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Anal. Chem. 1989, 6 1 , 577-580
Amperometric Sensor for Carbon Dioxide: Design, Characteristics, and Performance John Evans, Derek Pletcher,* and P. Richard G. Warburton Department of Chemistry, T h e University, Southampton SO9 5NH, England
T. Kim Gibbs Neotronics, Ltd., Parsonage Rd., Takeley, Bishops Stortford, Herts CM22 6PV, England A new sensor for atmospherlc carbon dloxlde Is described. I t Is an amperometrlc device based on a porous electrode in a three-electrode cell and the electrolyte Is a copper diamine complex In aqueous potasslum chloride. The platinum cathode, held at constant potentlal, Is used to detect the formation of Cu2+ following the change In the pH of the solutlon when the sensor Is exposed to an atmosphere containing carbon dioxide. The sensor descrlbed Is deslgned to monitor carbon dloxlde concentrations In the range 0-5 %, although with some modlflcatlons, other ranges would be possible. The response to a change In the carbon dloxlde content of the atmosphere Is rapid (about 10 8 ) while the monltored current Is strongly (but nonlinearly) dependent on carbon dioxide concentratbn. Unlike other amperometrlc devices for carbon dloxlde, there Is no interference from oxygen although other acld gases would lead to an lnterferlng response.
INTRODUCTION Traditionally, carbon dioxide in atmospheres has been determined by absorption in base (followed by back titration with acid), infrared spectroscopy, or thermal conductivity (sometimes coupled to separation by gas-liquid chromatography) (1-3). None of these methods is well suited to the development of cheap, portable sensors. Sensors based on a glass p H electrode covered by a gas permeable membrane are also available (4-11) but these potentiometric devices have a response to changes in carbon dioxide concentration that is small and nonlinear. More recently, Bruckenstein and Symanski (12, 13) have described a continuous sensor for carbon dioxide based on the measurement of conductance. None of these sensors has the convenience and favorable characteristics of the amperometric devices for oxygen where the measuring cathode is separated from the gas phase by a gas permeable membrane or porous barrier and usually the current is determined by the diffusion of oxygen to the electrode (14-19). Such oxygen sensors are now manufactured by many companies and, indeed, they have been modified to allow detection of other gases such as carbon monoxide and sulfur dioxide. Not surprisingly, in view of the commercial success of these amperometric devices, the possibilities for determining carbon dioxide by a similar sensor based on its cathodic reduction have been examined. Unfortunately, in aqueous solutions the reduction wave for carbon dioxide is insufficiently separated from hydrogen evolution. Hence, recent studies have emphasised the use of aprotic solvents (20, 21) but even then there is the serious problem of the interference from oxygen which always reduces at far less negative potentials. Despite ingenious attempts to overcome this interference from oxygen, we are aware of no commercial sensors based on an aprotic solvent cell. This paper describes an amperometric sensor for carbon dioxide that is based on quite different chemistry in an aqueous solution. A subsequent paper will report the electroanalytical studies that demonstrate conclusively the
mechanism of operation of this sensor (22).
EXPERIMENTAL SECTION All experiments in this paper were carried out in the three-
electrode sensor described below. The working and counter electrodes were both deposited onto porous PTFE supplied by W. M. Gore Associates. The PTFE was heated on a hotplate and the platinum electrode material was sprayed onto it. The mixture sprayed was 2 g of platinum black (Johnson Matthey Ltd.), 1.45 g of Fluon, and 1g of water. Fluon (Whiteford Plastics Ltd.) is a commercial preparation containing 30% PTFE and a surfactant in water. After preparation the coated PTFE was cured in an oven and disk electrodes could then be cut with a cork borer. All solutions were prepared from water distilled and then deionized by a Millipore system and AnalaR Potassium chloride (BDH Ltd.). The copper(I1) bis( 1,3-propanediamine) complex was prepared from Anal& cupric sulfate and the diamine (Aldrich Chemicals). Stoichiometric amounts of the diamine and copper sulfate were mixed in water and the water was then removed on a rotary evaporator; the solid isolated was recrystallized from ethanoljwater. Identical solutions were obtained by simply dissolving stoichiometric quantities of copper sulfate and 1,3propanediamine. The carbon dioxidelair and carbon dioxide/ nitrogen mixtures were all obtained from BOG Special Gases Ltd. The working electrode potential was controlled by using either a Hitek potentiostat or a battery-poweredhome-built potentiostat. Experiments were carried out at room temperature, 293 f 2 K.
