1152
Anal. Chem. 1983, 55, 1152-1156
Conductometric Sensor for Atmospheric Carbon Dioxide Determination J. S. Symanskl, G. A. Martlnchek, and Stanley Bruckenstein” Department of Chemistty, State University of New York at Buffalo, Buffalo, New York 14214
A novel conductometrlc sensor is described for atmospheric gaseous species that dissolve In water to form conducting solutions. The sensor consists of a hydrophobic gas porous membrane which separates the gas sample from a thin layer of water. The gaseous species, e.g., carbon dioxide, diffuses through the membrane and rapidly equilibrates in the water layer. Conductivity electrodes positioned within the thin layer measure its conductance. The theory relating the square of the equilibrium conductance to the partial pressure of carbon dioxide, PC4, is given. The configuration of the sensor as a CO, detector requires that chemical filters be employed to remove interferences from SO,, NO,, and NH3. Linearized calibration curves are obtalned over a CO, range of 0-3%, with response tlme on the order of 20 s or less. The portable conductometric sensor is a hand-held unit measuring 3 In. X 2.5 in. X 5 in.
interface where the COz dissolves and reacts with water to produce H+ and HC03- in amounts related to the partial pressure of COz in the gas phase. The conductance is measured between electrodes deposited on the membrane’s surface, where response time and sensitivity should be optimal. The conductivity measurement is made 16 s after the water previously within the thin layer has been flushed away with freshly deionized water. A rapid pulse of water about 20 times the volume of the thin-layer cell (- 10 pL) is used to flush the cell. The “pulsed water” thin layer cell demonstrates the same response characteristics as a thin stationary film of pure water. This cell design was adapted to construct a small, low power portable detector.
THEORY The conductometricsensor’s response is determined by the following equilibria ~
Conductometry is one of the oldest techniques of electroanalytical chemistry. Its application to the determination of gaseous carbon dioxide was first demonstrated by Cain and Maxwell (I). Their method along with other early methods relied on measuring the decrease in conductance of hydroxide solutions as they absorbed carbon dioxide (2, 3). More recently, the conductance of a stream of deionized water after carbon dioxide absorption has been employed for COz determinations. In the method described by James (4) the gas sample containing carbon dioxide is bubbled upward through a stream of deionized water moving in the opposite direction. The water stream passes through a flow conductivity cell where its conductance is measured. All of these types of methods involve the physical mixing of gross amounts of gas and water, making them difficult to adapt to portable applications. Van Kempen and Kreuzer have described a fast responding microsensor for Pco,measured based on the method of conductometry (5). Their sensor is constructed from a double lumen catheter, the tip of which is covered by a COzpermeable membrane. A separate set of conductivityelectrodes is placed inside each lumen and the catheter is flushed with a constant flow of twice distilled water. Carbon dioxide from the gas phase diffuses across the membrane and dissolves into the flowing stream. The difference in conductivity between the entering and exiting water is related to the PCO, in the gas sample. This sensor shows a marked decrease of sensitivity as the water flow rate increases and as the distance between the conductivity electrodes becomes larger. As the mean volume flow rate in the liquid boundary layer adjacent to the membrane is very large compared to the flux of COz through the membrane, the solution layer never approaches equilibrium with COz in the gas sample. Therefore, the maximum possible sensitivity cannot be achieved. In this paper, we describe a unique conductometric carbon dioxide sensor. It is based on a thin layer cell having a gas porous membrane separating pure water from the gas sample. The porous membrane allows rapid COztransport to the water 0003-2700/83/0355-1152$0 1.50/0
0
2=~ 0 2 ( a q ) 2
COz(aq) + H20
~
& H,CO,(aq)
& H+(aq) + HCO,-(aq)
HzC03(aq)
(1) (2)
(3)
The rate of reaction 1,the partition equilibrium describing the solubility of COP in water, is fast compared to the formation of H2C03,reaction 2, as is the dissociation of HzC03, reaction 3. Although reaction 2 is known to occur at a readily measurable rate (6),its kinetics are fast compared to the time scale of mass transport controlled equilibration of the water layer in our cell. Therefore, reactions 1-3 may be summed and the overall equilibrium reaction between COz and the ions formed in the water layer can be written as COz(g)
+ HzO -’Kdc.
