Table IV. Variation of Peak Current with Scan Ratea ip, U A
Scan rate,
mvisec b
C "27
P b2+
10 20 50 100
0.98 2.03 5.22 10.0
0.39 0.61 1.58 2.74
0 0.5M NaC1, 30 rps, 5-min deposition time at -1.0 volt, 6.6 x l O - 5 M HgZ+, 2.6 ppb Pb2-, 7 . 5 ppb Cuz+, 26 "C. * Measurements made on the first scan.
Table IV shows the variation of peak current with scan rate for copper and lead. Peak current amplitudes increased linearly with scan rate as would be expected with this film electrodes (6). We found that high potential scanning rates gave results of the same reproducibility as the low rates. For rapidity of the analyses, fast rates are to be preferred. Variations of peak current with deposition time are illustrated in Table V. Peak currents were extremely linear with deposition time. For the part-per-billion range, deposition times of approximately 5 minutes were suitable. CONCLUSION
The method described is simple and very fast for the determination of trace metals a t the part-per-billion level and below. Table V. Variation of Peak Current with Deposition Timea
ACKNOWLEDGMENT
ipr u A
Deposition time, sec b
C"2+
Pb2+
100 200 3 00
2.98 6.02 9.00
0.98 1.87 2.81
0.5M NaC1, 30 rms, 6.6 X 10-5M Hgz-, 2.6 ppb Pb2+, 7 . 5 ppb Cu*+, deposition at -1.0 volt, pH 4.1,26 " C . Measurements made on the first scan. (I
The authors express sincere appreciation to D. H. Howling of New College, for his constructive criticism and to E. N. Pollock, L. P. Zopatti, and J. C. Cornwell of Ledgemont Laboratory for furnishing the quartz cells and the high purity reagents used in this study. RECEIVEDfor review January 16, 1974. Accepted June 27, 1974. This work was partially supported by the National Science Foundation under Grant No. G.1.-31605. Barendrecht, in "Electroanalytical Chemistry, Vol. 2," A. J. Bard, Ed., Marcel Dekker, New York. N.Y., 1967, p 69.
(6) E.
Semiquantitative Potentiometric Method for Direct Measurement of Nitrogen Dioxide in Air G. G. Barna and R. J. Jasinski Texas lnstrurnents Incorporated, P. 0. Box 5936, Dallas, Texas 75222
The potentiometric measurement of electroactive gases, in contrast with amperometric measurements such as using a Field Lab 0 2 Analyzer (Beckman Instruments, Part No. 100800),a Faristor NO2 Sensor (Environmetrics, Marina del Rey, Calif.), or the method of Bay et al. ( I ) ,has the inherent advantage of simplicity and low cost, in that the only components required are the electrochemical transducer for the particular species, a reference electrode, and a high impedance voltmeter. There exist, however, only a very limited number of cases where such measurements are perfor:ned, and except for 0 2 ( Z ) , those involve the indirect measurement of the aqueous reaction product of the particular gas, via an ion-selective electrode. The determination of SO2 using an SO2 electrode (Model 95064, Orion Research Inc., Cambridge, Mass.) or the method of Young et al. ( 3 ) ,NH3 with an NH3 electrode (Model 95-10, Orion), and NO and NO2 ( 4 ) has been achieved by these means. All these techniques involve a preconcentration step to achieve the necessary sensitivity, obtained by purging the (1) H. W. Bay, K. F. Blurton, H. C. Lieb, and H. G. Oswin, Amer. Lab., No. 7, p 57 (1972). (2) P. B. Hanh, M. A. Wechter, D. C. Johnson, and A. F. Voigt, Anal. Chem., 45, 1016 (1973). (3) M.Young, J. N. Driscoll, and K. Mahoney, Anal. Chem., 45, 2283 (1973). (4) E. M. Kneebone and M. Freiser, Anal. Chem., 45, 449 (1973).
