Anal. Chem. 199466, 2197-2199
Pulse Electrolysis within a Solution Boundary Layer To Minimize Convective Effects Ken-lchl Morita' and Etuo Furuyat Materials Science and Technology, Toin Universiv of Yokohama, 16 14 Kurogane-cho, MidorNu, Yokohama, 225 Japan
Puise electrolysis within the solutionboundary layer to minimize the convective effect was carried out by using a microbole array electrode or a membrane oxygen sensor that has an additional spacer between the membrane and a platinum electrode. Currents for the reduction of oxygen became completelyflow-rateinsensitivewhen thedepthofthe cylindrical well over active microdisk electrodes was more than 40 pm, pulse steptime was 280- and time between pulses was greater than 30 s. Pulse electrolysis of a membrane oxygen sensor that has a spacer with a 40-pm thickness and a polypropylene fhas a membrane was studied as8 flow-rate insensitive sensor. When an electrochemical reaction is performed in a flow system and electrodes are used in the conventional steadystate manner, the electrode output varies with any change in the flow rate. The flow-rate requirement sometimes causes serious problems. It has been reported's2 that an electrode can be made insensitive to convection if pulse techniques are used in conjunction with a boundary layer that restricts interactions with the convective region. The boundary layer for a common electrode is not sufficiently thick to draw any clear distinction between the faradaic and charging currents. We have recently reported3that for assembledmicroholearray electrodes the thickness of the diffusion layer for the electrochemical reaction in a flow system is the sum of the depth of the microhole and the thickness of the boundary layer. This paper describes pulse electrolysis utilizing a microhole array ele~trode.~ Amperometric oxygen sensors have been widely used in the fields of fermentation, fish cultivation, water treatment, and other industries.516 Oxygen sensors are sensitive to the flow rates of a solution. A sensor which is insensitive to the flow rate has been developed by Leed & Northrop Company.' Caudill et a1.8 reported that a carbon-fiber microdisk array electrode, the microdisksof which were arranged 6 diameters apart from each other, was insensitiveto the flow rate. Pulse techniques for this purpose have emerged several times in the ~
t On leave from the Department of Analytical Technology, TOA Electronics Ltd., Kitairiso, Sayama-shi, 613 Japan. (1) Daviess P. W.; Brink, F., Jr. Reo. Sci. Instrum. 1942, 13, 524. (2) W". K. D.; Smart, R. B.; Mancy, K. H. Anal. Chim. Acta 1980,116,297. (3) Monta, K.; Shimizu, Y. Anal. Chcm. 1989,61, 159. (4) Preliminary account of the work was reported at the 13th Chemical Sensor Symposium, October 12, 1991, Nagoya, Japan. (5) Fatt, I. Polarographic OxygcnScnsors; Robert E. Krieger Publ. Co.: Florida, 1982. (6) Gnaiger, E., Forter, H.,Eds. Polarographic Oxygen Sensors; SpringerVcrlag: Berlin, Heidelberg, New York, 1983. (7) US. Patent 4076596. Feb 28, 1978. (8) Caudill, W. L.; Howell, J. 0.;Wightman, R. M. Anal. Chem. 1982,542532.
0003-2700/94/03682197~04.50/0 @ 1994 Amsrican Chemical Society
community of oxygen sensors.1-2$-12 We have studied the pulse amperometry of a membrane sensor which has an additional spacer between the membrane and a platinum electrode! EXPERIMENTAL SECTION Apparatus. A computer-controlled potentiostat (HECS 328B (Fuso Manuf. Co.)) was employed. Electrodes and Electrochemical Measurements. Platinized microhole array electrodes with a diameter of 2 mm were prepared from high-strength carbon fibers (Torayca T-300; the number of fibers in a single electrode, 1000; 6.93 pm in diameter) using a fabrication technique described in a previous paper. '3 The electrochemicalcell compriseda 100-mL glass beaker with three holes in a silicon rubber lid for a three-electrode system. A platinum wire served as an auxiliary electrode. The solutionwas stirredby a mechanical stirrer with a standard bar at a constant rotation speed of 200 rpm using a Chemy Stirrer B-200G (Tokyo Rikakikai Co. Ltd.). The velocity at the electrode was measured using a Fiberoptic Laser Velocimeter System 88 11 (Japan Kanomax Co.) and was around 0.5 ms-l. For pulse electrolysis for dissolved oxygen the working electrodes were stepped to -0.6 V vs a standard calomel reference electrode (SCE) for the pulse step time and floated at open-circuit potentials or held at 0 V vs SCE during the time between pulses. For pulse electrolysisof Fe(CN)& the electrode was stepped to 0 V. Membrane Oxygen Sensor. The sensor comprised a platinum working electrode of 1-mm diameter, silver as the counter electrode, polypropylene film (Celgard 2400, Daicel Chemical Industries, Ltd.; film thicknes, 25 pm; mean pore diameter, 0.02 pm, maximum diameter 0.2 pm; cavity ratio, 38%; total surface area, 50 m2 gl)as a membrane, and a nonwoven textile with a 40-pm thickness (H81015, Japan Vilene Co. Ltd.) as a spacer between the working electrode and the membrane. The electrolyte was a mixture of 0.1 M KOH and 0.2 M KCl. The response time (tm) was defined as the time it took to reach 90% of the steady-state current between an air-purged solution and an aqueous 5% NazSO3 solution. ~
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(9) Lilley, M. D.; Story, J. B.;Raible, R. W. J. Electrwnal. Chcm. 1969,23,425. (10) Scrak, L.; Jelinck, R.; Hauscr, V. J . Elecrroanal. Chcm. 1987, 226, 193. (11) Silver, I. A. Philos. Trans. R. Soc. London, B 1987, 316, 161. (12) Siu, W.; Cobbold, R. C. Med. Elol. Eng. 1976, 109. (13) Shimizu, Y.; Morita, K. J. Electrochem. Soc. 1992, 139, 1240.
