1254
Anal. Chem. 1981, 53, 1254-1257
Membrane Polarographic Detectors for Determination of Hydrogen and Oxygen Produced by the Photodissociation of Water Andrew Mllls,* Anthony Harriman, and George Porter Davy Faraday Research Laboratory, The Royal Institution, 2 1 Albemarle Street, London, W1X 4BS
Two-electrode membrane polarographlc detectors (MPDs) were used as detector units for monltorlng the concentration of H2 and O2 produced from the photodlssoclatlon of water. Responses Included good stability, short response times, and h e a r correlatlons with the concentratlon of H2 or O2 over 3 orders of magnitude. These units provlde rapld, facile, and Inexpensive methods for following the concentration of evolved gas from such photochemlcal systems.
There is considerable interest in the collection and storage of solar energy in the form of chemical potential. One approach toward this ideal involves the photosensitized dissociation of water into H2 and O2and, in recent years, considerable advances have been made in this field (1-5). In fact, there are numerous photochemical systems capable of generation of either H2 or 0, from water upon irradiation with visible light and, very recently, cyclic cleavage of water into both Hzand 02 has been reported (6). However, before these systems can be used in practical devices for the storage of solar energy it is necessary that the efficiency for production of the gaseous products be optimized. This optimization requires routine, repetitive experimental work involving only minor variations in reaction conditions (e.g., pH, pressure, type of catalyst, concentration) and involves accurate quantitative analysis of H2 and 02. In most systems, analysis for H2 and 0, has involved mass spectrometry (MS) (7), gas chromatography (GC)(8), or “End-0-Mess” (9) techniques, but, although these methods allow selective and sensitive analytical determinations, they require perturbation of the system under investigation. All three techniques necessitate removal of samples from the system and this perturbation presents problems, especially if several samples have to be removed from the same solution. As an alternative to the above techniques, membrane polarographic detectors (MPD) offer a high degree of specificity and sensitivity (10). The specificity arises from the use of (i) a preselected polarizing voltage so that only Hzor 02 is detected and (ii) a membrane, permeable to gases only, which isolates the working electrode from the reaction solution. The sensitivity arises from the use of electronics that can accurately measure small currents. MPDs for determination of O2in aqueous solution have been known since 1956 and have been developed to a high degree (10). The MPD can be used in a closed system to monitor continuously the O2concentration in the liquid phase and the sensitivity of such devices compares favorably with the more expensive MS, GC, and “End-0-Mess” techniques. However, MPDs for quantitative analysis of Hz in aqueous solution have received little attention although Green (11)has patented several such devices. More recently, Calzaferri et al. (12) improved the previous systems by developing a three-electrode H2-MPD that was particularly sensitive to low concentrations of Hz. In this paper, we describe a two-electrode MPD that can be used for the continuous and rapid
quantitative analysis of both O2and H2 in aqueous solution and we examine the potential use of such devices as detectors for the routine estimation of Hz and/or O2produced from the photodissociation of water.
EXPERIMENTAL SECTION As shown in Figure 1 the MPD (supplied by Rank Bros., Cambridge, England) consisted of a plastic base (2) containing both the Pt working electrode (9) and the Ag/AgCl counterelectrode (8). Over these, a Teflon membrane was situated (10) clamped firmly into position by the silicone rubber “0”ring (11). This “0” ring was held in position by the cell top (3) which, in turn, was clamped to the base (2) by two spring-loaded clips (7). The glass cell top (3) consisted of a central reservoir (14) (volume 37 mL), for the reaction solution, surrounded by a thermostated water jacket (15) and was equipped with suitable inlet and outlet tubes (13) for saturating the solution with a gas. A septum cap (4) allowed aliquots of the vapor phase to be withdrawn and analyzed by GC. The MPD was held in position (12) on a magnetic stirrer so that the solution could be agitated continuously throughout the experiment and the required polarizing voltage was applied via a potentiostat (6). The output was monitored with a Servoscribe 1s x/t chart recorder. Polarograms were recorded with a triangle wave generator (13) and the output monitored with a HR2000 x/y recorder. The response toward dissolved 0, or Hz was calibrated by using a number of 02/Nzor Hz/Nzgas mixtures obtained by means of the tangential gas mixer as shown in Figure 2. The percent 02 or H2 composition in these mixtures was determined by GC using the conditions recommended by Valenty (8). In the preparation of electrodes for either 0, or Hz detection, both the Ag counterelectrode and the Pt