minations gave linear first-order plots. The rates were unaffected (within experimental error) by changes in base concentration from 0.01 OOM sodium hydroxide down to nearly solvolytic conditions. Reactions were generally followed through three half-lives. A typical rate plot is shown in Figure 3. Note that a clearly linear plot is obtained and that even in the third half-life, the average deviation of points from the line is not large. This is a consequence of the good precision of the analytical method. As shown in Table I, the standard deviations for the rate determinations for four of the six compounds were less than 1%. For 3-chloro-1-ethyl-1-azacycloheptane hydrochloride (a), the standard deviation of 1.7% was probably due to uncertainty in time, since the compound had a half-life of only 61 sec. An important factor for obtaining first-order kinetic plots in the formation of aziridinium ions from P-chloro amines is that the aziridinium ions must not back-react with chloride either before or after quenching of the reaction with acid. We found that back-reaction with chloride was suppressed by the low substrate concentrations (5 X 10F4M). It was negligible under the conditions in which the kinetic measurements were made. This conclusion is drawn from the linear nature of the first-order plots and their insensitivity to changes in base concentration. It is also supported by the accurate infinity titers. Extrapolation of the rate plot to time zero gave a value for chloride concentration at that time. If no alkyl-bound chloride reacted with mercuric ions during the analysis, and if no undesirable side or back reactions occurred, then the
chloride concentration (or absorbance) at time zero should be half as great as that at infinity. Such a correlation was observed in most circumstances to within 1 %, in further support of the following general equation for the reactions under study :
Products
A "s",:,":
CONCLUSIONS
The mercuric chloranilate method of chloride ion analysis can be adapted for kinetic applications, having a dynamic range of > l o 3 down to 10-5M concentrations. Good precision (approximate size 1 %) was obtained in following the rate of appearance of chloride from P-chloroamines. A novel apparatus which permits rapid and precise sampling of kinetic solutions was also described. The techniques developed for obtaining this enhanced precision in the mercuric chloranilate procedure can be utilized for general analytical purposes. RECEIVED for review May 14, 1970. Accepted August 7, 1970. This work was supported by National Institutes of Health Grant CA 5180 (C.F.H.) and pre-doctoral fellowship G M 32,752 (J.H.C.).
In-Field Behavior of and Cumulative Effects on Certain Electrodes in a Gamma Field Hisashi Kubota Analytical Chemistry Diuision, Oak Ridge National Laboratory, Oak Ridge, Tenn. 37830 The effect of a gamma radiation field on the response of the saturated calomel, glass, fluoride specific, and nitrate specific electrodes was studied. All four electrodes gave reliable readings in gamma-radiation fields as high as 15,000 rad/min. The calomel electrode was unaffected by cumulative dose up to lo8 rad. The glass electrode was somewhat affected by large dose to give readings to the high pH direction. The potential response of the fluoride electrode shifted in one instance; however, the Nernstian slope remained unaffected. As a result, the affected electrode did not lose its effectiveness as long as calibrations were made periodically with standard solutions. The nitrate electrode began to give erratic readings as the cumulative dose approached 107 rad. Replacing the internal standard by fresh solution restored this electrode. The radiation stability of any internal reference solution is a prime factor that governs the behavior of an electrode in a radiation field. The calomel and glass electrodes were stable in solutions of high alpha activity at least up to dose of 108 rad.
THEREI S A PROGRAM at this laboratory which is directed specifically to radiation effects on chemical analysis with particular emphasis on hot cell analyses. This paper describes work that was conducted on the behavior of certain electrodes which are used as sensing elements in electrochemical analysis when placed in a gamma radiation field.
