1383
Anal. Chem. 1882, 64, 1383-1389
Equilibrium Studies on Colorimetric Plastic Film Sensors for Carbon Dioxide Andrew Mills,’ Qing Chang, and Neil McMurray Department of Chemistry, University College of Swanaea, Singleton Park, Swansea SA2 8PP, U.K.
The equilibrium responses of three new colorimetric plastic flim sensors for COzas a function of % C02 and temperature are descrlbed. The results fH a model In which there is a 1:l equilibrium reaction between the deprotonated form of the dye (present in the flim as an Ion pair) and COz. The 0-50% and 0 4 0 % response and recovery tlmes of each of these films when exposed to an alternatlng atmosphere of air and 5 % C02 are determined and in two cases are typically less than 3 8. The shelf IHe of the films is long (many months); however, prolonged use of the films leads to the permanent generatlon of the protonated form of the dye over a period of 20-100 h. A posslbie c a w of thls latter effect is discussed.
INTRODUCTION The qualitative and quantitative detection of gaseous carbon dioxide is an important feature in medicine, environmental monitoring, and monitoring in many industries.132 A popular method193 for the detection of carbon dioxide in the gas phase, i.e. CO,(g), utilizes the infrared absorption of C02(g). Devices which work on this principle are typically fast-acting (0-90?5 response in 99%) and included air, C02, and a 5% C02/95% air gas mixture, all of which were purchased from BOC, U.K. All other gas mixtures were generated using a gas blender (Model No. 852Vl-B, Signal Instruments Co., U.K.). Spectrophotometric measurements were carried out using either a Philips PUS620 UV/vis/NIR spectrophotometer or a Philips PU 8720 UVIvis scanning spectrophotometer. Methods. All the plastic films prepared had the general composition dye/phase-transfer agent/polymer/plasticizer/support. The methods of preparation of these films were as follows. Film Solution X: (m-Cresol PurplelTetraoctylammonium HydroxidelEthyl CelluloseITributyl PhosphatelGlass Slide). A solution (solution I) was prepared by adding 0.012 g of mcresol purple to 1 mL of a solution containing 0.5 mol dm-3tetraoctylammonium hydroxide in methanol. A further 2.5 mL of (20) Miller, I. T.; Springall, H. D. Sidwick’s Organic Chemistry of Nitrogen, 3rd ed.; Claredon Press: Oxford, U.K.1966; p 117.
methanol was then added to provide the appropriate concentration of dye for making absorbance measurements on the dried film. A second solution (solution 11) was prepared in which 10 g of ethyl cellulose was dissolved in a solution containing 20 mL of ethanol and 80 mL of toluene. The film solution X comprised 1mL of solution I, 10g of solution 11,lmL of tributyl phosphate, and 1 mL of a 0.5 mol dm-3tetraoctylammonium hydroxide in methanol solution. Film Solution Y: (Cresol RedlTetraoctylammonium HydroxidelEthy1Cellulose/TributylPhosphate/GlassSlide). The preparation of this film solution was identical to that used to prepare film solution X, with the exception that cresol red was used instead of m-cresol purple. Film Solution Z: (Cresol RedlTetraoctylammonium HydroxidelPoly(viny1butyral)/Tris(2-ethylhexyl) Phosphate/ Glass Slide). A solution (solution 111) was prepared by adding 0.025 g of cresol red to 1.78 mL of a solution containing 0.5 mol dm-3 tetraoctylammonium hydroxide in methanol. A further 6.3 mL of methanol was then added to provide the appropriate concentration of dye for making absorbance measurements on the dried film. A second solution (solutionIV) was prepared in which 10 g of poly(viny1 butyral) was dissolved in 100 mL of ethanol. The film solution,Z, comprised 1.2g of solution 111,lOg of solution IV, 2.4 g of tris(2-ethylhexyl) phosphate, and 1 mL of a 0.5 mol dm+ tetraoctylammonium hydroxide in methanol solution. In the preparation of the dried, plastic, colorimetric films for COn analysis, i.e. X, Y, and Z, used in the work described below, a sample of the film solution in question was placed on a microscope slide. A wet film of uniform thickness was created by drawing the microcope slide plus solution underneath a razor blade clamped ca. 100 pm above the glass of the slide. The ca. 100-pm wet film was allowed to dry under ambient conditions, and for all three films the final thickness of the dried film was ca. 20 Hm. In some cases a 100-pm-thick brass sheet with a rectangular hole (0.5 X 1cm) was used as a template to cast films which were typically 20 pm when dried. Film thicknesses were usually measured using a digital electronic micrometer with a resolution of f l pm (RS Components