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Anal. Chem. 1983, 55, 698-704
Differential Normal Pulse Voltammetric Oxidation of Mercury in the Presence of Iodide Timothy R. Brumleve,' Robert A. Osteryoung, and Janet Osteryoung" Department of Chemlstty, State Unlversiv of New York at Buffalo, Buffalo. New York 14214
The technlque of dlfferentlal normal pulse voltammetry wlth alternatlng sign of the second pulse Is applled to the determlnatlon of Iodide at a statlc mercury drop electrode. The greatest analytlcal utlllty Is found for low Iodide concentratlons and small pulse wldths, whlch llmlt the surface coverage of adsorbed product to a small fractlon of a full monolayer. Under these condltlons the dlfferentlal normal pulse responses are dlffusion controlled and provlde llnear analytlcal working curves. A dlffuslon-controlled, paftlal electron-transfer adsorption mechanlsm Is developed whlch accounts for the observed behavior at small coverage.
The technique of differential normal pulse (DNP) voltammetry employs a wave form which is a hybridization of the differential pulse (DP) and normal pulse (NP) wave forms. The working electrode spends most of the time at an initial potential, Ei,and the faradaic reaction takes place during the application of double potential pulses of short duration (typically 1-500 ms). The resulting voltammetric response is the differential display of currents sampled near the end of each step of the double pulse. This technique thus provides the advantage of peak shaped voltammograms (as obtained in DP voltammetry) while limiting the extent of the faradaic reaction (as in N P voltammetry). Recently DNP voltammetry has been applied to reversible reactions at mercury electrodes (1) and irreversible reactions at solid electrodes (2). A certain sequence of pulse application (referred to as the alternating pulse mode (1-3)) has been found to be advantageous for improving the voltammetric peak shape and reducing background responses. The alternating pulse modification involves subtraction of the voltammograms obtained from differential (second) pulses of alternating sign and yields symmetric (1) or nearly symmetric ( 2 , 3 )voltammetric peaks. This procedure resembles techniques involving differencing of NP responses (4,5),and equivalent information may result (at least in cases where kinetic complications are absent). The truly differential forward and reverse DNP responses, however, provide additional diagnostic information not available with subtractive N P methods. This paper is concerned with the application of the DNP technique in the alternating pulse mode of the determination of I- at a static mercury drop electrode (SMDE). While a number of cathodic stripping techniques for iodide and other halides have been reported (6-11),direct pulse voltammetric determinations have received little attention (12). Although direct pulse methods cannot be expected to attain the detection limits of cathodic stripping techniques (see especially ref 6)),they offer the advantages of fast analysis and freedom from such interferences as slow halide exchange reactions encountered in simultaneous halide stripping determinations (7,8).Although this study does not treat rigorously the subject of detection limits and chemical interferences, the applicability of the DNP technique is demonstrated at I- concentrations as low as 9.7 MM(1.2 ppm). Present address: Anderson Physics Laboratories,406 N. Busey
Av., Urbana, IL 61801.
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At low I- concentrations ( system by Peter et al. (31). A similar underpotential deposition (or adsorption) mechanism was found at low coverage, followed by a first-order phase transition and nucleation and growth of multiple layers. The resultg clearly indicate, however, that the dramatic shifts in peak and wave slope and position are consistent with a change in stoichiometry of the electrode reaction: from the partial electron transfer adsorption process of reaction 2 1 at low coverages to the formation of mono- or multilayers of HgJ, described by reaction 2. In the above the DNP technique, by virtue of providing a fixed potential where no reaction occurs as in NP voltammetry, but also yielding a differential response, has proven useful for both analysis and elucidation of mechanism. The results indicate that the technique should be valuable in the study of other adsorption and passivation phenomena.
ACKNOWLEDGMENT The authors thank John J. O'Dea for technical assistance and helpful discussions. Registry No. Hg,7439-97-6. LITERATURE CITED (1) Brumleve, T. R.; O'Dea. J. J.; Osteryoung, R. A.; Osteryoung, Janet Anal. Chem. 1081, 53, 702-706. (2) Brumleve, T. R.; Osteryoung, R. A.; Osteryoung, Janet Anal. Cbem. 1082, 5 4 , 782-787. (3) Brumleve, T. R.; Osteryoung, Janet Anal. Cbem. 1081, 53, 988-991. (4) Anderson, J. E.; Bond, A. M. Anal. Chem. 1881, 53, 504-508. (5) Lane, R. F.; Hubbard, A. T. Anal. Cbem. 1076, 48, 1287-1293.