Despite the great interest in radiation effects and the popular use of electrochemical methods in hot cell analysis, surprisingly little has been reported with respect to what effects take place when various electrodes are exposed to radiation. It has been assumed at this laboratory that most electrodes will be adversely affected by radiation. The glass electrode used to check the acidity of the highly radioactive homogeneous reactor solution was a disposable homemade affair which was discarded after a single determination. It had also been the practice to introduce calomel reference electrodes through the medium of a salt bridge t o minimize radiation effects to the calomel paste. Kinderman and Carson ( 1 ) placed various electrodes in a buffered phosphate solution containing 32P as the radiation source and followed the response of the electrodes as they were exposed to the beta radiation. They reported the silver-silver chloride and calomel electrodes to be stable over long periods within this radioactive medium while the glass and antimony electrodes were stable for shorter periods. Fedotov ( 2 ) placed lithium glass electrodes in a gamma field ( 1 ) E. M. Kiiidermaii and W. N. Carson, Jr.. U. S . A / . Energy CO/HW RP/>. ~ . TID-280 (1949). (2) N . A. Fedotov, A / . Energ., 8, 262(1960).
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l o BBB
A
Table I. Radiation Stability of pH Buffer Solutions Dose (rads) 0 5 x 106 2 x 108 2 x 107 4.000 4.253 4.861 5.802 PH 7.093 7.083 7.131 7.095 10 021 10.030 10.035 10.041 t
Figure 1. Arrangement for studying in-source response of an electrode A . Lead container B. Lead shield C. Salt bridge D. Electrode E. Cobalt
and measured the changes in sensitivity gradient, asymmetry potential, and resistivity after dose up to 2.5 x 107 rad. H e found the changes taking place to be minor and concluded that this electrode was usable for p H measurements in media of high gamma activity. None of the above work has described the behavior of the electrode in question when it is actually in the radiation field, and these studies have been concerned with cumulative effects. The behavior of a platinum electrode in a gamma field (3) has been described when a small current was drawn off in proportion to the production of certain products from the radiolysis of water. This present paper reports the behavior of the saturated calomel, glass, solid state fluoride, and membrane nitrate electrodes when in place in a gamma radiation field and covers the behavior of the electrodes when they are operating in a gamma field along with the cumulative effects from long term exposure. EXPERIMENTAL
Electrodes. Beckman GC glass electrodes and Corning triple purpose glass electrodes were selected for the study. The selections were made to include both mercury and silver-silver chloride internal glass electrodes. Beckman fiber tip saturated calomel electrodes were used. The fluoride electrode was the Orion solid state unit using a europium-doped lanthanum fluoride single crystal as the sensing unit. An Orion membrane type nitrate electrode was used. Radiation Source. The cobalt-60 source was made available by the courtesy of the Radiation Chemistry Group of this laboratory. The source has a fixed cavity for placing the sample to be irradiated, and the source is lowered into place after the sample is properly positioned. Dose rates were determined with the Fricke dosimeter. Two sources were used, both with the same geometric structure but differing in cobalt content and associated dose rate. Radiation Stability of Solutions Used In-Source. The radiolytic stability of the solutions to be used in-source was established first. Commercially available p H 4, 7, and 10 buffers were exposed in the cobalt source, and the changes in p H with dose were determined. The results are shown in Table I from which it is evident that the p H 4 buffer is unstable but the p H 7 and 10 buffers are stable. This behavior is indicative of the relative instability of the alkali salts of organic acids and the stability of phosphate and borate (3) F. S. Feates, Trans. Faraday SOC.,56, 1671 (1960). 1594
solutions to radiation. A solution of potassium dihydrogen phosphate was prepared (0.2M pH, 4.5) and irradiated. This solution showed no detectable change in p H with dose up to lo8 rad and thus was used as the solution for the insource runs with the calomel and glass electrodes. Standard fluoride solutions of 0.0001M, 0.001M, and 0.01M were prepared in phosphate buffer (KH2P04-Na2HP04,p H 6.9.) These showed no change in fluoride concentration up to 108 rad dose. Nitrate is decomposed by radiolysis to nitrite, and the rate of decomposition increases with increasing nitrate concentration and p H (4). No detectable change on a p H 6.9 buffered 0.001M nitrate solution was found up to 5 X lo5 rad after which measurable changes took place. As a result, in-source nitrate activity measurements were terminated by 5 X 10; rad dose. In-Source Measurements. The arrangement of Figure 1 was utilized for following single electrode behavior. The electrode in