (7) WILLIAMS, R. R., JR., J. P h y ~& Colloid Chern., 52, 603 (1948). See also Paper No. 3.2.2, Vol. 9B, Plutonium Project Record, National Nuclear Energy Series.
