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Anal. Chem. 1987,59, 2260-2263
Dual Porous Electrode Membrane Cell for Detection of Nonelectroactive Species in Flowing Streams Antonin Trojanek’ and Stanley Bruckenstein*
Chemistry Department, University at Buffalo, State University of N e w York, Buffalo, N e w York 14214
A dual electrode detector capable of being used in flowing streams Is described. One electrode generates a volatile reagent from a constituent present In the flowlng stream, which then reacts with a dissolved anaiyte also present In the flowlng stream. Excess volatile reagent diffuses through a nonwettlng porous membrane to a second electrode at which the unreacted volatlle reagent Is determined by constant potential electrolysis. The utility of this detector Is demonstrated by the determination of allyl alcohol with electrogenerated bromine In the range 23-697 ng of allyl alcohol.
The rotating ring-disk electrode generates an electroactive species at the upstream disk electrode and detects this species at the downstream ring electrode (1-3). This principle has been adapted to stationary electrodes positioned in flowing solutions. In flow-through analytical systems, the generating electrode is placed upstream and the collecting (or detecting) electrode downstream. The downstream collecting electrode gives the analytical signal, which may be used directly (4-6) or to control the upstream generating electrode (7). The ratio of the current a t the collecting electrode to the generating electrode is the collection efficiency, N . In the case in which the generated species and its parent are stable and electrochemically interconvertible, N depends upon the geometry of the electrode system and both electrode potentials. If the generated species undergoes a chemical reaction in transit to the collecting electrode, N depends also on the details of the homogeneous chemical reaction, and the convective-diffusion situation that exists in the cell. In a recent communication, we described a novel type of pneumatoamperometric detector for the direct analysis of volatile electroactive species in a flowing stream (8-10). It consisted of a gas permeable, hydrophobic membrane upon one face of which was deposited a porous electrode (GPME). The bare face of the membrane contacted a flowing stream containing a volatile, electroactive species that diffused through the membrane to the porous electrode. The porous electrode comprised the indicator electrode of a detecting electrochemical cell that was electrically and physically isolated from the flowing stream by the insulating membrane and the gas pores in the membrane. The porous electrode’s potential was selected to electrolyze the volatile species that diffused from the flowing stream through the membrane’s gas pores to the electrode. In this report we describe how two such GPME’s can be configured into a generating/collecting electrode configuration. This novel detector consists of two porous electrodes that are separated from each other by a porous, hydrophobic membrane (see Figure 1). These two electrodes are electrically insulated from each other by the membrane, as are both solutions on opposite sides of the membrane. One electrode, the generator, contacts the flowing aqueous stream that contains the analyte. A volatile, electroactive reagent is Permanent address: Jaroslav Heyrovsky Institute of Physical Chemistry and Electrochemistry, Czechoslovak Academy of Sciences, Jilska 16, 11000 Prague 1, Czechoslovakia. 0003-2700/87/0359-2250$0 1.50/0
generated at this electrode and reacts with the dissolved analyte. If the amount of the generated reagent is in excess of that required to react with the analyte, some of the reagent diffuses through the membrane to the second gas-porous electrode. This second electrode is the working electrode in an independent controlled-potential electrolysis cell. It serves as the collecting electrode as its potential is set to electrolyze the volatile reagent. Thus the current at the collecting electrode becomes a direct measure of the concentration of analyte in the flowing aqueous phase. The characteristics of this electrode configuration are evaluated. We also describe one analytical application involving the determination of allyl alcohol by reaction with electrogenerated bromine. EXPERIMENTAL SECTION Detector. The body of the detector, depicted in Figure 2, is made of Plexiglas and consists of two parts clamped together by three screws (not shown). The left part, D, is the flow-through cell with the generating electrode, F, deposited on one face of a 0.003-in.-thickporous, hydrophobic membrane. The right part, K, contains the collecting electrode, H, also deposited on an identical kind of membrane. The left part houses solution outlet, A, and inlet, B, Teflon capillaries (0.01-in. i.d.), and a saturated Ag/AgCl reference electrode, electrolytically connected to the auxiliary and generating electrodes via a salt bridge compartment and a porous Vycor glass tip, J, glued into the wall. A strip of platinum foil, C (0.005 in. thick), glued on the wall was used as an auxiliary electrode for the generating system. The chamber of the right part, I, contains the electrolyte that contacts the collecting electrode and the other components of a conventional three-electrode controlled-potential system. These include a Luggin capillary, a reference standard calomel electrode (SCE),and an auxiliary platinum-wire electrode in addition to the porous collecting electrode. Electrolytic contact to the porous collecting electrode is made via a 0.1 in. diameter hole drilled in the wall. The contact area between the Plexiglas body and the membrane is covered by layer of Densil pressure-sensitivesilicon adhesive (Denison Manufacturing Co., Framingham, MA). The horizontally drilled hole of large diameter allows for electrolyte replacement and is closed during operation. A 0.037-cm polyethylene spacer, E, with a 0.25-cm-wide channel separated the metalized generating membrane electrode from the inlet and outlet capillaries in the left-hand part. This channel defined the effective area and position of the generating electrode, F, to be directly opposite the collecting electrode, H, which was the same diameter as the channel. A 0.0075-cm-thick nonmetalized, porous, hydrophobic membrane, G, was inserted between the membranes supporting the generating and collecting electrodes, and both parts of the cell were clamped together with sufficient pressure to produce a liquid-tight assembly. The same membrane material was used to support both the generating and collecting electrodes (poly(tetrafluoroethylene) sheeting, specified as being 0.003 in. thick by its manufacturer, W. L. Gore and Associates, Inc., Elkton, MD). Gold generating and collecting electrodes were deposited on one face of the membrane as described earlier (8). The preparation technique sometimes produced membranes having a trace of gold on the face not in contact with solution, and these kinds of membranes had a continuous electrical path between the solution (electrode) side of the membrane and the other “bare” side. The insertion of membrane G eliminated the possibility of an electrical short between the “bare” faces of the membranes supporting the
a 1987 American Chemical Society
ANALYTICAL CHEMISTRY, VOL. 59, NO. 18, SEPTEMBER 15, 1987 Collecting Electrode Compartment With Electrolyte Gas Filled Pore
il ! Br- + X
2Br---
Collecting Membranes
L\ Generating @ '2e' 0 Brz X -. Y (+Br-) Electrode t
3 Carrier Stream Flow
+
Flgure 1. Principle of dual porous electrode membrane cell. X is the sample analyte plug in the carrier stream that reacts with electrogenerated bromine to form the product Y. Excess bromine diffuses through the membrane to the other side where it is reduced back to bromide ion. ,D
J
/E
Flgure 2. Dual porous electrode membrane cell: A, flowing solution exit; B, flowing solution inlet; C, generating system auxiliary electrode;
D,Plexiglas flow-through part of cell; E, 0.037-mm polyethylene spacer with flow channel; F, Goretex membrane with porous, platinized generating electrode on left hand face; G, unmetallzed Goretex membrane; H, Goretex membrane with platinized collecting electrode 'on right hand face; I, well for collecting electrode supporting electrolyte end of the electrodes; J, porous Vycor glass tip; K, Plexiglas body for collecting electrode part of cell. generating and collecting electrodes and did not introduce any significant increased resistance to diffusion of bromine in the gas phase. Gold is not a suitable electrode material for the generation of bromine by the oxidation of bromide in acid media, the conditions we employed for the determination of allyl alcohol, nor is it the most convenient material for use as the collecting electrode. Hence, both gold electrodes were platinized after the flow-through cell was assembled and placed in the flow system. First, the collecting electrode compartment was filled with platinizing solution and, while stirring with a stream of nitrogen, a potential of -0.3 V was applied to the collecting electrode for 1 min. The generating electrode was plated with platinum by a single injection of 200 pL of platinizing solution into the stream of mobile phase flowing at the rate 1.0 mL/min while the generating electrode was held at -0.3 V. Such a platinized electrode was very efficient for bromine generation for up to 5 days of continuous use. However, corrosion of the platinum occurred and a sharp decrease in the bromine generation efficiency was eventually observed. Replatinizing the generating electrode as described above by injection of platinizing solution brought its response back to within 5% of its original value. Cell Conditioning. Freshly prepared electrodes were platinized. The potential of the generating electrode wm set to +1.45 V vs. a saturated Ag/AgCl electrode and the potential of the collecting electrode to +0.20 V vs. SCE. After about 30 min, a newly platinized, dual porous electrode detector yielded a reproducible response to injected samples containing bromide. For a previously used cell, conditioning for about 10 min was sufficient. Instrumentation. Two potentiostats had to be used to control the potentials of the generating and collection electrodes and were constructed from operational amplifiers. Conventional threeelectrode circuitry employing current-to-voltage converters (current followers) was used. Simultaneous recording of current responses obtained at the generating and collecting electrodes was made using a XYY' 7046B Hewlett Packard (Hewlett Packard, San Diego, California) recorder. The constant current generation of bromine was accomplished by using a conventional operational amplifier circuit in which the cell was placed in the feedback loop and current injected into the summing point.
