Internal-Reference Solid-Electrolyte Oxygen Sensor David M. Haaiand Sandia Laboratories, Albuquerque, New Mexico 871 15
A new solid-electrolyte oxygen sensor has been developed that eliminates the conventional oxygen reference in previous solidelectrolyte oxygen sensor designs and is, therefore, ideally suited as an insertion device for remote oxygen monitoring applications. I t is constructed with two cells of stabilized zirconia sealed into a small unit using a new hlgh-temperature platinum-zlrconia seal. One electrochemical cell monitors the ratio of oxygen partial pressures inside and outside the sensor while the other solid-electrolyte cell is used for quantitative electrochemical pumping of oxygen. The internal oxygen reference is generated by initaiiy pumping ail oxygen out of the known internal volume of the sensor and then quantitathrety pumping oxygen back In untli oxygen partial pressures are equal inside and out. This information is used with the ideal gas law to calculate oxygen partial pressures. Tests were conducted from 400 to 1000 OC in mixtures of oxygen and nitrogen spanning -0.2-21 % oxygen concentration range. Sensors with sputtered platinum and porous platinum paste electrodes were compared.
Solid-electrolyte oxygen sensors have found wide applicability in gas monitoring, combustion control, metallurgy, petrology, chemical kinetics, and thermodynamic studies ( I , 2 ) . Galvanic cells constructed with stabilized zirconia electrolytes have generally been employed for direct oxygen analysis in gas environments. Within wide limits of temperature a n d oxygen content, the conductivity of this solid electrolyte has been shown to be due almost exclusively to oxygen ion transport (3-5). Therefore, the open circuit emf ( E )generated by an oxygen concentration cell using stabilized zirconia electrolyte is given by the Nernst equation
where R is the gas constant, T the absolute temperature, F is the Faraday constant, and Polf' and Po2' are the oxygen partial pressures (strictly speaking, the oxygen fugacities) on either side of the electrolyte. If Pop" is a suitable oxygen reference and is known, then the unknown value PO; can be determined from Equation 1. References used in oxygen gauges of this type have typically been air or pure oxygen a t a known pressure or on occasion a n equilibrium mixture of reactive gases (e.g., H 2 / H 2 0or CO/C02) for which oxygen partial pressures can be calculated. Because leakage of oxygen through the electrolyte is generally present a t high temperature (6) (due to physical or electrochemical permeability), the reference gas must be continuously flushed across the electrolyte. In a number of remote oxygen sensing applications such as in-situ fossil fuel recovery processes (e.g., oil shale retorting or coal gasification), delivering the reference gas to the cell can become a difficult or impossible task. T h e use of a metal/metal oxide oxygen reference can eliminate this problem, but the slowness of equilibration of the reliable buffers a t temperatures below 700 "C ( I ) limits their usefulness at lower temperatures.
In the present paper, the design of a new solid-electrolyte oxygen sensor is described which circumvents these problems. This new design offers a closed, measured volume used as an internal reference. Because no external reference is required, gas feeder and outlet tubes are eliminated, and only electrical leads are needed to the exterior. This feature makes this new probe ideally suited as an insertion device for remote oxygen sensing applications. Further advantages include compactness, elimination of cumbersome tube-type construction, lowtemperature operation, and null-point detection. This sensor does not have the high absolute accuracy nor sensitivity to extremely low oxygen contents of traditional solid-electrolyte sensors. It is, however, a novel internal reference design which allows it to be used as a working sensor in applications where conventional oxygen sensor designs are not feasible.
