A.J. Bard, L.R. Faulkner - Electrochemical methods - Fundamentals and Applications (794273), страница 79
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Contamination from the electrolyte canbe reduced by lowering its concentration from the usual 0.1 to 1 M range to 0.01 M or even0.001 M. The lower limit is fixed by the maximum cell resistance that can be tolerated, if it isnot set first by chemical considerations, such as the role of the supporting electrolyte in complexation or pH determination. In most analyses, aqueous media are used, both for convenience and for compatibility with the chemistry of sample preparation; however, othersolvents can provide superior working ranges and merit consideration for new applications.The working range for any medium is much narrower for trace analysis by differential pulsepolarography or square wave voltammetry than for conventional polarography, simply because the residual faradaic background becomes intolerably high at less extreme potentials.This point is clear from the data in Figure 7.3.17.Under some circumstances, pulse techniques can produce distorted views of a sample's composition.
Note that a fundamental assumption underlying analysis by normalpulse polarography is that the solution's composition near the working electrode at thestart of each pulse is the same as that of the bulk. This assumption can hold only if negligible electrolysis occurs at the working electrode during the waiting period before r'.300 P Chapter 7. Polarography and Pulse VoltammetryBackground-120 mV us. SCEFigure 7.3.17 Differential pulsepolarogram for 4.84 X 10" 7 M As(III)in 1 M HCl containing 0.001% TritonX-100.
r m a x = 2 s, AE = -100 mV.[From J. G. Osteryoung and R. A.Osteryoung, Am. Lab., 4 (7), 8 (1972),with permission.]2цА-0.4-0.6-0.8E (V vs. SCE)(a)-0.2-0.4-0.6-0.8-0.6E (V vs. SCE)-0.8E {V vs.(b)-0.2-0.4Figure 7.3.18 Polarograms at a DME of 1 0 " 3 M F e 3 + and 1 0 " 4 M C d 2 + in 0.1 M HCl.(a) Conventional dc mode, (b) Normal pulse mode, Еъ = -0.2 V vs. SCE. (c) Differential pulsemode, A£ = - 5 0 mV.7.4 References301Therefore, the base potential must be either the equilibrium value itself or in a range overwhich the electrode behaves as an IPE (46). Otherwise, electrolysis modifies the solution's composition near the electrode before the pulse begins.Figure 7.3.18 is a display of conventional and pulse polarograms for 1 mM Fe 3 + and410~ M Cd 2+ in 0.1 M HC1.
Since £ 0 ' for Fe 3+ /Fe 2+ is more positive than the anodic limitof the DME, no wave is seen for that couple, yet the diffusion-limited reduction current forFe 3 + is recorded from the positive end of the working range. In the conventional polarogram (Figure 7.3.18a), the limiting current for Fe 3+ is about five times greater than that ofCd 2+ , as expected from the ratio of concentrations and n values. This response contrastsmarkedly with that from a normal pulse experiment (Figure 7.3.18/?) in which Еъ was -0.2V vs. SCE. The limiting current for F e 3 + is actually smaller than that for Cd 2 + , becauseelectrolysis at the base potential depletes the diffusion layer of F e 3 + before the pulse has achance to measure it faithfully. Since the wave height for Cd 2 + is unaffected, this effectwould be alarming only if an accurate picture of the concentration ratio CFe3+/CCd2+ weredesired.
It can actually be useful for suppressing the response of a concentrated interferent.Differential pulse polarography also produces an ambiguous record for this kind of situation, as shown in Figure 7.3.18c. A peak is seen only for the Cd 2 + reduction, because thetrace covers potentials only on the negative side of the F e 3 + wave. We note again that thedifferential pulse polarogram approximates the derivative of the normal pulse record; hencedistinct peaks will not be seen in DPV (or in SWV) unless distinct waves appear in NPV.Aside from this type of problem, DPV and SWV are particularly well-suited to theanalysis of multicomponent systems because their readout format usually allows the separation of signals from individual components along a common baseline.
This point is illustrated in Figure 7.3.19. Note also from that figure that pulse methods are applicable toa much richer variety of analytes than heavy metal species.Although pulse techniques were developed specifically for the DME, they can be employed analytically with other kinds of electrodes. As important examples, one can citedifferential pulse anodic stripping at a hanging mercury drop or at a thin mercury film ona rotating substrate. See Section 11.8 for details.Analysis of mixture of antibiotics 0.1 M acetate buffer, pH4differential pulse mode4.20 ppmTetracycline • HCI0.2JIAT2.40 ppmChloramphenicol0-0.1 -0.2 -0.3 -0.4 -0.5 -0.6 -0.7 -0.8 -0.9 -1.0 -1.1 -1.2 -1.3E (V vs. SCE)-1.4Figure 7.3.19 Differential pulse polarogram for a mixture of tetracycline and chloramphenicol.AE = - 2 5 mV. [From Application Note AN-111, EG&GPrinceton Applied Research, Princeton, NJ, with permission.]302Chapter 7.
Polarography and Pulse Voltammetry7.4 REFERENCES1. J. Heyrovsky, С hem. Listy, 16, 256 (1922).2. I. M. Kolthoff and J. J. Lingane, "Polarography," 2nd ed., Wiley-Interscience, New York,1952, Chap. 17.3. J. J. Lingane, "Electroanalytical Chemistry,"2nd ed., Wiley-Interscience, New York, 1958,Chap. 11.24. A. J. Bard and H. Lund, Eds., "Encyclopedia ofthe Electrochemistry of the Elements," MarcelDekker, New York, 1973-1986.25. L. Meites and P.
