Anal. Chem. 1904, 56,2367-2360
2387
Influence of High y Radiation Fields on Response of Ion-Selective Electrodes J a n i s Gulens* General Chemistry Branch, Atomic Energy of Canada Limited, Chalk River Nuclear Laboratories, Chalk River, Ontario KOJ 1JO, Canada Steven J. West and James W. Ross, Jr. Orion Research Inc., 840 Memorial Drive, Cambridge, Massachusetts 02139
Exposure of glass (H' or Na') electrodes to hlgh y fields (3 X IO4 rad/mln) In a Gamma Cell at ambient temperatures shifts the electrode potentials by 20-100 mV, but the potential of chloride-selective electrodes is unaffected. The offset voltage decreases with decreasing fleid strength and decreasing electrode reslstance and can be ellmlnated by decreasing the resistance to 75 OC.
On-line chemical analyzers that are based on ion-selective electrodes are widely used in industry, including the power industry (1). Industrial analyzers have common problems such as sample pretreatment and delivery, calibration,maintenance, and reliability (2), but analyzers for the nuclear power industry have the added problem of having to operate in the presence of radiation fields. The analyzers may experience high radiation fields because of their particular location within the station and/or because the samples may be radioactive. In addition, radiation levels may change suddenly, and by many orders of magnitude, if equipment malfunctions or fails. Ion-selective electrodes are also used for routine determinations in hot cell facilities where high local fields may be encountered, for example, during the dissolution and processing of irradiated nuclear fuels or targets to separate and isolate fissionable isotopes or isotopes used for medical purposes. It is thus important to know the influence of high radiation fields on electrode response so that reliable and accurate results can be obtained. There is very little information in the literature regarding the effects of radiation on ion-selective electrodes (3). Kubota (4) has reported that, apart from the expected thermal effects, the saturated calomel and glass electrodes were unaffected by y fields as high as 1.5 X lo4rad/min and that the electrode behavior was determined by the stability to radiation of the internal reference solution. We have used higher y fields of 3 X lo4 rad/min and have observed that the response of ion-selective electrodes is significantly different from that reported by Kubota (4). This paper reports on our findings that the potentials of glass electrodes (both H+ and Na+ responsive) are immediately and significantly offset by exposure to the y field and that the offset can be minimized by decreasing the field strength and/or electrode resistance. EXPERIMENTAL SECTION Radiation Source. A Gamma Cell 220 (Atomic Energy of Canada, Ltd., Commercial Products Division, Ottawa, Ontario) with a cobalt-60 source was used. The source is in a fixed position and the sample is placed in a vertical cylindrical chamber that is lowered into the field. The field within the sample chamber is nonuniform;typical variations with height and radius, determined by Fricke dosimetry, are shown in Figure 1. An access port at the top of the Gamma Cell permits insertion of tubing 0003-2700/84/0356-2367$0 I.50/0
and cables into the sample chamber. Equipment, Chemicals. A variety of commercially available H+and Na+ glass electrodes were tested, either as combination electrodes or with separate conventional reference electrodes (saturated calomel or silver/silver chloride). The chloride ion selective electrode was a pressed AgCl membrane electrode. Coaxial cables, fitted with appropriate adapters and connectors, were used to connect the electrodes to the pH/mV meters that were located outside the Gamma Cell. Meters of different input impedance, ranging from 10l2il (Cole-Parmer Q (Orion Model 611,801, and Model 5986-10Digi-Sense) to 901) were used. A Keithley digital meter (K-172) was used for resistance measurements. Buffer solutions (borate, acetate, and sulfate with varying amounts of chloride) were prepared from reagent grade chemicals and distilled-deionized water. In-Source Measurements. Electrode potentials (and resistances) were measured in unstirred buffer solutions in glass beakers that were placed on a Styrofoam block at the bottom of the sample chamber. RESULTS AND DISCUSSION Voltage Offsets. The measured potential of a glass electrode changed immediately on exposure to the y field, but also reverted rapidly to its initial value on removing the electrode from the field. In initial experiments where the cables connecting the electrodes to the pH meter were taped to the sides of the sample chamber, the measured offset was large, 100-300 mV. The reproducibility of the offset during repetitive cycling of the electrodes in and out of the field in a given experiment was good, but the reproducibility from one experiment to the next was poor. However, when the cables were positioned in the center of the sample chamber by means of a wood stand, the offsets decreased in magnitude to 20-100 mV. The reproducibility of the offset also improved (f5mV) for any given experiment, but the magnitude of the offset could still vary by 10-15 mV from one experiment to the next depending on the actual positioning of the electrode leads and cables. The electrodes gave an approximate Nernstian response to changes in hydrogen ion concentration while being irradiated under these conditions, with the accuracy of the response being limited by the reproducibility of the offset. The offset also decreased as the field to which the electrodes and cables were exposed was decreased. When both the electrodes and cables were raised -30 cm above the bottom of the sample chamber, thereby decreasing the field strength by a factor of -3, the offset decreased from -40 to -5 mV. If, however, the electrodes were raised to the top of the cell but a loop of cable was left hanging in the high field area, the offset remained at -40 mV. Similar response was observed for all glass (H' or Na+) electrodes and all pH meters. The voltage offset was demonstrated to be due to the presence of the glass electrode, as negligible offsets were measured when the glass electrode was replaced by a second reference electrode, or a remote reference electrode located outside the Gamma Cell and connected to the sample solution Published 1984 by the American Chemical Society
2388
ANALYTICAL CHEMISTRY, VOL. 56, NO. 13, NOVEMBER 1984
I
I
IO I
d
I
I
I
I
I
I
20 30 HEIGHT,cm I
I
I
I
I
4c
I
W U
oz 2.5 5
0 R A D I U S , cm
5
Flgure 1. Typical variations of field in sample chamber of Gamma Cell with (a) height and (b) radius (at 8 cm from bottom).
by means of a liquid bridge. In addition, voltage offsets were not measured when a chloride ion selective electrode was exposed to the y field, and the electrode continued to give Nernstian response to changes in the chloride concentration. The observation that significant offsets are measured when glass electrodes are exposed to a y field is in contrast to Kubota's results (4), although the finding that the potentials of the reference electrodes are unaffected by the field is in agreement with Kubota's work (4). Influence of Electrode Resistance. The fact that voltage offsets were observed for glass electrodes but not for chloride ion selective electrodes suggested that the offset was related to the high resistance (- lo00 MO (5)) of the glass membrane. This conclusion was confirmed by measuring the voltage of resistors exposed to the y field; similar effects were observed for resistors as for glass/reference electrode pairs. Significant voltages (>2 mV) were measured for resistors >10 MO, with the measured voltage increasing approximately linearly with increasing resistance and decreasing with decreasing field strength. The resistance of glass membranes varies with the glass composition but compositions that produce suitable membranes of low resistance (
