981
Anal. Chem. 1986, 58, 981-982
13408-63-4;Ru(CN):-, 21029-33-4;IrCh2-, 16918-91-5;graphite, 7782-42-5; hydroquinone, 123-31-9; quinone, 106-51-4.
LITERATURE CITED Oyama, N.; Anson, F. C. J . Electrochem. Soc. 1980, 727, 247-250. Snell, K. D.; Keenan, A. G. Chem. SOC. Rev. 1979, 8 , 259-282. Murray, R. W. Electroanal. Chem. 1984, 73, 191-387. Majda, M.; Faulkner, L. R. J . Electroanal. Chem. 1984, 769, 77-95. Faulkner, L. R. Chem. Eng. News 1984, 27, 28-45. (6) Oyama, N.; Shlmomura, T.; Shlgehara, K.; Anson, F. C. J . Electroanal. Chem. 1980, 112, 271-280. (7) Oyama, N.; Anson, F. C. Anal. Chem. 1980, 5 2 , 1192-1198. (8) Oyama, N.; Sato, K.; Matsuda, H. J . Electroanal. Chem. 1980, 115, 149-155. (9) Shigehara, K.; Oyama, N.; Anson, F. C. Inorg. Chem. 1981, 2 0 , 518-522. (10) Anson, F. C.; Ohsaka, T.; SavBant, J. M. J . Phys. Chem. 1983, 87, 640-647. (11) Anson, F. C.; Ohsaka, T.; SavBant, J. M. J . Am. Chem. Soc. 1983, 105, 4883-4s90. (12) Anson, F. C.; SavOant, J. M.; Shlgehara, K. J . Electroanal. Chem. 1983, 145, 423-430. (13) Anson, F. C.; SavOant, J. M.; Shigehara, K. J . Am. Chem. Soc. 1983, 105, 1096-1106. (14) . . Rublnstein. I.: Bard, A. J. J . Am. Chem. Soc. 1980, 102, 6641-6642. (15) Henning. T. P.; Whlte, H. S.; Bard, A. J. J . Am. Chem. Soc. 1981, 703, 3937-3938. (16) Rubinsteln, I . ; Bard, A. J. J . Am. Chem. Soc. 1981, 703, 5007-5013. - - .. .- . -. (17) Buttry, D. A.; Anson, F. C. J . Electroanal. Chem. 1981, 730, 333-338. (18) Whlte, H. S.; Leddy, J.; Bard, A. J. J. Am. Chem. Suc. 1982, 704, 4sii-4816. (1) (2) (3) (4) (5)
(19) Martln, C. R.; Rubinsteln, I.; Bard, A. J. J . Am. Chem. SOC. 1982, 104,4817-4s24. (20) Buttry, D. A,; Anson, F. C. J . Am. Chem. Soc. 1982, 704, 4824-4829. (21) Martin, C. R.; Rhoades, T. A,; Ferguson, J. A. Anal. Chem. 1982, 5 4 , 1639-1641. (22) Buttry, D. A.; Anson, F. C. J . Am. Chem. Soc. 1983, 705, 685-689. (23) Ovama, N.: Ohsaka, T.; Sato, K.; Yamamoto, H. Anal. Chem. 1983, 55, 1429-1431. Rublnstein, I. J . Electroanal. Chem. 1985, 188, 227-244. Tsou, Y.-M.; Anson, F. C. J . Electrochem. Soc. 1984, 137, 595-801. Ohnuki, Y.; Ohsaka, T.; Matsuda, H.; Oyama, N. J . Electroanal. Chem. 1983, 158, 55-67. Delahay, P. "Double Layer and Electrode Kinetics"; Wlley: New York, London, and Sydney, 1965. Oyama, N.; Ohsaka, T.; Shimizu, T. Anal. Chem. 1985, 5 7 , 1526-1532.
Noboru Oyama* Takeo Ohsaka Takeyoshi Okajima Department of Applied Chemistry for Resources Tokyo University of Agriculture & Technology Koganei, Tokyo 184, Japan RECEIVED for review October 29, 1985. Accepted December 30, 1985. The present work was partially supported by Grant-in-Aid for Scientific Research No. 60211011 for N. Oyama, from the Ministry of Education, Science, and Culture, Japan.