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A Rheodyne Model 7120 injection valve was connected to the detector by a 6-in.-long Teflon capillary of 0.01-in. i.d. Other electronic equipment and the parts of the liquid manifold were the same as described previously (8). A 1mL/min flow rate of the mobile phase was used and 0.020-mL samples were injected into the mobile phase, if not otherwise stated. Other. Analytical reagent grade chemicals were used without further purification. The aqueous mobile phase, containing 0.1 M sodium sulfate and 0.05 M sulfuric acid was purged of dissolved oxygen by passing a stream of nitrogen through it and then it was degassed under vacuum in an ultrasonic bath prior to use.
RESULTS AND DISCUSSION Preliminary Studies. The bromine/ bromide couple was selected as the model volatile, electroactive reagent system because of the large number of analytes that react with bromine and bromine's volatility. Allyl alcohol was chosen as the model analyte because it reacts rapidly with bromine ( I I , 1 2 ) . Bromide was added to samples containing allyl alcohol (analyte) and the concentration of bromide adjusted to produce an excess of bromine upon electrooxidation. A portion of the sample was then injected into a flowing stream in a typical flow-injection apparatus. When the sample slug passed by the generating electrode of the dual porous electrode cell, bromine was produced by electrooxidation at constant potential or current. The bromine reacted with analyte present in the injected sample, and excess, unreacted bromine diffused through the gas pores in the membrane to the collecting electrode where it was detected by electroreduction back to bromide ion. The concentration of bromide ion and bromine-consuming analyte in the injected sample determined the bromine flux to the collecting electrode. In the preliminary experiments, the collecting electrode compartment was filled only with 0.1 M sulfuric acid. A severe tailing of the collecting electrode current peak was observed for samples containing high ( 2 5 mM) concentrations of bromide. When samples containing lower concentrations of bromide (1-5 mM) were injected, tailing was absent but the response peak heights depended on the time between sample injections and also upon the bromide concentration in foregoing samples. A study of this phenomenon revealed that the collecting electrode response depended upon the bromide ion concentration formed by electroreduction of the bromine that diffused through the membrane. This problem was solved by adding excess bromide to the electrolyte in the collecting electrode compartment. In the experiments reported below 0.05 M KBr was added to the 0.1 M sulfuric acid. Electrochemical Conditions for Detection of Bromine. The detection of bromine by reduction a t a porous, unplatinized platinum electrode (9) was successfully performed previously a t +0.7 V vs. SCE. However, with a platinized collecting electrode, the best sensitivity and reproducibility were obtained in a potential region where the electrode was free of surface oxides, +0.2 V. The presence of surface oxides on the generating electrode plays an essential role in obtaining 100% current efficiency for bromide oxidation. The production of bromine increased during successive injections of bromide samples a t 30-9 intervals while simultaneously increasing the collecting electrode potential in the range +1.2 to +1.5 V. The use of lower potentials in this range resulted not only in a lower collection of bromine, but also produced a dependence of the collection on the time between sample injections and bromide concentration in the samples. Also at less positive potentials, several minutes were necessary for the collecting electrode to recover its original response after a sample containing 0.005 M bromide was injected. These results suggested to us that bromide oxidation occurs via a chemical reaction involving the pool of surface oxidized platinum, whose formation rate depends on the electrode
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ANALYTICAL CHEMISTRY, VOL. 59, NO. 18, SEPTEMBER 15, 1987
Figure 4. Flow rate dependence of the generating and collecting electrode currents. Condtlons as in Figure 3. Curve 1, generating electrode; curve 2, collecting (indicating)electrode.
