Metal, Oxide
ISOTOPES OF NICKEL A. R. BROSI Chemistry Division, O a k Ridge National Laboratory, Oak Ridge, Tenn. T h e eleven known isotopes OF nickel have many special properties which make them valuable For a wide variety OF chemical studies. Electromagnetically concentrated samples of each of the five stable isotopes have been prepared. Six radioisotopes have been characterized. Among these are several isotopes which emit very penetrating gamma radiation that can be measured through the walls OF heavy process equipment. O n e radioisotope emits very soFt beta radiation and is suitable For studying reactions OF a relatively few atomic layers. The use of nickel isotopes in chemical studies has been reported in several publications. A more extensive use of these isotopes is Foreseen.
OTH the separated stable isotopes and the radioisotopes of most of the elements are being used to an increasing extent to aid in the solution of chemical and engineering problems. Many of these applications of isotopic techniques are possible only with some of the elements which have isotopes with special characteristics. The chemist or chemical engineer working with nickel is fortunate in t h a t there are 11 known nickel isotopes with many special properties. I n order t o facilitate discussion of the characteristics of the individual isotopes a few of the general types of applications of isotopic techniques t o chemical problems will be reviewed. The properties of the individual nickel isotopes will then be described in so far as they affect the feasibility of applying isotopic twhniques t o problems in nickel chemistry. Finally, examples will be given of the use of nickel isotopes in chemical studies which have been reported in recent publications. One of the important chemical applications of tracer techniques has been in the analysis of unknown samples for small amounts of a n element. Two general methods of analysis have been used. I n one method, known as isotopic dilution analysis (11), a known amount of the element with an isotopic composition different from t h a t occurring in nature is added to the sample. The element is then isolated and the abundance ratios redetermined. From the known change in abundance ratios the amount of the element in the unknown sample can be computed. Radioisotopes can also be used in making isotopic dilution analyses. In this case the specific activity of the radioisotope is measured before addition to the sample and again after recovery. One advantage of the isotopic dilution method is that quantitative recovery is not required. Activation analysis ( I ) is the second general method of analyzing for trace amounts of a n element in a n unknown sample. When this method is used the sample is bombarded in a cyclotron or nuclear reactor and the amount of a given radioisotope produced is measured. The amount of the element in the sample can be computed either from the cross section for production of the radioisotope and the time and flux of the bombardment or from a previously determined calibration curve. byhere repeated quantitative analyses are required in a chemical experiment, i t is often convenient to add a radioisotope with known activity t o a definite mass of the element. Quantitative analysis for the element then involves only a n activity measurement. This technique has been used in determining the distribution of an element or compound between phases as in solubility determinations. It has several advantages over the usual methods of analysis. One of these is speed, a second is sensitivity, and a third is automatic recording where concentration histories are being measured as in ion exchange or solvent extraction experiments.
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Another type of application of isotopic techniques is in eschange reactions. I n these experiments the tracer isotope, either stable or radioactive, is in one chemical form and the natural isotopes are in another form. I n this wag the rates of exchange between different oxidation states or between different complex ions can be measured. Experiments of this kind furnish evidence for reaction mechanisms and in some cases shed light on the type of bonding in molecules. Isotopes have also been used t o determine diffusion coefficients in solids, liquids, and gases. Where self-diffusion is being studied only those techniques involving isotopes can be used. Another large area of study where stable isotopes are valuable is in the field of molecular structure, The differences in mass, nuclear spin, and magnetic and electric moments of the isotopes cause shifts and splitting of energy levels which are invaluable in the interpretation of spectra. This brief resume of general applications of isotope techniques leads to the question of whether such experiments are feasible with the nickel isotopes. Where there is only one stable isotope as in the case of cobalt or none as in the Gase of technetium or promethium, the stable isotope experiments are not possible. Nickel, however, has five stable isotopes with mass numbers 58, 60, 61, 62, and 64. Nickel 64 is about 10% heavier than nickel 58, which is enough t o cause appreciable shifts in the energy levels of molecules. The natural abundances are Nib*, 67.7%; NPo, 26.2%; Ni61, 1.25%; Ni6*, 3.66y0; and NP4, 1.16% (9). The large differences in natural abundances are very favorable for isotopic dilution analyses and for exchange experiments. When the nickel in one chemical form has the natural ratios and the other chemical form is highly enriched in Ni61 or Ni", there is a factor of almost 50 between no exchange and complete exchange. If all of the isotopes had the same abundance the factor would be only three. One of the nickel isotopes, mass 61, has an odd number of neutrons and is probably