PACIFIC SOUTHWEST ASSOCIATION OF CHEMISTRY TEACHERS RADIOCHEMICAL EXPERIMENTS IN THE PHYSICAL CHEMISTRY LABORATORY COURSE CLIFFORD S. GARNER University of California, Los Angeles, California
As POINTED
out in an earlier paper (1) describing the Lauritsen quartz-fiber electroscope, the low cost and simple maintenance of this instrument permit most universities and colleges to utilize it for instruction in some aspects of radiochemistry and allied subjects. Moreover, the availability (8) of pile-produced radi* isotopes allows those institutions which do not have facilities for the production of such isotopes to engage in research and instruction involving radioisotopes. Several papers (e. g., 3,4,5) and a book (6) have been published mhich give laboratory exercises in radiochemistry, some of which may be included in the physical chemistry laboratory course. I t is this course which many universities will probably find most adaptable to the introduction of laboratory work n-ith radioactive substances. Indeed, me have found that several radiochemical experiments can be included without appreciable curtailment of the more conventional laboratory exercises. Many radiochemical exercises are possible and the selection of such experiments depends upon the interests of the instructor and the students and upon the time vhich may be allotted. The experiments outlined below are typical of those which we have found suitable and mhich do not demand excessive time for their preparation by the instructor or their execution by the students. The basic principles of radiochemistry have been adequately described elsewhere, particularly in the excellent book by Friedlander and Kennedy (7). "National Bureau of Standards Handbook 42" (8) and the laboratory manual of Schweitzer and Whitney (6) may be consulted regarding health considerations and hazard control. National Bureau of Standards Circular 476 (9)will be found helpful as a general reference on the measurement of radioactivitv
DETERMINATION OF HALF-LIVES
One of the simplest types of radiochemical experiment is the determination of the half-life of a radioisotope by direct observation of its decay over a period of time. The instructor may prepare "unknowns"
consisting of one or two radioisotopes in some stable chemical form mounted on labeled cards or aluminum plates; an activity of about 0.1-1 division per second on the second or third step of a conventional Lauritsen electroscope (I) is appropriate, although less activity may be used per sample. We find half-lives hetween about one week and four months convenient, inasmuch as we give the "unknown" to the student during the first week of classes and-instruct him to follow the decay a t suitable intervals throughout the semester. The observed readings, after being corrected for background (taken once each class day by one student assigned on a rotating basis to a given electroscope) and for nonlmearity of scale if necessary, are plotted on semilogarithmic graph paper. The student may be required to find in a table or chart of isotopes (1M4) those radioiso-. topes which are compatible with the experimentally found half-life, thus acquainting the student with the variety and some of the systematics of nuclei. This simple experiment may serve to introduce the student to the Lauritsen electroscope, the radioactive decay lam (including decay of a mixture of radioisotopes), and the statistical treatment of data (analysis of complex decay curve into components, least squares analysis, etc.). The instructor may take up briefly in quiz section the determination of half-lives too long or too short to be readily measured directly, or this material may be provided through reading assignments. Health hazards are negligible in this type of experiment, provided the samples are covered with cellophane or some other protective covering. ABSORPTION AND SCATTERING O F NUCLEAR RADIATIONS
In addition to the determination of the half-life of a radioisotone., the identification of the t w e s of emitted radiations (principally alpha, negatron, positron, gamma, X-ray, bremsstrahlung, Auger electron, and neutrino) and the measurement of their energies are of great value in the characterization of a radioisotope and