1834
air sample through a scrubbing solution; consequently, they do not yield real-time analyses. They are also, in the case of the p H measuring devices, amenable t o interference by a range of p H altering gases. This paper describes the direct potentiometric measurement of NO2 in air by means of a properly activated Fechalcogenide glass electrode. This system avoids the complication of a preconcentration step while still maintaining high sensitivity; achieves selectivity directly via the nature of the chemical interaction between the gas and the sensor material; while it maintains the advantage of simplicity in measurement. EXPERIMENTAL The active element of the electrochemical 9 0 2 sensor is a nonoxide chalcogenide glass with the approximate composition of Sefi&e&blz doped with 1.7-2% Feo (the subscripts refer to the mole percentages of the elements). This material is prepared by the direct fusion of the elements, as described in a previous publication ( 5 ) ,and will subsequently be designated as "Fe 1173." The electrode was constructed by attaching a glass disk (7 mm in diameter and 1 mm thick) cut from the melt to a brass screw fitted inside a cylindrical rod of paper-based phenolic resin (A-1 Plastic Supply, Dallas, Texas). This material was chosen for its compatability with the necessary high-temperature air-oxidation (5) R. Jasinski and I. Trachtenberg, J. Hectrochem. SOC.,120, 1169 (1973).
ANALYTICAL CHEMISTRY, VOL. 46, NO. 12, OCTOBER 1974
Gas
Air NO? (350 ppm in N?) NO (100 ppm in N!) CO (1% in N2) so, (100%) CH, (1%in N?)
Gas
Gar Inlet
Table I. Sensor Potentials us. G a s Composition
&!let
a
Potential shift (us. N:), mV
+4
+324 1-3 +6 +7 +3
llernorane hlembrane Holder
activation required by the sensing material (see below) and with the electrolytes. Other details related to the construction of the electrode have been described before ( 5 ) . The necessary and sufficient procedure required for making the Fe 1173 material reproducibly active toward NO:!, has been established to be a 3 min/300 "C air oxidation of the electrode material. Although various slices of the non-activated material have shown a limited degree of activity towards NOz, these responses of the nonactivated sensors were not reproducible either in rate or in magnitude. The necessary liquid-solid interface was obtained via the cell configuration, hereafter to be designated as "inverted," shown in Figure 1. The rationale behind this construction will be discussed below. A wick was taped along the body of the electrode such that it was in close contact with the sensing surface of the electrode, which protrudes above the surface of the electrolyte. In this way, the sensing element was directly exposed to the gas being monitored. The electrolyte brought to the exposed surface by the capillary action of the wick, completed the solution circuit between the sensing and reference electrodes. A Beckman Perma-Probe electrode was used as a reference. A piece of %-mil perforated Teflon membrane was placed between the gas stream and the sensor, a t approximately 3 mm above the surface of the sensor, to retard the evaporation of the electrolyte while allowing the gas an access to the sensor. The electrolyte, unless otherwise mentioned, was a 0.1M solution of (NH4)zHPOd a t its natural pH of 8. Identical sensor responses were obtained with 0.1M solutions of KNOs, CsC1, NaC104, NaNO:j, KC1, and NH4C1, in the pH range of 3.7 to 8.0. The calibrating gases were pre-analyzed mixtures of NO:! in air, purchsed from Matheson Company. The flowrate of the gas stream was generally held a t 120 cm3/min; however, a twofold increase in this rate made no appreciable difference. No attempt was made a t controlling the relative humidity in the sensing cavity. However, no differences were observed when the fluid electrolyte was substituted by the gelled electrolyte, which necessarily provided a lower partial pressure of water around the sensor. The response of the system was monitored by measuring the open-circuit voltage between the sensor and reference electrodes by means of a Keithley 610B Electrometer the output of which was fed into a Hewlett-Packard 4340A digital voltmeter, and simultaneously into a T I ServoiRiter I1 strip-chart recorder.
RESULTS AND DISCUSSION
The chalcogenide glass described in the experimental sectZionhas been shown ( 5 ) to form the basis of an electrode sensitive to ferric ions. With the proper pretreatment, this material also gave a positive potential shift when equilibrated with NO2 in the presence of air and was insensitive to NO, SOz, CO, and CH4. The results of these selectivity experiments are shown in Table I, where potentials (us. Ag/ AgzSO4) are listed for a properly activated Fe 1173 electrode exposed to a series of gases. The electrolyte was 7 N H"O4; however, similar results were obtained with other electrolytes. Persuant to exploiting this selective activity of the Fe 1173 material to N02, a gas cell, similar in design to commercial 0 2 cells, was constructed. This configuration, however, was unsatisfactory for the potentiometric measurement of N02, because of the unacceptably slow response and purge rates of the device. The remainder of this paper will be devoted to a description of a viable structure and the resulting measurements.