the only stable isotope which has a nuclear spin different From zero. Although the nuclear spin, magnetic moment, and electric quadrupole moment of nickel 61 have not been measured, it would seem probabIe t h a t this isotope will be particularly valuable in molecular structure studies using microwave techniques. The stable isotopes would have little value as research tools if they could not be separated. I n the case of the nickel isotopes the technique has been developed (6) and electromagnetically concentrated samples of each of the isotopes have been produced. The abundances in the enriched samples are usually greater than 90% for a particular isotope. Samples ranging from a few milligrams t o several hundred milligrams are available for distribution through the Isotopes Division of the Atomic Energy Commission at Oak Ridge. The characteristics of the radioisotopes of nickel (9) are also favorable for applications t o a wide variety of chemical problems. Six radioisotopes are known, having mass numbers of 56, 57, 59, 63, 65, and 66. All of these have been well identified from the standpoint of element and mass assignments. The radiations of all except the recently discovered isotopes with mass numbers 56 and 66 have been studied in detail by several different groups with good agreement. Nickel 56 has been produced ( 8 , I O ) by an a,2n reaction on iron It decays with a half life of about 6 days by orbital electron capture followed by the emission of a complex spectrum of gamma rays. T h e convenient half life and the presence of high energy
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ICKEL-METAL,
OXIDE
gamma rays will make N P a useful isotope in tracer analyses and exchange studies. Detection equipment can be chosen which will minimize the complications caused by the presence of the Cob6 daughter. Nickel 57 has been produced by cyclotron bombardments using an or,n reaction on iron and a n n,2n reaction on nickel. Only small amounts are produced in low specific activity by bombardments in uranium reactors. Nickel 57 has a half life of 36 hours and decays by positron emission to Co57, which has a 270day half life. The positron has a maximum energy of 845 k.e.v. and is followed b y the emission of a 1.9 n1.e.v. gamma ray. Xi57 should be a valuable tracer for experiments extending over periods of several days. Because of the energetic gamma radiation it could be traced through the walls of heavy equipment. The fact that it decays t o a radioactive cobalt isotope might be an inconvenience with some kinds of detection instruments. The radiations of KT7 differ enough from those of Co57 so that Xi5’ can be detected in the presence of the daughter isotope. Nickel 59 has been made by cyclotron bombardments using an or,n reaction on iron and a d,2n reaction on cobalt. It has been produced in low specific activity by bombarding nickel in a uranium reactor. Nickel 59 has a half life of the order of 100,000 years and decays by orbital electron capture. Cobalt x-rays are therefore emitted. Nickel 59 should be valuable in experiments extending over long periods. The cobalt x-rays are readily absorbed and hence this isotope cannot be counted through heavy walls or in massive samples. I n laboratory samples weighing only a few milligrams corrections for absorption in the sample are small. Nickel 63 can be made by bombardment of nickel in a uranium reactor. It has a half life of about 85 years and emits a low energy beta particle. Since this beta particle has a range of about 6 mg. per square Centimeter, samples must be thin and uniformly spread. They should be counted with a very thin window or windowless counter. It has been found t h a t Xi63 samples can be electroplated and counted reproducibly. The labor involved is greater, of course, than when energetic beta radiation is counted. I n spite of this Ki63 will probably be one of the most valuable radioisotopes; although the softness of the radiation is an inconvenience in some experiments it is a valuable property in others. Some of the possibilities for using Sic3, as well as some of the difficulties in working with it, might be illustrated by observations made in experiments to determine the half life (9). I n one case the half life was computed from the neutron absorption cross section of Ni62 measured by Pomerance and the amount of activity produced by a known neutron flux. This gave 85 years with an uncertainty of about 20%. I n another case the activity of samples electroplated onto platinum was measured a t intervals over several years to observe the decay directly. I n all cases the activity decreased more rapidly than would be expected for an 85-year half life. Some of the samples had apparent half lives of only 35 years. It is believed that at least some of the discrepancy resulted from changes in the samples. Further evidence for this was found in changes with time in the energy distribution of the electrons emitted. All of the samples stood in air a t room temperature. I n some the nickel plate was thicker than the range of the beta radiation and the changes may have resulted from oxidation. I n other samples the plate was only a few micrograms per square centimeter in thickness and complete oxidation could not have added enough absorber t o reduce the counting rate as much as was observed. Possibly diffusion into the platinum was occurring at room temperature. Ni63 is counted in a 5070 geometry counter, 10 pg. geryWhen square centimeter of absorber will reduce the counting rate 2y0. With sufficiently active sam les counting rates can be
determined with a precision of 0 . l Z If a substance with a density of 10 grams per cc. were deposited on a nickel surface, 10 pg. per square centimeter would correspond to a 100-A. thickness and this could be determined within 5%.