Ltd., U.K.). All the dried films (Le. X, Y, and Z) appear blue in transmitted light in air, indicatingthat the dyes are primarily in their deprotonated forms, i.e. as D-(seeeq 6). In addition, of the film preparations described above, none showed any tendency to lose indicator dye upon exposure to water even over periods of many days. The absorbance spectrum of each of the films as a function of the % COz in the gas phase was measured using a Philips PU8720 UV/vis scanning spectrophotometer. In this case the filmmicroscope slide sample was incorportated into a 1-cm plastic cuvette (Hughes and Hughes Ltd, U.K.; cutoff h < 300 nm) in which itself was incorporated a 1-mm plastic tube, glued to one of the curvette’scorners and used to conveythe gas stream which set the composition of the gas phase in the cell; the whole arrangement is illustrated in Figure 1A. From Figure 1A it can be seen that the film under test (on a microscope slide) was positioned midway down the length of the 1-cm plastic cuvette, parallel to the cell’s optical faces and perpendicular to the interrogating light beam from the spectrophotometer; the plastic cell had been previously split midway along its length in order to provide a mounting for the film-microscope slide combination. The same cell as illustrated in Figure 1A was used for the routine measurement of the variation of absorbances of the deprotonated forms of the dyes (and, therefore, R;vide infra) in the different films as a function of % COz(g),at ambient temperature. However, a different system was used to make the same measurements as a function of temperature. In this case the film-microscope slide combination was glued to one of the faces of a cylindical thermostated quartz cell and covered with half of a 1-cmplastic cuvette glued into place and incorporated with the necessary gas feed pipe. The plastic film-thermostated cell arrangement was positioned in the spectrophotometer so that the film was perpendicular to the interrogating light beam, and the whole arrangement is illustrated in Figure 1B. In this work a Philips PU8620 UVIvislNIR spectrophotometer was used to make the absorbance measurements and the wavelength of the interrogating light used was 600 nm for film X and 590 nm for films Y and Z.
ANALYTICAL CHEMISTRY, VOL. 64, NO. 13, JULY 1, 1992
A
Table I. Dye Characteristics in Aqueous Solution and in the Films X, Y, and Z
aqueous solution m-cresol purple cresol red D-
DH
\
1385
film
X
Y
Z
593 406
592 422
591 431
A(”) 572 429
574 437
Dye PKa 8.9
B
WATER IN 1
,(a)
8.2
A common feature of each of the films prepared is use of a phase-transfer agent {Q+OH-)to extract, as the ion pair (Q+D-J (or, (Q+D-*xH,O);vide infra) the ionic deprotonated form of the dye, D-, from a highly polar protic medium (Le. methanol/ethanol) into the less polar medium of the polymer/plasticizer. Evidence for such ion pair formation, and not solubilizationof the dye in microdroplets of methanol or water present in the plastic film, is provided by the clear bathochromic (red) shift in the UV/vis absorption maximum of the deprotonated form of the dye on going from aqueoussolution to the film, as can be seen from the results given in Table I.
RESULTS AND DISCUSSION
.f G A S IN
The observed changes in absorption spectrum as a function of PCO,are illustrated in Figure 2a-c for films X, Y, and Z, respectively. These changes were always completely reversible and highly reproducible. From these results, therefore, it does appear that the prepared plastic films X, Y, and Z will act as colorimetric sensors for CO,(g). The general properties of these films as reversible colorimetric film sensors can be illustrated very well by considering just one of these films, film X, in detail. In general,the results of parallel experiments carried out using each of the other two films (i.e. Y and Z) are then summarized in table form.
Y t
WATER OUT
Figure 1. Optical arrangement used for measuring the absorbance of an (a) film and (b) microscope slide combination, for each of the films, as a function of % Con. The optical system (A) was used in the measurement of the absorbance spectrum of each of the films as a function of % COdg)at ambient temperature. The plastic cell is split midway along its length in order to provide a mounting for the filmmicroscope slide combination. The optical system (B) was used in the measurement of the absorbance of the basic form of the dyes in each of the films as a function of (i) YOC02(g)at various temperatures and (ii) response and recovery time measurements in which the YO C02(g) was varied.