DESIGN OF THE SENSOR The cell is based on a design described in a British patent (23) and the essential components are shown in Figure 1. The working and counter electrodes are both porous PTFE disks coated with a platinum layer as described above. The counter electrode has a wick passing through its center to allow electrolyte to pass from a reservoir (where it is absorbed onto a glass wool pad) into the interelectrode spaces. The working and counter electrodes are separated by two filter pads between which is placed a silver wire to act as the reference electrode. The potential of the sensing electrode is held by using a potentiostat at a controlled value, usually about +0.16 V vs the Ag/AgCl reference electrode, chosen so that the current is zero when the cell is exposed to carbon dioxide free air. The electrolyte throughout the cell is 1 M aqueous potassium chloride containing 2-3 mM copper(I1) bis( 1,3propanediamine) sulfate. The chemistry of the cell is as follows (22). When carbon dioxide free air is fed to the cell, the copper(I1)bis(1,3-propanediamine) ion is stable in solution. Since neither this complex whose reduction potential is -0.57 V vs SCE nor oxygen are reduced at +0.15 V, the sensor current is zero. On the other hand, when the solution is contacted by an atmosphere containing carbon dioxide, its pH decreases due to the reactions C02
+ H20 e H+ + HC03- + 2H+ + C032-
(1)
and Cu2+is formed as the free proton competes for the ligand Cu(NH2CH2CH2CH2NH2)2+ + 4H+ Cu2+ + 2NH3+(CH2)3NH3+ (2)
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ANALYTICAL CHEMISTRY, VOL. 61, NO. 6, MARCH 15, 1989 P ~ M O S P H E R E
POROUS P T F E 9ISC 60 -
-7
I
/ W i C K TO T RA N SFER ELECTRCLY TE
Figure 1.
COUNTER ELECTRODE POROUS C A R B O N OR PLATINUM ELECTRODES
Schematic diagram of the sensor.
Since the Cu2+/CuC12-couple has a standard potential of +0.18 V vs SCE, the reduction Cu2+
+ 2Cl- + e-
-
CuC12-
(3)
takes place at +0.15 V vs Ag/AgCl and a current is observed from the sensor. It should be noted that both Cu(I1) and Cu(1) exist in chloride solutions as mixtures of aquo and chloro complexes. In practice, however, i-E curves show a single reduction wave indicating that the complexes are in equilibrium and, hence, for convenience, in this paper the couple is written in terms of only the major species. The current for reaction 3 depends on the percent carbon dioxide in the atmosphere. The chemical changes (eq 1 and 2 ) are completely reversible so that when the atmosphere is changed back to one free of carbon dioxide, the current reverts to zero. The choice of ligand is, of course, critical. It is necessary to match the pH for the chemical change (2) to that caused by different concentrations of carbon dioxide in solution. Simple computations of the equilibrium concentrations demonstrate that the ligand should be a weak base and a good ligand for copper(I1). 1,3-Propanediamine was found to be the most suitable. The copper complexes of ammonia, primary amines, and other diamines all either had unsuitable stability constants or pKB values or led to precipitates in the cell (22). Indeed, even with the preferred ligand it was necessary to use a relatively low concentration of the complex to avoid such precipitation. Several potassium or sodium salts were investigated for the base electrolyte but only chloride and bromide had all the desired properties. A response could be obtained with many salts, for example perchlorate, nitrate, and sulfate, but in these media, the electrode reaction after exposure to carbon dioxide is Cu2+
+ 2e-
-
Cu
(4)
This could lead to undesirable changes in the cathode surface as well as irreversible loss of copper(I1). In addition, reaction 4 occurs at a slightly more negative potential where interference from oxygen reduction is a problem. If iodide is used as the electrolyte, the copper(I1) species produced when the solution contacts carbon dioxide spontaneously reduces and it is obvious that iodine and insoluble cuprous iodide are being formed. In contrast, the choice of chloride or bromide has several advantages. In addition to having an influence on equilibrium (21, since Br- and C1- are weak ligands for Cu(II), such electrolytes allow the use of a sensing potential where the reduction of oxygen is minimized. It also ensures that the electrode reaction leads only to copper(1). Moreover the small amount of copper(1) formed a t the monitoring electrode is slowly reoxidized to copper(I1) by the oxygen in the gas feed or the halogen formed at the anode, thereby maintaining a
Figure 2. Steady-state i-E curves for a sensor with a Pt electrode and an electrolyte of 2.5 mM copper(1I)bis(l,3-propanediimine)sulfate and 1 M KCI: (a)carbon dioxide free air (b) 5% carbon dioxide in air.
constant copper(I1) concentration in solution. Potassium chloride is the electrolyte used in most sensors to date. Other transition metals were also considered but only copper(I1) was considered to have all the necessary properties-suitable stability constants, a solution soluble, lower oxidation state, and appropriate redox potentials for the species to be detected without interference from oxygen reduction. Overall the performance of the sensor is clearly very dependent on the choice of an appropriate complex and electrolyte, as well as their concentrations. Moreover we believe that there is surprisingly little freedom in their selection.
