Herbeti M. Clark
Rensseloer Polytechnic Institute Troy, New York
Criteria for Experiments in Radiochemistry
W h e n Becquerel discovered radioactivity in 1896 (I), a new phenomenon was added to those which were then considered to constitute the science of chemistry. Today the various aspects of radioactivity, as they relate to chemistry, are identified with the area of specialty known as radiochemistry. During the first four decades of radiochemistry, radiochemists were concerned mainly with the discovery (t), identification, separation, purification and uses of the naturally occurring radioelements (3, 4). A few experiments utilizing the new radioelements as indicators were introduced into college chemistry courses (5). Following the discovery of the neutron (6) by Chadwick in 1932 and of artificial radioactivity (7) by Curie and Joliot in 1934, the scope of radiochemistry expanded to encompass the entire periodic table, as it mas then known. Within a few years several hundred radionuclides had been identified and characterized (8, 9 ) . As the number of chemists with experience in the production and, in particular, the utilization (10) of radioisotopes grew, the interest in introducing experiments in radiochemistry into chemistry courses in colleges and universities also grew. Apart from photographic film or plates, the instruments available for radioactivity measurements for educational purposes a t that time were often limikd to various types of electroscopes (6,11-14). These were and still are (e.g., pocket dosimeters) extremely valuable detectors although limited with respect to sensitivity. Geiger tubes and scalers were in many cases homemade and were nursed along primarily for use in research.
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The current era of radiochemistry began about 1939 with the discovery of nuclear fission by Hahn and Strassmann. This was follon-ed in 1940 by the discovery (15-17) of elements beyond uranium in the periodic table, namely, neptunium by McMillan and Abelson and plutonium by Seaborg, McMillan, Kennedy, and Wahl. The impact of these discoveries on college chemistry curricula did not begin, however, until after World War 11. As a result of the Manhattan Project, there came into being a relatively large community of scientists who were experienced in a wide variety of new techniques and methods of producing, separating, and using radioactive materials. Many scientists returned to academic institutions with interests stemming from their wartime experiences. Declassification of reports began and the facilities of some of the national laboratories became available for the education of college faculty members. Centers for training in radioisotope techniques were established (18,19). Such wide-scale dissemination of information would not have been effective had there not been a simultaneous improvement in the availability of radioisotopes and instrumentation for radioactivity measurement. Reactor-produced radioisotopes, including fission products, were made available as byproduct material in various forms a t a nominal cost. Research reactors and plutonium-beryllium neutron sources capable of inducing radioactivity became available to colleges and universities. Concurrently, the nuclear instrumentation industry grew in magnitude from its small pre-u-ar size to the comparatively large and com-
petitive industry which exists today. To assist in the acquisition of irradiation facilities and nuclear instrumentation for use in college courses in science and engineering, the U. S. Atomic Energy Commission and the National Science Foundation have made equipment grants to educational institutions and have supported the preparation and publication of teaching experiments (80). Several boo& (81-51) containing procedures for experiments in radiochemistry have been published and numerous articles describing specific experiments, the teaching of nuclear chemistry or radiochemistry (19, 52-42), or uses of radioisotopes in chemistry (10, 43-48) have appeared in THIS JOURNAL.The Subcommittee on Radiochemistry of the Committee on Nuclear Science of the National Academy of SciencesNational Research Council has issued two reports (49, 50) relating directly to the teaching of radiochemistry and a series of monographs on the radiochemistry of the elements and on radiochemical techniques (51). These monographs contain ideas, methods, and procedures which could be adapted for use in teaching experiments. Today, then, with an increasing number of college faculty members having some knowledge of and excerience with radioisotope techniques, and with instruments, radioisotopes and published experiments being relatively available, the introduction of one or more aspects of radiochemistry into the chemistry curriculum is becoming commonplace. Two distinct ways are used to introduce radiochemistry into the chemistry curriculum. One is by means of a course in radiochemistry. Such a course deals with the more sophisticated methods and techniques described in a number of special laboratory textbooks and is usually taught by a specialist. The second way is by introducing one or more experiments in radiochemistry in some of the regular chemistry courses. It is the latter approach which is given the emphasis in this paper. By drawing attention to certain aspects of experiments in radiochemistry the author hopes to encourage others to design new experiments, modify published experiments, and in a general way extend the usefulness of their nuclear instruments. The Nature of Radiochemistry