question was placed within the lead container at the corner of the smaller source cavity, and lead shields were placed between this container and the source itself. This geometry represented the position of the minimum dose rate of 40 rad/min. Removing the three lead plate shields upped the dose rate to 90 rad/min, complete removal from the lead container but still in the corner resulted in dose rate of 250 rad/min, and placement at the source center gave 1000 radimin. A large source was used to obtain the maximum dose rate of 15,000 rad/min. A salt bridge was connected between the solution in which the electrode in question was immersed and the complementary half cell placed outside the source cavity. This bridge was a tygon tube plugged with terminal agar plugs and filled with saturated KC1 solution. The source was lowered into place, and the necessary readings were made. This arrangement allowed following the behavior of an individual electrode with very little complicating effects. Effect of Cumulative Dose. Each electrode immersed in a suitable solution was placed in the center of the larger of the two sources and irradiated at the rate of 1.5 X l o 4 rad/min. At appropriate intervals the electrode was removed for a measurement us. a n unirradiated complementary electrode. RESULTS AND DISCUSSION
The response of the calomel electrode was found t o be unaffected by a gamma radiation field at least as high as 15,000 rad/min. A slight shift of the order of 5-8 millivolts with time was observed when this electrode was placed at the center of the larger source. It was demonstrated that this shift was a temperature effect resulting from the in-source heat buildup by opening the source cavity t o allow the system t o cool down t o ambient temperature, whereupon the reading dropped back to the original and remained there even when the source was again lowered into place. The good stability of the calomel electrode is particularly gratifying t o us because we had expected adverse reaction from radiation in the past and had been resorting to elaborate arrangements to protect this electrode as much as possible. The results here indicate that (4) M. L. Hyder, J . Phys. Chem., 69, 1858 (1965).
ANALYTICAL CHEMISTRY, VOL. 42, NO. 13, NOVEMBER 1970
I\
I
1
-BEFORE IRRADIATION ---- AFTER 2 x 106 rad DOSE -- AFTER REPLACING I N T E R N A L
-BEFORE IRRADIATION -- AFTER 5 x 106rad DOSE
----- AFTER
'F
2 x 107rad DOSE
-
10-2
Figure 2. Effect of y-radiation on the fluoride (Orion solid state) electrode placing the calomel electrode directly within a gamma or beta field will cause no change t o its potential. No changes in electrode behavior were observed with cumulative dose up t o lo8 rad. The glass was turned brown by the radiation, but there were no other signs of change or deterioration. Kinderman and Carson ( I ) reported that the saturated calomel electrode showed no effects of the exposure to beta radiation in aqueous solution where the total dose was estimated to be about 5 X loGrad. It has been reported also that a 106-rad dose to saturated calomel electrodes caused shifts in potential of about 2 mV compared t o unirradiated electrodes and this difference vanished with time (5). This shift was indicated t o be possibly a thermal effect. We conclude that the potential of a saturated calomel electrode is not affected by a gamma radiation field at least up to 1.5 x 104 radimin nor by cumulative dose up to lo8 rad. The potential of the glass electrode was found to be unaffected at dose rates between 40 and 15,000 rad/min. The same temperature effect that was associated with the calomel electrode also was seen here. Ag-AgCI and mercury paste type glass electrodes were tested, and no difference was found between them. There was a small cumulative effect which amounted t o about 6-15 millivolt after 10' rad dose, and this shift was to the high pH side. The magnitude of this shift in potential as a result of large dose exposure is consistent with the change in asymmetry potential reported by Fedotov (2). Assigning the major portion of the shift in glass electrode potential t o changes in asymmetry potential appears reasonable since the glass membrane is the most likely candidate to suffer radiation effects. The fluoride electrode also maintained steady readings in the range of gamma field intensity mentioned above. There was, however, some difference between the long term behavior of two fluoride sensitive electrodes after exposure t o high dose. The response of one electrode remained essentially unaffected even up to lo7 rad. The readings of the other electrode studied shifted with dose t o the negative direction (Figure 2). The plot of the potential readings in and 10-'M fluoride solutions after each dose was found t o shift parallel t o the initial calibration curve before irradiation. In other words, the Nernstian slope remained essentially the same. Thus, the fluoride electrode can be affected by gamma radiation, but its usefulness can be maintained by periodic checks with standard fluoride solutions. (5) J. McNicholas and W. Horn, BNL-50149.