\',
,--Covei
\
nla!e
L-Electrolyte
ick
0-rtig
Sensor Electrode
Reference Electrode
Figure 1. A prototype gas sensor system in the "inverted" configuration
There are two reasons for the poor (equilibrium potential not reached in 30 minutes) response characteristics of the system assembled with a relatively large volume of electrolyte between the gas-permeable membrane and the sensing electrode. First, unlike 0 2 , NO2 reacts with aqueous electrolytes as shown in the following equation
2 NO2 + HzO
"02
+ HNOn
Since the potential determining process is some interaction of the N02, not NO2- or Nos-, with the electrode material (6),it can be seen that this solution chemistry of NO2 serves only as an interference in as far as it necessitates the establishment of a gas electrolyte equilibrium prior to the measured equilibrium between the NO2 and the sensor electrode. Second, in contrast to a commercial amperometric NO2 sensor, in which NO2 is reduced electrochemically, the potentiometric measurement allows for no process, other than diffusion, for the elimination of NO2 from the electrolyte. To overcome these drawbacks, the inverted configuration was developed. The principle behind this inverted configuration is as follows. In the potentiometric mode of operation, the measured variable is the open-circuit voltage between the sensor and reference electrodes. This voltage is determined by the potentials of the two electrodes. However, the potential is an intensive property of an electrode-Le., it does not depend on the size of the electrode or, what is more to the point, on the size of the electrode-electrolyte interface. This is in direct contrast to the amperometric mode of operation, where the measured variable is a current. This is an extensive property and is thus directly proportional, at a given gas concentration, to the size of the electrode-electrolyte interface. Thus, while the kinetics of the response, as limited by the solution chemistry of N02, are anticipated to improve if the volume of electrolyte between the gas and the sensor is reduced, thermodynamics indicates that such a reduction would have no bearing on the magnitude of the response. This is demonstrated in an experiment where the gas cell was assembled in a non-inverted configuration: with a 45mil spacer between the gas permeable membrane and the (6)R . Jasinski, G. Barna, and I. Trachtenberg, J. Electrochem. Soc., in press.
A N A L Y T I C A L CHEMISTRY, VOL. 46, NO. 12, OCTOBER 1974
1835
Table 11. Response of the Cell as a Function of the Volume of Electrolyte between the Electrode and the Membrane Rate of change Sequence
45-Mil spacer
Wick
Response (NO2 On) +10 mV/3 min +127 mV/3 min Purge (NO?Off, Ais On) - 7 mV/3 min - 70 mV/3 min
Table 111. Response of the Sensor to NO2 in Air Figure 2. Characteristic response curve
sensor and with only as much electrolyte in this interface as was conducted there by the capillary action of a wick. Compared in Table I1 are the rates of response of these two systems to a mixture of 290 ppm NO2 in air. These data clearly illustrate the advantage of minimizing the volume of electrolyte. Through a minor alteration of the cell configuration, the entire cell was inverted, and the wick thus became the sole conduit for the electrolyte to reach the exposed surface of the sensor electrode. This configuration, however, suffered the problem common to all such gas sensing electrochemical cells-Le., the drying of the electrolyte in contact with the gas stream. This problem was essentially eliminated by the placement of a piece of perforated Y4-mil Teflon membrane between the gas and the electrode. This membrane prevents the crystallization of the electrolyte on the exposed surface of the electrode, and has been capable of maintaining a “clean” electrode surface during weeks of intermittent operation, while still allowing an equilibrium to be established between the sample and the gas cavity. It is worth noting that while a membrane has been employed in the final prototype of this cell, the inverted configuration eliminates the inherent need for the use of a membrane with nonporous electrochemical cells. Generally, such a membrane serves as the gas permeable boundary enclosing a limited volume of electrolyte between itself and the electrode surface. By necessity, this membrane can only be a negative contribution to the overall response time of the sensor system. Whatever the couple a t the electrode, the rate and amount of response will be a function of the permeability of the gas through the membrane. The response can vary from zero, in the case where the membrane is impermeable to the gas, to a value that is essentially determined by the electrode reaction-Le., where the membrane is so permeable that permeability ceases to be the limiting factor. With the membrane