For the sake of discussion, radiochemistry is taken to encompass both the chemistry of radioactive substances and the application of radioisotopic tracer techniques to the study of phenomena associated with the various branches of chemistry. Since radiochemist~yis a blend of the classical subdivisions of chemistry, it is not surprising that the techniques of radiochemistry are utilized in connection with the research and teaching activities in these subdivisions, e.g., analytical, bio-, general, inorganic, instrumental, organic, and physical chemistry. It is important to recognize that radiochemistry differs from stable-isotope chemistry in several ways. For example, since radioactive decay is a non-equilibrium process, time becomes an important experimental variable. I n extreme cases the half life may be so short that the radionuclide cannot be isolated and studied radiochemically; or the half life may be so
long that the radionuelide, e.g., Thy", can be considered to be "stable" in terms of chemical manipulation. In radiochemistry it is common to encounter a separation problem involving a mixture of elements having atomic numbers differing by only one or two units. Such mixtures may be produced during the irradiation of a target or may arise from decay of a radionuclide to form a radioactive daughter a5 in the fission product series or in the naturally occurring series. Although a parent-daughter relationship between radioelements or between isomers of the same element may introduce undesirable radiochemical complications, it may have practical advantages in some cases. Thus, a long-lived parent is often a convenient source of a very short-lived daughter or the radiation emitted by the daughter may enhance the detection and usefulness of the parent radionuclide. Under suitable conditions the means of detection and measurement of the radiation emitted by a radioactive substance are so sensitive that chemical phenomena may be studied a t extremely low concentrations or for extremely small quantities of material. Examples of the uses of radioisotopic tracers are well known. It should be noted, however, that at exceedingly low concentrations, chemical behavior, as observed radiochemically, may differ significantly from that at macro concentrations (52). There are at least two additional ways in which radiochemistry differs from the chemistry of nonradioactive substances. The first is with respect to chemical purity. I n terms of the detectable number of atoms of a contaminant, radiochemical purity can be much more stringent than ordinary chemical purity, and the radiochemical purity of a sample may increase or decrease with time. Secondly, the nuclear radiation which provides the basis for the methods of instrumentation used in radiochemistry also creates the problem of radiological; it may, by ionization and excitation processes, induce chemical reactions, e.g., radiolysis, which can complicate the chemical behavior of a sample of radioactive material. I n the design, selection, or evaluation of teaching experiments in radiochemistry several obvious criteria should be considered. The objectives of the experiment should be consistent with the objectives of the course, the experimental techniques required should be consistent with the level of the course and the previous experience of the students, and the experimental procedures must be consistent with safe handling techniques. I t is generally tme that the introduction of an experiment or experiments using radioactivity into an established course can be done only by substitution. The difficulty and the justification of such substitution vary with the level and type of course. Although the difficulty may be greatest a t the first-year level, an elementary course may be the only chemistry course which a student takes, and it may, therefore, be the course which provides a basis for his understanding of the phenomenon of radioactivity in our nuclear age. This is particularly true in a small liberal arts college. I n any case, the introduction of experiments in radiochemistry into the chemistry curriculum should not be based on fashion but rather on recognition of the educational value of such experiments. Volume 40, Number 12, December 1963
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Objectives of Laboratory Experiments in Radiochemistry
