rated with 100% oxygen the current stabilizes at 15.0 pa.; for pure oxygen gas, at 19.1 pa. I n its present form the electrode can be used on gas samples within the concentration range from 0.01 to 100% by volume. Similar current-time curves are obtained for sulfur dioxide gas with a n applied potential of -0.7 volt us. S.C.E. Although the curves stabilize in about 3 minutes, the stabilized current for different runs is not nearly as reproducible as for oxygen; for sulfur dioxide it ranges from 45 to 65 pa. for various runs of pure sulfur dioxide gas. Oxygen, on the other hand. gives currents reproducible to less than 1% deviation. The anodic wive for sulfur dioxide gives current-time curves similar to those of Figure 4, which stabilize in about 3 minutes for an applied potfntial of f0.7 volt vs. S.C.E.Different runs are not very reproducible and the stabilizcd current for various runs of pure sulfur dioxide gas ranges from 25 to 80 pa. Saturated water solutions of sulfur dioxide are much more reproducible and stabilize in about 1.5 minutes to a current of 48.6 pa. for the cathodic portion of the wave. Because of erratic behavior, electrode attack, and low sensitivity, current-time studies a t constant applied potential have not been made for the other gases. The erratic behavior of sulfur dioxide
indicates the need for a more satisfactory electrode material than platinum. If a stable and reproducible response can be obtained for this gas, it should be possible to devise a system that could continuously determine the percentage of sulfur dioxide and oxygen simultaneously in a stream of gases, by using two gas-electrode systems, one \T ith a n applied voltage of $0.7 volt us. S.C.E. to determine sulfur dioxide, and one a t -0.7 volt us. S.C.E. to detcrmine both sulfur dioxide and oxygen. The amount of sulfur dioxide found by the first electrode would be subtracted froni the total signal of the second electrode, leaving a net current representing the amount of oxygen in the stream, Additional work is planned in this laboratory on other electrode materials and membranes. It is hoped to defelop satisfactory methods for determining hydrogen, hydrogen sulfide, hydrogen cyanide, carbon monoxide, nitrous oxide, nitric oxide, and other electrode-active gases. The effect of various solyents, supporting electrolytes, and buffers will be investigated also. The selective nature of a gas-membrane electrode has many advantages for the continuous analysis of gaseous mixtures and mixtures of dissolved gases. Potential applications for oxygen determination include: process streams, oxygen tents, flue gases, sewage, water,
blood, and biological systems. Similar applications can be suggested for other electrode-active gases. ACKNOWLEDGMENT
The authors wish to thank the Beckman Instruments Co. for a grantin-aid supporting this work. They are also indebted to John E. Leonard of this company for helpful advice and assistance in obtaining some of the materials and equipment. LITERATURE CITED
( 1 ) Clark, I,. C., Jr., Trans. A n . Soc. $rtificial Inlernal Organs 2, 41 (1956). (2) Clark, I,. C., Jr., Kold, R., Granger, D., Taylor, F., J . A p p l . Physiol. 6, 189 (1953). (3) Kolthoff, I. AI., Lingane, J. J , “Polarography,” 2nd ed., pp. 405, 406, 552, Interscience, Sew York, 1052. (4) Meites, L., “Polarographic Techniques,” pp 277, 282, Interscience, Xew Tork, 1955. (5) Reeves, R. B., Rennie, D. W., Pappenheimer, J. R ., Federation Proc. 16, 693 (1057). (6) Watanabe, H., Leonard, J. E., Be+-
man Instruments Co., Fullelton, Calif., unpublished data, presented at Pittsburg, Conference on Analytical Chemistry and Applied Spectroscopy, March 1957.
RECEIVEDfor Ieview April 18, 1058. Accepted September 29, 1958.
An Electrode System for Measuring Dissolved Oxygen DAYTON E. CARRITT and JOHN W. KANWISHER’ Department of Oceanography and the Chesapeake Bay Institute, The Johns Hopkins University, Baltimore, Md.
b A continuous measure of oxygen tension in fluid systems having the wide range of properties found in natural fresh and sea waters, blood, sewage, etc., has not been practicable. Polarographic devices using solid electrodes have had limited applicability to such systems because of electrode poisoning and high temperature coefficient. The device described overcomes the major objections to previous oxygen-measuring instruments. It is temperature-compensated and, because of the selectivity of the membrane separating the electrodes from the environment, will function satisfactorily in a wide variety of aqueous systems. It can b e used with continuous recording devices or as a completely portable instrument with readout on a microammeter.