I
N ' O;
Figure 3. Effect of y-radiation on the nitrate (Orion membrane) electrode The response of the nitrate electrode was tested for short periods in a gamma field, and there was no effect of any of the dose rates employed on the potential readings. Doses of the order of lo5rads resulted in small parallel shifts of the response curve, but this shift became nonlinear from about 106 rad. This is shown by the lowest line in Figure 3. The linearity was restored with a slight displacement in potential from the initial when the internal reference solution was replaced. It is difficult to establish unambiguously any changes taking place to the membrane independent of the liquid ion exchanger. On the other hand, the nearly complete recovery of the electrode by the replacement of the internal reference solution indicates that the major radiation effect is on this reference solution. The liquid ion exchanger became dark colored after large dose, and the white electrode body took on a n amber color. These were the only other signs of change after a 5 x lo7 rad dose. The results described above lead us to conclude that the behavior of a n electrode is, in general, little affected by the presence of a gamma radiation field. The electrodes studied here behaved normally, and there is good reason to expect that most electrodes also will d o the same. The possible exception is the family of metal electrodes like platinum which is reported t o be sensitive to the hydrogen peroxide buildup within the system ( 3 ) . Even in this case, it is not the electrode response but the solution composition that is affected. Thus, we feel that the type of electrodes commonly used can be used in a gamma radiation field of up to at least fairly high intensity (lo4rad/min) with little abnormal behavior. A junction box was used during the early phase of the work to connect the end of the electrode lead to the meter. While the readings of the calomel electrode were not noticeably affected by this box, the glass electrode readings were highly erratic. This interference could be reduced somewhat by placing lead shielding around the box, but the most satisfactory solution was t o use a glass electrode with a lead long enough to extend outside the source shielding. The major cumulative change brought about by gamma radiation t o these electrodes on large dose exposure is associated apparently with the stability of any internal reference standard solution. The nitrate electrode was the only one of the electrodes studied here which showed the adverse effects of cumulative dose, and its internal reference solution is known to change in nitrate concentration when irradiated. Fluoride solution in the millimolar concentration level was found to be
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unaffected by prolonged exposure, The 0.1N HC1 which is the usual internal reference solution of a glass electrode is relatively stable (6),and neutral alkali chloride solutions are unaffected. Thus, one criterion for the behavior of an electrode after large dose is the stability of its internal reference solution. Solid state changes may also affect the performance of a n electrode. A good example here is the asymmetry potential of the glass electrode which undergoes minor but permanent changes. It is possible that the europium-doped lanthanum fluoride single crystal which is the sensing unit of the fluoride electrode also undergoes some change in internal structure. Stability of the Glass and Saturated Calomel Electrode to Alpha Radiation. The glass and calomel electrodes were also tested for their stability t o alpha radiation. The radiation source was Z4Cm in 3M HCI that was prepared from the metal oxide, and its activity was close to lo9 dps/ml. No dose rate measurement was made, but the dose rate calculated from ~
(6) H. Kubota, ANAL.CHEM., 40, 271 (1968).
the disintegration rate was of the order of lo4 rad/min. Microelectrodes were used to keep down the volume of the alpha solution required. Since this solution was kept acidic to retain the curium in solution, each electrode was removed from the hot solution, and p H readings were taken in p H 4, 7, and 10 buffers using a counter electrode that was not irradiated. The counter electrodes were checked against each other before each set of measurements. Readings taken at various intervals over a two-week period (cumulative dose close t o lo* rad) showed no significant change. From the experience with gamma radiation, little effect to these electrodes was expected from pure alpha emitters. It is felt that adverse effects are more likely only when there is relatively energetic gamma associated with the alpha decay; the type of effects described for gamma radiation then are expected to take place.