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Time between Pulses / 8 Figure 1. Ratlo of the currents In the stirred state to the statbmy state for a reduction of oxygen In an aqueous 0.9% NaCl ~~IMOII at 30 O C by pulse ekctrdysls as a functbn of tlme between put3e8 for microhole electrodes wlth depths of 0 (O), 10 ( a ) , 20 (X), 40 (0)and 80pm(O).Conditions:-0.6VvsSCE;potentlaidvlngthetimebaw~n pulses, open potential; pulse step time, 280 ms.
Depth of microhole / um Flgu. 2. Ratio of the currents In the stirred state to the stationery state for the reductkn of oxygen in an aqoeous 0.9% NaCi adutlon at 30O C bypulseeb&dythasa hnctknofihedepthofthemlaohoks. Condltkn~: -0.6 V vs SCE; potentlelduingthethW bOtWWP&eS, open potentkl; pulse step tknes,280 (0),840 (X), or 1140 m (*)*
RESULTS AND DISCUSSION Pulse Electrolysis Using Microhole Array Electrodes. According to the Cottrell equation14 the thickness of the diffusion layer (t)is given by
fromconvectionwhen the step time was 280 ms and the depths of the microholes were deeper than 40 pm. After carrying out calculations using the above equation and280msforthesteptime,2.5X 10-5cm2s-1forthediffusion coefficient of oxygen in water, and a value of 10 pm for the thicknessof the solution boundary layer (1 value), we obtained 37 pm for thenecessarydepthofthemicrohole. Thecalculated value agreed approximately with the experimental value (40 pm). A range of 1 values from 4 to 13 pm was observed for the reduction of dissolved oxygen or Fe(cN)6% in previous ~tudies.~J5The discrepancy may result from a possible variation of the stirring conditions in the solution. We used 10 pm as an 1 value in the calculation. In the case of the reduction of 1 mM Fe(CN)6> in an aqueous solution containing 0.4 M Na2S04 at 0 V vs SCE,the experimental value was about 20 pm, compared to the calculated value of 18 pm when 9.1 X 1V cmz s-l was used for the diffusion coefficient15of Fe(CN)&. Here again, a close agreement between the observed and calculated values was obtained. A linear calibration curve was obtained between the pulse and the steady-state measurements. It is now clear that one can measure the currents of electrochemical reactions without experiencing any effect of convectiondue to the pulse technique using a microhole array electrode which possesses cylindrical wells of about 40-pm depth over active microdisk electrodes. Pulse Electrolysis Using M Amprometric Membrane Oxygen Seasor. The sensor used in these experiments had a spacer with 40-pmthickness (nonwoven textile) between a Pt electrode (1-mmdiameter) and a polypropylene membrane. The counter electrode was silver, and the electrolyte was a mixture of 0.1 M KOH and 0.2 M KCI. Figure 3 shows current-potential curves for normal pulse voltammograms for the reduction of dissolved oxygen in pure water at 30 OC. Pulse potentials were applied every 20 mV, and the waiting period was fixed at 5 s. The potential step time changed to 40 ms (1). 120 ms (2), and 280 ms (3). Figure 3a shows the result of electrolysis when the cell was not connected to an external power supply during the time between pulses; Figure
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wheret isthe potentialsteptimeandDisa diffusioncoefficient. For the microhole array electrode the current should become flow-rate insensitive if the potential step time is less than
or the depth of the microhole (h) is more than (uDt)'.'
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where I is the thickness of the solution diffusion boundary layer. Reduction of dissolved oxygen was carried out by pulse electrolysis using microhole array electrodes having various depths. The working electrodes were stepped to -0.6 V vs a standard calomel reference electrode for the pulse step time and floated at open-circuit potentials during the waiting time between pulses. The ratio of the currents in the stirred state to the stationarystate asa functionof the waiting timebetween pulses at different microhole depths (0-80 pm) is shown in Figure 1. The deeper were the depths of the microholes, the greater was the waiting time between pulses required to obtain a constant value of the ratio. The currents for the reduction of oxygen became completely flow-rate insensitive when the depths were more than about 40 fim and the waiting time was greater than 30 s. The dependence of the ratio on the depth of the microholes is shown in Figure 2. The currents were measured at step times of 280,840, and 1140 ms, respectively. The shorter were the potential step times, the smaller was the ratio observed, and the reduction currents became insensitive (14) Cottrell, E G.2.Physik. Chcm., Stoechiom. Verwandtschafffsl.1902, 42, 385.
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(15) Tokuda, K.;Morita,
K.;Shimizu, Y.A
d Chcm. 1989.61, 1763.
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Time / min. Flgure 4. Currents of the membrane sensor with the 40-pm spacer in air-saturated water (1) and In an aqueous 5 % NazSO. solution (2). Conditions: -0.6 V w) SCE; tkne between pulses, 5 s; potential step times, 120 ms; potential between pulses, open potential (a) or 0 V (b).
3b shows the result when the cell was poised at 0 V vs SCE during the time between pulses. A plateau was observed when the potential step time was 120 or 280 ms. The effects of the potentials during the pulse waiting time on the charging currents were investigated. Figure 4a shows the current-time curves when the potential was floated at the open-circuit potentials during the pulse waiting time, and Figure 4b shows the curves when the potential was fixed at
Received for review June 9, 1893. Accepted February 21, 1904.O Abstract published in Advance ACS Abstracts. May
IS, 1994.
Analyticel Chembtty, Vd. 66, No. 13, July 1, 1994
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