Any experiment in radiochemistry provides the student with a laboratory experience in the handling of radioactive material and in the measurement of radioactivity. These are general objectives. I n addition, any experiment has one or more specific radiochemical objectives which relate the experiment to a topic or topics in the course. The specific objectives will be used in this paper as a basis for classifying experiments in radiochemistry. One specific objective, for example, would be to illustrate an aspect or several aspects of the chemistry of radioactive substances, e.g., the preparation, separation, purification, characterization, identification, or assay of radionuclides in various chemical forms. In an experiment having this objective the major emphasis would be on the phenomenon of radioactivity and such an experiment would he introduced on the basis of a belief that radioactivity per se is sufficiently important in the field of chemistry to share in the already crowded schedule. Included in this category would be radiochemical separation of two or more radionuclides followed by an evaluation of the effectiveness of the separation; determination of the half life of a radionuclide by decay measurement or specific activity measurement; characterization of the radiation emitted by a radionuclide; investigation of the properties of the radioelements, including the transuranics (53); preparation of carrier-free radionuclides by the Szilard-Chalmers reaction (54, 56); and utilization of radioactivity for age determination.' A second specific objective would be to illustrate the use of radioisotopic tracers to provide a more convenient or a more sensitive means for determining the amount or concentration of a substance. Examples of such experiments would be the determination of solubility, common ion effect, and heat of solution (56-58); measurement of the distribution between immiscible solvents (59, 60); study of chemical reaction rates; evaluation of effectiveness of chemical separation methods (61, 6%); determination of stability constants for complexes (63); radiometric analysis (64); activation analysis; study of diffusion; study of adsorption; measurement of surface area; determination of atomic weight, measurement of blood volume (65); study of biosynthesis (66); etc. A 'third specific objective would be to illustrate one or more of the unique applications of isotopes (although not necessarily radioisotopes). Examples of effects which may be measured by the use of isotopes are exchange reactions between different chemical species of an element (e.g., species differing with respect to the oxidation state of a given element) (67, 68) ; exchange reactions between species in different phase states (e.g., a solid salt and its ions in solution) (69); certain types of reaction mechanisms, self-diffusion, the isotope effect, and isotope dilution (e.g., isotope dilution analysis). "Tagged-atom" experiments would fall in this category. In general the reaction, process, or effect is studied by measurements u-hich reflect a change in the isotopic ratio of the labeled element. The papper by Fairhall appearing in thisissue, J. CHEM.EDUC., 40, 626 (1963) illustrates this topic.
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A fourth specific objective, which is related to the first one. would be to show bv an error analvsis how the resuits obtained in the experiment are ikuenced by the statistical nature of radioactive decay and nuclear radiation detection (70, 71). This aspect of radioactivity should receive some attention in any teaching experiment in radiochemistry. The relative importance assigned to it will depend on the background of the students and on how complete an analysis of experimental data is required of the students. I n the foregoing discussion it is implied that an experiment having the second or third objective would be used by an instructor to emphasize the principles involved in the use of radioisotopes in the study of chemical phenomena. To the extent that it is feasible to do so, the radioisotope would probably he selected so that radioactive decay would not complicate the experiment and the radiation measurements would be relatively simple. Although the particular chemical phenomenon ~vouldprobably be familiar to the student, an objective of the experiment would be to improve the student's understanding of the chemistry involved. On the other hand, the primary educational aim of the instructor might be to introduce a fundamental concept or principle of chemistry more effectively through the use of radioisotopes. A related aim would be to use radioisotopes to effect a saving in teaching time. The appropriate experiments would be similar to those having the second or third objective, but the emphasis would be different. There would be less emphasis on radioisotope methodology. Finally, there are two areas of chemistry, namely, nuclear chemistry and radiation chemistry, in which the experimental research can involve radioactive materials and can require the use of radiochemical procedures. In some cases, then, a teaching experiment in either of these areas could also be considered an experiment in radiochemistry. For example, a specific objective, which is becoming important as theories of life become more detailed, would be to study the chemical effects of radiations from radioactive material, e.g., radiolysis, Szilard-Chalmers effect, etc. Such experiments in radiation chemistry often require relatively large amounts of activity. When this is the case, they would best be performed as part of a course in radiochemistry, with adequate hazard control equipment and shielding facilities. Experimental Techniques