H+(aq)
+ HCO,-(aq)
(4)
where K , is the “apparent” dissociation constant of COz(aq) and is equal to KhKd. When the thin water layer is in equilibrium with gas phase COP [H+] = [HC03-]
KpK =
(5)
[H+l [HC03-lfH+fHCOa-
pco,
(6)
and the specific conductance, K , is given by K
= 10-3(X~+[H+] i- XHco,-[HCO3-])
(7)
Bracketed terms represent equilibrium concentrations,f terms the activity coefficients, and X terms the equivalent ionic conductances of the specified species. In using eq 5-7, we assume the equilibrium concentrations of H&03 and COZ- are negligible and the ion concentrations are sufficiently small so that the activity coefficients of H+ and HC03- approach unity and the equivalent ionic con@ 1983 American Chemical Society
ANALYTICAL CHEMISTRY, VOL. 55, NO. 7, JUNE 1983
A-
WATER RESERVOIR
BC-
-G
F-
--D
L,
/I HIXED
BED
EXCWWGER
PULSE PUMP
__-__-
CELL
Flgure 1. Model diagram of the pulsed water thin layer cell: (A) mixed bed ion exchange columnl, (B) Lucite cell body, (C) thin water layer, (D) gold electrodes, (E) porous Teflon membrane, (F) activated carbon filter, (G) thermistor.
ductances at infinite dilution (hoi) apply. These conditions are met for C02even at Pco, = 1 atm. Substituting eq 5 and 6 into eq 7 yieldelas the expression for the specific conductance K
= 10-"(XoH+
1153
RTMOSPHERE Figure 2. Block diagram of packaged thin layer cell: (A, B) miniature one-way check valves. Arrows indicate direction of intermittent water flow.
+ X o ~ ~ ~ ~ - ) ( K a K ~ ) l ~(8) z(P~~~)l~z
The actual cell conduct,ance,S, is proportional to K , therefore where B is the cell constant and A is equal to 10-3(X0~+ + XoHco,). The exlpression relating the square of the equilibrium conductance to the partial pressure of C02given by eq 9 holds for any gas that dissolves to form an acid or a base which is weakly dissociated. For example, NH, and SOz also obey relations analogous to eq 8 and 9. Thus, the ratio of the square of the conductance for another gaseous species (i) with respect to that of COz is given by eq 10.
[si/Sco,12 = I : A ~ / A C O , I ~ [ ( K ~ K ~ ) ~ [pi/P~0,1 /(K~~)CO~I (10) CELL DESCRIPTION Figure 1shows a schematic representation of the sensing region of the thin layer conductometric cell. It consists of a nonwetting, gas porous Teflon membrane (E) separating the thin water layer (C) from the gas samplle. The thickness of the water layer is approximately 0.004 in. (0.010 cm). Its volume is approximately 13 pL. Gold conductivity electrodes (D) are deposited on the porous membrane's surface. Water is passed through a small mixed bed ion exchange column (A) to remove any ionic species. This water displaces the COzcontaining water from the thin layer, preparing the cell for a new C02 determinatilon. Gaseous carbon dioxide diffuses through an activated carbon filter mat (IF) before dissolving into the thin water layer. A thermistor (G) is embedded in the cell adjacent to the thin layer. This thermistor makes possible temperature compensation of the conductometric response. A block diagram of the complete thin layer sensor is shown in Figure 2. A carbon dioxide determination is initiated by manually depresoing a plunger which operates a low volume, water pulse pump that simultaneously activates the electronics and flushes the thin layer with a quantity of water corresponding to several thin layer volumes. The pulse pump consists of a positive displacement chamber having one rigid wall and one flexible rubber wall. Attached to the entrance and exit of the pulse pump are one-way check valves (A, B) which are arranged so as to permit only unidirectional water flow when the rubber diaphragm is displaced (pressed) by the operator's finger. The water reservoir is positioned above the thin layer cell and the pulse pump in order to maintain constant water pressure on the Teflon sensing membrane and to maintain a positive back-pressure on the check valve (A).