Flow- Injection Analysis of Volatile, Electroinactive Organic Compounds at a Platinum Gas Diffusion Membrane Electrode by Use of a Redox Mediator Sir: Pneumatoamperometric analyses described to date have involved a volatile, electroactive species. This species either is the original sample component or is generated by a chemical reaction. It can either be in a gas (1-6) or liquid (7, 8) carrier stream and is determined after it passes through a gas-permeable, hydrophobic membrane to an electrode. This scheme is shown in Figure 1 of ref 7), where Y is the volatile species. The usual detection technique a t the porous electrode of the gas-permeable membrane electrode, GPME, is amperometry at constant potential (1-5, 7, 8). Cyclic voltammetry has also been employed for the cathodic stripping analysis of silver sulfide films produced by reaction of hydrogen sulfide with a porous silver electrode (6). In this report we describe the determination of a volatile, nonelectroactiue species through its reaction with one component of a redox couple in contact with the porous electrode. One realization of this scheme is shown in Figure 1 for the case where the volatile, nonelectroactive species, Z, reacts with Ox to form Red and products, P. By use of the appropriate electrode potential, Red is oxidized back to Ox at the electrode surface. The current required for this redox process becomes a direct measure of the concentration of the volatile species. The Ox/Red redox couple acts as mediator in the determination of Z. Its concentration and solution environment can be optimized for the chemical and electrochemical steps because the electrochemical cell is isolated from the carrier stream by the hydrophobic gas porous membrane. The Os(VIII)/Os(VI) redox couple in alkaline media reacts rapidly with a number of organic compounds and has excellent redox behavior. For this reason, we illustrate the scheme in Figure 1using the latter redox couple as mediator in the determi0003-2700/86/0358-098 1$01.50/0
nation of acetone, methanol, and ethanol.
EXPERIMENTAL SECTION All apparatus (pump, flow injection assembly, detector cell, electronics, and recorder) have been described previously (7,8). Reagent grade chemicals were used throughout and water was prepared by use of a Millipore Mill-& system. Os04 (0.5 g) was dissolved in 250 mL of 1 M KOH to prepare a 0.0078 M Os(VII1) solution. This solution was used as the supporting electrolyte in the electrochemical cell. The porous platinum electrode on the electrochemical cell side of the GPME was held at +0.3 V vs. the saturated calomel electrode. At this potential Os(VI), produced by the reaction of Os(VII1) with acetone, methanol, or ethanol, is quantitatively reoxidized back to Os(VII1). This potential was chosen on the basis of cyclic voltammetric studies and previous literature (9).
RESULTS AND DISCUSSION Twenty-microliter volumes of aqueous acetone solutions were injected into the carrier stream (air saturated, deionized water) flowing at a rate of 0.5 mL/min. Anodic current peaks proportional to the concentration of acetone studied (4.3 X lo4 M to 3.4 X M) were obtained. The amounts of acetone corresponded to 5-4000 ng. The slope of this linear calibration line was 1.85 nA/ng of acetone. Twelve successive injections of 157-ng samples of acetone yielded peak heights with a relative standard deviation of 0.8%. Typical peak responses are shown in Figure 2 for sample injections made at 1.5-min intervals. A sample analysis rate of 40 samples/h is easily achieved. Analogous studies were performed with methanol (1.58-158 pL) and ethanol (1-500 pg) and a linear relationship between peak current response and sample size was found. The slopes 0 1986 American Chemlcal Soclety
g82
ANALYTICAL CHEMISTRY, VOL. 58, NO. 4, APRIL 1986 Electrolytic Vessel 7-
0 +Ox+
Red t P
-
Metalized
6-
Membrane -----\,
Mobile Phose Flow
Figure 1. Schematic representation of pneumatoamperometrywith a redox mediator. Z is the volatile, noneiectroactivespecies that is oxidized by Ox to Red. Red is electrooxidized at constant potential back to Ox.