The problems of introducing experiments in radiochemistry into a first year chemistry course are certainly different from those encountered for an upperclass course in, for example, instrumental analysis. For the latter, an experiment in radiochemistry may simply provide laboratory experience in one of a wide variety of specialized instrumental methods of analysis. At the first year level, however, one cannot expect the student to follow a procedure requiring sophisticated techniques. The problem is further complicated by the fact that the student may not have had any high school chemistry experience. Furthermore, there is always the possibility that the purpose of the experiment may be obscured by a procedure unnecessarily complex for the level of the course. Simple procedures and techniques are preferable from the point of view of
both the student and the instructor. At this level and even through the senior year one is dealing primarily with ideas, concepts, and principles rather than techniques. These should be introduced as simply and directly as possible but without misleading the student by oversimplification. This does not imply that a student should not be required to learn techniques, and it does not overlook the fact that the success of an experiment depends upon the procedure and techniques used. However, the educational value of an experiment is not lost if, for example, a simplified radiochemical separation procedure which takes only 10 min instead of 30 min and requires relatively inexpensive chemical apparatus results in a yield and purity of only 50 and 75%, respectively, instead of 85 and 90%. There are many ways in which the procedures and techniques for undergraduate radiochemical experiments can be simplified by making use of techniques used for non-radiochemical experiments. For example, it is questionable whether mastery of the technique of using a micropipet and micropipet control is essential in an experiment in an elementary course. Such precision apparatus is properly associated with radiochemistry and its use is not questioned when the success of an experiment depends upon it. It is not sacred, however, and can often be replaced by the humble and more familiar medicine dropper. The latter is far less delicate than a micropipet and is less costly (about '/mu of that for a micropipet plus its control). If desirable, a dropper can be calibrated rapidly in fractions of a milliliter and marked with a file. Furthermore, the tip can be adjusted in a flame to give volumes in the range of 10 to 40 X drop and, of course, solution concentration can be adjusted for further control. Finally, the placing of a contaminated rubber bulb in a waste storage vessel is less difficult than the decontamination of a pipet control. A wide variety of other commonly available inexpensive apparatus may also be used effectively in radiochemical experiments. The familiar dropping bottles, test tubes, and centrifuges used for semi-micro qualitative analysis are suitable for experiments involving low levels of radioactivity. Two criteria should be used in the selection of such apparatus. Obviously, the first'is that it must be suitable for the proposed chemical manipulation. The second, which narrows down the choice, is that it must be suitable for use in the safe handling of radioactive materials. As a conservative guide in visualizing safety problems it is often helpful to apply the same stringent tests that would be applied to apparatus used for the safe handling of bacteria. This implies that use of the apparatus will not result in the contamination of the student's hands, will not release airborne activity, will not lead to ingestion of activity, will not result in a spreading of activity throughout the working area, and will not result in contamination of the counter. Accordingly, in handling radioactive liquids it is best to avoid tall, top-heavy vessels and vessels having glass stoppers or stopcocks through which leakage is very likely to occur. A simple screw cap vial can often serve as a mixing vessel such as might be used for solvent extraction. A soft metal foil liner (e.g., lead, tin, or aluminum, depending on possible reactions involving
the foil) in the cap helps to make a tight seal. Obviously care must be used in venting volatile solvents when a vessel containing a radioactive material is opened. Pipetting of liquids of any kind by mouth should be strictly forbidden (as should be the case in any chemistry laboratory). Rather inexpensive rubber bulbs can be used with normal size pipets. Radioactive liquids should not be caused to spatter and should not he boiled in open vessels. The latter includes planchets or dishes used for counting. A liquid sample can be evaporated to dryness in a planchet by means of heat lamp which is placed a t a distance that is reasonable in terms of the volatility of the liquid. I n general, if one avoids handling radionuclides in a volatile or potentially volatile form, there is little need to require a hood for a radiochemical experiment involving a few hundredths of a microcurie of activity. Even with a hood, one would avoid releasing radioactive halogens, volatile radioactive organic compounds, and radioactive gases such as Cot and H2S which might be released by thermal dissociation or acidification of radioisotopically labeled carbonates or sulfides. Planchets containing