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Because Ni63 radiation is so readily absorbed this isotope should be extremely valuable in studying surface reactions, diffusion, and homogeneity in solids containing nickel. It should be possible t o detect differences in nickel concentration in regions of a few microns using radioautographic techniques. Nickel 65 has been produced by cyclotron bombardments using a n n,p reaction on copper and an n,a reaction on zinc. I t is also produced in high specific activity in a uranium reactor. Nickel 65 has a half life of 2.6 hours and decays by beta and gamma emission to copper 65. dlthough the decay scheme is complicated, it is well understood. Because beta and gamma radiations are energetic, the technique of sample preparation and measurement is simplified. This is a convenient isotope for tracer experiments when these last for only a few hours. The short half life and energetic radiations of Xi65 make it particularly suitable for measurement in activation analyses. With presently available neutron fluxes it should be possible to determine the amount of nickel in samples containing considerably less than a microgram with good precision. Sickel 66 has been produced by high energy spallation and fission reactions. It decays with a 56-hour half life by beta emission to Cuss, having a half life of 4.3 minutes. It may become a valuable tracer but a t present it is not widely available. Because of the differences in the half lives and radiations of thc nickel isotopes it is possible to assay moat of them in the presence of the others. This makes possible multiple tracer experiments, whereby nickel in several different chemical forms can be followed through a reaction or process. Although the characteristics of the nickel isotopes are favorable for a wide range of applications to chemical problems they have not as yet been used extensively. Johnson and Hall ( 5 ) used Kiss, having a 2.6-hour half life, in a study of the exchange between nickel perchlorate and several different organic complexes in organic solvents. Because of the short half life of Ni65 their experiments ran a t most for 90 minutes. Some of the compounds exchanged completely within minutes after mixing whereas no exchange was found in other cases. Johnson and Hall were able to correlate their results with evidence from other types of experiments and could draw conclusions about the type of bonding in their complex compounds. This study has been extended by Hall and Willeford ( 4 ) to 14 other four-coordinated nickel complexes using Ni63 (85-\.ear half life). They correlated their exchange results with published magnetic susceptibilities and other measurements indicative of bond types. Long ( 7 ) has studied exchange reactions of tetracyanonickclate ion in aqueous solutions using ?Jia3. One conclusion was that solid nickel cyanide contains two nonequivalent kinds of nickel, suggesting that the solid nickel cyanide is really nickel tetracyanonickelate. I n another study Cook and Long ( 3 ) have used Ni63 in the determination of the dissociation constant of the complex formed from nickel ion and ethylenediaminetetraacetic acid. Work is known to be in progress where the nickel isotopcs are being used in diffusion, ion exchange, and further isotopic exchange studies. Because of the availability of the stable isotopes and the special properties of many of the radioisotopes these uses will undoubtedly be extended in the future. LITERATURE CITED
(1) Boyd, G. E., Anal. Chem., 21,335 (1949). (2) Brosi, A. R., Borkowski, C. J., Conn, E. E., and Griess, J. C., Phys. Rev.,81, 391 (1951). (3) Cook, C. M., and Long, F. A., J . Am. Chem. Sac., 73,4119 (1951). (4) Hall, N. F., and Willeford, B. R., Ibid.,73, 5419 (1951). ( 5 ) Johnson, J. E., and Hall, N. F., Ibid.,70,2344 (1948). (6) Keim, C. P., NucZeonics, 9, No. 2, 5 (1951). (7) Long, F. A., J . Am. Chern. Sac., 73,537 (1951). (8) Sheline, R. K., and Stoughton, R. W., Phys. Rev.,in press. (9) Way, K., Fano, L., Scott, M. R., and Thew, K., Natl. Bur. Standards, Circ. No. 499 (1950). (IO) Worthington, W. J., Phys. Rev.,in press. (11) Yankwich, P.E., Anal. Chern., 21,318 (1949). for review October 17, 1951. RECEIVED
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ACCEPTEDMarch 13, 1952.
Vol. 44, No. 5