THEORY The new internal-reference oxygen probe takes advantage of the principle that a solid electrolyte, such as stabilized zirconia which is a specific oxygen ion conductor, can operate either as a n oxygen concentration cell or as a quantitative electrochemical oxygen pump. By taking advantage of both these features, a compact two-cell internal-reference oxygen sensor was constructed as illustrated in Figure 1. This sensor consists of both an electrochemical pumping cell (Faraday cell) and a separate measwing or galvanic cell. Each is a thin disk of stabilized zirconia to which porous platinum electrodes have been applied. Separating these cells is a ring of stabilized zirconia which also serves to form a closed, internal volume. The three ceramic pieces are hermetically sealed using a recently developed high-temperature platinum-zirconia seal (7,8). The resulting unit is compact with the enclosed center volume serving as an adjustable reference. The emf generated by the disk selected as the measuring cell is constantly monitored to provide the ratio of oxygen partial pressures on either side of the disk (see Equation 1). T h e second zirconia disk serves as an oxygen pumping ce11. Oxygen is pumped into or out of the sealed internal volume by applying an appropriate potential to this cell. In this configuration, the two cells can be operated as an internal-reference oxygen sensor in the following manner. First, oxygen is electrochemically pumped out of the internal volume of the sensor by applying a positive potential to the outside of the pump cell. The pumping is continued until the oxygen level inside the probe is negligible compared to that outside (e.g., Po, (inside)/Po, (outside) 50.01 as determined by the emf of the measuring cell). Reversing the polarity of the applied potential then causes oxygen from the surrounding gas to be pumped back into the internal volume of the sensor. Pumping is continued until the oxygen partial pressures are equal on both sides of the measuring cell (i.e., the potential of the measuring cell is zero). Assuming 100% Faraday current efficiency for oxygen pumping, the integrated current required to achieve this condition will then yield the number of moles of oxygen pumped back into the cell. If the internal volume of the sensor is known and the temperature measured, then the partial pressure of oxygen inside the sensor can be determined from the ideal gas law. This value is also the oxygen partial pressure of the gas to be monitored since the pumping procedure has ANALYTICAL CHEMISTRY, VOL. 49, NO. 12, OCTOBER 1977
1813
A
".
3
d l -,.,-
-
*
i ectrxe,
Figure 1. Schematic diagram of new internal-reference solid-electrolyle oxygen sensor
made t h e oxygen contents t h e same within and outside t h e sensor. Assuming t h e total gas pressure outside t h e sensor is known, then t h e percentage of oxygen in t h e gas can also be readily calculated.
EXPERIMENTAL The solid electrolyte materials tested were 4 wt % CaO-stabilized ZrOp (Le. (Zr02)0.92~(Ca0)0.08) and 16.9 wt 90Y203-stabilized Z r 0 2 (Le., (Zr02)o.so~(Y203)o,lo) supplied by the Zirconium Corporation of America. The CaO-stabilized ZrOl offers high thermal shock resistance while the Y203-stabilizedmaterial offers higher ionic conductivity but at a reduced resistance to thermal shock. The material was machined into either disks 2.5 cm (1.0 in.) in diameter and 0.64 cm (0.025 in.) thick or into rings 3.2-cm (1.25-in.) o.d., 1.6-cm (5/8-in.)id., and -0.15 cm (0.060 in.) thick. All parts were polished flat and parallel to within 0.0005 cm. Platinum electrodes were applied to the zirconia disks by either of two techniques. Initially the electrodes were prepared by painting two coats of DuPont 7919 platinum paste on both sides of the ceramic disks. The first coating was fired to 800 "C to burn off the organic binder and to sinter the platinum ink particles. The second coating was fired to 1200 "C. This higher firing temperature produced a better bond to the ceramic and yielded a more suitable surface for the spot welding of lead wires. Platinum electrodes were also prepared by sputtering a 5500-A thick layer of platinum directly onto the stabilized-zirconia disks. The electrodes were 1.6 cm in diameter and centered on the disks. This resulted in a 2.0-cm2 electrode area on each side of the disk. The two disks and ring of zirconia were then fashioned into a leak-tight package (see Figure 1) using a recently developed platinumzirconia sealing technique (7, 8). The seal was obtained using 1.90-cm (3/4-in.)diameter O-rings of 0.025-cm (0.010-in.)platinum wire. Two platinum O-rings were sandwiched between the three zirconia parts as shown in Figure 1 and the entire unit was placed in a hot press facility. The temperature was programmed to reach 1050 "C in 3 h at which time a load was applied to the unit at the rate of 65 N/min (15 lb/min) until the load reached a value of 2200 N (500 lb). After 2 h, the load was removed at the same
