Anal. Chem. 1982, 5 4 , 2105-2107
similarly deactivates the electrodes. In these cases, regeneration is only partially possible; both the slope and detection limit suffer although the selectivity is relatively unaffected. Under normal use conditions, the electrodes are stored in 0.1 M HCl. They may be used for a full working day without acid treatment and the repetitious use of the electrode does not noticeably degrade electrode response in this period. Day to day drift is slight; calibrations differing by fl mV may be obtained over a week, with overnight acidic storage between each working day. Drift during use is also slight. Under these “normal” conditions, electrodes in periodic use have retained their slope, detection limits, and selectivity factors over a 6-month period. ISE’s based on ion-exchange principles have been widely recognized for many years and there is %I sound theoretical basis describing the electrode behavior of ion exchange membranes ( I , 18). The electrodes reported here are examples of biionic bathing of an ion exchange membrane which sustains equal and opposite fluxes of the two ions (cations, protons) under net zero current conditions (18). However, the additional feature of these BE’S is the apparent failure of the membranes to respond except when in a “pumping” configuration. Simple cation-proton ion exhanglers without specific binding sites (carboxylic or phosphoric acids) exhibit the expected response to cations at zero pH gradient across the membranes (1S21). However, previously reported membrane electrodes incorporating monensin have the “pumping” configuration (inside acidic with respect to outside) although this is not explicitly stated as an essential condition (7,9). The origins of the anomalous behavior exhibited in our system are not clear but may reflect the more demanding nature, hence slower rate, of ion extraction to a specific site as opposed to rapid, simple ion exchange. Of interest in this regard as well are other synthetic ionophores bearing carboxylic acids on macrocycles (22-24). Continuing studies are addressing the origin of the effect and the generality of the observation that the electrode behavior olf these carboxylate ionophores depends on the “pumping” configuration.
LITERlATURE CITE11 (1) Helfferich, F. “Ion Exchange”; McGraw-Hit New York, 1962; Chapter 8.
2105
(2) Morf, W. E.; Simon, W. I n “Ion-Selective Electrodes in Analytical Chemistry”; Frieser, H., Ed.; Plenum: New York, 1978;Vol. 1. (3) Morf, W. E.; Ammann, D.; Simon, W. Chimia 1974, 28,65. (4) Ovchinnikov, Yu. A.; Ivanov, V. T.; Shkrob, A. M. “Membrane-Active Complexones”; Elsevier: Amsterdam, 1974. (5) Westiey, J. W. Adv. Appl. Microbiol. 1977, 22, 177. (8) Kedem, 0.; Loebel, E.; Furmansky, M. German Patent No. 2027 128, Dec 23, 1970;Chem. Abstr. 1970, 74,70998. (7) Kraig, R. P.; Nicholson, C. Science 1976, 194, 725. (8) Covington, A. K.; Kumar, W. Anal. Chim. Acta 1978, 8 5 , 175. (9) Kotera, K.; Satake, N.; Honda, M.; Fujimoto, M. Membr. Biochem. 1979, 2, 323. (10) Pressman, B. C. Fed. froc., Fed. Am. SOC. Exp. Biol. 1968, 27, 3283. (11) Pfeiffer, D. R.; Read, P. W.; Lardy, H. A. Biochemistry 1974, 79, 4007. (12) Choy, E. M.; Evans, D. F.; Cussier, E. L. J . Am. Chem. SOC. 1974, 9 6 , 7085. (13) Frederick, L. A.; Fyles, T. M.; Gurprasad, N. P.; Whitfield, D. M. Can. J . Chem. 1981, 5 9 , 1724. (14) Fyies, T. M.; MaiikDiemer, V. A.; Whitfieid, D. M. Can. J . Chem. 1981, 59, 1734. (15) Izatt, R. M.; Eatough, D. J.; Christensen, J. J. Struct. Bond. (Berlin) 1973, 16, 161. (16) Fiedler, V.; Ruzicka, J. Anal. Chim. Acta 1973, 6 7 , 179. (17) Ryba, 0.; Petranek, J. J . Nectroanal. Chem. 1973, 4 4 , 425. (18) Buck, R. P. I n “Ion-Selective Electrodes in Anaiytical Chemistry”; Frieser, H., Ed.; Plenum: New York, 1978;Voi. 1. (19) BotrQ, C.; Mascini, M.; Memoli, A. Anal. Chem. 1972, 4 4 , 1371. (20) Brown, H. M.; Pemberton, J. P.; Owen, J. 0.Anal. Chim. Acta 1976,
85,261. (21) Harrell, J. B.; Jones, A. D.; Choppin, G. R. Anal. Chem. 1969, 4 1 , 1459. (22) Strzelbicki, J.; Bartsch, J. Anal. Chem. 1981, 5 3 , 2251. (23) Gokel, G. W.; Dishong, D.; Diamond, C. J. J . Chem. Soc., Chem. Commun. 1980, 1053. (24) Heigeson, R. C.; Weisman, G. R.; Toner, J. L.; Tarnowski, T. L.; Chao, Y.; Mayer, J. M.; Cram, D. J. J . Am. Chem. SOC.1979, 101, 4928.
Thomas M. Fyles* Cynthia A. McGavin Department of Chemistry University of Victoria P.O. Box 1700 Victoria, British Columbia, Canada V8W 2Y2
RECEIVED for review April 5, 1982. Accepted June 4, 1982. This work was initiated by a grant from the Research Corporation (Cottrell Foundation). The project continues to be supported by the National Sciences and Engineering Research Council of Canada and the University of Victoria.
Residual Currents at the Static Mercury Drop Electrode Sir: The static mercury drop electrode (SMDE) (1) represents the first radical alteration to the dropping mercury electrode (DME) of Heyrovsky since its inception in the early 1920s. The wide bore capillary of this electrode permits rapid drop formation from a minimal head of mercury. Drop formation is controlled by a solenoid valve which opens for 50, 100, or 200 ms to form ”small”, “mediumn, or “large” drops. The analytical performance of the SMDE has been described by Bond and Jones (2). In particular Bond emphasized the considerable improvements in the faradaic cirrentxapacitative current ratios due to the stationary nature of the drop at the time of measurement. In sampled dc polarography (sdcp), normal pulse polarography (npp), and differential pulse polarography (dpp), improvements in detection limit were reported of two to ten times, depending on the technique (2). Similar improvements in detection limits have been noted elsewhere (3). Capacitative current is not the sole contributor to the residual (background) current of polarography. Faradaic current arising from the presence of more easily electroreducible
species may contribute a substantial proportion of the residual current. It is the purpose of this paper to describe the behavior of the SMDE where faradaic current is a major contributor and to compare this behavior with that of the DME. Acid extracts of soil, in which Mo6+ is to be determined in the presence of a large excess of Fe3+,were used to illustrate the problems that can arise in these circumstances.
EXPERIMENTAL SECTION Apparatus. The apparatus has been described previously (3, 4). Data were acquired on a single scan basis and were filtered by using a 15-point window. The DME was used at a reservoir height of 40 cm and a mercury flow rate of 1.2 mg s-l. All potentials are referred to a Ag/AgCl reference electrode. Reagents. “Aristar” (B.D.H) acids were used to prepare the electrolytes and to digest the soil samples. All other reagents were reagent grade. Distilled, deionized water was used for all solutions. Oxygen-free nitrogen (B.O.C.) was passed through vanadous chloride solution prior to purging. Procedures. Soil digests were prepared from 10 g of finely ground (