radioactive material should be handled with forceps such as the type having medium fine, curved points. The forceps alone should not, however, be used to carry the planchet about the laboratory. To reduce the chance of dropping the planchet, a source holder, i.e., the holder used to position the source beneath an end-window detector, or a small flat plate or tray, should be used to support the planchet when it is being moved. Consistent with good housekeeping, the laboratory working surface should be kept clean with an easily removed covering of plastic or paper. Inexpensive trays or rectangular glass dishes of the household type are helpful for confining a spill of liquid. All bottles and stnrage vessels containing radioactive material should be properly labeled in accordance with governmental regulations. This includes, of course, the containers for radioactive liquid waste and for solid waste. Suitable labels, tags, and printed adhesire tape are commercially available from a number of suppliers of laboratory equipment for radiochemistry. The safe-handling problem varies depending on the chemistry of the experiment, the quantity of radioactivity involved and the particular radionuclide or radionuclides being used. Thus, in separating solids and liquid, centrifugation is generally preferable to filtration because of the greater difficulty in controlling filtration with respect to recovery of material, spread of contamination, etc. Filtration is sometimes unavoidable as, for example, in the preparation of certain sources for counting. Special filtering apparatus (of which several types are commercially available) and careful techniques are required because of the nature of this mechanical separation process. Quantity of Radioactivity and Choice of Radionuclide
Several factors combine to determine the quantity or radioactivity used in a teaching experiment. Although millicurie quantities of a radionuclide might be available as material produced locally nith an accelerator or a research reactor, or as licensed by-product material, or as natura,lly occurring material, the quantity Volume 40, Number 7 2, December 1963
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handled by a student in a teaching experiment should be l i i t e d by radiological safety requirements to the minimum necessary to obtain meaningful results. Neither an experiment requiring the use of an exceptionally hazardous radionuclide nor one for which the minimum quantity is of the order of a millicurie would be an attractive experiment in a regular undergraduate chemistry course. In the case of radionuclides produced with a small ( q n ) source or available as "generally-licensed" or "exempt" quantities of byproduct material, the limitation in quantity used is often controlled by the limited supply. Generallylicensed quantities, commonly limited to a few microcuries, do not require a specific license. They are specified by law and are under the regulation of either the federal or the state government, depending upon Table 1. Radionuclide
Quantity, wc
t
l
~
the state. Generally-licensed quantities together with the half life and information about the radiation emitted by each radio-nuclide are listed in Table 1. A number of these radionuclides are available in generally-licensed quantities from commercial snppliers2 as indicated in Table 1. It should be noted that the use of generally-licensed quantities does not entitle one to ignore the practices of safe handling. 'A list of suppliers may be obtained from the U. S. Atomic Energy Commission, Division of Isotopes Development, Washington 25. D. C. Most of the comnanies that sell the instruments and laboratory equipment used for work with radioactive materials also sell the radioisotopes. In addition, there are companies that specialize in radiochemicals. Suppliers are also liated in "The Isotope Index," published by the Scientific Equipment Co., P. 0.Box 19086, Indianpolis 19, Ind.
Generally-Licensed or Exempt Quantities of Radionuclides~~' ff
7 0
Radionuclide
Quantity, IIC
t l h
ff
9
10
All other 6, remitting material: 1 uue' Alpha emitting msterid: 0.1 pc
Not as a sealed source. "or by roduct material, based on the Federal Register Title 10, Chapter 1, Part 30 (October 17, 1957) and the New York State Sanitary &de Chapter 1, Part 1,6 (October 15, 1962). No person is s,ll.llowed at m y one time to transfer, receive, possess or use more than a total of ten generally-l~eensedquantities. E , in Mev for only the predominant beta radiation-of interest in selecting a counting procedure. A number in parenthesis indicates low intensity. Negatron emitters unless otherwise indicated. E + emitter. " a emitter. J Radionuclide liated in the New York Sanitary code but not in tbeFederalRegister. Characteristic X-rays are indicated when electron capture is not acY indicates emission of 7 rays; (y) indicates low intensity. companied hy y emission. Radionuclide is one of those readily available commercially; often sold as part of a set or source kit of generally-licensed material. "xsmples of other radionuclides available as generally-licensed, 1 pc quantities are Sbl", Balsa, Cow, Mnw, HgPo: Ag'IDm,and SP. PhUO (a source of Bizlo and Poal0) is availahle in 5 po quantities.