not essential for the short term operation of the system, studies could be carried out for the permeability of a given membrane for a specific gas, the electrochemical measurement of a gas for which a permeable membrane can not be found, and fundamental gas-electrode interactions, by being able to separate the gas-membrane and the gas-electrode effects. Having developed a gas cell in which the necessarily adverse effect of the volume of electrolyte has been minimized, it was possible to study the sensor response after various types of chemical pretreatment--i.e., activation. The necessity for the high-temperature air-oxidative activation of the electrode material, as described in the Experimental section, is shown by the data in Table 111. Summarized there is the nature of the response for the gas cell assembled under conditions that were identical in every respect other than the treatment of the electrode. Thus, the activated electrode yields a much quicker and larger response to NO2 than one that has not been so treat1836
*
Result Nonactivated electrode
Sequence
Air 290 ppm N02/Air
Air-oxidized e 1ectrode
Steady potential Steady potential 3-min delay 30-sec delay + 5 8 mV/3 rnin +115 mV/3 rnin
~~
Table IV. Calibration Data for the NO2 Sensor (Potentials Relative to Air) 1.2 ppm, mV
25 ppm, mV
392 ppm, mV
Electrode 46-9
1 2
3 6 7 8 9 10
13 14 15 16
Average Std dev
6, 10 20 9 17, 20
11,11 8, 8 14, 14 13, 13 16, 20 14, 14 20, 23 12, 1 3 14 4.3
118, 138 90, 106 91 100, 101 94, 103 79, 80 109,110 86, 96 106 104, 107 97,99 95,97 100 1 3 .O
197, 212 200 157, 186 213, 220 205, 210 202, 205 196, 208 210, 222 217, 221 207, 209 205, 207 218, 225 199 13.7
Electrode 46-A Day
1 2
Average Std dev
17, 19 25, 26 21.7 4.0
103, 105 124, 127 114.7 14.8
178, 183 213, 207 193.9 15.7
ed. In line with previously described activation treatments ( 5 ) for Fe3+ sensing, various modes of chemical oxidation were evaluated. A sequential treatment of a freshly etched electrode with H202, Fe3+ and NO2 did give a response to NO2. However, both the amount (potential shift for a fixed NO2 concentration) and the rate of response were very poor; typically, a 150 ppm mixture of NO2 in air gave an increase in potential of 5 mV in a period of 8 minutes. None of the other combinations of this activation sequence, such as an H202 plus NO2 or an Fe3+ plus NO2 treatment, improved the previous results. The nature of the response to varying NO2 concentrations obtained from the described prototype of a properly activated NO2 sensor is summarized in the following Tables and Figures. A typical tracing of the potential us. time, for the system in the inverted configuration, is shown in Figure 2. After a steady state potential had been obtained for air going through the system, a 23-ppm NO2 mixture was introduced. The vertical bars in Figure 2 represent the
ANALYTICAL CHEMISTRY, VOL. 46, NO. 12, OCTOBER 1974
times when the NO2 was turned On and Off. When the NO2 mixture was turned On, there was an initial 10-second delay, partially due to the time it takes for the gas to travel through the approximately 6 feet of tubing associated with the gas delivery system. Including these initial periods of delay, 90% of the final response to the NO2 mixture, as well as to the subsequent air purging, was obtained in 1.5 and 2 minutes, respectively. The magnitudes of the response of two separate sensors to NO2 in air mixtures in the range of 1.2 to 392 ppm, are demonstrated in Table IV. The numbers represent the differences in potential between air and the specified mixture. While the data with electrode 46-9 were obtained with a fluid electrolyte [0.1M (NH&HPOd], the electrolyte in the cell containing sensor 46-A was immobilized by gelling the above mentioned solution with 0.5 Agar/100 cm3 electrolyte. The magnitudes of the response are essentially the same for the two sensors. In the case of the system employing the gelled electrolyte, the advantage of portability has been gained since the possibility of flooding the sensing surface, due to any movement of the cell during the course of the measurement, has been eliminated. The averages and standard deviations from Table IV are plotted in Figure 3. The curves exhibit slopes of 70 mV/ decade. Precedents do exist ( 2 , 5 ) for such a "super-Nernstian" response of an electrochemical process not involving
' i I
I
/
.-
I-
I
I
I
I I I I I I
I
I
1
I I I I l l
'W
in @,
Colre"t'al*r
1
I
I
l
l
Wrr
Figure 3. Calibration curves obtained with: (A) electrode No. 46-9 and (E) electrode No. 46-A, curve displaced by $60 mV
a classical type of redox reaction. Considering the standard deviations, this system, in its current state of development, yields a semiquantitative means for the direct, real-time analysis of NO*. RECEIVEDfor review January 21, 1974. Accepted May 3, 1974.