The response and recovery absorbance vs time profiles for each of the films were recorded using the plastic film-thermostated cell arrangement illustrated in Figure 1 B and an optical bench system, comprising a quartz-iodide light source, monochromator, photomultiplier, amplifier, and microcomputer,the details of which have been reported previously.21 The principal absorption peaks of the dyes m-cresol purple and cresol red in aqueous solution and in the different films are given in Table I, along with the experimentallydetermined pK, values for the two dyes in aqueous solution. The absorption spectra of the protonated forms of the different dyes in the films were generated by exposing the films to an atmosphere of pure CO2 gas. (21)Mills, A.; Russell, T. J. Chem. SOC.,Faraday Trans. 1991, 87, 313-318.
As noted earlier, in a conventional C02colorimetric sensor the presence of water is vital. However, it is well established that when a phase-transfer agent is used to extract an anion from an aqueous solution into an organic medium, in many cases a small number of molecules of water of solvation22 are found to be associated with the ion pair. Thus, in our plastic films, X, Y, and Z, it is probably more correct to envisage the ion pair of D- as (Q+D-*xH20)rather than (Q+D-). A simple general explanation of how the films may work as colorimetric sensors for CO,(g) can be summarized by the following equilibrium reactions:
(Q+D-*xH,O) + CO,(g)
K7 ~t
(Q+D-*xH,O*CO,(aq)) (7)
K9
(Q+D-*(x- 1)H20*H2C03) FI (Q+HCO?(x - l)H,O*HD) (9)
If it is assumed that K, and KSare very small and Kg is very large, it follows from the equilibrium reactions 7-9 that the relationship between PCO,and the concentrations (and, therefore, absorbances) of the two main species present, (Q+D-*xH20)and (Q+HCO3-*(x- l)H20*HD),which are likely (22)Dehmlow, E. V.; Dehmlow, S. S. Monographs in Modern Chemistry: Phase Transfer Catalysis; Verlag Chemie: Weinheim, 1980; Chapter 1 (see also references therein).
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ANALYTICAL CHEMISTRY, VOL. 64, NO. 13, JULY 1, 1992
A
I 200
300
500
400
600
700
800
900
700
800
900
0
5
08
-
07
-
06
-
2 t o 5 P
20
-
04
-
03
-
02
-
01
15
10 %COS
Wavelenglh(nm)
I
I 200
300
500
400
000
0
Wevelenglh(nm) 13
2
4
6
%COS
Flgure 3. Plots of (a, top) the absorbance of film X at 600 nm vs % COz and (b, bottom) R vs % COP(see eq 12). Data are from Figure
I
2a.
I 200
300
500 600 Wauelengih(nn)
400
700
800
900
Figure 2. UV/visibie absorption spectra of the different plastic films as a function of % C o n . (a, Top) film X: 65 pm thick; % COz levels used(fromtoptobottom)0.03% (air),0.3%,0.4%, 1 % , 5 % ,l o % , and 100%. (b, Mlddle) film Y: 30 pm thick; % COz levels used (from top to bottom)0.03% (air), 1 % , 3%, 5 % , l o % , 20%, and 100%. (c, Bottom) film 2: 20 pm thlck; % COz levels used (from top to bottom) 0.03% (air),3%, l o % , 20%, 50%, and 100%.
to be differently colored, will be given by the expression
a’Pco2= R = [{Q+HCOc(x- l)HzO.HD)]/ [(Q+D-*xHzO)l(10) where a’ = K7Kag. Under these conditions the overall process can be summarized as follows {Q+D-.xH,OJ + CO,(g) color A
a‘
F!
