2396
Anal. Chem. 1980, 52, 2396-2400
periments produce fewer changes in the electrochemical response than do scanning experiments such as semidifferential DPV (7), or CV (7). voltammetry (8), Determination of DA in the presence of ascorbic acid is clearly facilitated with microelectrodes, since contributions from catalytic chemical reaction are minimized. This result is predicted by eq 6 for hemispherical electrodes of identical radius. However, this result can also be rationalized by considering the rate of the catalytic reaction. Under the conditions employed here, the oxidized DA has a 2.1-ms half-life. Assuming that the diffusion coefficient for DOQ is 1X cm2sd, it will diffuse a little over 1.4 pm before being reduced to DA. This distance is much less than the radius of our carbon paste electrode (790 pm) but is only about half the radius of a carbon fiber electrode (2.8 pm). As has been shown, the minimization of catalytic reactions, t.he recognition of the irreversibility of DOPAC, and the shape of the current-voltage curves for well-behaved electrochemical systems can all be readily deduced from existing electrochemical theories. These relationships should lead to an optimization of the use of carbon fiber electrodes in specific applications. I t is important to note that the observations reported here reflect the time scale of the measurements; faster measurements should result in conventional time-dependent currents a t microelectrodes.
LITERATURE CITED Dayton, M. A.; Brown, J. C.; Stutts, K. J.; Wightman, R. M. Anal. Chem. 1980, 5 0 , 946-950. Pletcher, D.; Fleishmann, M. University of Southampton, Southampton, England, personal communication, 1980. Adams, R. N. Anal. Chem. 1978, 48, 1126A-1137A. Wightman, R. M.; Strope, E.; Plotsky, p. M.; Adams, R. N. Nature(L0ndon) 1978, 262, 145-146. Kissinger, P. T.; Hart, J. B.; Adams, R. N. Brain Res. 1973, 55, 209-213. Marsden, C. A.; Conti, J.; Strope, E.; Curzon, G.; Adams, R. N. Brain Res. 1979, 171, 85-99. Cheng, Y.-Y.; Schenk, J.; Huff, R.; Adams, R. N. J. Electroanal. Chem. 1979, 100,23-31.
(8) Lane, R. F.; Hubbard, A. T.; Blaha, C. D. Bloelectrochem. Bioenerg. 1978, 5 , 504-525. (9) Lane, R. F.; Hubbard, A. T.; Blaha, C. D. J . Nectroanal. Chem. 1979, 95. 117-122. (10) Lane, R. F.; Hubbard, A. T.; Fukunaga, K.; Blanchard, R. J. Brain Res. 1978. 114. 346-352. (1 1) GOnOn, F.; Cespuglio, R.; Ponchon, J.-L.; Buda, M.; Jouvet, M.; Adams, R. N.; PujOl, J.-F. C.R. h b d . Seances, Acad. Sci., Ser. D 1978, 286, 1203-1206. (12) Ponchon, J.-L.; Cespuglio, R.; Gonon, F.; Jouvet, M.; Pujol, J.-F. Anal. Chem. 1979, 51, 1483-1486. (13) Loach, P. A. "Handbook of Biochemistry and Molecular Bblogy", 3rd ed.; Fasman, G. D., Ed.; CRC Press: Cleveland, 1976; Vol. 1, pp 122-130. (14) Evans, J. F.; Kuwana, T.; Henne, M. T.; Royer, G. P. J . Electroanal. Chem., 1977, 80, 409-416. (15) Abel, R. H.; Christie, J. H.; Jackson, L. L.; Osteryoung, J. G.; Osteryoung, R. A. Chem. Instrum. (N.Y.) 1978, 7 , 123-138. (16) Burrows, K. C.; Brindle, M. P.; Hughes, M. C. Anal. Chem. 1977, 49, 1459-1461. (17) Soos, 2. G.; Lingane. P. J. J . Phys. Chem. 1984, 68, 3821-3828. (18) Sarangapani. S.; DeLevle, R. J . Electroanal. Chem. 1979. 102, 165-174. (19) Galus, 2. "Fundamentals of Electrochemical Analysis"; Halsted Press: New York, 1976. (20) Shain, I.; Martin, K. J.; Ross, J. W. J. Phys. Chem. 1981, 65. 