Zuman, "ElectrochemicalData," Wiley, New York, 1974.26. I. M. Kolthoff and J. J. Lingane, op. cit.,Chap. 9.4. L. Meites, "Polarographic Techniques," 2nded., Wiley-Interscience, New York, 1958,Chap. 2.27. I. M. Kolthoff and J. J. Lingane, op. cit.,Chap. 11.5. J. J. Lingane, Anal. Chim. Acta, 44, 411 (1969).29. J. Koutecky, Chem. Listy, 47, 323 (1953); Coll.Czech.
Chem. Commun., 18, 597 (1953).6. A. Bond, "Modern Polarographic Methods inAnalytical Chemistry," Marcel Dekker, NewYork, 1980.7. W. M. Peterson, Am. Lab., 11 (12), 69 (1979).8. Z. Kowalski, K. H. Wong, R. A. Osteryoung,and J. Osteryoung, Anal. Chem., 59, 2216-2218(1987).9. J. Koutecky, Czech. Cas.
Fys., 2, 50 (1953).10. J. Koutecky and M. von Stackelberg in"Progress in Polarography," Vol. 1, P. Zumanand I. M. Koltoff, Eds., Wiley-Interscience, NewYork, 1962.11. J. J. Lingane and B. A. Loveridge, /. Am. Chem.Soc, 72, 438 (1950).28. L. Meites, op. cit., Chap. 4.30. J. Weber and J. Koutecky, Coll. Czech. Chem.Commun., 20, 980 (1955).31. L. Meites and Y. Israel, /. Electroanal. Chem.,8, 99 (1964).32. J. Koutecky and J. Cizek, Coll. Czech.
Chem.Commun., 21, 863 (1956).33. J. E. B. Randies, Can. J. Chem., 37, 238 (1959).34. J. E. B. Randies in "Progress in Polarography,"I. M. Kolthoff and P. Zuman, Eds., Wiley-Interscience, 1962, Chap. 6.35. J. Osteryoung, Accts. Chem. Res., 26, 77,(1993).36. E. Wahlin, Radiometer Polarog., 1, 113 (1952).12. P. Delahay, "New Instrumental Methods inElectrochemistry," Wiley-Interscience, NewYork, 1954, Chap. 3.37.
E. Wahlin and A. Bresle, Ada Chem. Scand., 10,935 (1956).13. I. M. Kolthoff and J. J. Lingane, op. cit.,Chap. 4.39. R. Bilewicz, K. Wikiel, R. Osteryoung, and J.Osteryoung, Anal. Chem., 61, 965 (1989).14. L. Meites, op. cit., Chap. 3.40. P. He, Anal. Chem., 67, 986 (1995).15. D. Ilkovic, Coll. Czech. Chem. Commun., 6, 498(1934).41. G. С Barker and A. W. Gardner, Z. Anal.Chem., 173, 79 (1960).16. D. Ilkovic, J. Chim. Phys., 35, 129 (1938).42. E. P. Parry and R. A. Osteryoung, Anal.
Chem.,37, 1634 (1964).17. D. MacGillavry and E. K. Rideal, Rec. Trav.Chim., 56, 1013 (1937).18. I. M. Kolthoff and J. J. Lingane, op. cit.,Chap. 18.19. L. Meites, op. cit., Chaps. 5 and 7.38. L. Meites, op. cit., Chap. 10.43. J. G. Osteryoung and R. A. Osteryoung, Am.Lab., 4 (7), 8 (1972).44. J.
B. Flato, Anal. Chem., 44 (11), 75A (1972).20. J. J. Lingane, Ind. Eng. Chem., Anal. Ed., 15,588 (1943).45. R. A. Osteryoung and J. Osteryoung, Phil.Trans. Roy. Soc. London, Ser. A, 302, 315(1981).21. I. M. Kolthoff and J. J. Lingane, op. cit.,Vol. 2.46. J. L. Morris, Jr., and L. R. Faulkner, Anal.Chem., 49, 489 (1977).22. L. Meites, op. cit., Appendices В and C.47. J.
Osteryoung and E. Kirowa-Eisner, Anal.Chem., 52, 62-66 (1980).23. L. Meites, Ed., "Handbook of Analytical Chemistry," McGraw-Hill, New York, 1963, pp. 4-43to 5-103.48. K. B. Oldham and E. P. Parry, Anal. Chem., 42,229 (1970).7.5 Problems < 30349. L.
Ramaley and M. S. Krause, Jr., Anal. С hem.,41, 1362 (1969).50. J. Osteryoung and J. J. O'Dea, Electroanal.Chem., 14, 209 (1986).51. G. С Barker and J. L. Jenkins, Analyst, 77, 685(1952).52. G. C. Barker in "Proceedings of the Congress onModern Analytical Chemistry in Industry," St.Andrews, Scotland, 1957, pp.
209-216.53. J. H Christie, J. A. Turner, and R. A. Osteryoung, Anal. Chem., 49, 1899 (1977).7.5 PROBLEMS7.1 The following measurements were made on a reversible polarographic wave at 25°C. The processcould be written О + ne ^ RE(volts vs. SCE)-0.395-0.406-0.415-0.422-0.431-0.445id = 3.2/(MA)0.480.971.461.942.432.92Calculate (a) the number of electrons involved in the electrode reaction, and (b) the formal potential(vs.
NHE) of the couple involved in the electrode reaction, assuming Do = Z)R.7.2 The following data were obtained for an apparently totally irreversible polarographic wave:£(volts vs. SCE)i QiA)-0.419-0.451-0.491-0.515-0.561-0.593-0.680-0.7200.310.621.241.862.482.793.103.10The overall reaction is known to be О + 2e —> R; and m = 1.26 mg/s, fmax = 3.53 s (constant at allpotentials), and C* = 0.88 mM. Assume the initial step in the mechanism is a rate-determining oneelectron transfer, (a) Use Table 7.2.1 to determine kf at each potential.
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