T
reduction of molecular oxygen a t the dropping mercury electrode and a t macro and micro solid electrodes has been reported by many investigators. The phenomenon can be either a nuisance to the polarographer who is working with substances that are reduced a t potentials that also reduce oxygen, or under some circumstances, can be turned to a n advantage as the basis for the determination of dissolved oxygen. In the aquatic sciences, a knowledge of the distribution of dissolved oxygen in space and time is the basis for qualitative and quantitative deductions regarding the behavior of parts of the systems. For example, Worthington (10) evaluated the oxygen data obtained in the North Atlantic Ocean during the past 25 years and concluded that the water in the deep North HE
Atlantic is about 140 years old-that is, it has been about 140 years since that part of the Atlantic has received water from the surface. Information about deep ocean circulation has present-day, practical importance in evaluating the feasibility of using the deep oceans as a durnping ground for radioactive wastes. Pritchard and Carpenter ( 7 ) combined knodedge concerning oxygen distributions and current velocities measured in existing empoundments to predict the behavior of empoundments not yet constructed. Such considerations are important in regulating the extent to which dissolved oxygen may be depleted by users of streams and rivers. Present address, The Woods Hole Oceanographic Institution, Woods Hole, Mass. VOL 31, NO. 1, JANUARY 1959
5
In oceanography and limnology, dissolved oxygen measurements are used for evaluating the production potential of various waters. These are usually done by dividing a sample of the water of known initial oxygen content between two bottles, one a clear bottle, the other completely covered to exclude light. After exposure of both bottles to light for a known length of time the oxygen concentration in each is determined. The difference in oxygen concentrations between the exposed light bottle and the initial concentration is a measure of the difference between photosynthetically produced oxygen (equivalent to fixed carbon dioxide)
and the oxygen used during respiration. The dark bottle gives a measure of respiration only. The supply of fixed organic material to all aquatic food chains is obtained from measurements of this kind. I n most studies involving dissolved molecular oxygen, the measurement of dissolved oxygen has been by the lengthy and, under some conditions, nonspecific titrimetric method-the Winkler titration. To obtain a simpler, specific, more rapid, and continuous method of measuring dissolved oxygen, the device described was designed, constructed, and tested. PREVIOUS STUDIES
RING
-AGIO
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/
1 PLAT, N uM INDICATOR ELECTRODE
Figure 1. assembly
Cross section of electrode
30k 25
Polarographic systems for the measurement of dissolved oxygen are not new. .Gigu&re and Lauzier (3, 4) described the use of the dropping mercury electrode, and stationary and rotating platinum electrodes for the determination of dissolved oxygen in samples of natural waters. None of their systems are usable as in situ devices because of uncompensated temperature, salinity, and flow characteristics. Muller (5) in a study of a bypass electrode detailed more of the factors that govern the behavior of a stationary electrode immersed in a floxing test solution. He noted that results were reproducible as long as certain conditions, such as the rate of flow of solution past the electrode, the rate and direction of change of applied voltage, the temperature, the pretreatment of the
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electrode, and the kind and concentration of the supporting electrolyte were kept constant. Although the device described by Muller is not directly applicable to in d u measurements, his results provided a convenient guide in the design of such an instrument. Olson, Brackett, and Crickard (6) overcame some of the limitations of solid electrodes by applying a 5- to 10-cycle-per-minute square wave with interposed shorting periods to the cell and observing the current flow a t the end of each negative pulse. Such an arrangement would appear t o remove substances that are reversibly deposited on the electrode, essentially setting up a constant pretreatment of the electrode during use. Clark (1) appears to have been the first to cover a solid microelectrode with a membrane permeable to oxygen, but much less so to other dissolved constituents. He measured the oxygen tension directly in blood in the aorta of a dog, and for short time intervals a t least, the platinum microelectrode appeared to maintain constant diffusion current characteristics. I n this arrangement the reference electrode and the indicator electrode mere separated by the semipermeable membrane; thus the device suffered from high impedance characteristics, a feature that limited its use with conventional recording equipment. Although not important in the direct measurement of oxygen tension in vivo, the uncompensated temperature coefficient does limit the general utility of the instrument.