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When there are several radionuclides suitable for use in an experiment, it is advantageous to choose one with a half life not over a few weeks. The apparatus becomes selfdecontaminated in a reasonable length of time and the accumulation of radioactive waste does not become a problem. If needed, however, radioactive waste disposal service is available commercially and it is usually feasible to cooperate with other users of radioactivity on campus in employing such service. For experiments illustrating a method of radiochemical separation, the generally-licensed quantities are usually adequate. There are also several naturally occurring radionuclides which are suitable. One might use perhaps 1-5 times as much as is needed for counting, i.e., a total of about 0.03pc, to obtain a final counting rate of about 1000 cpm. If a radiochemical separation is to be followed by half life determination in the same laboratory period, the time required for the separation and subsequent source preparation must be short. Commonly, but not necessarily, this means a separation by precipitation or solvent extraction. Separation by ion exchange can be rapid when volumes are small. Ion exchange, adsorption, solvent extraction, electrolysis, or precipitation have all been used in experiments involving the separation of a short-lived daughter from its long-lived parent, e.g., UX, from U238(12, 14, 44, 72-76) ; UX2from UX, (72, 74, 75) ; Bala'm from Cs13' (76,77) ; Pb2mmfrom Biz" (78) ; ThB from ThX (79,80) ; ThC from ThB (79); La1" from Bal4O(81); PrlP4from CelP4(89); and RaE and RaF from RaD (85, 84). Thin layer chromatography and filter paper chromatography (85-85) are worthy of serious consideration in experiments illustrating radiochemical separation. Although filter paper chromatography, for example is relatively slow, an adequate separation of two radionuclides can often be obtained in less than an hour if AR, for the elements involved is large. Chromatographic methods are inherently simple, and since they require no attention, the students can be conducting other experimental work concurrently. The chromatogram can be sectioned for counting or it can be scanned with an end-window detector (or a cylindrical detector if the radiation is sufficiently penetrating). A simple scanner can be made with a ruler, a scanning slit (e.g., 2 mm) cut in a piece of metal such as lead and interposed between the chromatogram and the detector, adhesive tape, a piece of wood or heavy cardboard, a laboratory clamp with holder, and a ringstand. Relatively few of those generally-licensed radionuclides which are currently readily available are suitable for the determination of a half life by decay during a single 3-hr laboratory period. I n general the activity should be followed for a t least two half lives for a sample known to contain only a single radionuclide. For a sample containing longlived activity as well as the short-lived one to be measured, the decay should be followed until the longerlived component or components predominate. Allowing for time to prepare a source for counting, the upper limit of half life for measurement during a single 3-hr laboratory period is about 1 hr. The lower limit, assuming a relatively simple counter consisting of a Geiger tube in combination with a scaler or a ratemeter, is about 1 min. Both generally-licensed and naturally
occurring radionuclides require radiochemical separation of the desired short-lived nuclide from a long-lived parent. When an accelerator, nuclear reactor, sub-critical assembly, or (qn) source is available, several radionuclides suitable for half-life determination can be produced. An (a+) source, e.g., Pu-Be or Ra-Be (86), is convenient for the production of a limited number of radionuclides for low level work. By judicious choice of target material, one can often use the induced activity without radiochemical separation. A selected list of short-lived radionuclides, including two which are naturally occurring, is given in Table 2. This list is limited to those radionuclides for which there is no special problem in readily obtaining target materials or long-lived precursors. Vorres (87) has given a detailed description of neutron activation with a Pu-Be source and has listed the initial activities attainable for a number of radionuclides. Table 2.
Selected Beta and Beta-Gamma Emitters with Half Lives Between 1 Min. and 1 Hr.