-
rate a t which it was applied, and the unit was allowed to cool to room temperature overnight. The resulting platinum-zirconia seal strength was found to exceed 30 MPa (4400 psi), and the seal was capable of thermally cycling above 1100 "C without damage. The completed sensors had a sealed internal volume of -0.3-0.4 cm3. The volume of each sensor was calculated from measurements of its components before and after assembly. These calculated volumes were accurate to 12%. Of course, the dimensions of the sensor and its internal volume could be modified as desired. Leads to the outer electrodes were made by spot welding 0.025-cm (0.010-in.) diameter platinum wire to the platinum electrodes. Leads to the inner electrodes were made by either of two methods. The first involved spot welding a 0.013-cm (0.005-in.) diameter platinum wire across the inner electrodes. These wires were then brought out across the platinum O-ring between the zirconia ring and disk before the seal was made. This lead wire then becomes a part of the seal where it crossed the O-ring. The deformation of this thin wire a t the seal, however, made it very fragile and subject to breakage. A more convenient method of making contact with the inner electrodes was to extend painted or sputtered platinum tabs on the inner electrodes out beyond the platinum O-ring seal. Thin platinum strips were spot welded to the tabs on the zirconia rings, and electrical connection to the inner electrode was then provided through the platinum seal. Of course any lead material which survives the test environment without reacting with the electrode or electrolyte can be used if care is exercised to avoid the creation of a situation where thermoelectric emf s could be generated. The cells were tested in a controlled temperature environment within a 5-cm diameter quartz tube inside a Lindberg model 542334 two-zone tube furnace with a 10-cm constant temperature zone. Temperatures were controlled to f l "C. Ultra-high purity Matheson nitrogen and oxygen or dry air were metered through calibrated Matheson flow tubes (600 series) and mixed to form gas mixtures of varying O2 content. These gas mixtures were passed through the furnace over the oxygen sensor at a total flow rate of -2500 standard cubic centimeters per minute (sccm). Oxygen levels were reproducible to f 6 9 0 of the oxygen content. The electronics used in monitoring oxygen levels with this new sensor are illustrated in Figure 2. Included in the pumping circuit on the right are a regulated 2-V dc power supply, an ammeter, a digital stopwatch, and a current integrator constructed in this laboratory. The measuring circuit consisted of a Keithley 616 digital electrometer and a comparator circuit for automatic shut off of the current in the pumping circuit. After the desired test temperature and oxygen partial pressure had been set, oxygen was electrochemically pumped out of the sensor by applying a potential (Eappl) to the pumping cell. Typically ICappl= 0.1 V. After equilibrium had been achieved, the potential of the measuring
Flgure 2. Schematic diagram of electronics used to monitor oxygen partial pressure
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ANALYTICAL CHEMISTRY, VOL. 49, NO. 12, OCTOBER 1977
1000
Table I. Test Results of Several Oxygen Sensors Sensor No.
Temperature, ' C
1
1020 920 830 730
2
700
3
640 590 530 630 570 540
490 4
750 500
650 600 800
700 640 600 520
O 2 in
TEMPERATURE
=
565.C
0:
gas phase measured, ( + 8 % ) ,5% '% Error, % 5.2 5.2 5.2 5.2 0.20 0.20 0.20
4.7 4.9 4.9 5.2 0.21 0.21 0.20
0.20
0.18
4.8 4.8 4.8 4.8 4.8 4.8 4.8 5.1 0.50 0.50 0.50 0.50 0.50
4.5 3.8 4.7 4.8 5.1 5.1 5.0 4.0 0.60 0.59 0.53 0.56 0.59
-9 -6 -6
SPUTTERED
Pt ELECTRODES
800
0
+5 -5 0 - 10
-6 -21 r3 0
r6 -6 -4 - 22 +2 0 t18 +6 +12 +18
600
I / t
20b/::jl' 400
FI
RESULTS AND DISCUSSION The test results of several oxygen sensors constructed with platinum paste electrodes are presented in Table I as a function of temperature and oxygen concentration. The first sensor listed was constructed of (Zr02)o92.(CaO)o which exhibits relatively low oxygen ion conductivity while the remaining sensors were made from high-conductivity (Zr02)o 90-(Y203)o The results are generally within or only slightly exceed the -*8% experimental uncertainty attributed to gas flow meter accuracy and reproducibility, and the measurement of sensor volume and temperature. Without changing flow tube settings, it was found that the precision of the 3-5 measurements taken a t any given set of conditions was 5 f l % . This is simply a result of the random errors generated during the integration of the ion current. The relative accuracy of these results suggests that Faraday's law is closely followed for the conditions employed, and the oxygen
