Anal. Chem. lS88, 6 0 , 226-230
226
04-1; sodium diethyldithiocarbamate, 148185; cycloate, 113423-2; vernolate, 1929-77-7;triallate, 2303-17-5;monolinuron, 1746-81-2; difenoxuron, 1421432-5;metoxuron, 19937-59-8;metobromuron, 3060-89-7;chlortoluron, 15545-48-9;diflubenzuron, 35367-38-5; fenuron. 101-42-8.
LITERATURE CITED Moye, H. A.; Scherer, S. J.; St. John, P. A. Anal. Lett. Ig79, 10,
1049-1073. US. Environmental Protection Agency, Method 531, 1985,EPA 600/4-
a51054. Moye, H. A.; St. John, P. A. ACS Symp. Ser. 1980, No. 136,
89-102. Move. H. A.: Miles, C. J.: Scherer, S. J. J , Aorlc. Food Chem. 1083, 31; 69-72. Krull, I. S.;Lacourse, W. R. I n Reaction Detection in Li9uM Chromatography; Krull, I. S., Ed.; Marcel Dekker: New York, 1986. LuchtefeM, R. G. J. Chromatogr. Sci. 1985,23,516-520. Engethardt, H.; Neue, U. D. Chromatographia 1982, 15, 403-408. Crank, G.;Mursyidl, A. Aust. J. Chem. 1982, 35, 775-764. Federal Register, 1984,Friday, October 26,Part V I I I , Environmental Protection Agency, 198-199.
(10) Scholten, A. H. M. T.; Brinkman, U. A. Th.; Frei, R. W. Anal. Chim.
Acta 1080, 114, 137-146. (1 1) Scholten, A. H. M. T.; Frei, R . W. J. Chromatogr. 1979, 176,
349-357. (12) Freeman, P. K.; McCarthy, K. D. J. Agric. Food Chem. 1984, 32, 873-677. (13) Freeman, P. K.; Ndip, E. M. N. J. Agric. Food Chem. 1984, 32, 877-aai. (14) Mazzocchi, P. H.; Rao, M. P. J. Agric. food Chem. 1972, 20, 957-959. (15) Kotzias, D.; Korte, F. Ecotoxicol. Environ. Saf. 1981, 5 , 503-512. (16) Poulsen, J. R.; Birks, K. S.; Gandelman, M. S.; Blrks, J. W. Chromatographia 1986,22,231-234. (17) Batley, G. E. Anal. Chem. 1984,5 6 , 2261-2262. (18)Hancock, K. G.;Dickinson, D. A. J. Org. Chem. 1975,4 0 , 969-970. (19) Krause, R. T. J. Chromatogr. 1979, 185, 615-624.
R~~~
for review March 30,1987, Resubmitted September 2, 1987. Accepted September 2, 1987. This study was supported by Grant No. WM 151 from the Florida Department of Envkommdal Regulation. Florida Agricultural Experiment Station Journal Series No. 8138.
Kinetics of Ion Pair Extraction Frederick F. Cantwell*' and Henry Freiser Department of Chemistry, University of Arizona, Tucson, Arizona 85721
The rate of extractlon into chloroform of tetrabutyiammonlum picrate (OP) In a rapldly stirred mlxture of chloroform and aqueous phases Is studled, with the ald of a porous Teflon membrane phase separator, by measurlng the Increase In absorbance of ihe chloroform phase ( A , ) with thne ( t ) . Because of the very high extractlon rates generated In these experknents, mathematical deconvdutlon Is necessary to eliminate the band broadening effects due to flow through the system. The firstorder extractlon rate constant (0.4, f 0.1, 8 - ' ) calculated from the steady-state portion of the deconvolved A , vs t curve Is Independent of the concentratlon of excess Ion 0' or P-, over the 20-fold concentratlon range studied. Extractlon rate is controlled by dlffusional mass transfer through the Nernst dmuslon layers rather than by the rate of the ion pairlng chemical reaction Itself.