RESULTS AND DISCUSSION Figure 2 shows steady-state i-E curves, obtained by using a point by point technique, for a sensor with a porous Pt sensing electrode and an electrolyte consisting of 2.5 mM copper(I1) bis(l,3-propanediamine) sulfate in aqueous 1 M potassium chloride. Curve a is the response when the gas fed to the cell is carbon dioxide free air and curve b is the response when the atmosphere is changed to 5% carbon dioxide in air. It can be seen that there is a very clear difference between the two curves. For a sensor there is the choice between operating at a potential where the current due to air is close to zero, i.e. +0.16 V vs the Ag/AgCl reference electrode, or accepting a background current but increasing the sensitivity by using a more negative potential, e.g. +0.10 V; in most experiments, the former choice was made. While these responses are clearly suitable as a basis for a sensor, there is no well-formed cathodic wave for the reduction of Cu2+after contacting the solution with carbon dioxide. This is in complete contrast to the cyclic voltammetry seen at a polished vitreous carbon disk electrode (22). The difference must arise either because of the use of a porous electrode or because of the increased catalytic activity of the high surface area cathode for oxygen reduction. Figure 3 shows the i-t response of the sensor at +0.16 V vs Ag/AgCl when it is exposed to atmospheres with different carbon dioxide contents. Initially the feed is carbon dioxide free air when the current is zero and then the carbon dioxide content is increased at intervals. It can be seen that the current increases strongly with percent carbon dioxide although a current-concentration plot is not quite linear. Also the lowest concentration of carbon dioxide used, O . l % , does not represent the lowest detection limit and it can be seen that the response time to a 95% change in signal is typically 1 2 s. The recovery time to zero from the higher carbon dioxide levels is longer, ca. 30 s, and on some occasions there is some overshoot when the sensor may take a few minutes to recover the original zero level. The reproducibility
ANALYTICAL CHEMISTRY, VOL. 61, NO. 6, MARCH 15, 1989
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579
c
+
I+ 6n
I
- ,12
I
10
6
1
I
6
1
Time/min
Flgure 3. Sensor response to a series of changes of gas feed, sensor as Figure 2 potential +0.16 V vs Ag/AgCk gas flow rate 150 mL/mln.
gas
contents in N2
20
Timo/min
-5
* % Corbon Diozide
Figure 4. Plots of sensor current vs percent carbon dioxide in the gas feed: 2 mM copper(I1)bls(l,3-propanediamlne)sulfate In 1 M KCI; Pt electrode; potential +0.16 V; gas flow rate 150 mL/mln; current read after 5 min of exposure to each atmosphere: 293 K; (a) 0-2% COz, (b) 0-10% COP.
of the i-t response during increases in carbon dioxide levels is good and sensors have been run successfully for upward of three weeks without decay in performance. Figure 4 shows plots of response vs percent C02 for two carbon dioxide concentration ranges (0-2% and &lo%); again, it can be seen that the current is a strong function of carbon dioxide in the atmosphere but the relationship is not quite linear. It is possible to produce sensors with very similar current/percent C 0 2 characteristics and for each sensor the response to any percent C 0 2 atmosphere changes by less than 5% during the test period of the sensors (Le. >3 weeks). It is believed that the lifetime of the sensor, as currently manufactured, will be determined by water evaporation, and future designs will include features to minimize water loss. The sensor response is also dependent on temperature and commercial sensors will require a temperature compensation circuit. The sensor described above shows some excellent characteristics and good overall performance. Some thought was, however, given to the possible advantages of changing some of the conditions. Firstly, the use of other copper(I1) complexes was considered during the study of the mechanism for +.heoperation of the sensor. No ligand had all the favorable properties of l,&propanediamine and only the use of substituted ethylenediamine for sensors to monitor very high carbon dioxide levels seems to merit further attention (22). A t first sight, an alternative strategy for increasing the sensitivity of the sensor would be to increase the concentration of the complex; at any pH, the concentration of Cu2+in solution is close to being a fixed fraction of the initial complex concentration. Unfortunately, however, at higher concen-
response/
MA
0 2
c02 1% 20% CO 100 ppm
-4 -0.2 +0.23
NO
+0.04
100ppm SO2 100ppm NO2 100ppm CHI 10ppm H2 1%
-10
2
16
Figure 5. Sensor response to a change in gas feed for a sensor with a copper complex concentration of 10.6 mM in 1 M KCI: Pt electrode; potential 4-0.15 V; gas flow rate 200 mL/mln.