TIMING CONTROL
'
DIGITRL PANEL ME:TER
REFlllATED SUPPLY VOLTRGES
BRTTERIES
SMJRRING ClRUllT
SRIJRRE HAVE GENERRTOR
THERMISTOR
CELL
M L MRVE RECTIFIER
CURRENT TO VOLTAGE CONVERTER
I
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ANALYTICAL CHEMISTRY, VOL. 55, NO. 7, JUNE 1983
generate regulated voltages for the LCD digital panel meter (DPM) and the square wave generator. The square wave voltage generator imposes a 256-Hz square wave across the electrodes of the thin layer cell. The magnitude of the square wave is controlled by the resistance of the thermistor embedded adjacent to the thin layer. The output of the current to voltage converter is a 256-Hz square wave proportional to the temperature compensated conductance of the thin layer cell. This output is rectified and then squared by a precision squaring circuit, giving a dc voltage proportional to the square of the thin layer conductance. The DPM displays directly % COz in the gas sample. After a COz determination is initiated, the DPM reading increases rapidly as the water sample in the thin layer approaches equilibrium with the gas sample. Sixteen seconds after the start of the determination, the DPM display latches and a latch indicator appears in the display. The display now presents the gaseous COz concentration. This value is displayed for an additional 16 s, after which the sensor's power is switched off automatically. All timing control functions are derived from a 32 768-Hz crystal oscillator used to support the DPM timing functions. The circuit operates only during a 32 s period, thus conserving the two batteries. Over 3000 determinations can be made before battery replacement. A complete description of the electronic circuitry and mechanical configuration of the sensor is given elsewhere (7). Sensor Configuration. The thin layer cell (Figure 1)and the packaged conductometric sensor (Figure 2) are constructed from Lucite stock. The components of the water flow system (Figure 2) are joined with 1/8 in. diameter silicone rubber tubing and minature brass fittings (Clippard Minimatic Fittings, Clippard Instrument Laboratory, Inc., Cincinnati, OH). The porous membrane (E) (Figure 1)is 0.004 in. Gore-Tex poly(tetrafluoroethy1ene) sheet (W. L. Gore and Associates, Elkton, MD). The gold conductivity electrodes (D) are deposited by painting a gold resinate solution (Engelhard Industries, East Newark, NJ) on the surface of the memblrane in a pattern of "interlocking fingers". Alternate fingers are joined to form each conductance electrode. The resinate is heat cured to produce porous gold electrodes (8). The porous membrane (E) is affixed to the body of the thin layer cell (B) with using a pressure-sensitive silicone transfer adhesive (Densil, Dennison Manufacturing Co., Framingham, MA). Contact wires are affixed to pads painted on the outside edge of the Teflon membrane with a conductive silver impregnated epoxy resin (Eccobord Solder 57C, Emerson and Cumming, Inc., Canton, MA). A piece of 0.007-in. polypropylene screen (Tetko, Inc., Elmsford, NY) attached to a 0.060 in. thick perforated Lucite sheet (not shown in Figure 1)is interposed between membrane (E) and filter (F) to support the membrane against flexing, thus maintaining constant thickness of the thin water layer. The filter material (F) is supplied as in. thick activated carbon mat (Dexsan, Dexter Corp., Windsor Locks, CT). Prior to use the filter disks are soaked in 0.1 M sodium bicarbonate and are dried a t 100 "C under vacuum. The filters are held in place by a removable screw fitting, therefore allowing easy replacement of expired filters in less than 60 s.