PASTE
ELECTRODES
0
0 10
cell E,,,,, was noted t o assure that sufficient oxygen had been removed from the sensor. The limiting current I h t was also noted and is at least partially related to the rate at which oxygen leaks back into the cell. A large value of' I,,,,, or a sudden increase in its value is an immediate indication of a crack in the cell. After the pump-out operation had been completed, the applied potential was reversed and oxygen pumped back into the fixed internal volume of the sensor. The emf of the measuring cell was monitored continuously and the current of the pump cell integrated until the potential reached a null point. A t this time. the comparitor circuit activated a relay to open the pump circuit and automatically discontinue the oxygen pumping and the integration. The integrated current, total pressure, and temperature (obtained from a Pt/Pt.lO'% Rh thermocouple attached to the sensor) were recorded. These data and the previously measured internal volume of the sensor were used with the ideal gas law to calculate oxygen partial pressure and % oxygen in the gas mixture. In addition, the decay of the measuring cell potential in open circuit was obtained for several sensors on a strip chart recorder after the pump-out operation had been completed. These potential vs. time curves were useful in obtaining information about the rate at which oxygen leaked back into the cell. Finally, data were also obtained on the polarization characteristics of both sputtered platinum and platinum paste electrodes on yttriastabilized zirconia. These current vs. potential curves were made using two probe techniques and by applying an increasing dc voltage in 0.02-V steps at 1-min intervals. The current was measured at the end of each minute. This process was similar to that used and described by Brook et al. ( 9 ) .
?
0 30
u 0 50
0 70
E(VOLTS1
Figure 3. Polarization characteristics of sputtered platinum and platinum paste electrodes on 0.063-cm thick disks of 16.9 wt YO yltria-stabilized zirconia in air
pumping efficiency is close to unity. T h e verification of Faraday's law has been previously obtained by Fouletier et al. (6, IO) in their tests of a solid-electrolyte oxygen pump. However, both Fouletier e t al. (10) and Agrawal et ai. (11) observed inaccuracies when an electrode was common to both pump and measuring cells in units which were used to simultaneously control and measure oxygen contents in a flowing carrier gas. Therefore, the four-electrode two-cell approach shown in Figure 1 was selected rather than a possible three-electrode arrangement. The low temperature operation of the various sensors was found to be limited either by electrolyte ionic conductivity or by electrode polarization. Sensor No. 1 (see Table I), constructed with (ZrOJo 92.(CaO)o elclctrolyte and platinum paste electrodes, was found t o be limited in operation to temperatures above 700 "C because of the low ionic conductivity of this material. By constructing similar sensors with high-conductivity (Zr02)o90.(Y203)o and platinum paste electrodes, the lower temperature limit of sensor operation was reduced to -500 "C. However, this limit is apparently affected by electrode polarization. This first became evident from the fact that the conductivity of (Zr02)o90-(Y203)o calculated from Strickler and Carlsson's conductivity measurements (12) does not become less than t h a t of (Zr02)oga4CaO)oo8 at 700 "C (13)until the temperature decreases to 450 "C. Based on electrolyte conductivity, therefore, the yttria-stabilized zirconia should be operable to temperatures as low as 450 "C. However, reports in the literature have shown that the rate of transport of oxygen through the cathode becomes rate limiting a t low temperatures (9, 12-16). T h e use of thinly sputtered platinum electrodes has been shown to exhibit greatly improved oxygen transport over other forms of platinum ( 9 , 16). For this reason, the polarization characteristics of thin-film sputtered platinum and thick-film platinum paste electrodes on yttria-stabilized zirconia disks were compared a t several temperatures. As expected, the new sputtered electrodes exhibited significantly reduced polarization. Figure 3 shows the results of this study for 850-A thick electrodes a t 565 "C in air. T h e platinum paste electrodes exhibit large polarization effects while the sputtered electrodes ANALYTICAL CHEMISTRY, VOL. 49, NO. 12, OCTOBER 1977