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Current-voltage curves
Solutions air soturated at stated Platinum, 12-mm. diameter
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ANALYTICAL CHEMISTRY
temperatures
Figure 3.
-
Temperature functions
1.1 volt vs. silver-silver oxide a. Current at b. Dissolved oxygen, milliliters per liter in air-saturated fresh water
membrane prevents the passage of substances that would poison or otherwise change the characteristics of the electrode. This arrangement then permits the device to operate under most conditions, as a selective electrode for dissolved oxygen. It has performed satisfactorily in sea water, fresh water, sewage, blood, and gas streams when a thin film of liquid is maintained on the surface of the membrane. Chlorine, hydrogen sulfide, acids, and bases, the latter in sufficient quantities to change the pH in the range between 2 and 12, show no measurable effect w, hen added to the external environment, When used to furnish a continuous record of dissolved oxygen, the potential drop across the thermistor is measured with a recorder having the proper impedance characteristics. A needle and scale microammeter can be used in a portable field-type instrument, in which case the thermistor is used to shunt the meter, and calibration to give direct reading is achieved with a second, manually varied, shunting resistor. The silver-silver oxide reference electrode and potassium hydroxide internal electrolyte mere chosen because hydroxide is a reaction product formed a t the platinum. The hydroxide added during the initial assembly starts the electrode operating under conditions similar to those that obtain after long periods of use. I n early experiments in which a silver-silver chloride reference and potassium chloride electrolyte were used, the current-voltage curves shifted after long use, presumably the result of depletion of chloride from the electrolyte and slow conversion of the reference system to the hydroxide form. I n practice, the silver-silver oxide reference electrode is preformed by
More recently Reeves, Rennie, and Rappenheimer (8) described the use of a membrane electrode in a study of the oxygen tension in urine. Their instrument had no provision for temperature compensation and had to be used with careful control of temperature. FZyn (2) has adapted the streaming mercury electrode and a zinc reference electrode to the measurement of dissolved oxygen in N'orwegian fjords. This device, in which the electrodes are lowered into the water from a ship and the diffusion current read on a meter on deck, appears to be the first successful attempt to obtain a continuous indication of dissolved oxygen in situ in subsurface waters. As originally described, however, the temperature coefficient, which is probably approximately 2% per degree, was not compensat'ed. IMPROVED ELECTRODE SYSTEM
The present electrode system, shown in Figure 1, is a combination of a silversilver oxide reference electrode, a thermistor which provides temperature compensation, 0.5N potassium hydroxide as an internal electrolyte, and a polyethylene membrane pulled tightly over both electrodes. Principle and Practice of Operation. The principle of operation is simple. The polyethylene membrane being permeable t o dissolved molecular oxygen, b u t much less so to other dissolved substances, permits the passage of oxygen to the platinum electrode, where i t is reduced to hydroxyl, under operating conditions. The
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electrolysis in potassium hydroxide solution, without a membrane covering over the cell. After initial assembly of the device, or after a change of potential, equilibrium is established rather slowly-10 to 20 minutes, depending upon the magnitude of the potential change or the concentration of the oxygen in the solution in which the new electrode is placed. However, once an electrode has been equilibrated, the time response as described below, is adequate for many purposes, provided the operating potential is maintained on the cell and the cell is immersed in water when not in use. The five properties of the electrode system having practical importance are the temperature coefficient, the time constant, the flow characteristics, the current-oxygen concentration relation, and the stability. These properties have been measured and some of the factors that control them have been examined. Temperature Compensation. When used without temperature compensation, a marked temperature dependence is found. Figure 2 shows the current-voltage curves obtained for a n uncompensated system. I n this experiment, the test solutions \yere saturated with air a t 1-atm. pressure and a t the stated temperatures. The curves, therefore, reflect a temperature as well as a n oxygen concentration effect. Anomalies are present a t potentials below the plateau. There appears to