RESULTS AND DISCUSSION Cell Response. Figure 4 illustrates typical conductancetime transients for different values of Pco,. The traces were obtained from a cell with a water layer of thickness 0.004 in. Following the water pulse, the conductance falls to a near zero value and then quickly rises as equilibrium with the gas sample is achieved. The equilibration time is rapid and, in the absence of complications such as kinetics or gas phase depletion of CO,, is controlled by diffusion of CO, in the water layer. This
1
I
HRTER PULSE
6
W 0
"
U
Y
s 8
t I/
I
I li 2
l!i
B
I
\ /
;lV/
C
\
0 F;' 0
10
20
30
40
50
60
TIRE ISECONLXI
Figure 4. Conductancevs. time for thin layer conductometric cell: (A) 1.0% CO,, (B) 0.05% COP, (C) nitrogen blank. Thickness of water layer is approximately 0.010 cm. Table I. Square of Cell Conductance vs. Pco, a Range of CO, Concentrations in Gas 0-0.8%
pco, , 0.00 0.025 0.050 0.10 0.15 0.20 0.25 0.40 0.60 0.80
0-3%
s2,!JS*
pco, ,
0.0400 1.14 2.00 3.77 5.29 6.86 8.64 13.2 20.1 27.2
0.00 0.23 0.43 0.65 0.87 1.07 1.73 2.18 3.10
slope 33.4 * 0.260 p S 2 / % CO, inter- 0.228 t 0.0937 pSz cept a T = 26.5 "C.
s2,PS, 0.0400 8.70 16.0 22.9 30.3 37.8 56.8 75.1 108.0
34.3 i 0.454 p S 2 / % CO, 0.402 i. 0.675 pSz/% CO,
diffusion model has been treated by Crank (9),and the time required to reach 98% of the equilibrium uptake of CO, after a water pulse is calculated as
t = 1.5OLwZ/D,
(11)
Where L, is the thickness of the water layer in the thin layer cell and D, is the diffusion coefficient of C02in water. Using values of L, = 0.004 in. and D, = cm2/s yields an approximate value of t equal to 15 s. This time agrees satisfactorily with the data shown in Figure 4. The shape of the conductance vs. time trace for a given Pco, is independent of flow rate of the sample gas. Even a t zero gas flow, response times are identical with those seen in Figure 4. This insensitivity to gas flow rate is the result of the relatively small value of Kpand K , (eq 1-3). Gaseous diffusion even in the absence of convection is sufficient to equilibrate rapidly the thin water layer with CO,. This effect obviates the need for a gas sampling pump in the conductometric sensor. The validity of the relationship between S2 and Pcol (eq 9) is verified by the data given in Table I. Confirmation of eq 10 is given in Table I1 for SO, and NH3 vs. COz. Of the three gases, only C 0 2 exhibits conductance vs. time responses that are independent of sample gas flow. Both SOz and NH, require substantial gas flow ( 1.0 L min-l) to achieve a response time in the range of 60-120 s. This effect is characteristic of gases with large values of K , and K,. In such instances mass transport in the gas phase becomes the rate N
ANALYTICAL CHEMISTRY, VOL. 55, NO. 7, JUNE 1983
Table 11. Ratio of thle Square of Cell Conductance vs. Partial Pressure of Gas for SO, and NH3 vs. COza K
zilPg~
K2COZIPCOZ
theoreticalb observed
CO, 1.0
SO,
1.3 x 10'' 1.2 x 10"
"3
3.1 X lo4 4.5 x 104
a T = 25.0 "C. Equation 8 used for theoretical ratios. Thermodynamic constants from ref 10-15.