While the equilibrium aspects of ion-pair extraction have been extensively studied (1-4), the kinetic aspects have received much less attention. Under conditions of slow stirring with neither the aqueous nor the organic phase dispersed, the ion-pair extraction of dextromethorphan hydrobromide and of quaternary ammonium bromides had forward first-order s-l (5,6). The rate of extraction rate constants of to was governed by diffusional mass transport rather than by the rate of the chemical reaction between cation and anion to form the ion pair. Later workers noted that slow mass transfer resulting from the slow stirring used in these earlier studies makes it impossible to observe the rate of even moderately slow chemical reactions. In order to obtain faster mass transfer, they employed the ascending/descending drop technique (7-12). For Permanent address: Department of Chemistry, University of Alberta, Edmonton, Alberta, Canada T6G 2G2.
0003-2700/88/0360-0226$01 SO/O
some of the systems studied it was claimed that the ion-pair formation reaction was, in fact, the slow step (8, 12). Although the occurrence of a slow chemical reaction is well-known in the extraction of metal-ligand complexes (13, 14), the observation of a slow ion-pairing reaction is unexpected. Because of this interesting observation and because so few studies have been performed on the kinetics of ion-pair extraction, we have undertaken an investigation of the rate of extraction of tetrabutylammonium picrate using a rapid stirring technique (15) in which the interfacial area between chloroform and aqueous phase is large (16)and mass transfer is very fast.
THEORY The extraction rate of tetraalkylammonium picrate ion pairs (QP)is governed by both mass transfer rate and chemical reaction rate (ion pair formation), with the slower of the two making a greater contribution (14, 17, 18). Chemical Reaction Regime. The ion-pairing chemical reaction could conceivably be occurring in the aqueous phase, at the liquid-liquid interface or in the organic phase, as shown in eq 1-3. Species without a subscript are in the aqueous
M.T.
kl
Q+ + P- eQ + I , ~+ P - I , ~ M.T.
Q+ + P- a Q+,
(QP),,,
+ P-,
M.T.
(&PIo
kl k-1
(QP),
(3)
phase, those with the subscript I,a and I,o are in the very thin (=lo-' cm) equilibrium layers of solution on either side of the actual interface (5,14,20) and those with the subscript o are in the organic phase. M.T. indicates a mass transfer step and k , and k_l are forward and reverse chemical reaction rate constants. When mass transfer is much faster than the chemical reaction, the species connected by M.T. are, nec62 1988 American Chemlcal Society
ANALYTICAL CHEMISTRY, VOL. 60, NO. 3, FEBRUARY 1, 1988
227
Table I. Pseudo-First-Order Rate Constants, kobdin Equation 4, for the Ion-Pair Extraction of QP under Chemical Rate Control When Qt Is Present in Excess. ion-pairing phase
ko~'~*
aq int
kJCQ k J(Ad/V)Ce
0%
klfKI(
vO/ v)
Obviously, by analogous reasoning, eq 6 is valid also when the ion P is in excess, rather than Q'. Thus, for mass transfer control the rate constant is independent of the concentration of excess ion.
cQ
"Equilibrium constant: K I = [Q+]o[P-]o[Q+]-l[P-]-l. * A is area of liquid-liquid interface; d is the thickness of the interfacial equilibrium layer on the aqueous side of the interface (=lo-' cm); V and Vo are volumes of aqueous and organic phases; f is the constant fraction of excess ion Q that is present in the aqueous phase. essarily, at equilibrium with one another. Since the extraction is quantitative at equilibrium, the rate law will be pseudo first order when one of the reactants Q' or P- is present in excess. For each of the three cases represented by eq 1-3 the integrated extraction rate equation has the form