" Gamma only
Many applications of radioisotopes which one might like to exemplify in a teaching experiment normally require amounts of radioactivity of the order of a few tenths to several millicuries. Appropriate experiments are to he found in laboratory courses in radiochemistry since it is in such courses that the techniques for handling millicurie amounts are presented. When the level of radioactivity is limited entirely to generally-licensed quantities and when many students are involved, the design of a suitable experiment becomes a challenge. In fact, one must often be prepared to tailor the teaching experiment to fit the radionuclide. I n research one tries to avoid this limitation. Consider, for example, the use of radioisotopes in the determination of the solubility of a slightly soluble substance. The advantage of the tracer method for analysis in this case can be readily understood in terms of its sensitivity. An experiment dealing with solubility, ionic equilibrium, the common ion effect, etc. is appropriate in a variety of courses. There is no difficulty in preparing a list of substances which are slightly soluble in a common solvent such as water. Yet, relatively few are attractive for a teaching experiment with generally-licensed quantities. A few simple calculations will show why. One must first estimate the quantity of radioactivity required in order that significant result,s can be obtained with the counting equipment available. Volume 40, Number 12, December 1963
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In its simplest form, the experiment consists of (1) measurement of the radioactivity concentration of the tracer solntion to be used, (2) preparation of a few milligrams of labeled solid of known specific activity usually predetermined by the amount of tracer solntion used, (3) equilibration of the labeled solid with a solvent (or solntion containing a common ion, for example), and (4) measurement of the radioactivity concentration of an aliquot of the saturated solution. Measurements of the activity of the original tracer solntion and of the final saturated solution are made in an identical manner, i.e., as a solid after plating or evaporation of the solvent in a planchet, or as the liquid using a suitable Geiger or scintillation counter. Taking as an example an unsbielded, thin-window Geiger counter having a background counting rate of about 3&50 counts per minute, one finds that counting errors associated with the statistical nature of radioactive decay have a strong influence on the results when measurement times are limited to a few minutes and when the counting rate in excess of background for the sample is much less than 300-500 counts per minute, i.e., about ten times background. If the connting conditions are favorable, i.e., high sensitivity of the counter for the radiation, high energy radiation attenuated to a negligible degree by the window, and negligible self-absorption in the source, then it is feasible to expect an overall detection efficiency of 1&20%. Assuming that the lower limit is more realistic, the sample of saturated solution taken for counting should have a t least 3000-5000 disintegrations per min, corresponding to about 0.002 PC. If the volume of liquid in equilibrium with the solid is as small as 10 ml and if a 1ml sample is taken for connting, the 10 ml of solution would contain abont 0.02 pc. If the solubility of the substance is 1 mg per 100 ml of water, for example, the specific activity of the substance must be 0.2 pc per mg. Preparation of a sample of the labeled solid weighing only 25 mg will require 5 pc of activity. The lower the solubility of a substance is, the higher is the specific activity necessary because only a fraction of the total activity in the system is in solntion and available for measurement. This is quite apart from any aliqnoting involved in sampling the saturated solution. In actual applications of the tracer method, higher counting rates are desirable, and the necessary quantity of radioactive material is used. The solubility of calcium sulfate in water is sufficiently high (300 mg per 100 ml) to make its determination feasible (within the limits of reaching equilibrium, etc. in a three-hour period) with generally-licensed quantities. Similarly, the solubility of lead iodide (40 mg per 100 ml of water) can be measured by the use of I13'. Sometimes a solubility-type experiment becomes feasihle when a complexing agent is present or when the temperature is somewhat above room temperature. The latter variation, of course, introduces the added complication of requiring a temperature-controlled apparatus. Starting with an estimate of the amount of radioactivity needed for measurement and following a similar approach based on a knowledge of the chemical phenomenon involved, one can estimate the total amount of activity needed for any type of experiment. For radionuclides emitting low-energy beta radiation, e.g., C", S35, losses in detection efficiency resulting 624