259-261. (21) Nicholson, R. S. Anal. Chem. 1985, 3 7 , 1351-1355. (22) Delahay, P. "New Instrumental Methods of Anaiysls"; Wiley-Interscience: New York, 1954 p 103. (23) Tse, D. C. S.; McCreery, R. L.; Adams. R. N. J . Med. Chem. 1978, 19, 37-40. (24) Delmastro, J. R.; Smith, D. E. J. Phys. Chem. 1987. 7 1 , 2138-2149. (25) Vetter, K. J. Z . Electrochem. 1952, 56, 797-806. (26) Perone, S. P.; Kretlow, W. J. Anal. Chem. 1988, 38, 1760-1763. (27) Lindaulst. J. J. Electroanal. Chem. 1974. 52. 37-56. i28j Wightman,RI M.; Paik, E. C.;Borman, S.;'Dason, M. A. Anal. Chem. 1978. 50, 1410-1414. (29) Lane, R. F.; Hubbard, A. T. Anal. Chem. 1978, 48, 1287-1293.
RECEIVED for review June 9, 1980. Accepted September 19, 1980. This research was supported by the National Science Foundation (Grant NO.BNS 77-28254). M.A.D. is a combined Medical-Ph.D. candidate, Indiana University. R.M.W. is the recipient Of a Research Career Award from the National Institutes of Health (Grant No. 1 KO4 NS 00356).
Determination of Sulfur Dioxide by Reaction with Electrogenerated Bromine in a Thin-Layer Cell Having a Gas-Porous Wall Stanley Bruckenstein," Kevin A. Tucker, and Paul R. Glfford Department of Chemistv, State University of New York at Buffalo, Buffalo, New York 14214
An electrochemical sensor is described for the determination of ambient sulfur dioxide by reactlon with electrochemically generated bromine in a thin-layer cell. Sulfur dioxide in the gas phase diffuses through a porous, hydrophoblc membrane into a thin layer of solution in which bromlne is generated electrochemically. The quantity of bromine at the gas-solution interface is determined by reduction at a gold cathode sltuated on the solution side of the porous wall separating the gas phase from the solution phase. The dlffusion of sulfur dioxide through the porous membrane into the thin-layer cell decreases the amount of bromine reaching the gold cathode. This decrease produces a change In current that Is proportional to the concentration of sulfur dioxide in the gas phase. The devlce was evaluated for sensitivity and stabllity for the determination of parts-per-million levels of sulfur dioxide.
Electrochemical analyzers have been extensively used for
monitoring ambient sulfur dioxide levels, and several commercial instruments using the "coulometric" principle are presently available. These instruments provide a sensitive method for the determination of sulfur dioxide and the technique has been defined in ref 1 as an equivalent method to the manual reference colorimetric procedure. The method was initially developed by Shaffer, Briglio, and Brockman for the detection of mustard gas (2) and was later extended to sulfur dioxide determinations. The principle behind these analyzers is based on the oxidation of sulfur dioxide to sulfate by electrogenerated halogen (iodine or bromine). A gas stream containing sulfur dioxide is bubbled through a stirred electrolyte containing halide ion (Br- or I-) in an electrochemical cell, and the halogen is generated by electrooxidation of the halide ion. The cell contains two pairs of electrodes. One pair consists of an indicator and reference electrode, and the emf between these two electrodes is determined potentiometrically; this emf is a measure of free halogen concentration in solution. The other
0003-2700/80/0352-2396$01,00/00 1980 American Chemical Society
ANALYTICAL CHEMISTRY, VOL. 52, NO. 14, DECEMBER 1980
E
Pt ANODE’
CRREON ELECTRODE
GRS PHRSE
K
