the interface circuit. The spectrum would be presented on the vertical amplifier or accumulated o n paper tape for later processing. Numerous other combinations are possible depending o n the type of equipment which is available. ACKNOWLEDGMENT The author thanks John B. Grutzner of Purdue University for helpful comments during the development of this circuit.
The circuit was constructed in the instrument laboratory of Purdue University’s Department of Chemistry. The instrument laboratory is directed by Jon W. Amy.
RECEIVED for review December 11, 1970. Accepted February 10, 1971.
Galvanic-Coulometric Monitoring of Oxygen and Hydrogen in Glove Boxes Wanda Bahmet’ and Paul A. Hersch2 Gould, Inc., St. Paul, Minn. 5516.5 WHEN OPERATING under inert atmospheres one wishes to follow the progress of purging the enclosure, to keep an eye on the quality of the atmosphere, and to be warned of inadequate purification, leaks, or sudden accidental ingress of air. The literature offers but crude means such as turning a tungsten filament into a “puff of smoke” ( I , 2). Yet more precise implements are at hand. This paper describes how to adapt a known coulometric monitor for oxygen and hydrogen (3, 4 ) to glove box work. The sensor cathode is a fleece of graphite or porous silver exposed on one side to the sample gas stream while the other side is lined by a separator imbibed with caustic alkali. The separator in turn contacts the cadmium “negative” of a nickel-cadmium battery, acting as anode. The two electrodes are bridged by a microammeter. Any trace of oxygen in the gas stream creates electric current owing to the processes 1 / 2 0 2 H 2 0 2e- + 2 0 H - and Cd 2 0 H - -+ Cd(OH)? 2e-. From Faraday’s law, the maximum current attainable is 10.0 PA for each volume 0 2 per million in 37.4 ml/min inert gas (flow being measured at 20 O C and 1 atm). Hydrogen is indicated in terms of oxygen consumed when the gas stream is first provided with oxygen and then passed over a room temperature combustion catalyst. Workers in various fields have applied earlier versions of the galvanic sensing element to determine residual oxygen in inert gas streams, for example, in the study of the role of oxygen during the irradiation of tissues (5-7). However, there seems to be no reference to the application of the principle in work with enclosed atmospheres. The earlier cells had low and, more importantly, variable, coulombic yields. With the more recently described sensor, yields close to the theoretical are attainable, leaving little margin for variability with temperature or cell individuality.
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Present address, 1725 James Street, St. Paul, Minn. 55105. Present address, 910 Franklin Terrace, Minneapolis, Minn. 55406.
(1) 1. D. Eubanks and F. J. Abbott, ANAL.CHEM.,41, 1708 (1969). (2) Specialty Gases and Equipment Catalog, Air Products and Chemicals, Inc.. Allentown, Pa., 1968, p 85.
(3) P. A. Hersch, “Galvanic Analysis” in “Advances in Analytical Chemistry and Instrumentation,” Vol. 3, Interscience Publishers, Inc. New York, N. Y . , 1964, pp 209-19. (4) P. A. Hersch, U. S . Patent 3223597 (1965). (5) E. J. Hall, J. S. Bedford, and R. Oliver, Brit. J . Rndiol., 39, 303, 896 (1966). (6) E. J. Hall and J. Cavanagh, ibid., 40, 128 (1967). (7) Zbid., 42,270 (1969).
A
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Figure 1. Routine system for determining traces of oxygen in a glove box atmosphere Blower Wall of box; inside at left 3, 4. Needle valves 5. Blow-out 6. Sensor 7. Flowmeter 1. 2.
The adaptation of the coulometric cell to glove boxes requires only simple, generally available accessories, whether the glove box operates above, at, or below atmospheric pressure, and whether the sample gas stream can be allowed to run to waste or, carrying toxics, must be returned to the box. EXPERIMENTAL
Simple Routine System. Only a few essentials are needed for oxygen readings when these are to be taken only intermittently, to f5 %, and the sampled gas can be allowed to go to waste. The essentials are, then: a blower, such as a fishbowl pump, inside the glove box; two needle valves; a “blow-out’’ column of water; the sensor, with a four-way dual channel stopcock permitting insertion and by-passing ; a multirange microammeter; a flowmeter (Figure 1). The needle valves are adjusted to allow a flow rate of F = 40 ml/min, plus 1-2 bubbles/sec rising through the blow-out ANALYTICAL CHEMISTRY, VOL. 43, NO. 6, MAY 1971
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Figure 2. System for determining oxygen and hydrogen in a glove box, with selfchecks 1. 2. 3. 4. 5.
6. 7. 8. 9. 10. 11.
Blower Wall of box Drier Blow-out Humidifier Wick 0% absorber Dual electrolyzer Catalyst Sensor Flowmeter
column. Its water head, together with the flow resistance of the second needle valve, plus that of the flowmeter, determine the flow. Normally the sample flow bypasses the sensor. To take oxygen readings, the sensor is inserted for a few minutes. Each vpm (volume per million) 0 2 is indicated by a deflection of approximately 10 PA. This figure is exact for a coulombic yield q = 0.935 and a flow F = 40.0 ml/min a t any point in the gas train where the temperature is % = 20 “C and the pressure p = 760 Torr. The general relationship is
X
=
ki/(qF) vpm O2
(1)
where k = 3,739.1 X
T 760 X - N 3,739.1 293 P, -
(0 - 20)
+ 12.76 X
X vpm Os + 4.92 (760 - P) ml mA min
(2)
and i is the galvanic effect in milliampere. (Verification: Assuming theoretical yield, i.e. q = 1, one mA derives from a stream of 60 X 10-a/(4 X 96,487) mole Oa/min, or from 24,053 times this number, i.e. 3.7391 X IOp3 ml 02/min, measured at % = 20°, p = 760 Torr, in F ml gas/min. Thus the concentration is X = 3.7391 X X 106/F = 3,739.1/F vpm.) The time periods for the response and recovery of the sensor are less than two minutes for 95 deflection provided the microammeter has low resistance (