4
1 % Curbon Dioxide
12
Table In
Current/pA
0
8
4
+1.4
-5.6 +0.25
comments depends on conditions no long-term influence on C02 response serious interference serious interference
+10
a Response of sensor to various components of common atmospheres.
trations of the complex, precipitation of a copper hydroxide or carbonate species can be seen to occur on contacting the solution with carbon dioxide. Indeed, even with slightly increased copper(I1) bis( 1,&propanediamine) concentrations, other problems arise. Figure 5 shows the i-t response to an increase in percent carbon dioxide when the sensor solution contains 10.6 mM complex. A peak is observed that leads to a greatly increased response time if the steady-state current is to be used as a measure of percent carbon dioxide. These peaks are most prevalent at high concentrations of complex and/or carbon dioxide and this seems to be compatible with the idea of some copper(I1) precipitation within the cell. Carbon electrodes have also been used during this study. Similar current vs percent carbon dioxide plots are obtained and the potential for zero current with carbon dioxide free air is very similar. The carbon electrodes are, however, more fragile during handling and for this reason platinum is strongly preferred. The interference effects from other gases likely to be found in atmospheres were considered; the response of the sensor to various gases in nitrogen is reported in Table I. The response to oxygen is always small and can always be reduced to a negligible level by appropriate choice of conditions. It is, however, also complex since the response to oxygen is typically doubled when the electrolyte pH changes from 9 to 6 (Le. when the solution is contacted by carbon dioxide). Carbon monoxide, nitric oxide, methane, and hydrogen also do not constitute serious interferences since, at levels likely to be met in atmospheres in practice, their presence does not lead to a significant current. Sulfur dioxide and nitrogen dioxide, as well as presumably other acidic or basic gases, are serious interferences. If present in significant quantities they would have to be removed by chemical methods before the
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Anal. Chem. 1989, 6 1 , 580-583
atmosphere enters the sensor. These interferences are, however, almost universal with carbon dioxide sensors. The authors believe that the sensor described in this paper represents a novel and useful approach to monitoring carbon dioxide levels in atmospheres. Although the response of the sensor results from a pH change of the electrolyte, its chemistry, the nature of the response, and the design of the sensor all differ totally from the Severinghaus type carbon dioxide electrode. It is, therefore, not surprising that the characteristics and performance of the two types of sensor are quite different. As the amperometric sensor is developed further, we believe that in some applications, it will show marked advantages. It should further be noted that with an appropriate selection of metal ion and ligand it should be possible to design sensors for other acid gases based on the same principles. Registry No. COz, 124-38-9; KCl, 7447-40-7; Cu(NH2(CH2),NH2)2+,18007-72-2.
LITERATURE CITED Strauss, W. Industrial Gas Cbaning; Pergamon Press: London, 1966. Cheremisinoff, P. N.;Morresi, A. C. Air Pollution Sampling and Analysis Deskbook; Ann Arbor Science: Ann Arbor, MI, 1978. , , Chemical Detection of Gaseous Polluianfs; Ruch, W. E., Ed.; Ann Arbor Science: Ann Arbor, MI, 1968. (4) Ion Selective Nectrode Methodology; Covington, A. K., Ed.: CRC Press: Cleveland. OH, 1979.