1815
Table 11. Test Results for a Platinum Paste Electrode Sensor (No. 5 ) and a Sputtered Platinum Electrode Sensor (No. 6 )at 700 Casb O? in Probe gas phase, No. D
6 6c 5 6 6c 5 6
7%
0.50 0.50 0.50 4.8 4.8 4.8 21.0 21.0
0, measured, %
0.58 0.62 0.50 5.0
5.1 4.7 21.4 20.7
Error, %
Pump times, s
+16
9.5
0
1.3 1.0
+4 i 6 A2 1.3 -1
73 9.3 8.6 255 43.6
+ 24
Sensors were constructed using ( Z r 0 2 ) o ,.9( Y 2 , 0 3 ) ,,, , solid electrolyte. These data were collected with a conThese stant 0.1-V potential applied t o the pump cell. data o n sensor No. 6 were obtained after correcting for the overshoot caused by the rapid oxygen pumping rates. are fairly well behaved with a nearly ohmic response. Similar curves were obtained a t 500 and 610 "C with electrode polarization decreasing and current increasing a t the higher temperature. At any given applied voltage, the current achieved with the sputtered electrodes is a t least an order of magnitude greater than that obtained with the paste electrodes. Both 850-8, and 8000-8,thick sputtered electrodes were also compared. Although the 850-A electrode exhibited slightly better electrode characteristics, the greater ease of lead wire attachment to the thicker electrodes made them more suitable for oxygen sensor applications. On this basis, a sensor using 5500-A sputtered platinum electrodes was fabricated and tested. This oxygen sensor exhibited two greatly improved characteristics over the previous designs using platinum paste. First, oxygen pumping times required to make each oxygen determination were reduced by almost an order of magnitude. Second, minimum operating temperatures for this probe decreased to 400-450 "C depending on oxygen concentration and applied voltage. Because oxygen pumping rates become so rapid a t higher temperatures, desorption of oxygen from the inner pump electrode and its diffusion to the measuring cell become slow compared to the oxygen pumping rate, and noticeable overshoot (positive) errors become evident with this sputtered electrode probe. However, the overshoot could be greatly reduced by alternately switching the pump current on and off to allow equilibrium to be achieved during the measurement or by increasing the resistance of the external pump circuit to decrease the pump rate. The lower electrode polarization also minimizes the electronic conductivity that can be generated with highly polarized electrodes and allows higher pump voltages to be applied without the introduction of additional errors in accuracy. This latter point means that greater pumping speeds can be achieved a t lower temperatures, thereby reducing measurement times still further. The operation, accuracy, and pumping times are presented in Table I1 for sensors using platinum paste electrodes (sensor No. 5) and 5500-A sputtered platinum electrodes (sensor No. 6). These results were obtained at 700 "C with an applied voltage of 0.1 V. Note that by eliminating the overshoot with sensor No. 6, errors in measurement become negligible a t this temperature. Sources of error with these new sensors include the leakage of oxygen into the internal volume. Independent of the source, oxygen leakage will result in negative measurement errors. T h e platinum-zirconia seals have been leak tested at room temperature and were found to be leak-tight to helium within the limits of our detection of 5 x lo-'' sccm. Oxygen permeability through the solid electrolyte itself is possible at high temperatures as a result of molecular diffusion or 1816
ANALYTICAL CHEMISTRY, VOL. 49, NO. 12, OCTOBER 1977
electronic conduction (16). However the greatest source of oxygen leak may be the presence of the platinum seal in contact with the solid-electrolyte. Differences in oxygen concentration inside and outside the sensor can result in a flow of oxygen ions through the solid electrolyte in combination with the return of electrons through the conductive platinum seal. A nonconductive seal would eliminate this latter source of leakage, and work has been initiated to implement the use of a glass-ceramic seal. The oxygen leak rate has been determined for a few of these sensors a t several temperatures. This was accomplished by first applying a pump-out potential and allowing equilibrium to be reached at PO, (outside)/ Po2 (inside) L 50. By opening the circuit and following the potential as a function of time, the rate a t which oxygen enters the