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as the latter is formed from the radioactive disintegration of the uranium. It may be shown by mathematical analysis and the law of radioactive decay (7) that the ThZa4 will grow back with its own half-life until the amount of it present is such that it is disappearing by its own disintegration at the same rate as it is being formed from the long-lived U238(this state of affairs is termed "secular radioactive equilibrium"); i. e., after 24.1 days one-half the final amount of ThZa4will be present, after another 24.1 days three-quarters of the ThZa4 will have been grown, etc. Generally, a hottle of a chemically purified uranium compound is sufficiently old so that the ThZS4 and Paza4in it have gron-n into secular equilibrium following the chemical separation of the uranium in the process of its production from ores. Accordingly, a sample of such a uranium compound (or metallic uranium) 'will exhibit alpha, beta, and gamma radioactivity. Inasmuch as the U alpha particles are filtered out completely by a sheet of ordinary paper (which has essentially no effect on the beta radiation), insertion of a piece of paper between the sample and the detector (electroscope, GM counter, etc.) will permit measurement of the intensity of the beta and gamma radiations from the ThZa4and Paza4. If now the uranium is separated away from the ThZa4Pa234 the latter will decay (with the half-life of ThZa4) since UZa8is no longer present to replenish the ThzS4Pa234, and the separated u r a n h will grow fresh Thza4-Pal34 in it. Actually, the ThZs4betas are rather soft to be detected efficiently under ordinary conditions, hut since the UX2 grows almost instantly from the RADIOCHEMICAL SEPARATIONS Th23&its hard beta radiation may be used as a tracer Chemical separations of radioisotope mixtures may for the Th234. The branching decay of Th234gives be carried out if desired. We have found,th& the UXZprimarily, only a small fraction (about 0.4 per separation of the thorium daughter, ThZa4(UXI), of cent) of the Th234decaying to UZ. Because of the U238alpha decay from ordinary uranium salts makes a very long half-life of UZa4the decay chain is essentially good experiment, inasmuch as substantially no hazard blocked a t that isotope for all time periods shorter is involved, the uranium salts are readily available at than those of geological magnitude. The chemical separation of Thza4and Paza4from low cost, the separation is readily carried out by any of several simple methods, and activity measurements may uranium may be made by many procedures (16). One be made on the separated ThZa4to show its decay and of the best of these is ether extraction of the uranium on the separated uranium fraction to show the growth as uranyl nitrate hexahydrate (UNH), UOz(N03)a.6He0, from an aqueous nitric acid solution saturated of Th234hack into the sample. UZasis the long-lived parent (half-life = 4.5 X lo9 with a soluble nitrate such as calcium or ammonium years) of the so-called uranium-radium (4% 2) dis- nitrate, the thorium and protactinium remaining principally in the aqueous phase (16, 17). The purified integration series: in the selection of appropriate devices and conditions for detection. Although specialized equipment is required for a complete study of such radiations, much can be accomplished, both in identifying certain radiations and in measuring their energies, through the determination of the extent of absorption of the radiations by various thicknesses of absorbing material interposed between the radioisotope sample and the detector. A good discussion of absorption methods relating to beta particles and photons has been given by Glendenin (15), whose paper contains range-energy curves for betas and energy-half-thickness curves for photons. A radioisotope sample may be given to the student for absorption analysis with aluminum and lead absorbers. Commercial 2s aluminum and quality lead sheet and foil in thicknesses from about inch down to about 0.005 inch may he cut into absorbers and the thicknesses calculated from the measured weight and area of each absorber.' The corrected electroscope activity readings' may he plotted against absorber thickness, and the range of the beta particles or halfthickness of the photons found. Reference to appropriate relationships or curves, such as the Glendenin curves, gives the upper energy limit of the betas and the gamma or X-ray energies. If time permits, such effects as self-absorption, scattering, and back-scattering of nuclear radiations may be investigated. This type of experiment is likewise essentially free from radioactivity hazards and health problems.
+
A sample if initially pure U238 m4l "grox" ThZa4in it
' Calibrated sets of abbsorbers may be purchased from Radiation Counter Laboratories, Inc., 1844 W. 21st Street, Chicago 8, Ill., and Tracerlab, Inc., 55 Oliver Street, Boston 10, Mass.