working electrode were polished with alumina. The clean Ag counterelectrode was plated with AgCl following the galvanostatic procedure outlined by Janz and Ives (14).For Hz analysis, the Pt electrode was platinized according to the procedures recommended by Calzaferri et al. (12) and by Janz and Ives (15). The final appearance of the platinized electrode was dark gray with the original metal sheen being clearly visible (16). The H,-MPD was then set up as follows: (i) Sufficient 1M KCl electrolyte was added to wet the Ag/Agcl and Pt electrodes. (ii) A 1-cm2lens tissue, with a 1-mm2hole in its centre, was placed over the P t electrode. (iii) A 1-cm2piece of Teflon membrane (0.0005 in. thickness) was placed over the two electrodes. (iv) A silicon rubber “0’ ring was placed over the Pt electrode so as to hold in place the Teflon membrane when the plastic base and the glass cell top were clamped together. (v) The platinized Pt working electrode was polarized at +0.4 V vs. Ag/AgCl reference/counterelectrode. In setting up the 0,-MPD an identical procedure was followed except that a polarizing voltage of -0.7 V vs. Ag/AgCl was used. Solutions under investigation were thermostated at 25 “C and stirred continuously. In all cases, samples of the vapor were analyzed by GC in order t o maintain an independent check on the MPD output. The calibrated MPDs were used as detection units for the photochemical production of H2 and O2from water using welldocumented photochemical systems. For the photoproduction of H2,a system devised by Gratzel(3) and by Kagan (4) and their co-workers was used. This involved irradiation of Nz-purged
0003-2700/81/0353-1254$01.25/00 1981 American Chemical Society
ANALYTICAL CHEMISTRY, VOL. 53, NO. 8, JULY 1981
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WATER
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N2 O2 or H2 GAS FLOW
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N 2 GAS FLOW
Flgure 2. Tangential gas mixer (A) front view and (6)top view: (1) plastic body; (2) gas mixture outlet; (3) rubber septum cap: (4)plastic
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GAS MIXTURE OUT
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connector of plastic tangential mixing jets (5)and glass needle valves (6); (5) tangential mixing jets; (6) and (7) glass needle valve, which regulates the flow of one component of the gas mixture.
remove light of X 400 nm resulted in the continuous production of 02,as monitored by the MPD, in the liquid phase. ?Lpical O2concentrations were in the order of -0.33% O2 saturation in the liquid phase for a 15-min irradiation period, and, under these conditions, no O2 could be detected in the vapor phase, as monitored by GC. The total concentration of 02 that can be formed by this system is limited, since prolonged irradiation leads to the destruction of the chromophore (12) and thus irradiation is restricted to short times. The use of a sensitive MPD which measures O2concentrations in the liquid phase has proved invaluable in following the course of reaction in the above system.
ACKNOWLEDGMENT We are greatly indebted to David Madill, Stephen Pavlou, Peter Barron, and W. J. Albery for their technical assistance and helpful advice. In particular, we thank David Madill who designed and constructed the tangential gas mixer. Also we thank Rank Brothers, Cambridge, for help in the design and construction of the MPD. LITERATURE CITED (1) Koryakin, B. V.; Dzhaliev, T. S.; Shilov, A. E. Chem. Absff. 1977, 86, 166117. Dokl. Akad. Nauk SSSR 1977, 233, 620. (2) Lehn, J. M.; Sauvage, J. P.; Ziessel, R. N o w . J . Chlm. 1979, 3 , 423. (3) Kalyanasundaram. K.; Kiwi, J.; Gratzel, M. Helv. Chlm. Acta 1978, 61, 2720. (4) Moradpour, A.; Amouyal, E.; Keller, P.; Kagan, H. Nouv. J. Chlm. 1978, 2, 547. (5) Krasna, A. I.Photochem. Photobkl. 1979, 29, 267. (6) Kalyanasundaram, K.; Gratzel, M. Angew. Chem., rnt. Ed. En@. 1979, 18, 701. (7) Keller, P.; Moradpour, A.; Amouyal, E.; Kagan, H. B. Now. J. Chim. 1880, 4 , 377. (8) Valenty, S. J. Anal. Chem. 1978, 50, 669. (9) Kalyanasundaram, K.; Micic, 0.; Pramauro, E.; Gratzel, M. Helv. Chlm. Acta 1979, 62, 2434. (10) Hitchman, h.l. L. "Measurement of Dissolved Oxygen"; Wiley-Interscience: New York, 1978; Chapter 5. (11) Green, M. W. U S . Patent 3325276, 1967. (12) Gruniger, Von H. R.; Sulzberger, B.; Calzaferri, G. Helv. Chlm. Acta 1978, 61, 2375. (13) Huntingdon, J. L.; Davis, D. G. Chem. Insfrum. (N.Y.) 1970, 2 , 83. (14) Janz, G. J.; Ives, D. J. G. Ann. N.Y. Acad. Sci. 1988, 148, 210. (15) Janz, G. J.; Ives, D. J. G. "Reference Electrodes"; Academic Press; New York and London, 1961; p 106. (16) Hill. G. J.: Ives. D. J. G. J. Chem. SOC. 1951. 305. Neumann-Spallart, M.; Kalyanasundaram, K.; Gratzel, C.; Gratzel, M. Helv. Chlm. Acta 1980, 63, 1111. Fatt, I."Polarographlc Oxygen Sensors"; Chemical Rubber Co. Press: Cleveland, OH, 1976; Chapter 2. Mancy, K. H.; Okun, D. A.; Reilley, C. N. J. Necfroanal. Chem. 1982, 4 , 65.
RECEIVED for review January 26, 1981. Accepted April 13, 1981. We thank the Science Research Council (S.R.C.) for financial support of this work.