(4) in which kobed is a constant. In Table I are presented the Q is the excess ion. The first expressions for kobsd when ' column in Table I indicates the phase in which the ion-pairing reaction between Q' and P- takes place. In every case the pseudo-first-order rate constant depends on the analytical concentration of the ion in excess, CQ.The quantity f is the ' present in the aqueous phase during the exfraction of Q periment. This will have a value lower than one only if QX is extractable, where X- is the anion of the inert salt such as Br- or H2P04-. A completely analogous set of expressions (not shown) can be derived for the cases in which P- is the ion in excess. In those cases the rate depends on the analytical concentration
CP. Mass Transfer Regime. In stirred liquid-liquid systems the two-film theory of Lewis and Whitman, though simplistic, has proved to be a useful model for describing mass transfer rates (5,6,14,20-22). In the model it is postulated that two stagnant layers exist, one on either side of the interface and extending into the liquids for a distance 6, usually several tens of micrometers. The rate of mass transfer to and from the interface is associated with the rate of diffusion across these so-called Nernst films. The system is assumed always to be a t steady state, with linear concentration gradients between the interface and bulk phases. Mass transfer across the interface itself, which usually is much faster than mass transfer to and from the interface (20),will not be a rate-determining step and is ignored in the present work. In the mass transfer regime the steps labeled M.T. in eq 1-3, are slow, while the species connected by k l / k l are at equilibrium with one another. For the case in which Q' is present in large excess, if it is assumed that the diffusion coefficients of QP and P- in the aqueous phase are equal to one another, with the value Daq,and that the volume of liquid in the aqueous Nernst layer is small compared to that in the bulk phase, then the extraction rate equation is
where A is the area of the liquid-liquid interface. Equation 5 has the same form as a pseudo-first-order rate law in which the term (A.10-3Daq/V06,,) is the rate constant. Integration of eq 5 and replacement of concentrations with absorbances yields
EXPERIMENTAL SECTION Apparatus. The principal components of the extraction apparatus, including a 200-mL Morton flask, high-speed stirrer, porous Teflon membrane phase separator, and peristaltic pump, have previously been described (15). The Morton flask was thermostated at 20 1 0.5 "C by immersion in a water bath. The chloroform phase was continuously circulated through an 80-pL flow cell having a 1-cm path length (part 178-QS, Hellma Corp.) in a Cary 219 double beam spectrophotometer (Varian Instruments). The spectrophotometer was interfaced to an Apple 11+ microcomputer. The beam chopping frequency for the spectrophotometer is 15 s-l. Each absorbance value taken as a data point was the average absorbance from seven chopper cycles, giving a sampling frequency of 2.14 s-l. These absorbances were stored on disk for subsequent processing. Solvents and Chemicals. Water used throughout was first distilled and then deionized by passing it through a mixed-bed ion exchanger. Chloroform was reagent grade and was washed by shaking with twice its volume of water immediately before use. A solution of pH 6.5 phosphate buffer with an ionic strength of 0.1 was prepared by dissolving 7.59 g of NaHzP04-H,0 and 1.90 g of Na3P0,.12Hz0 in water and diluting to 1.000 L. Sodium picrate (NaP) solutions in phosphate buffer were prepared from reagent grade picric acid (Mathesen, Coleman and Bell) at conM. Because solid centrations ranging from 6 X to 6 X picric acid contains up to 15% moisture as supplied, the picrate concentrations in these solutions were determined spectrophotometrically at 355 nm (19). Tetrabutylammonium bromide (QBr) solutions in phosphate buffer were prepared from the reagent grade chemical (Aldrich, 99%). o-Nitroaniline solutions were M concentrations in both chloroform and prepared at 6 x water using the reagent grade chemical (Eastman Organic, White Label). Extraction Procedures. In a typical extraction of tetrabutylammonium picrate, 50.0 mL of water-washed chloroform was pipetted into the Morton flask which already was fitted with the porous membrane phase separator. Next to