Electrochemical Technique for the Measurement of Carbon Monoxide H. W. Bay, K. F. Blurton, J. M. Sedlak, and A. M. Valentine Energetics Science, lnc., 85 Executive Boulevard, Elmsford, N.Y. 10523
Recently a portable instrument for the measurement of carbon monoxide was described ( I , 2) and independently evaluated (3, 4 ) . Its operating principle is based on electrochemical oxidation o f c a r b o n ~ ~ o n o x at ~ dae Teflon-bonded diffusion 15). This paper reports a analysis of the mass transfer and electrochemical Processes occurring in this electrochemical sensor and describes the experimental data justifying these equations.
Analyzed mixtures of CO in air in the range 21-385 ppm C o were obtained from Matheson Gas Products Go. The mixtures were certified to f 2 % Of the stated concentration. During sensor operation, a CO/air mixture was passed a t a constant flow rate over the back (gas side) of the sensing electrode. The current generated due to the electrochemical oxidation of CO was determined by measuring the potential drop across a standard resistor. The values of the current were reuroducible to &10/0. All measurements were made in triplicate a t a temperature of 22 + 1 ~
oc.
EXPERIMENTAL The sensor consisted of a three-electrode electrochemical cell containing 3.4M HzS04 solution (Figure 1). Each of these electrodes (sensing, counter, and reference) were Teflon-bonded diffusion electrodes ( 6 )which were catalyzed with Pt. The sensing electrode (anode) was maintained a t the potential of 1.2 V ( u s . SHE) by a conventional potentiostat. This potential was selected to maximize the electrode sensitivity toward CO ( 7 ) and to minimize the current due to 0 2 reduction and water oxidation. (1) H. W. Bay, K. F. Blurton, H. C. Lieb and H. G. Oswin, Amer. Lab., 4(7),
57 (1972). (2) K. F. Blurton and H. W. Bay, Amer. Lab., in press. (3) N. Yamate and H. Inoue, J. Environ. Contr. (Japan), 9,No. 3 (1973). (4) C. F/ Smith and G. C. Ortman, Paper presented to 20th Meeting of Instrument Society of America, May 1974. (5) H. G.Oswin and K. F. Blurton, U.S. Patent 3,776,832(1973). (6) J. P. Hoare, "Electrochemistry of Oxygen," Interscience, New York, N.Y., 1968. (7) K. F. Blurton and J. M. Sedlak. J. Electrochem. Soc., in press.
THEORETICAL ANALYSIS OF SENSOR RESPONSE
A schematic of the sensing electrode face plate is shown in Figure 2. The air stream enters the sensor through ACBD and exits through A'R'C'D'. The sides and top of the sensor element (ACA'C', BDB'D', and CDC'D') are impervious to mass flow. AA'BB' is the back side of the diffusion electrode through which CO molecules must pass to reach the electrocatalytic surface-ie., the gas migrates to the catalytic sites in a direction perpendicular to convective flow through the sensor. The reaction at the sensing anode is
CO
+
H,O
-
A N A L Y T I C A L CHEMISTRY, VOL. 46,
COz
+ 2H' +
NO. 12.
2e
OCTOBER 1974
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