TW I%)
Figure 5. Equilibrium conductance vs. temperature for 1 % COP. Data are normalized about 25 "C. Theoretical conductance Is given by line. Experimental data are shown by points.
limiting step in equilibrating the thin water layer. The PCo, of the nitrogen blank (Figure 4) is less than 0.01% COz. Thus ambient COz levels (-(0.03%) are easily determined. Interferences. Any dissociable species partitioning from the gas phase inl;o the thin water layer will produce a response in the thin layer cell. Gases of this sort include SOz, NOz,and NH,. These gases give a significant conductometric response even in the parts-per-billion range (Table 11). NH3 can be removed from ZI gas sample by flowing the gas through a column containing a solid support (crushed firebrick) on which saturated KHSO, has been dried. However, it is unlikely that NH3 would be encountered as an interference in routine COz determinations, (andour detector omits this feature. Effective removal of SO2 and NOz in a COz containing gas sample is accomplished by interposing chemically treated activated carbon filter ma1B between the gas samplle and the gas porous membrane. Sulfur dioxide is removed by the reaction S02(g) + NaHC03(s) NaHS03(s) i t COz(g) (12) The quantity of COz produced by reaction 12 contributes negligibly to the' total PCO, response under normal ambient conditions. NOz removal by chemisorption occurs on carbon
-
v R1
v1 - b c
1155
filter mats not treated with NaHCO,. However, NaHC03 treatment substantially increases the filter's capacity for NOz removal. Using two filter mats in the conductometric sensor gives a filtering capacity of about 75 ppm h for each gas. The kinetics of the rate of removal of SOz and NO2 by the filter appear to be fast. A nitrogen gas sample containing as much as 60 ppm of SO2 or 20 ppm of NO2 produced no apparent C02response until the filter capacity was exhausted. Upon exhaustion, these filters are easily removed and replaced with new ones. Temperature Compensation. Equation 8 gives the specific equilibrium conductance as a function of well-defined thermodynamic constants and Pco,.Variation in these consants with respect to temperature predicts a temperature dependence of the thin layer conductometric response for a given value of Pc0,.The temperature coefficients of these constants (11, 13, 15) were used to calculate the theoretical curve presented in Figure 5. Compliance of the experimental COz data to theory is shown in the same Figure. The data demonstrate that the conductometric sensor calibrated at 25 "C will show its major response errors at lower temperatures. The magnitude of this error is within the range of compensation attainable by using simple thermistor circuitry. A model of the electronic circuit used for temperature compensation in the conductometricsensor is shown in Figure 6. The gain equation of this circuit is
where a t temperature T , S(T)ce,~ is the equilibrium conductance of the thin layer cell and R2(T) is the resistance of the thermistor adjacent to the thin water layer. V2(T)is the temperature-dependent voltage proportional to S( T)ce,l and R2(7'). The square of V Z ( nis proportional to Pco,. From the temperature response of the cell in the absence of temperature-compensating circutry and the known temperature response of the thermistor, it is possible to determine a value of R3,such that [ V2(T)l2 (~PCO,) varies no more than *2% over a temperature range of 7-35 "C. This was done by choosing R3 such that Vz(Tj/Vlyielded identical values for T = 11 OC and T = 31 "C. A plot of [Vz(T)l2 vs. temperature for a given thermistor, Rz, is shown in Figure 7. The figure shows that the conductometricsensor, when calibrated at 11 "C, will exhibit maximum temperature response errors of +2% and -2% at 21 "C and 35 "C, respectively. Long-Term Stability. For calibration of the COz sensor to remain stable it is important that the conductance-time transient, shown in Figure 4, be reproducible. It was found that when a calibrated sensor is not used for a period of a few days, the shape of the transient curves changed, and the device demonstrates an apparent high sensitivity to COz. In addition, the response varies upon successive pulsing. A conductance vs. time plot for this situation is shown in Figure 8. The c w e s labeled 1,2, arid 3 are the first three transients obtained from a sensor which had not been used for 1 week. One
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ANALYTICAL CHEMISTRY, VOL. 55, NO. 7, JUNE 1983
precise reason for this effect is unknown; we believe it is associated with adsorption of impurities on the conductance electrodes and that these impurities may come from the materials of construction (Lucite, brass, and rubber). There is also the possiblity of bacterial growth in the plumbing, and as is the case for eliminatingthis problem in the Milli-Q water purification system, periodic recirculation of the water through the exchange bed and plumbing will minimize this problem (16). 0
10
20
30
VD
90
TEMP (%I
Figure 7. Calculated values of [ V,(T)]* vs. temperature for the temperature-compensated conductometric sensor. Plot is normalized about 11 OC.