1
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Figure 3. Generating and collecting (indicating)electrode responses to KBr Injections. Successhe 2O-kL injections of a sample containing 5 mM KBr into 0.1 M sodium sulfate i- 0.05 M sulfuric acid flowing at 1.O mL/min. The generating electrode was held at -t1.45 V for the first two injections, and open circulted thereafter. potential. Evidence for this mechanism comes from an experiment in which the generating electrode, potentiostated at +1.4 V until a steady-state residual current had been obtained, was disconnected from the generating potentiostat. Even under these conditions, bromine was formed upon injection of bromide sample. As seen in Figure 3, the collection current decreases with successive injections of bromide. This result is consistent with gradual consumption of surface oxides. Such a chemically reduced surface requires about 3 min to recover its original activity on being repotentiostated at 1.4
+
V. The reproducibility of the determination of the amount of injected bromide was estimated from 20 replicate injections of samples containing 5 mM KBr. Constant current generation of bromine was also used. When the time between successive samples was reduced to 20 s and the sample contained 5 mM bromide, a minimum constant generation current of 25 pA was required. This current level maintained the potential of the generating electrode at i-1.35 V between sample slugs. Increasing the constant current to 100 MAhad no significant effect on the collection of bromine. Detector Response Rate. The detector’s response rate was estimated from the generating and collecting electrodes’ steady-state response obtained after injection of 200-pL samples containing 5 mM KBr. The response time constant was taken as the time to attain 63.2% (1- l/e) of the steady-state current. The response time constant reflects the geometry of the detector’s internal volume as well as the fluid’s flow pattern. At a flow rate of 1.0 mL/min, response time constants of 1.9 s for bromine generation and 2.2 s for bromine collection were obtained. At a flow rate of 0.5 mL/min, the respective time constants were 2.8 and 3.4 s. There is only a short time delay of 80 ms, independent of the flow rate, between the initial rising parts of the response curves at both generating and collecting electrodes. Hence, under our conditions the response at the collecting electrode closely follows the response of the generating electrode and the slight delay is caused by the time of transport of bromine through the membrane pores. Flow Rate Dependence of the Peak Current Responses. When 20-pL samples containing 5 mM KBr were in-
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Figure 5. Collection efficiency as a function of flow rate. Conditions as in Figure 3. Curve 1, steady-state current response; curve 2, transient current response. jected, the transient peak height response of the generating electrode changed with the flow rate in the range 0.2-0.65 mL/min (Figure 4). The subsequent slight decrease of peak current response with increasing flow rate may be caused by changes in the flow pattern in the detector. The collecting electrode’s peak response changed modestly with flow rate in the range 0.3-0.65 mL/min. At higher flow rates, the collection current showed a slight decrease as compared to the generating electrode current. In the allyl alcohol experiments that are described below a higher flow rate of 1 mL/min was used. This decreased the wash-out time of the detector, which then allowed more determinations per unit time, with hardly any loss in sensitivity. Collection Efficiency. The collection efficiency was calculated as a ratio of current responses of the collecting and generating electrodes. Figure 5 shows that the collection efficiency is high and is inversely proportional to the flow rate. Plots of the collection efficiency vs. flow rate for transient (20-pL injection of 5 mM KBr) and steady-state signal conditions (2WpL injections) are two parallel straight lines with slopes of 0.0358 (SD 0.00052) and 0.0357 (SD 0.00075) mL/min. The intercepts, which represent the collection efficiencies extrapolated to infinite flow rate, were 0.19 (SD 0.035) and 0.307 (SD 0.0013) for the transient and steady-state signals, respectively. The difference in the intercepts reflects the finite response rate of the detector to transient generating electrode processes. Determination of Allyl Alcohol. Twenty-microliter samples containing allyl alcohol and potassium bromide dissolved in the liquid phase were injected into the flowing liquid stream while the generating electrode was potentiostated a t +1.45 V. As can be seen in Figure 6, the presence of allyl alcohol in the samples decreased the collecting electrode’s current response. The current decrease was propor-
ANALYTICAL CHEMISTRY, VOL. 59, NO. 18, SEPTEMBER 15, 1987 a
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Figure 6. Effect of allyl alcohol concentration on the generating and collecting currents: 1, blank: 2, 5 X IOd M; 3, 1.00 X lo4 M; 4,2.00 X lo-' M; 5, 3.00 X IO4 M; 6,4.00X 1 0 4 M 7,5.00 X lo4 M; 8 , 6.00 X M allyl alcohol. The samples contained 5 X lo4 M KBr, the flow rate was 1.0 mL/min, and the generating electrode was potentiostated at 4-1.45 V.