(5) Stow, R. W.; Baer, R. F.; Randall, B. F. Arch. Phys. Med. Rehabil. 1957, 5 9 , 646-650. (6) Gertz, K. H.; Loeschke, H. H. Naturwissenschaften 1958, 45, 160-161. (7) Severinghaus, J. W.; Bradley, A. F. J. Appl. Physiol. 1958, 13, 515-520. (8) Severinghaus, J. W. Ann. N . Y . Acad. Sci. 1958. 148, 115-137. (9) Czaban, J. D. Anal. Cbem. 1985, 5 7 , 345A-356A. (10) Jensen, J. A.; Rechnitz, G. A. Anal. Chem. 1979, 5 1 , 1972-1977. (11) Kobos, R. K.; Parks, S. J.; Meyerhoff, M. E. Anal. Chem. 1982, 5 4 , 1976- 1980. (12) Bruckenstein, S.;Symanski, J. S . Anal. Chem. 1988, 5 8 , 1776-1777. (13) Bruckenstein, S.; Symanski, J. S. J . Chem. SOC. faraday Trans. 7 1986, 82, 1105-1116. (14) The Polarogragbic Oxygen Sensor; its Theory of Operation and its Applications in Biology, Medicine and Technology; Fatt, I., Ed.; CRC Press: Cleveland, OH, 1976. (15) Hitchmann, M. L. Measurement of Dissolved Oxygen; John Wiiey and Sons: Chichester, 1978. (16) Dietz, H.; Haeker, W.: Jahnke, H. Adv. Nectrochem. Electrochem. Eng. 1977, 10, 1-90, (17) Clark, L. C.; Wold, R.; Granger, D.; Taylor, 2 . J. Appl. Physiol. 1953, 6 189-193 .1
(18) Bergman, I.; Windle. D. A. Ann. Occup. Hyg. 1972, 15, 329-339. (19) Bergman, I.Ann. Occup. Hyg. 1975, 18, 53-65. (20) Zook, J. D.; Venkatasetty. H. V. Proceedings of "Transducers' 85" 1985, 326-328. (21) Albery, W. J.; Barron, P. J . Hectroanal. Chem. 1982, 138. 79-87. (22) Evans, J.; Pletcher, D.: Warburton, P. R. G.: Gibbs, T. K. J . Nectroanal. Chem., in press. (23) British Patent 2,094,005.
RECEIVED for review July 20, 1988. Accepted December 12, 1988.
Fluorescence Lifetime Selectivity in Excitation-Emission Matrices for Qualitative Analysis of a Two-Component System David W. Millican and Linda B. McGown* Department of Chemistry, P. M . Gross Chemical Laboratory, Duke University, Durham, North Carolina 27706
Steady-state fluorescence excitation-mission matrices (EEMs), and phase-resolved EEMs (PREEMs) collected at modulation frequencies of 6, 18, and 30 MHz, were used for qualitative analysls of mixtures of benzo[k#luoranthene ( T = 8 ns) and benzo[b#iuoranthene( T = 29 ns) in ethanol. The EEMs of the individual components were extracted from mixture EEMs by means of wavelength component vectorgram (WCV) analysis. Phase resolution was found to be superior to steady-state measurements for extraction of the component spectra, for mixtures in which the intensity contributions from the two components are unequal.
INTRODUCTION Fluorescence spectral information can be fully represented by means of the excitation-emission matrix (EEM) ( I ) . Studies have demonstrated the resolution of EEMs for twocomponent mixtures into the EEMs of the individual components ( 2 ) ,based on linear transformations of eigenvectors that are obtained from the covariance matrices of the EEMs. Predictably, the ability to resolve EEMs of mixtures into the EEMs of the individual components decreases as the degree of overlap between the component spectra increases. Resolution can be improved by means of a third, independent dimension of information. Our approach has been to use phase-resolved fluorescence spectroscopy (PRFS) to incorporate fluorescence lifetime selectivity into EEM data ( 3 , 4 ) . 0003-2700/89/036 1-0580$01.50/0
The resulting data format, referred to as the phase-resolved EEM (PREEM), is analogous to the steady-state EEM in that intensity is represented as a function of both emission wavelength and excitation wavelength. Because intensity in the PREEM is also a function of fluorescence lifetime, the contribution of each component to the total intensity at a is a function of the fluorescence lifetime of the given (A,, )A, component; therefore, the experimental parameters of PRFS, excitation modulation frequency and detector phase angle, can be used to selectively enhance or reduce the fluorescence contribution of individual components in the PREEM as a function of fluorescence lifetime ( 5 ) . In this paper, we compare the PREEM approach with the steady-state EEM approach for the qualitative analysis of mixtures of benzo[k]fluoranthene (B[k]F) and benzo[b]fluoranthene (B[b]F). The PREEMs were collected at three modulation frequencies, with the detector phase angle adjusted to suppress scattered light (3). The EEMs of the individual components were extracted from the PREEMs and EEMs of the mixtures, by means of the wavelength component vectorgram (WCV) technique (6). Uncorrected matrix correlation was used to compare the extracted EEMs with steady-state EEMs of the individual components.
THEORY The principles of PRFS have been thoroughly discussed elsewhere (7, 8). Briefly, the sample is excited with sinusoidally modulated light, resulting in fluorescence emission that is modulated at the same frequency, but phase-shifted @ 1989 American Chemical Society