be delivered were volumes of phosphate buffer and either NaP solution in buffer or QBr solution in buffer, depending on which was to be in excess, such that the total volume of the aqueous phase was 50.0 mL. While the mixture was stirred at 83 rps the peristaltic pump was turned on to continuously circulate chloroform phase through the spectrophotometer (wavelength = 375 nm) and back to the flask, at 5.0 mL/min. After a stable absorbance base line was established (e.g. 10.001 absorbance unit), the experiment was initiated by rapidly injecting an accurately known volume of 1-2 mL of an aqueous solution of either QBr or Nap, using whichever one was not already in the flask. The computer data acquisition was initiated simultaneously with the injection. Experiments involving o-nitroaniline (0-NA)instead of QP were performed in a similar manner. When the extraction of o-NA was being measured, an aqueous solution of o-NA was injected. When data needed to generate the impulse response function were being collected, a chloroform solution of o-NA was injected. RESULTS AND DISCUSSION All experiments are performed with an excess of a salt of either the tetrabutylammonium cation Q' or the picrate anion P- and with a much larger excess of inert salt, MX, in the aqueous phase. In the experiment the chloroform and aqueous phases are stirred vigorously and an aqueous solution of a salt of the ion that is not in excess (i.e. either P- or Q+) is rapidly injected. The increasing concentration of ion pair is observed by continuously monitoring the absorbance of the chloroform
228
ANALYTICAL CHEMISTRY, VOL. 80,NO. 3, FEBRUARY 1, 1988
Table 11. Observed First-Order Rate Constants (Slopes) for the Extraction of Tetrabutylammonium picrate (QP) into Chloroform from pH 6.5 M Phosphate Buffer (Ionic Strength = 0.10) expt no. 1 2 3 4 5 6 7 8 9 10 11 12 13
0.00 0
2
4
6
8
10
12
CQ,M 1.0 x
lo-'
1.0 x 10-3
1.0 x 10-3 1.0 x 10-3 5.0 x 10-4 1.0 x 10-5 2.0 x io-5 2.5 x 10-5 5.0 x 10-5 1.0 x io-, 1.0 x 10-5 2.6 x 10-5 2.0 x 10-5
CP, M 3.5 x 1.2 x 3.5 x 5.8 x 3.5 x 1.2 x 1.2 x 1.2 x 1.2 x 1.2 x 1.2 x 3.5 x 5.8 x
io-5 10-5 10-5 10-5 10-5 10-3 io-3 10-3 10-3
io-3
10-4 10-4 10-5
slope 0.36 f 0.1, 0.4, i 0.1, 0.4, f 0.1, 0.4, i 0.1, 0.3, i 0.1, 0 . 3 ~i 0.1, 0.36 f 0.10 0 . 4 ~i 0.1, 0 . 3 ~i 0.1, 0.46 & 0.1, 0 . 4 ~i o.io 0.42 f 0.1, 0.42 i 0.1,
14
TIME (SEC)
Flgure 1. Absorbance of organic phase vs time: (A) for extraction M and C , = 5.8 X M; (E)for inJection of QP, with C, = 2 X of o-NA in chloroform; (C) after deconvoiutlon of curve A with the IRF obtained from curve 6. The true extraction rate curve is curve C.
phase. At 375 nm the molar absorptivities of the species Pand QP in the organic phase are identical (19). At equilibrium the extraction of QP is quantitative. In the two-phase liquid mixture the aqueous phase is dispersed as droplets (16,23), the average density is 1.24 g/mL and the average viscosity is 0.016 P (24). Previous studies with the same rapid-stir extraction apparatus showed that extraction rate no longer increases with stirring speed above about 80 revolutions per second (rps) (13,16,25). Therefore a stirring speed of 83 rps was used in the present work. The impeller diameter was 1.6 cm giving an impeller Reynolds number of 2 X lo4 (26). Data Treatment. Because a time of 11.7 s as required for chloroform to travel from the membrane phase separator to the flow cell, there was a delay time which was readily compensated in the calculations by taking zero to be 11.7 s from the instant of injection. In Figure 1, curve A is a typical plot of absorbance vs time for the extraction of QP, corrected in this way for delay time. A second, and more difficult, correction of the absorbance vs time data was necessitated by the band broadening imposed on the concentration-time distribution by the following processes (27): (i) finite duration of syringe injection of the aqueous sample solution (=0.25 9); (ii) time required for uniform mixing of the injected aqueous sample with all of the bulk aqueous phase in the extraction flask; (iii) convective mixing in