(Q+HCOP(x- l)HzO-HD) (11) color B
It is possible to test the scheme proposed above for each film using the appropriate data illustrated in Figure 2. To begin with, for each film, the absorbance at the X(max) of the
deprotonated dye ion pair (Le. Abs(Q+D-.xH20))can be plotted against % COZ in the gas stream (total pressure 1 atm), using the data illustrated in Figure 2; for film X this plot is illustrated in Figure 3a. From the data in Figure 2a it appears that in film X the protonated form of the dye, i.e. (Q+HC03-.(x- 1)HZO.HD) does not absorb significantly at h(max)for (Q+D-.xHzOJ.Assuming(Q+D-*xH20) obeys Beer’s law it follows that the term R = [deprotonated dye in the film]/[protonated dye in the film] = [(Q+HC03-4x 1)H20.HD)I/[(Q+D-.xHzO)l (see eq 10)can be calculated from the associated experimental data using the following expression a t any 7% COSused:
R = (Abs(Q+D-.xH,OJ0- AbS(Q+D--xH,O))/ (AbS(Q+D-.xH,O) - Abs(b1ank)) (12) where Abs((Q+D-.xHzO)ois the value of Abs(Q+D--xHzO)for 76 COz = 0 andAbs(b1ank)is the absorbance of the microscope slide alone. A plot of R vs 76 COSfor film X is illustrated in Figure 3b and appears to be a good straight line (see Table I1 for the results of a least squares analysis of the data). The linear relationship between R and 7% COz was also confirmed for the other two films (Le. Y and Z), and the results of a least squares analysis of the R vs % COz plot for each of the three films, X, Y, and Z, is given in Table 11. The straight-line relationship between R and 5% COS observed for each of our films is a typical characteristic of a direct sensor in which there is a 1:l equilibrium reaction between the analyte and the immobilized reagent,= a situation which is proposed in the reaction scheme described by eqs 7-9 and summarized by eq 11. These findings, therefore, (23) Seitz, W.
R.A n d . Chem. 1984, 56,16A-34A.
ANALYTICAL CHEMISTRY, VOL. 64, NO. 13, JULY 1, 1992
Table 11. Results of Least Squares Analyses' of Various Plots of Experimental Data Associated with the Different Films plot n m C r Film X Rb vs % COz 8 0.75 f 0.02 0.04 f 0.05 0.9971
1387
0 20
~
In a' vs
T1 6
(9.3 f 0.3) X lo3
-30 i 1
0.9976
Rb vs % CO2
Film Y 0.25 f 0.01 (8.5 i 0.3) X lo3
(8.7 f 5.2) X -30 i 1
0.9937 0.9969
Rb ~8 % CO2 9
Film Z (6.4 0.1) X (3.9 f 0.3) X 103
7 In a' vs T1 6
In a' vs
T1
6
*
c
f 0 15
:
z
0 10
(2.6 i 0.6) -15 f 1
X
0.9996 0.9849
n = number of points, m = gradient, c = intercept, and P = correlation coefficient. For R values D- > OH- and this is consistent with the findings of others.22 From eqs 10 and 12 of the model, when R = [(Q+HC03-+ - 1)H20.HD]]/ [{Q+D--xH20]]= 1,the colorimetric film will be posied a t the halfway in its color change. Thus, from the results in Table 11, it would appear that films X, Y and Z are halfway through their overall respective color changes when the 5% COz is 1.25 7% , 3.65 ?6 , and 15.2 7% , respectively. The gradients of the R vs 7% COZplots (from eq 10, m = a') also provide an indication of the sensitivity of these different films to 7'% COS. From either set of data it appears that film Z exhibits a much lower sensitivity toward C02 than the other two films. From the results for film Y, which uses the same dye (cresol red) as film Z but encapsulated in a different plastic medium, it appears that the COZ insensitivity of film Z is associated with the change in plastic medium. It is likely that with a change of reaction medium there will be an associated changed in a number of physical characteristics, such as dielectric constant and hydrophobicity, which define the environment encapsulating the dye and each or all of which could effect reactions 7-9 and, therefore, the value of
005
I
I 5
0
20
15
10
ZCO, 50
a: 2 5
0 0
0
2
E
4
zco,
Flgure 4. (a, Top) effect of temperatureon the change of absorbance (at 600 nm) vs % C02for film X (18 Mm thlck). (b, Bottom) subsequent plot of R vs % COz using the data in (a). Temperatures used were (0) 50 O C , (A)45 OC,(0)40.5 OC, (0)34 OC, (X) 28 O C and (+) 18.8 O C , respecttveiy.
a'.