be a definite shortening of the plateau with increasing temperature, as well as a shift of the potassium discharge curve to higher potentials. Figure 3, a, shows the variation of diffusion current, a t 1.1 volt us. silversilver oxide, as a function of temperature. Again, there is an oxygen concentration as well as temperature factor in this curve. Figure 3,6, shows the variation in oxygen concentration in air-saturated solutions a t the stated temperatures. The two curves are shown together to demonstrate the magnitude of the temperature compensation problem. If complete compensation could be achieved, the diffusion current curve would have the same shape and slope as the oxygen saturation curve. The major problem is to change the sign of the slope of the diffusion current curve and then to do some fitting. The first part of the problem could be solved by using the negative temperature coefficient characteristic of thermistors. The temperature coefficient of the cell itself (neglecting nom the effect of changing oxygen concentration with temperature) is a function of temperature, varying from about 4y0 per degree a t 10' C. to about 8.5% per degree a t 25" C. The temperature coefficient of thermistors is also a function of temperature. I n using a thermistor for temperature compensation of the cell, the first reVOL. 31, NO. 1, JANUARY 1959
* 7
Figure 6. characteristics
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Behavior of electrode
quirement is that the coefficient of the thermistor be as large as or larger than the coefficient of the cell a t all temperatures. If it is larger, a fit can be achieved by using an appropriate series-parallel network, of fixed resistors around the thermistor. The best solution to the first part of the problem so far achieved has been by using a Veco (9) 52822 thermistor, for which the ratio of resistances a t O D and 50' C. is 10.3. Figure 4 shows the extent to which the arrangement of thermistors imbedded in the cell in contact with the reference electrode achieves compensation. Figure 4,a, is the temperature coefficient of the cell a t constant oxygen concentration. Complete compensation should produce a curve parallel with the abscissa. Figure 4,b, is the compensated curve. The ordinate is per cent of the 20" C. value. The average departure from perfect compensation is about 2% per degree; the maximum, at low temperatures is about 4%, and a t high temperatures, compensation is complete. Other thermistors with higher temperature coefficients have not been tested. However, experience with the unit and with ones having low average temperature coefficients suggests that that problem can be solved if higher precision is needed. Time Constant. I n the electrode arrangement described, there are several diffusion barriers-in the film of the internal electrolyte, in the membrane itself, and probably in a thin film of liquid on the outside of the membrane. It is to be expected t h a t the time constant of such a device will be larger than in the case of a bare electrode immersed in the test solution. Figure 5 shows the behavior of an electrode system having a 0.5-mil (0.0005-inch) thick polyethylene mem8
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ANALYTICAL CHEMISTRY
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Photosynthesis and respiration of phytoplankton Culture immediately after removal from culture rack Culture after adaptation and dilution
40
brane and a 5-mil thick filter paper disk (its function described below) covering the platinum electrode. The electrode initially \vas in m t e r through which air was being bubbled, and was transferred to water through which nitrogen was being bubbled. The minimum current did not represent the residual current for the electrode, because the system contained a small quantity of oxygen, which had been carried into the nitrogen system with the electrode. I n this case 50y0 of full scale response \vas achieved in about 2 seconds, 90% in 18 seconds, and 99% in 60 seconds. A quantitative study of the effect of membrane thickness on response tinie was not made. It has been noted, however, that with 1.0- and 1.5-mil membrane, 90% of full scale response is achieved in 20 seconds, but about 2 minutes are required to reach 9970. Flow Characteristics. Previous electrode arrangements have shown a marked dependence of diffusion current on thc velocity of flow of the solution past the electrode. 1Iuller (5) obtained a quantitative expression for the effect. In the present system f l o ~dependence is present, but has a much smaller effect than in other cells. Figure 6 shows the relationship for the electrode described above. The filter paper disk n a s included in this case to reduce the effect-that is to make the diffusion current less dependent on flow. In addition, the filter paper disk also reduces the diffusion current Constant. Twenty - four - hour Stability. records obtained with a n electrode continuously immersed in a n airsaturated solution shon no zero drift, for electiodes t h a t have been in operation 1 hour or more. I n one such experiment, a periodic variation was noted which \\-as of the order of magnitude expected from vaiiations in saturation values with the changes in temperature that normally occur in the laboratory. One of the first uses of this electrode system was in a n attempt to obtain vertical oxygcn profiles in the Chesaprake Bay. The electrode was inserted into a pipeline through which water was pumped (Figure 7 ) . Putting the pump a t the bottom of the line n.as