0'5------1
1
2
3
4
5
v-1 [min mi-'] Flgure 3. Dependence of the peak current on the recirocai of carrier stream flow rate. Amount of InJectedacetone was (a) 1570 ng and (b) 471 ng.
I-
300 Seconds
&TIME
Flgure 2. Successive current peak responses, 157-ng lnjectloms of acetone into carrier stream, Y = 0.5 mL/min.
were 5 X nA/ng of methanol and 1.2 X nA/ng of ethanol. The responses are in the inverse order of these compounds volatilities and reflect the rates of the chemical reaction of the alcohols and acetone with Os(VII1) ( 1 0 , I I ) . Figure 3 depicts the effect of carrier stream flow rate, u, on current peak height. Two levels of acetone were studied, 471 ng and 1570 ng, and in both cases the current peak height was inversely proportional to u. This result is consistent with a model in which the rate-determining step is the rate of oxidation of acetone. The peak current is proportional to the total time a sample contacts the bare face of the hydrophobic membrane. Changes in the thickness of the convective-diffusion layer play little or no part since a convective-diffusion controlled response would produce an increase in peak current height depending approximately on u1I3. Thus there is little concentration polarization at the carrier stream side of the membrane, and only a small fraction of the acetone in the convective-diffusion layer is consumed by oxidation with Os(VII1). This conclusion suggests that the scheme shown in Figure 1 is capable of much greater sensitivity than has been obtained for the particular examples examined in this report. Even for these cases, it seems likely that several strategies could be employed to increase the sensitivity, e.g., higher concentrations of Os(VII1) could be used as the rate of these sorts of oxidation processes has been found to be proportional to its concentration or the detection cell could be operated at an elevated temperature to increase the rate of reaction.
CONCLUSION The use of a redox mediator to determine volatile species by the pneumatoamperometric technique has considerable promise. It can be used not only for totally electroinactive species but also for species whose electrochemistry is unsatisfactory for a variety of reasons. The approach is suitable for both gaseous and liquid carrier streams. The separation of the indicator electrode and mediator system from the carrier stream by the hydrophobic membrane permits optimizing the choice of carrier stream flow rate, supporting electrolyte, mediator system, and electrode potential. Registry No. Os, 7440-04-2;Pt, 7440-06-4;acetone, 67-64-1; methanol, 67-56-1; ethanol, 64-17-5. LITERATURE CITED Gifford, P.; Bruckensteln, S. Anal. Chem. 1080, 5 2 , 1024-1027. Gifford, P.; Bruckensteln, S. Anal. Chem. 1080, 5 2 , 1026-1031. Beran, P.; Bruckensteln. S. Anal. Chem. 1080, 5 2 , 1183-1166. Beran, P.; Bruckensteln, S. Anal. Chem. 1880. 5 2 , 2207-2209. Beran, P.; Bruckens2eln. S. Anal. Chim. Acta 1081, 136, 369-393. Opekar, F.; Bruckensteln, S. Anal. Chem. 1084, 5 6 , 1206-1209. Trojanek, A.; Bruckenstein, S. Anal. Chem. l 9 8 & 58, 866-869. , Trojanek, A.; Bruckensteln, S. Anat. Chem. 1886, 5 8 , 983-985. (9) Katvoda, R. J . Electroanal. Chem. Interfaclal Electrochem. 1970, 2 4 , 53. (10) Rashid, Abdui; Straka, P.; Kalvoda, R. J . Electroanal. Chem. Interfaclal Electrochem. 1971, 2 9 , 383. (11) Trojanek. A. Chem. Listy 1875, 6 9 , 34.
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'
Permanent address: Jaroslav Heyrovsky Institute of Physlcai Chemistry and Electrochemlsty, Czechoslovak Academy of Sciences, Jllska 16, 11000 Prague 1, Czechoslovakia.
Antonin Trojanek' Stanley Bruckenstein* Chemistry Department University at Buffalo State University of New York Buffalo, New York 14214 RECEIVEDfor review August 14,1985. Accepted November 8,1985. This work was supported by the Air Force Office of Scientific Research under Grant No. 83-0004.