For each of the films, a series of experiments were carried out in which the equilibrium absorbance due to the deprotonated dye anion-quaternary cation ion pair, i.e. Abs{Q+D-.xH20},as a function of ?6 COSwas itself studied as a function of temperature. The observed absorbance vs ?6 COZcurves for a range of different temperatures for film X are illustrated in Figure 4a, along with the subsequent replotting of each of these Abs({Q+D--xHzO)vs 7% COZprofiles in the form of R vs ?6 COZ profiles, illustrated in Figure 4b. As noted earlier, the gradients of these latter profiles are taken to be equal to a' (see eq 10). Since a' = K&&10 and if we assume that the enthalpy and entropy changesassociated with each of the equilibrium contants involved are invariant over the temperature range studied, from simple thermodynamics it follows that a plot In a' vs T1 should be a good straight line. Such a plot for film X is illustrated in Figure 5 and appears to confirm this model prediction, and the same was found to be true for the other two films. The results of a least squares analysis of data arising from this type of plot for each of the three films are summarized in Table 11. It can be shown that the gradient (m)of this plot should be -AH/R and the intercept ( c ) ASIR - In 100 (if Pco2 in eq 10 is taken to be in units of atmospheres). Thus, in Table 111, we also report for each of the films values for AH and A S for the overall process summarized by the equilibrium process (12).
30
31
32
33
34
35
T ' / ( l o ' K-')
Figure 5. Plot of in a' vs T-' where a' is the gradlent of an R vs % COzplot and is a functionof temperature. Values for a' were obtained from least squares analyses of the plots illustrated in Flgure 4b.
The response and recovery time associated with each of the prepared films were studied using an alternating gas supply of air and 5 % COzin air with exposure times of 60 and 15 s, respectively. The alternation of the two gas supplies was achieved using two solenoid valves each connected to a separate air actuator (Nupro Co.); and the valves were controlled electronically. The observed changes in Abs({Q+D-.xH20)vs time for each of the three films, X, Y and Z, are illustrated in Figure 6a-c, respectively, and the 0-5076 and 0-9096 response and recovery times are given in Table IV. From the results of this work it appears that films Y and Z exhibit response and recovery times which are almost
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ANALYTICAL CHEMISTRY, VOL. 64, NO. 13, JULY 1, 1992
Table IV. Film Resuonse and Recovery Timesa
Table 111. Thermodynamic Data Associated with the Eauilibrium Resuonses of the Three Colorimetric Films thermodynamic parameterhnits AHa/kJ mol-' ASb/J mol-' K-'
film X
film Y
film Z
71 f 2 -211 f 7
77f2 -211 f 7
responsels
50 %
90 %
50 %
90 ?A
x
0.8 0.8 1.3
2.6 3.0 2.6
9.4 1.3 0.8
31.0 3.2 2.9
~
32 f 2 -86 f 6
Y 2
Calculated using the results of a least squares analysis of the plot In d v s T1data (see Table 11),taking m = -AH/R. Calculated using data (see the results of a least squares analysis of the plot In a' vs T1 Table 11),taking c = -AS/R - In 100. 0
*
0.400
recovery/s
film tvDe
a
Results taken from the data illustrated in Figure 6a-c.
with the dye (m-cresol purple) rather than the nature of the plastic film. This observed difference in response and recovery times may be associated with the parallel set of equilibria involving COZand the excess phase-transfer agent present, i.e. (Q+OH-.xH,O} + CO,(g)
a"
G
(Q+HCOpxH,O}
(13)
coupled to the exchange reaction
-
( Q + H C O ~ ~ H ,+ O (Q+D-.~H,o} } color A
(Q+HCOp(x- l)H,O.HD} color B
0.220 I 0.0
100.0
200.0
tis 0.620
,
0.380
1
0.260 0.0
100.0
200 0 US
0.435
I
0.345 I 0.0
I
200.0
100.0
tis
Figure 6. Absorbance vs time plots for each of the three flims, (a, top) X, (b, middle) Y, and (c, bottom) 2 (each 20 Nm thick), when exposed to an alternating gas supply of air (60 s) and 5 % C02 (15 8 ) at ambient temperature. The absorbances were monkored at 600, 590, and 590 nm for films X, Y, and 2 , respectively.