The previously described effects are virtually eliminated when the sensor is pulsed periodically. Presumably the impurities are scrubbed out by the ion exchange bed. A simple solenoid operated device was constructed to periodically activate the sensor. A sensor that was actuated every 16 min, when not in use, yielded a reproducible conductance-time transient and ita calibration sensitivity varied by no more than 5% over a 30-day period. The combination of conductance techniques with a thin layer cell having one gas porous wall produces a simple, accurate, and convenient gas sensor. The sensor's physical size and supporting electronic requirements make possible construction of a lightweight, low-power COz sensor (7). Application to the detection of other gaseous constituents that dissolve to produce conducting solutions is feasible. Registry No. COz, 124-38-9.
LITERATURE CITED (1) (2) (3) (4) (5) (6) (7)
(6) (9) I
~~
0
10
20
30
UO
SO
(10)
TIME [SECONDS)
Figure 8. Conductance vs. time for Idled thin layer conductance cell, cell not used for 1 week: curve 1, first transient; curve 2, second transient 1 mln after first; curve 3, third transient, 1 mln after second.
minute elapsed between each curve. Upon additional pulsing, the sensor recovered and gave the "normal" behavior shown in Figure 4; however, the magnitude of the response had decreased. The time for recovery to the curve shape in Figure 4 is proportional to the time the sensor had not been used. When idled overnight, the sensor's transient response also demonstrated the behavior shown in Figure 8, but the normal response curve is restored within a few pulses. The magnitude of the square of the equilibrium conductance decreases by about 10-20% after a week of idling. The
(1 1) (12) (13) (14) (15) (16)
Caln, J. R.; Maxwell, L. C. J. Ind. Eng. Chem. 1919, 7 7 , 852. Maffly, R. H. Anal. Biochem. 1966, 2 3 , 252. HolmJensen, I . Anal. Chlm. Acta 1960, 23, 13. James, D. B. Biomed. Eng. 1969, 4 ( 3 ) , 126. Van Kempen, L. H.; Kreuzer, F. Respir. Physiol. 1975, 2 4 , 89. Keln, D. M. J. Chem. Educ. 1960, 37, 14. Symanski, J. S.; Martinchek, G. A.; Bruckensteln, S., unpubllshed work, SUNY at Buffalo, 1977-1982. Sherwood, W. G.; Martinchek, G. A.; Gal-Or, L.; Bruckenstein, S. First Semi-Annual Technlcai Progress Report, United States Bureau of Mines, Grant No. 155007, March 1, 1975. Crank, J. "Mathematics of Diffuslon", 1st ed.; Oxford University Press: London, 1956; Chapter 4. "Internatlonal Critical Tables", 1st ed.; McGraw-HIII: New York, 1928; Vol. 111, p 259. "Internatlonal Crklcal Tables", 1st ed.; McGraw-Hill: New York, 1929; VOI. V I , pp 260-261. Rabe, A. E.; Harris, J. F. J. Chem. Eng. Data. 1963, 8 , 334. Harned, H. S.; Owens, B. B. Physical Chemlstry of Electrolytlc Soiutlons", 2nd ed.; Relnhold: New York, 1950; p 617, pp 589-591. Robinson, R. A.; Stokes, R. H. "Electrolyte Solutions", 2nd ed.; Butterworths: London, 1959; p 463. Shedlovsky, T.; Mac Innes, D. A. J. Am. Chem. SOC. 1935, 57, 1705. "Milil-Q Type I Reagent Grade Water System Operator's Manual"; Continental Water Systems, 1981; pp 17-19.
RECEIVED for review October 15, 1982. Accepted February 4,1983. This work was supported, in part, by the Department of Interior, Bureau of Mines, Division of Health and Safety by Grant No. GO177109 and the Air Force Office of Scientific Research by Grant No. 78-3621.