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from self-absorption in the counting source and window absorption must be compensated for by using more activity. I n the case of C14,the losses of beta radiation by absorption in a counter window are approximately 23, 40, 55, and 65% for window thicknesses of 1.0, 2.0, 3.0 and 4.0 mg/cm2, respectively. Tritium, although available as a generally-licensed radionuclide, emits such low-energy radiation that relatively sophisticated techniques and rather special detection instruments are required. The design of experiments requiring only small quantities of radioactive material has been receiving attention, e.g., ($0,28, ?24,41, 57,85), and progress has been made in this direction. Further effort is needed. As already pointed out, however, the level of radioactivity required is not the only factor which determines the usefulness of an experiment. An experiment requiring only a low-level quantity of radioactivity is not a good experiment for a regular undergraduate chemistry course if it involves procedures, techniques, and apparatus used by a specialist who has a wellequipped radiochemistry laboratory a t his disposal. There is need for improvement in this direction. Many of the experiments which have been published over the years can be modified for use with reduced quantities of radioactive material and with simplified radiochemical procedures. I n some cases the counting procedure can be improved by counting liquid samples of gamma emitters with a well-type scintillation detector of the type which has become available for teaching purposes. Other Aspects
The design of an experiment in radiochemistry can be strongly influenced by the way in which the experiment is scheduled in a course. I n much of the foregoing discussion it is implied that the experiment is to be completed in a single laboratory period. Such is the case when the apparatus and instruments are used by a different student or team of students each period as in a course involving hundreds of students. When the scheduling permits a student to use the apparatus and instruments for more than one period, certain advantages accrue. Somewhat longer-lived radionuclides (88) can be used for half life determinations, filter paper chromatography can be combined with antoradiography, etc. Despite the limitations of a single three-hour laboratnrv .." nnriod. much can be accomdished. At Rensselaer, for example, the students in first year chemistry currently conduct two experiments in a single period. One consists of a rapid separation of Prl" from Ce14' followed by a determination of the half life of The second consists of a tracer experiment in which BizLois used in determining the distribution of bismuth between two solvents. It is difficult to relate the dollar cost to the value of an experiment but the relationship cannot be overlooked. The cost of nuclear instrumentation suitable for basic experiments, e.g., detector with scaler or ratemeter, is rather uniform. Sometimes savings can result from choice of detector. If one is not using a radionuclide such as C14 or 535, there is little jnstification for employing an end-window detector with an especially thin window. These special detectors are more costly and will require replacement more often r - - ~~7 ~
because they are more fragile. Glassware and apparatus normally carried in the stockroom should be used whenever possible. Radiochemistry does not inherently require the use of costly apparatus. Students should be impressed by the function of the equipment, not its elegance.
(16) "Symposium on the New Elements," J. CKEM.EDUC.3 6 . 2 (1959). (17) WALLMANN, J. C., J. &EM. EDUC.,36, 340 (1959). (181 OVERMAN. R. T.. J . CHEM.EDUC..28. 2 (19591.
Demonstration Experiments
Most of the foregoing also applies directly to radiochemical experiments designed for lecture or classroom demonstration. New problems arise, however, because of the limited time availahle for the demonstration and because of the challenge of conveying the nature and progress of the experiment to the spectator. The second complication becomes severe when there are several hundred students in the lecture hall. Where availahle, closed circuit television is helpful, of course. In general the requirements are (1) a simple and yet striking illustration of the phenomonon (2) a clearly visible or audihle indication of the phenomenon and (3) relatively inexpensive apparatus. The counting equipment which probably comes closest to meeting these requirements consists of a Geiger tube and a ratemeter equipped with a loud speaker. The counting rate can he recorded on the blackboard as a quantitative confirmation of the qualitative audihle signal experienced by the students. In designing a lecture experiment one has the advantage of being able to incorporate techniques consistent with the skill of the lecturer. The classification of experiments previously mentioned still applies. Thus the combination of a radiochemical separation followed by determination of a half-life by decay, e.g., the separation of the element protactinium as PaP34(UXJ from uranyl nitrate and determination of its half life (1.18 miu) has been used by the author. Tracer experiments such as those involving isotope exchange or dynamic equilibrium as for a solid salt in contact with a solution can also be used effectively for lecture demonstration. Acknowledgmenl
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