RECEIVED for review June 26,1970. Accepted August 19,1970. Research sponsored by the U.S. Atomic Energy Commission under contract with Union Carbide Corporation.
Sources and Prevention of Recoil Contamination of Solid-State Alpha Detectors Claude W. Sill and Dale G . Olson Health Services Laboratory, U . S . Atomic Energy Cottittlission, Idaho Falls, Idaho Most recoil contamination of the solid-state detectors used in alpha spectrometry results from radioactive chains in which short-lived, alpha-emitting daughters are produced either directly from alpha-emitting parents or indirectly by decay of a short-lived betaemitting daughter which in turn was produced from an alpha-emitting parent. Spectra are given showing the recoil products resulting from various combinations of exposure and decay time for each of the four main radioactive series. Recoil contamination of the detector can be reduced by a factor of at least lo3 with a loss in resolution of only 1 or 2 keV by leaving enough air in the counting chamber to produce 12 pg/cmz of absorber between the source and detector, and applying a negative potential of 6 volts to the source plate. Polonium-210 gives a marked pseudorecoil effect, probably due to its own inherent volatility, and is by far the most serious long-term contaminant encountered. Its spontaneous volatilization from a stainless steel support can be virtually eliminated by heating the plate for 5 minutes at about 300 O C without significant loss of resolution.
BECAUSEOF THEIR HIGH RESOLUTION and low backgrounds, solid-state surface-barrier detectors permit exceptionally high sensitivity and reliability in the detection and identification of alpha-emitting radionuclides by alpha spectrometry. However, the detectors are expensive, particularly in the larger sizes, and are susceptible to contamination during use that is virtually impossible to remove without damaging the detector. Consequently, both sensitivity and reliability decrease with use. The present work was carried out in an attempt to identify the most common sources of contamination and to find simple means of minimizing their effect. Emission of a particle during radioactive decay results in recoil of the residual nucleus in the opposite direction because 1596
of the fundamental requirement that momentum be conserved in each individual radioactive disintegration. Hahn and Meitner ( I ) were among the first to call attention to the usefulness of recoil collection in radiochemistry. Since then, the recoil technique has been used extensively for preparing extremely thin sources for alpha spectrometry (2), for making rapid radiochemical separations (3), and in “hot atom” chemistry in general ( 4 ) . The technique has been very useful in establishing genetic relationships among the members of both the natural radioactive series ( 5 ) and the new collateral series resulting from deuteron bombardment of thorium (6, 7), particularly in determining the half-lives of the short-lived members. Emission of a beta particle produces too little recoil energy to cause significant contamination under usual conditions. However, emission of an alpha particle can result in contamination of the detector due to the accompanying recoil atoms. A 5-MeV alpha particle produces recoil energy of about 100 keV in a residual atom of mass 200, or about 2 % (1) 0. Hahn and L. Meitner, Phys. Z . , 10, 697 (1909). (2) B. G. Harvey, “Introduction to Nuclear Physics and Chem-
istry,” Prentice-Hall, Inc., Englewood Cliffs, New Jersey, 1962, p 318. (3) Y . Kusaka and W. W. Meinke, “Rapid Radiochemical Separations,” Nat. Acad. Sci.-Nat. Res. Cororc., Nucl. Sci. Ser. NAS-NS 3104 (1961). (4) A. C. Wahl and N. A. Bonner, “Radioactivity Applied to Chemistry,” Wiley, New York, N. Y., 1951, Chapter 8. (5) C. M. Lederer, J. M. Hollander, and I. Perlman, “Table of Isotopes,” Sixth Ed., Wiley, New York, N. Y., 1967. (6) M. H. Studier and E. K. Hyde, Phys. Rec., 74, 591 (1948). (7) W. W. Meinke, A. Ghiorso, and G. T. Seaborg, ibid., 81, 782 (1951).
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