tional to alcohol concentration within a range of allyl alcohol concentrations. The upper concentration limit of allyl alcohol determination is reached when the current minimum observed in the presence of allyl alcohol can no longer be distinguished from the residual current. Increasing the amount of KBr added to the sample will raise this upper limit. The lower concentration limit for allyl alcohol determination is reached when there is no longer a difference between the currents observed in the presence and absence of allyl alcohol. It follows that for each allyl alcohol concentration range an optimum bromide concentration exists and should be selected. When the concentration of KBr in the added analyte sample was 0.5 mM, the difference in the collection current between the blank and a sample containing analyte was a linear function of allyl alcohol concentration in the range 0.02-0.6 mM, i.e., 23.2-697.2 ng of injected alcohol. The calibration curve slope was 3.47 (SD 0.07) nA/ng allyl alcohol and its intercept was 13 (SD 4) nA. The useful concentration range for samples containing 5 mM KBr was 0.2-6 mM allyl alcohol. In this case the calibration curve slope was 3.69 (SD 0.089) nA/ng of allyl alcohol and its intercept was 47 (SD 37) nA. The relative decrease of the collection current minimum in the presence of allyl alcohol was found to be independent of the flow rate. At a flow rate of 1mL/min, 90 samples/h can easily be analyzed. Identical results to those obtained above under potentiostatic conditions were obtained when constant currents of either 50 or 10 FA were used to oxidize bromide at the generating electrode.
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There is an increase in the current at the generating electrode with increasing allyl alcohol concentration. Since allyl alcohol is electroinactive at the applied generating electrode potential, this current increase is probably caused by electrooxidation of the brominated derivative. It is interesting to compare the response for collection of Br- as Br2 in the presence and absence of allyl alcohol. In the absence of allyl alcohol, the response is 272 nA/mmol Br-. In the presence of allyl alcohol, the response is decreased by -200 nA/mol allyl alcohol. Assuming that the monobromo derivative of allyl alcohol is the sole reaction product, (12), then 75% of allyl alcohol within the convective-diffusion layer reacts with the electrogenerated bromine.
CONCLUSIONS The dual porous electrode membrane configuration has considerable promise for applications to titrations performed by volatile electrogenerated reagents. This is illustrated for the case in which the sample is present in a flowing stream and the potential of the generating electrode is held constant. In the particular case of bromine generation from bromide ion at a platinized platinum electrode, programming the current to carry out an in situ titration (13) is not satisfactory because of concomitant slow oxide formation under our conditions. These conditions were chosen to minimize corrosion of the injector we used. However, a higher acid concentration could be used with a more inert injector. This would overcome this difficulty (13),which is peculiar to the generation of strong oxidants at platinum. The dual porous electrode membrane configuration has the unique advantage of allowing the optimum conditions to be used for both reagent generation and detection. Furthermore, the physical, electrical, and electrochemical isolation of the collecting electrode from the generating electrode and flowing stream eliminates virtually all detection problems caused by the presence of other electroactive species present in the analyte sample. Registry No. HOCH2CH=CH2, 107-18-6. LITERATURE CITED (1) Ivanov, Yu. B.; Levich, V. G. Dokl. Akad. Nauk. SSSR 1959, 126, 1029. (2) Frumkin, A. N.; Nekrasov, L. I . Dokl. Akad. Nauk SSSR 1959, 126, 115. (3) Bruckenstein, S.Nectrokhimla 1988, 9 , 1088. (4) Bruckenstein, S. Trans. Faraday Soc. 1968, 6 2 , 1920.
(5) Albery, W. J.; Bruckenstein, S.;Johnson, D. C. Trans. Faraday Soc. 1968, 62, 1938. (6) Tlndall, G.W.; Bruckenstein, S. Anal. Chem. 1968, 4 0 , 1044. (7) Albery, W. J.; Wood, P. UK Patent Application 2045943; Chem. Abstr. 1981, 9 4 , 167 192. (8) Trojanek, A,; Bruckenstein, S.Anal. Chem. 1988, 58, 866. (9) Trojanek, A.; Bruckenstein, S. Anal. Chem. 1988, 5 8 , 981. (IO) Trojanek, A,; Bruckenstein, S.Anal. Chem. 1986, 58, 983. (11) Albery, W. J.; Hitchman, M. L.; Ulstrup, J. Trans. Faraday Soc. 1989,
65(4),1101. (12)Atkinson, J. R.; Bell, R. P. J . Chem. Soc. 1983 (June), 3260. (13) Nagy, G.;Feher, 2s.Anal. Chim. Acta 1977, 9 1 , 87.
RECEIVED for review January 20,1987. Accepted May 21,1987. This work was supported by the Air Force Office of Scientific Research under AFOSR Grant No. 83-0004.