the porous membrane and in the membrane support; (iv) nonuniform flow profiles in connecting tubing and peristaltic pump tubing; (v) mixing in the detector flow cell. These processes have all caused curve A in Figure 1to be more spread-out along the time axis than is the actual concentration vs time profile in the bulk chloroform phase in the extraction flask, and they make it appear that the extraction rate is slower than it actually is. The data in curve A can be corrected for these band broadening effects by mathematical deconvolution. Deconvolution requires the use of an impulse response function (IRF) which is an absorbance (concentration) vs time distribution which has been produced by all of the processes (i-v) operating on an impulse function (28). With less than a hundred data points, the deconvolution is readily performed on a microcomputer using so-called discrete deconvolution (29-31). A suitable IRF was obtained by injecting a chloroform solution of o-nitroaniline (0-NA). Since this compound remains quantitatively in the chloroform phase (32), no chemical
reaction and no interphase mass transfer are involved. Also steps i and iii through v are nearly identical in this case to what they are when an aqueous solution is injected, while step ii is replaced with a very similar process, the mixing of the injected chloroform solution with all of the bulk chloroform phase. Curve B in Figure 1 is a typical A , vs t plot observed when a chloroform solution of o-NA is injected. The IRF (not shown) is obtained by inverting curve B. It is the hypothetical curve that would be observed if o-NA in chloroform had been injected a t t = 0 s as a spike of zero width. Comparison of curve A in Figure 1with curve B reveals that band broadening actually makes a greater contribution to curve A than do the chemical reaction or mass transfer which are the object of the experiment. When curve A is deconvolved with the IRF, the result is curve C which is the true A , vs t profile for the ion-pair extraction of QP. The scatter in curve C is due to the uncertainty introduced by the small differences between curves A and B. Extraction of QP. Extraction rate experiments for tetrabutylammonium picrate (QP) were performed both with an excess of tetrabutylammonium ion (asbromide salt) and with an excess of picrate (as sodium salt). The ion in excess was varied over a 20-fold range of concentration in each case, as can be seen in Table 11. Curve A in Figure 1 and its deconvolved form, curve C, are from one of the replicate experiments used to generate the results in experiment 13 and are typical of the A , vs t curves obtained in the experiments cited in Table 11. The upward concave shape of curve C a t t 5 2.8 s suggests the nonsteady-state behavior (lag time) expected in the early stages of a mass transfer controlled process (33,34) but not expected in a chemical rate controlled process. Shown in Figure 2 is - A,)) vs t. For a mass transfer cona plot of In (Ao,m/(Ao,m trolled process this type of plot should become linear (Le., rate becomes first order and eq 6 became applicable) only after steady state diffusion has become established. This appears to be the case for points at t 5 2.8 s. The solid line was obtained from a linear least squares fit to the points in the steady-state region. The slope of the line is 0.45 O.lo s-l and represents the first-order rate constant. The correlation coefficient of the line is 0.9989. Similar evaluation of the data from all of the experiments yielded the slopes presented in Table 11. Experiments 1 to 5 were done with excess Q' while experiments 6 to 13 had excess P-. In experiments 2-4 CQ was held constant and the nonexcess ion P was varied. Conversely, in experiments 6-10 Cp was held constant and the nonexcess ion Q' was varied. From experiments 2-4 and 6-10 it is evident that the firstorder rate constant is independent of the concentration of nonexcess ion. Since this result is predicted for both chemical
*
ANALYTICAL CHEMISTRY, VOL. 60, NO. 3.0
I
1
1 : 1 2.5
P
2.0
0
I
6
2
4
6
8
TIME (SEC)
Figure 2.