uranium can be recovered by washing the ether phase with water. Carriers for the ThZa4and Pa234are not required. An alternative procedure (16), satisfactory but some-
MAY, 1950
what more time-consuming, involves the fact that an iron (111) hydroxide precipitate will act as a "scavenger" for many substances present in tracer concentrations (such as the Th234-PaZJ4), carrying them out of solution. The uranium can he kept in solution as a carbonate complex. The UNH is dissolved in water, a few milligrams of Fe(II1) carrier added and dilute NH4OH added in very slight excess to precipitate "Fe(OH)3" and yellow ammonium diuranate (NH,);IT,Or. (NH&C03 solution is added in excess and the mixture warmed to 60°C. for 20-30 minutes; the diuranate dissolves to form a carbonate complex (perhaps UOI(C03)3----), leaving the "Fe(OH)," containing the Th2s4-Pa234,which precipitate may be washed vith warm (NH&C08 solution. The filtrate and washings may he acidified, Fe(II1) carrier added, and the procedure repeated. The combined "Fe(OH)," precipitates may be dissolved in RNOs and reprecipitated as before with NH40H and (NH4)&03. About 1 to 2 g. UNH is a convenient amount to take for the separation. The volumes of solution should be kept small for more rapid work. Precipitates may be filtered onto 1-in. diameter filter paper discs and mounted on cardboard for activity determination as a function of time; solutions may be evaporated onto 1-in. diameter vatch glasses which can be affixed to cardboards for decay measurements. Samples should be covered with cellophane or scotch cellulose tape to prevent contamination of the detectors (students should be cautioned to wash all glassware thoroughly after use and to wash the hands before taking activity readings on the samples). These samples are best measured on the top step of the electroscope ( 1 ) . Students may work in pairs, one of the pair fixing, mounting, and measurin~ the purified uranium fraction and the other handling the Th234Paza4fraction. Each sample should be measured about once T,.eek throughout the semester, F~~each sample the corrected activity may be plotted on the loganthmic scale and the time in days since the separation on the linear scale of semilogarithm paper (e. g., Keuffel and Esser paper, No. 358-51, 358-72L, 358-73L, etc.1. The half-life of the ThzS4can be computed from measurements on the decay of the Th2akPazs4 fraction, and the half-life for the growth of beta activity back into t,he separated U fraction can be derived from the measurements on it. The growth and nicely a discussion of the rates of first-order and con-
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secutive reactions, and analysis of the data provides an opportunity for the application of graphical methods. OTHER EXPERIMENTS
Some of the better students may be encouraged to undertake some minor research problem of simple nature, developing the details of the experiment by consultation with the instructor. Problems such as determination of solubilities of very insoluble substances, coprecipitation phenomena, distribution coefficients, transference experiments, and other physicochemical measurements may be undertaken with the aid of radioisotopes. LITERATURE CITED (1) GARNER,C. S., T m s JOURNAL, 26, 542 (1949). (2) "Isotopes," Catalogue and Price List No. 3, Isotopes Di-
(3) (4) (5) (6)
(7)
vision, U. S. Atomic Energy Commission, P. 0. Box E, Oak Ridge, Tennessee, July, 1949; Supplement No. 1, September, 1949; Supplement No. 2, December, 1949; Supplement No. 3, February, 1950. \VILLIAMS, R. R., JR., W. H. HAMILL, AND R. H. SCHULER, 26,210 (1949). THE JOURNAL, HAMILL,W. H., R. R. WILIJAMG,JR., I N D R. H. %~UHLER, ibid., 26, 310 (1949). SCHULER, R. H., R. R. WILLIA~~S, JR., AKD \i'. H. HAMILL, ibid., 26, 667 (1949). SCHWEITZER, GEO.K., AND IRAB. WHITXEY,"Radioactive Tracer Techniques," D. Van Kostrand Co., Ine., New York, 1949. FRIEDLANDER, G., AND J . W. KENNEDY, "Introduction to Radiochemistry," John Wiley & Sons, Inc., New York,
1949. (8) "National Bureau of Standards Handbook 42," "Safe Handling of Radioactive Isotopes,'' issued September, 1949 (Supt. of Documents, Washington 25, D. C., 80.15). (9) National Bureau of Standards Circular 476, "Measurements of Radioactivity," issued October, 1949 (Supt. of Documents. Washindon 25. D. C.. SQ.35). (10) SEABORQ, G. 'f.,AND'I. P E ~ L M A N , ' R ~Mod. U S . Phgs., 20, 585 (1948). (11) FRIEDLANDER, G., AND M. L. PERLMAN, "Chart of the 150topes " General Electric Research Laboratory, April, 1948. (12) S E G R ~E., , "Isotope Chsst," Addison-Wesley Press Inc., Cambridge, Massachusetts, 1947. (13) SULLIVAN, W. H., .Trilinear Chart of Xuclear Species," John Wiley & Sons, Ine., New York, 1949. (14) J, W,, H. M, CLARK, of
Isotopes Arranged According to Half-Lives," U. S. Atomic Energy Comm. document MonC-400, Oet. 30, 1947. (15) GLENDENIN, L. E., Nucleonics, 2, No. 1, 12 (1948). (16) MELLOR,J. W., "A Comprehensive Treatise on Inorganic and Theoretical Chemistry," Longmans, Green and Co., London, 1923, Vol. N, p. 119. (17) NORSTR~M, A,, AND L. G. S I L L ~ N8 ,v e n ~ kKern. Tid., 60, 227 (1948) (in English), through C. A., 43,3267 (1949).