Anal. Chem. 1083, 55, 704-708 Propst, R. C. Anal. Chem. 1977, 49, 1199-1205. Colovos, G.; Wilson, G. S.; Moyers, J. L. Anal. Chem. 1974, 46, 1045-1050. Colvos, G.; Wilson, Q. S.; Moyers, J. L. Anal. Chem. 1974, 46, 105 1-1054. Perchard, J. P.; Bwet, M.; Molina, R. J. Electroanal. Chem. 1967, 14, 57-74. Kemula, W.; Kubllk, Z.; Taraszewska, J. Mlcrochem. J. Symp. Ser. 1962, 2 , 865-877. S h a h 1.; Perone, S. P. Anal. ChSm. 1961, 33, 325-329. Turner, J. A.; Abel, R. H.; Osteryoung, R. A. Anal. Chem. 1975, 47, 1343-1347. Schuitze, J. W.; Vetter, K. J. J. Electroanal. Chem. 1973, 44, 63-61. Schultze, J. W.; Koppltz, F. D.Electrochlm. Acta 1978, 21, 327-336. Bouit, E. H.; Thlrsk, H. R. Trans. Faraday SOC. 1954, 50, 404-412. Aokl, K.; Osteryoung, Janet; Osteryoung, R. A. J. €lectroanal. Chem. 1980. 1-18 ...-, 110 . .. .-. Bewlck, A.; Fleischmann, M.; Thlrsk, H. R. Trans. Faraday SOC.1962, 58, 2200-2216, Flelschmann, M.; Thirsk, H. R. J. Electrochem. SOC. 1963, 110, 686-698. Flelschmann, M.; Thirsk, H. R. Electrochim. Acta 1964, 9 , 757-771. Armstrong, R. D.;Fieischmann, M.; Thlrsk, H. R. Trans. Faraday SOC. 1965, 61, 2238. Thlrsk, H. R.; Harrison, J. A. ”A Qulde to the Study of Electrode
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Kinetics”; Academic Press: New York, 1972; pp 115-135. Astiey, D. J.; Harrison, J. A.; Thlrsk, H. R. J. E/ectroana/. Chem. 1966, 19, 325-334. Conway, B. E.; Angerstein-Kozlowska, H. Acc. Chem. Res. 1981, 14, 49-56. Delahay, P. “New Instrumental Methods in Electrochemistry”; Interscience: New York, 1954; pp 278-281. Deiahay, P.; Trachtenberg, 1. J. Am. Chem. SOC. 1957, 79, 2355-2362. Relnmuth, W. H. J. Phys. Chem. 1961, 65, 473-476. Armstrong, R. D.;Harrison, J. A. J. Nectrochem. SOC. 1969, 116, 328-33 1. Relnmuth, W. J.; Balasubramanlan, K. J. Electroanal. Chem. 1972, 36, 271-281. Grahame, D. C. J. Am. Chem. SOC. 1958, 8 0 , 4201-4210. Kolthoff, I. M.; Lingane, J. J. “Polarography”; Interscience: New York, 1952; Vol. 11, pp 577-581. Peter, L. M.; Reid, J. D.;Scharlfker, B. R. J. €/ectroanal. Chem. 1981, 119. 73-91.
R E C E ~for D review July 21, 1982. Accepted January 5, 1983. This work was supported in part by the National Science Foundation under Grant No. 7917543.
Faradaic Response in Derivative and Differential Normal Pulse Voltammetry Mlllvoy Lovrle, John J. O’Dea, and Janet Osteryoung” Department of Chemlstty, State Unlversl@of New York at Buffalo, Buffalo, New York 14214
The peak-shaped voltammograms arising from differentlal pulse, dlfferentiai normal pulse, and normal pulse dlfference modes of pulse voltammetry are compared for reversible systems and for systems wlth heterogeneous charge transfer control. The effects of variatlon of klnetlc parameters, k , and a,and the dependence on experlmental parameters of time and potentlal are examlned. For the reverslble case the current-potentlal curves are the same for the appropriate choke of experlmentai parameters. For quasi-reverslble cases the differences depend strongly on klnetlc parameters. Maxlmum differences are obtalned for the totally Irreversible case. The dlfferences between the differential normal pulse and normal pulse difference currents are less slgniflcant than the striking relatlve dlmlnutlon of the differential pulse response for irreversible systems.
The advent of computer-controlled experiments has made possible and encouraged the proliferation of potential-time wave forms and current measurement schemes in pulse voltammetry. Variations on normal pulse voltammetry are especially interesting, for in the normal pulse mode the indicator electrode is maintained most of the time at a potential at which no faradaic reaction occurs. Thus for each pulse the same initial conditionsobtain at the time of pulse application. A “derivative” voltammogram can be obtained by using the difference between the currents arising from successive pulses. This option was available on the PARC Model 170 electrochemical system (1).Auerbach et al. did the same in the form of “incremental derivative polarography” (2). Jackson et al. (3)and Abel at al. (4) have described modifications of the PARC Model 174 polarographicanalyzer which also have this feature. Lane and Hubbard employed essentially the same technique but called it “differential double pulse voltammetry” 0003-2700/63/0355-0704$01.50/0
(5). Albery et al. have presentec an extensive theoretical description of charge-transfer irreversibility for “differential double pulse” (6). Anderson and Bond have described “pseudoderivativenormal-pulse polarography”,again a technique in which the normal pulse wave form is used but the currents are subtracted in a fashion which produces a peakshaped current-potential response (7). In an elaboration of this work Evans and Hanck have verified again the simple theory for the reversible case and presented an example of the response for an irreversible case (8). In all of this work, the current response arises from application of a series of single pulses, each applied from the same initial conditions. In contrast with these approaches, a differential technique may be used to give a peak-shaped response. So-called differential normal pulse voltammetry, in which a series of double pulses are applied, is an example (9-13). Traditional differential pulse voltammetry developed by Barker (14)is from the theoretical viewpoint a special case of differential normal pulse voltammetry in which the time duration of the fiit pulse is long in comparison with the time duration of the second pulse. The result of subtracting currents from successive double pulses gives peak-shaped voltammograms which resemble the various derivative normal pulse responses described above (10,12,13). There are several factors which should influence the choice of pulse wave form and current measurement scheme for either analytical or mechanistic studies. The pulse wave form should control conditions a t the electrode surface to enhance reproducibility, discriminate against unwanted capacitative or faradaic currents, and provide sensitivity appropriate for the application. Differential normal pulse voltammetry and normal pulse voltammetry have in common the characteristic that most of the time is spent at a potential at which no current flows. This potential may be chosen to affect adsorption or, especially at solid electrodes, surface reactions. 0 1983 American Chemical Society