Firstorder plot of the points in curve C of Figure 1. The straight line is fit to the points from 2.8 to 8.4 s in the steady-state region.
rate control (eq 4) and mass transfer rate control (eq 6), it gives no information about the rate-controlling process. Such information, however, is revealed by the fact that the rate constant is the same even for those cases in which the excess ion, either Cp or C,, is varied. For example in experiments 1,3, and 5 the concentration of nonexcess picrate M while the excess tetrabutylammonium ion ion is 3 X is varied from 1 X to 5 X lo4 M, with no effect on the rate constant. Equation 4, for chemical rate control, predicts a 20-fold change in rate constant in this case, while eq 6, for mass transfer control, predicts no change in rate constant. A similar situation can be seen by comparing experiments 6,11, 12, and 13 in which the excess picrate ion is varied from 1.2 X to 5.8 X 10" M but the rate constant remains unaffected. These results constitute strong evidence that the ion-pair extraction rate is mass-transfer controlled. The rate of the ion-pairing chemical reaction is too fast to measure even by this rapid-stir technique in which, because of the large area, A, mass transfer is quite rapid. Furthermore, it is not powible to tell from the present experiments where the ion-pairing chemical reaction occurs (aqueous phase, interface, or organic phase) because eq 6 is the same for all three cases. Extraction of o-NA. o-Nitroaniline is a simple molecule, un-ionized a t pH 6.5 (35),whose extraction rate must occur under mass transfer control (eq 6) because no chemical reaction is possible. When an aqueous solution of o-NA is injected, the observed plot of A, vs. t is similar to curve A in Figure 1. A curve very similar to curve C is obtained after deconvolving to eliminate band broadening effects. For each of three runs with o-NA the data were evaluated as described - A,)) vs t in the above for QP. Plots of In (Ao,m/(Ao,m steady-state region were linear with correlation coefficients 20.995 and an average slope of 0.51 f O.lo c', which is within experimental error of the average first-order rate constant for extraction of QP (0.41 f O.lo s-l). Assuming that the diffusion coefficients for o-NA, P-, and QP are not very different from one another, this gives further strength to the argument that the rate of ion-pair extraction of QP is purely mass transfer controlled. The thickness of the Nernst diffusion layer, 6aq, can be estimated from the parenthetic term on the right-hand side of eq 6. A previous study performed in the same apparatus showed that the average droplet diameter of the dispersed phase is 0.020 f 0.003 cm (16)which corresponds to an average droplet volume of 4.2 X 10s cm3. With 0.050 L of each phase, this corresponds to a liquid-liquid interfacial area, A, of (1.5
3, FEBRUARY 1, 1988
229
f 0.2) X lo4cm2. If the diffusion coefficient is given a typical value (20)of 5 X lo4 cm2/s, then 6, is about 0.003 cm. This value is in agreement with the thickness of Nernst diffusion films that have been reported for various diffusion controlled phenomena (36, 37). When experiments are done by using slow stirring with no dispersed phase, 6,, is predicted to decrease with increasing stirring speed (6,14,20)and is observed to do so (5,6). Near the upper limit of stirring, just before the interface became disturbed, Higuchi (5)found 6, = 0.005 cm. The similarity of this value to the one presently reported for a vigorously stirred liquid-liquid dispersion suggests that once stirring is made fast enough to create a liquid-liquid dispersion, then further increase in stirring speed increases the extraction rate mainly by producing smaller droplets which creates a larger interfacial area, A, rather than by markedly decreasing &,. At very fast stirring speeds (above about 80 rps in the apparatus used here) further increase in stirring speed no longer changes either A or Finally, it may be noted that when the extraction of QP is studied using larger excess concentrations of P than those employed in this work, the rate of extraction actually decreases with increasing Cp. This phenomenon has been studied extensively and will be the subject of a future communication.