cell can be determined. Because leak rates varied from sensor to sensor and with temperature and absolute oxygen levels, corrections for oxygen leakage would be difficult to make. At 600 "C the initial leak rate (i.e., the maximum leak rate since the difference in oxygen inside and outside the sensor is greatest) was 10 sccm of O2 in a 0.5% oxygen environment. Oxygen pump currents at this temperature and oxygen concentration corresponded to pump rates 10 to 150 times this leak rate. Thus the error due to oxygen leakage was minimized although not always negligible. T h e fact that measurement errors are not solely negative indicates that oxygen leakage is not always the dominant source of error. Error can also be introduced at high pumping speeds if the rate of desorption of oxygen from the inner pump cell electrode and its diffusion to the measuring cell is not fast compared to the rate of oxygen pumping. This results in an overshoot or positive error which becomes more significant a t higher temperatures, larger applied voltages, and lower oxygen concentrations. Electrode polarization caused by the slow transport of oxygen through the electrodes can also limit the accuracy as well as the low temperature limit of this oxygen sensor. Severe polarization has been shown to introduce electronic conductivity in the zirconia (9) which will result in decreased oxygen pumping efficiency. This is especially true if oxygen is severely depleted a t the external pump electrode. Increased measurement errors will then occur a t very low oxygen contents. Independent of source, positive error will be introduced if oxygen pumping efficiency drops below 100%. Of course, the use of this sensor is limited to oxygen concentrations which are sufficiently high for accurate current integrations to be obtained. This fact combined with the larger positive errors experienced at very low O2concentrations due to rapid pumping rates and decreased pumping efficiency does limit the low oxygen concentration range of this sensor. T h e presence of larger positive errors a t very low oxygen partial pressures was confirmed with oxygen measurements in flowing nitrogen gas containing 12 ppm 02.Measurements at 700 "C yielded oxygen contents of 100 ppm under these conditions. This is not a major limitation in many applications. For example, during air-fired oil shale retorting, only excess oxygen (>0.1%)is of interest. Small but rapid temperature changes have been found to have very little influence on the operation of these sensors. A 25 "C increase in temperature at a 40 "C/min rate changes the accuracy of the sensor by only 1-2%. Therefore this sensor yields meaningful results even in a rapidly changing temperature environment. As with any solid-electrolyte oxygen sensor with platinum electrodes, the use of this sensor is limited to nonreactive gas mixtures or equilibrium gas environments. This is because the platinum electrodes rapidly catalyze the oxidation of combustible gases in the presence of oxygen. However, silver
-
-
has been shown not to catalyze the reaction of methane and oxygen to a t least 800 "C in solid-electrolyte oxygen gauges ( I 7). Gold also has a greatly reduced catalytic behavior as compared to platinum (18). Therefore silver or possibly gold electrodes might be used if the oxygen content of nonequilibrium gas mixtures of oxygen in the presence of combustible gases is desired. Studies of this nature are in progress.
(5) D. A. J. Swinkels, J . Electrochem. Soc., 117, 1267 (1970). (6) J. Fouletier, H. Seinera. and M. Kleitz, J . Appl. Electrochem., 5 , 177 ( 1975). (7) G. S. Snow and P. D. Wilcox, U.S. Patent 3,951,327, April 20, 1976. (8) G. S. Snow and P. D. Wilcox, SAND76-0329. July 1976. (9) R. J. Brook, W. L. Pelzmann, and F. A. Kroger, J . Electrochem. Soc., 118, 185 (1971). (10) J. Fouletier, G. Vitter, and M. Kleitz, J . Appl. Electrochem., 5 , 11 1 (1975). f 1 1) Y. K . Aarawal. D. W. Short. R. Gruenke. and R. A. RaDD. J . Nectrochem. Soc., i21, 354 (1974). (12) D. W. Strickler and W. G. Carlsson, J . A m . Ceram. Soc., 48, 286 (1965). (13) W. M. Boorstein, R . A. Rapp, and G R. St. Pierre, Tech. Rept., AFML-TR-73-67, April 1973. (14) H. Yanagida, R. J. Brook, and F. A. Kroger, J , Electrochem. Soc., 117, 593 (1970). (15) T. H. Etsel!,and S. N. Flengas, J . Nectrochem. Soc., 118, 1890(1971). (16) L. Heyne, Mass Transport in Oxides", J. B. Wachtman. Ed., Natl. Bur. Stand. ( U . S . ) , Spec. Pub/.. 296, 149-164 (1968). (17) Y. L. Sandier, J . Electrochem. Soc., 118, 1378 (1971). (18) J. O'M Bockris and A. K. N. Reddy, "Modern Electrochemistry", Vol. 2, Plenum Press, New York, N.Y., 1973, p 1161.