necessary, because a pumping arrangement which pulled the water from the depths caused much cavitation in the line, with a resulting decrease in the oxygen tension in the water passing the electrode. A t the time of this test the bay was well mixed vertically, so that n-ide variations in oxygen concentration would not be found. However, a comparison of the electrode response x i t h the Winkler titration gave satisfactory arrangement over the range encountered. The departure from a straight line fit to four calibration points is no greater than the expected error in the Winkler titration. The measurement of production potential by the light and dark bottle technique, using the Kinkler titration for the measurement of oxygen concentration, is laborious and frequently produces questionable results. If it is necessary to expose the bottles for extended periods to produce a significant difference in oxygen concentration in the two bottles, as in relatively nonfertile waters, there also is a marked increase in the bacterial population in the tlvo bottles which gives a marked and unnatural increase in the respiration in the two bottles. According to sonir critics of this technique, the increase in bacterial populations will be differelit in the two bottles, so that the measure of photosynthesis, obtained as the difference between the two bottles, is not reliable. Six to 8 hours’ exposure is not uncommon and in some cases experiments have been run for as long as 7 2 hours. The sensitivity and stability of the present electrode are such that a light and dark bottle type of measurement can be done with continuous monitoring of the system, can be performed in much shorter time than previously, and can be done in one bottle. Figure 8 shows the results of tivo such experiments. The total time for the longer experiment was 40 minutes. The experimental material was a pure culture (initially) of a chlorella containing about 2,000,000 cells per nil. The top curve, A , shows the growth of the culture immediately after being removed from the culture rack where it had been gronn under continuous light saturation conditions for several days. ilt this time the culture
had reached maximum growth. 1111mediately after being placed in the dark, the start of A , respiration, is evident by the decrease in oxygen. After about 15 minutes in the dark, the light was turned on and A now shows the difference between photosynthesis and respiration; in this case the oxygen concentration continued to decrease, indicating that respiration exceeded photosynthesis. -4 second dark period indicates a n even greater rate of respiration than the first. The bottom curve s h o w the behavior of the same culture after about 3 days’ adaptation to normal day and night illumination and dilution with about 10% new culture medium. The start of the curve is inimediatply after dark adaptation. The slope of the curve shows an incrcase in oxygen a t the rate of 8 x 10-4 ml. per minute. In the dark the respiration rate is -12 X 10-4 ml. per minute. LITERATURE CITED
(1) Clark, I,, C., Jr., Wold, Richard,
Granger, Donald, Taylor, Zena, J . :lppl. Physiol. 6 , 180 (1953). (2) FZyn, E., Rept. Soruegian Fish Jliarkst Incest. 11. 13) 11955). (3) GiguBre, P. A , , Lauzier, Louis, Can. J . Research. B23, 87 (1954). (4) I b i d . , p. 223. 151 RIuller. 0 . H.. J . .4m. Chena. Soc. 69, 2992’(1947). ‘ (6) Olson, It. A , , Brackett, F. S., Crickard, R. G., J . Gen. Physiol. 32, 681 11949). ( 7 ) Pritchard, I). W,, Carpenter, J . H., \
,
private communication, January 1958. B., Rennie, D. W., Rappenheimer, J. R., Federation Proc. 16, 693 (1957). (!I) Victory Engineering Corp., Vnion, N. J., Technical Catalogue of Veco Products, 7th ed. (10) Korthington, L. V.>Deep-Sea lie-
(8) Reeves, R.
search 1, 244 (1!154). RECEITEI)for review May 20, lY58. Accepted September 29, 1958. Contribution KO, 39 of the Chesapeake Bay Institute of the Johns Hopkins University, Baltimore, l l d . Field observations supported by the State of Maryland (Department of Research and Education) and the Commonnealth of Virginia (llrginia Fisheries IAoratory) under contract with the Johiis Hopkins University.
VOL. 31, NO. 1, JANUARY 1 9 5 9
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