ACKNOWLEDGMENT Interfacing of the Apple 11+ microcomputer to the spectrophotometer was done by Edward Aprahamian, Jr. Registry No. QP, 914-45-4; chloroform, 67-66-3.
LITERATURE CITED (1) Schill, 0. I n Ion Exchange and Solvent Extractlon; Marinsky, J. A., Marcus, Y., Eds.; Marcel Dekker: New York, 1974; Vol. 6,Chapter 1. (2) Mohammed, H. Y.; Cantwell, F. F. Anal. Chem. 1980, 52, 553-557. (3) Cantwell, F. F.; Carmichael, M. Anal. Chem. 1982, 5 4 , 697-702. (4) Carmichael, M.; Cantwell, F. F. Can. J. Chem. 1982, 60, 1286-1290. (5) Higuchi. T.; Michaelis, A. F. Anal. Chem. 1986, 40, 1925-1931. (6) Lippold, B. C.; Schneider, G. F. Arzneim.-Forsch. 1975, 25, 843-852. (7) Jansson, S. 0.; Nordgren, T.; Schlll, G. Acta Pharm. Suec. 1977, 14, 435-450. (8) Nordgren, T.; SJoden, E.-K. Acta Pharm. Suec. 1978, 15, 241-254. (9) Nordgren, T.; Kolstad, A.-K. Acta Pharm. Suec. 1979, 16, 125-134. (10) Nordgren, T.; Hackzell, L. Acta Pharm. Suec. 1979, 16, 135-143. (11) Nordgren, T. Acta Pbarm. Suec. 1979, 16, 215-221. (12) Kolstad, A.-K.; Nordgren, T. Acta Pharm. Suec. 1980, 17, 327-332. (13) Freiser, H. Acc. Chem. Res. 1984, 17, 126-131. (14) Danesi, P. R.; Chiarizia, R. CRC Crff. Rev. Anal. Chem. 1980, 10, 1-126. (15) Watarai, H.; Cunningham, L.; Freiser, H. Anal. Chem. 1982, 5 4 , 2390-2392. (16) Aprahamian, E., Jr., Cantwell, F. F.; Freiser, H. Langmulr 1985, 1 , 79-62. (17) Hanson, C. Recent Advances in Solvent Extraction; Hanson, C., Ed.; Pergamon: New York, 1971; Chapter 12. (18) Laddha, 0. S.; Degaleesan, T. E. Transport Phenomena in Solvent Extfactlon; McGraw-Hill: New York, 1976; Chapter 15. (19) Gustavii, K.; Schill, 0. Acta Pharm. Suec. 1988, 3 , 241-258. (20) Davies, J. T., Rideal, E. K. Interfacial Phenomena; Academic: New York, 1961; Chapters 4 and 7. (21) Lewis, W. K.; Whitman, W. G. Ind. Eng. Chem. 1924, 16, 12 15-1220. (22) Laddha, G. S.; Degaleesan, T. E. Transport Phenomena In Solvent Extraction; McGraw-Hill: New York, 1978; Chapter 3. (23) Persaud, 0.; Tlan, X.-M.; Cantwell, F. F. Anal. Chem. 1987, 59, 2-7. (24) Goldberger, W. H.; Robblns, L. A. I n Chemical Engineer's Handbook, 6th ed.; Perry, R. H., Green, D. W., Eds.; McGraw-Hill: New York, 1984; Chapter 21, p 61. (25) Watarai, H.; Frelser, H. J. Am. Chem. SOC. 1983, 105, 191-194. (26) Miller, S. A. I n ChemicalEnglneer's Handbook, 6th ed.;Perry, R. H., Green, D. W., Eds.; McGraw-Hill: New York, 1984; Chapter 19. (27) Sternberg. J. C. I n Advances in Chromatography, Giddings, J., Keller, R. A., Eds.: Marcel Dekker: New York, 1966; Vol. 2, Chapter 6. (26) Horiick, G.; HieRje, 0. M. I n Contemporary Topics in Ana/ytical and Cllnlcal Chemistry; Hercules, D. M., Hieftje, 0. M., Snyder, L. R., Evenson. M. A., Eds.; Plenum: New York, 1978; Voi. 3. Chapter 4. (29) Valentlnuzi, M. E.; Montaldo-Volachec, E. M. Med. B o / . Eng. 1975, 13, 123-125. (30) Diffy, B. C.; Hall, F. M.; Corfield, J. R. J. Nucl. Med. 1976, 17, 352-355. (31) Diffy, B. L.; Corfield, J. R. Med. Mol. Eng. 1978, 17, 478. (32) Sandeli, K. B. Naturwlssenschaften 1966, 5 3 , 330-331. (33) Crank, J. The Mathematics of Diffusion, 2nd ed.; Oxford University Press: Oxford, 1975; Chapter 4. (34) Guy, R. H.; Fleming R. J. Colloid Interface Sci. 1981, 8 3 , 130-137.
Anal. Chem. 1988, 60,230-235
230
(35) Perrln, D.D. Dissociation Constants of Organic Bases in Aqueous So-
iution; Butterwodhs: London, 1965; p 90. (36) Bastow, S. H.; Bowden, F. P. Roc. R . SOC.London, A 1935. 151,
220-233. (37) Schulman, J. H.; Teorell, T. Vans. Faraday SOC. 1938, 3 4 , 1337-1 342.
RECEIVED for review March 24, 1987. Resubmitted June 22,
1987. Accepted October 15,1987. This work was supported by grants from the Science Foundation (H*F*)and by the Natural Sciences and Engineering Research Council of Canada (F.F.C.). Presented in part at the International Chemical Congress of Pacific Basin Societies, Hawaii, Dec 1984. Experimental work was performed while on study leave (F.F.C.).