..
~I
ACKNOWLEDGMENT T h e author acknowledges the very valuable assistance of James M. Freese in the physical design and construction of the oxygen sensors as well as the electronics used in this work. He also aided in acquiring experimental data and along with G. S. Snow developed the platinum-zirconia seals. In addition, thanks are given to E. E. Komarek, Jr., for preparing the sputtered platinum electrodes used in this work.
RECEIVED for review May 31, 1977. '4ccepted ,July 18, 1977. This work was supported by the U S . Energy Research and Development Administration and was presented in part a t the 173rd National Meeting, Americam Chemical Society, New Orleans, La., March 20-25, 1977, Analytical Division, Paper 173.
LITERATURE CITED (1) M. %to, "Research Techniques For High Pressure and High Temperature". G. C. Ulmer, Ed.. Springer Verlag, Berlin-New York, 1971, Chap. 3. (2) K. S . Goto and W. Pluschkell. "Physics of Electrolytes", Vol. 2, J. Hladik, Ed., Academic Press, London-New York, 1972, Chap. 13. (3) H. Schmalzried. Z . Elektrochem., 66, 572 (1962). (4) R. Baker and J. M. West, J . Iron Steel Ind.. 204, 212 (1966).
On-Column Reaction Gas Chromatography for Determination of Chloromethyl Methyl Ether at One Part-per-Billion Level in Ambient Air G. J. Kallos" Analytical Laboratories, Dow Chemical U.S.A., Midland, Michigan 48640
W. R. Albe Instrument Applications and Communications, Dow Chemical
U.S.A., Midland, Michigan
48640
R. A. Solomon Hydroscience Associates, Inc., 904 1 Executive Park Drive, Knoxville. Tennessee 379 19
A gas chromatographic method capable of selectively measuring one part-per-billion(v/v) or better of chloromethyl methyl ether (CMME) in air has been developed. This method utilizes the direct derivatiration of CMME as a vapor with an alkali salt of 2,4,6-trichlorophenoI. Subsequent analysis is performed without any enrichment. The volatile 2,4,6-trichlorophenoxy methoxy methane derivative is determined by gas chromatography with an electron capture detector. The cycle time per determination is 90 s.
Chloromethyl methyl ether (CMME) used as a reaction intermediate in chloromethylation reactions has been identified ( I ) as a suspected carcinogen. Recent reports have implicated CMME as an alleged carcinogen because of the development of lung cancers to workers who were occupationally exposed ( 2 , 3 ) . Although CMME is not stable in aqueous solutions (4,it has been found to be significantly stable ( 5 ) in humid air and consequently becomes a major concern. All these implications clearly dictate the need to
measure airborne concentrations of CMME a t very low parts-per-billion levels. The determination of CMME by the colorimetric technique (6) is tedious and not highly specific. Solomon and Kallos (7) reported a derivative procedure for the determination of CMME a t levels less than 1 ppb. This method utilizes the derivatization of CMME in a trichlorophenate solution to a more stable derivative, extracting the derivative with hexane and subsequent analysis by gas chromatography using an electron capture detector. A new gas chromatographic procedure is described which stabilizes CMME as a vapor through derivatization on column with an alkali metal salt of 2,4,6-trichlorophenol and immediately determined by gas chromatography using an electron capture detector.
EXPERIMENTAL Reagents. Sodium hydroxide and potassium hydroxide, Baker analyzed reagents, were obtained from the J. T. Baker Chemical Company, Phillipsburg, N.J. 2,4,6-Trichlorophenol, mp 67-68
"C, was obtained from the Eastman Kodak Company, Rochester, N.Y. Methanol, distilled in glass, was obtained from Burdick and Jackson Laboratories, Muskegon, Mich. Chloromethyl methyl A N A L Y T I C A L CHEMISTRY, VOL. 4 9 , NO 12, OCTOBER 1 9 7 7
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