symmetrical, whereas for film X this is clearly not the case. In the case of film X, the observed lack of symmetry between the response and recovery profiles appears to be associated
+ {&+OH-.xH,O} (14)
which, if its occurs, is likely to be slow. With some film formulations it is possible that the (Q+HCO~-.XHZOJ generated, via reaction 13, upon exposure of the film to COz, can act as a pool of COZ,via reaction 14, and lead to an observed slower recovery than response time for the film. Further work is in progress in an attempt to elucidate the factors which govern the response and recovery times of these films. The longevity of these films both in use and on the shelf represents an important aspect with respect to their possible future widespread application. Our work shows that these films may be stored in air, in sealed glass bottles, for many months with no loss in color or in response and recovery time. It has been found, however, that prolonged exposure to bright sunlight (particularly if the dye is in its protonated form) can lead to dye bleaching; however, this appears to be due to an inefficient photochemical process and under ambient room light the dyes in the films are stable for many weeks. As with most materials containing a dyestuff, exposure to strong, gaseous oxidizing agents (e.g. ozone or NOz) will also bleach the films. The longevity of each of the films in use was tested using the same system for providing an alternating environment of gases, containing different levels of COZ,as was used for generating the data illustrated in Figure 6. For each of the films the Abs(Q+D-aHzO}vs time profiles were recorded over a period of days using this system and the results for film X are illustrated in Figure 7. Similar results were obtained for film Y, but film Z appeared to only show a loss in color after ca. 100 h of constant use, rather than ca. 20-30 h for films X and Y. As illustrated in Figure 7, prolonged use of any of the films eventually leads to the disappearance in the absorbance associated with the deprotonated form of the dye ion pair and the permanent appearance of the protonated form of the dye. Thus, for each film, film fading due to prolonged use is associated with an acidification of the film. The original color of each of the films (due to (Q+D--xHzO})can be readily regenerated through the addition of a small amount of the tetraoctylammonium hydroxide phase-transfer agent. From the results of our work so far it appears that film fading with prolonged use is associated with the irreversible loss of the tetraoctylammonium hydroxide which in turn may be associated with the loss of water in the film. (It is known that tetraoctylammonium hydroxide will degrade with time, via a Hofmann elimination reaction with the formation of a tertiary amine and an olefin, and that the rate of this process may be enhanced by decreasing the amount of water
ANALYTICAL CHEMISTRY, VOL. 64. NO. 13, JULY 1. 1992
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low that the dye is permanently held in its protonated (yellow) form and the films will give the appearance of being COz insensitive.
CONCLUSION A pH-sensitive dye such as m-cresol purple or cresol red
01
::A++
+
+
+
t
I
present.24) In support of this suggestion, we have found that the deprotonated form of the dye can be permanently generated in a film if it is heated above 90 "C, placed in a vacuum, or exposed to very dry nitrogen over a period of >12 h. It has also been found that the lifetime of a film is extended by increasing the excess of tetraoctylammonium hydroxide present in the film. Although the greater longevity of film Z, compared with films X and Y, indicates that the nature of the plastic medium is an important factor, this finding may simply be associated with a greater water content or ability to retain water for film Z compared with the other two films. Further work continues in this area. Gases, such as NO2 and S02, which react with water to form acids will also interfere with the response of the film sensors. If these gases are present at high enough levels in the ambient air, the steady-state pH of the films may be so (24) Gordon,J. E.;Varughese,P.J.Chem. Soc., Chem. Commun. 1971, 1160-1161.
can be incorporated into a plastic film using a phase-transfer agent and function as a sensing element for COZin a test system. The response of such a film appears to be similar to that of a direct sensor in which there is a 1:l equilibrium reaction between the analyte and the immobilized dye. The films are not water soluble and have a long shelf life; however, prolonged use leads to the permanent generation of the protonated form of the dye over a period of 20-100 h, possibly due to the decomposition of the phase-transfer agent which, in turn, may be related to the amount of water present in the film. The 0-50% and 0-90% response and recovery times of each of these films when exposed to an alternating atmosphere of air and 5% C02 are in two cases typically less than 3 s. These films are cheap to prepare, fast-reacting, reversible sensors for COz. Recent work by our group has shown that it is also possible to incorporate into plastic films flourescent dyestuffs which are also PCO,or 02 sensitive. The novel use of a phase-transfer agent in these plastic films, which is responsible for many of the film's favorable properties, may lead to a wealth of cheap, near-solid-state colorimetric sensors of gases of clinical and/or industrial and environmental importance.
ACKNOWLEDGMENT We gratefully acknowledge support of this research by Abbey Biosystems (U.K.).
RECEIVED for review December 5, 1991. Accepted March 16, 1992. Registry No. Carbon dioxide, 124-38-9;cresol purple, 230301-7; cresol red, 1733-12-6; tetraoctylammonium hydroxide,
17756-58-0;ethyl cellulose, 9004-57-3; tributyl phosphate, 12673-8; tris(2-ethylhexyl) phosphate, 78-42-2.