Metallophthalocyanines as Chemical Interfaces on a Surface Acoustic Wave Gas Sensor for Nitrogen Dioxide Maarten S. Nieuwenhuizen,* Arnold J. Nederlof, and Anton W. Barendsz
Prins Maurits Laboratory TNO, P.O. Box 45, 2280 A A Rijswijk, The Netherlands
The response of a SAW (surface acoustlc wave) gas sensor for NO2 has been studied extenslvely by uslng NO2 and a number of other gases (CO, COP, CH,, NH,, SO2, H20, and toluene). Different metakphthaiocyanlnes (MPC; M = H2, Mg, Fey Coy NI, Cu, Pb) have been tested as a chemlcal Interface on one delay-line of a dual delay-llne oscillator. These compounds offer two sites for interaction, one at the metal ion (coordlnatlon complex formatlon) and one at the electron cloud In the periphery of the mdecule (charge transfer complex formatlon). The results are discussed In terms of general performance characteristics. At 150 *C CoPC Is preferred for Selectivity and sensitlvlty and CuPC Is preferred for response tbne. For CuPC the relation between sensltlvlty, layer thickness, and frequency (39-98 MHz) was calculated and also the effect of temperature (30-150 "C) was measured. PbPC cannot be used due to Irreversible effects. The transductlon mechanisms of these SAW chemosensors are a combination of changes In mass and in conductivity caused by several chemical and physical processes at the chemical Interface.
Surface acoustic wave (SAW) devices are attractive for chemical microsensor applications because of their small size, low cost, sensitivity, and reliability. Furthermore, SAW technology is compatible with planar integrated circuit technology. Chemosensors based on piezoelectric crystal using bulk acoustic waves (BAW) have been known for quite some time (1)and numerous examples of such sensors for various gases are known (2,3). The first gas sensor using surface acoustic waves (SAW) was reported by Wohltjen et al. (4,5 ) . They placed a SAW delay line, covered with a polymer, in the feedback loop of an amplifier and measured the oscillator frequency. Since then, Bryant et al. (6, 7) reported a SAW dual delay line and D'Amico (8)a three transducer type SAW device. The latter structures showed a better temperature stability because the second delay line is used as a reference compensating for such nonspecific effects as the variations of temperature and pressure on the substrate. The first use of a SAW resonator configuration as gas sensor was reported by Martin e t al. (9). For the adequate measurement of a specific gas in a mixture of gases the measuring delay line of the SAW device must be coated with an appropriate chemical interface. So far, many
chemical interfaces have been proposed (1-15). The interactions occurring at the interface will be responsible for the performance characteristics of the gas sensor such as selectivity, sensitivity, reversibility, and response time. Recently, a study dealing with these various aspects has been published by the authors (15). In our laboratory SAW chemosensor research is concentrating on the development of a SAW gas sensor for NO2 and CO (16-19) to be used in automotive exhaust systems, process control, or environmental monitoring. The interaction with these gases has to be sensitive, selective, reversible, and fast. Additionally, to prevent condensation of water and to reduce response times, the sensor will have to be operated at elevated temperatures, thus requiring a highly stable chemical interface. As early as 1972 it was recognized in our laboratory (20) that metal-free phthalocyanines and other polyaromatic molecules preferentially adsorb gases with high electron affinities such as NO2 and chlorine and therefore offered an interesting possibility for the detection of these gases. Phthalocyanines (PC) are p-type organic semiconductors with which the electronegative gases interact strongly resulting in a change in the electrical conductivity of these substances. Since organic semiconductors do not interact as strongly with water or oxygen as inorganic semiconductors and because organic semiconductors can easily be modified chemically to obtain tailor-made conductive properties, many papers have been published since, dealing with the development of socalled chemiresistors. For the detection of NOz or chlorine both hydrogen and metallophthalocyanines (MPCs) were investigated (21-32) as well as other compounds containing a-electron systems (20, 33-35). MPCs have also been used as sensitizers on solid electrolyte sensors (36) and as a chemical interface for piezoeledric crystal sensors (37). In previous papers (16,17) we reported on our first results using H2PC as the chemical interface on a quartz-based SAW gas sensor for NO2. Since then only Ricco et al. (14,36) reported on a single experiment using PbPC on a LiNb03-based SAW sensor. In this paper the results of a study are reported that used HzPC and a number of MPCs (M = Mg, Fe, Co, Ni, Cu, and Pb) to investigate the effect of different metal ions on important sensor performance characteristics as sensitivity, selectivity, reversibility, and response time.
EXPERIMENTAL SECTION Gases. The gases were bought from Matheson (NO2,CO, and SO,) and HoekLoos (NH3, C02, and CHI) and used
0003-2700/88/0360-0230$01.50/00 1988 American Chemical Society
