A.J. Bard, L.R. Faulkner - Electrochemical methods - Fundamentals and Applications (794273), страница 67
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This curve resembles that seen as the faradaic background scan for metal-solutioninterfaces. In general, the available potential window in which the ITIES behaves as anideal polarized interface depends upon the values of Д^ф? for the ions present in the aque0.012 |0.010.008-TPB0.006TBuA+0.0040.002-750-250ДВФ (mV)250750(a)0.0120.010.0080.0060.0040.002-1000-5000дРф (mV)(b)5001000Figure 6.8.2(a) Distribution of ions in thesystem in Figure 6.8.1 as afunction of the interfacialGalvani potential difference,А^ф. In the middle range, LiClis largely in the aqueous phase,and TBuATPB is in thenitrobenzene, (b) Distributionfor 0.01 M TMA + in the samesystem.6.9 References100ITMeA+(aq)+-> TMeA (nb) f~*J50 -0 --50TMeA+(nb) --> TMeA+(aq)-100100I200I300Acp, mVI400500255Figure 6.8.3 Voltammetry with the cellshown in (6.8.4).
(A) Current-potentialcurve for 0.1 M LiCl in the aqueous phaseand 0.1 M TBuATPB in the nitrobenzene;+(B) with addition of 0.47 mM TMeA tothe aqueous phase. Scan rate, 20 mV/s. Inthis plot /\(p is the potential of the referenceelectrode in the aqueous phase vs. that inthe nitrobenzene and includes junctionpotentials at the liquid junctions of bothreference electrodes, so that A(p ~300 - А%ф mV. Thus, the aqueous phasebecomes more positive as the scan extendsto the right, and a positive currentrepresents transfer of TMeA + fromaqueous phase to nitrobenzene. [Reprintedfrom P.
Vanysek, Electrochim. Acta, 40,2841 (1995), with permission fromElsevier Science.]this window isous and organic phases. Given typical maximum values of Agenerally smaller than 0.6 to 0.7 V (29).If an ion with a smaller Gibbs energy of transfer than that of the other ions is introduced into one of the phases, it will transfer at smaller values of Д^ф within the potentialwindow governed by the other ions. For example, when tetramethylammonium ion(TMA + ) is added to the aqueous phase, its A ^ f value is such that it transfers more readily than TBuA + between the water and nitrobenzene (Figure 6.8.2/?).
When the concentration of this ion is small (typically 0.1 to 1 mM), the rate of transfer across the interface isusually governed by the rate of mass transfer of the ion to the interface. Under these conditions, a scan of current vs. potential drop across the interface resembles a typical cyclicvoltammogram (Figure 6.8.35).Such voltammograms show the same characteristics as those of nernstian faradaicwaves at the metal/solution interface, that is, peak potential independent of u, peak curl/2rent proportional to v , peak splitting of 59 mV/|zi|, and proportionality of peak currentwith concentration.
Such measurements are often complicated by uncompensated resistance effects because of the high resistance of the organic phase. In almost all cases investigated, the voltammograms do not show effects of slow ion transport across the interface;that is, they are reversible. Thus, measurements like these can be used to find Gibbs energies of transfer, diffusion coefficients, and solution concentrations.
By exploiting ITIES,voltammetric electrodes can be prepared for ions that are not electroactive in the faradaicsense. For example, a voltammetric lithium ion electrode can be fabricated based on Li +transfer between water and an immiscible oil (o-nitrophenylphenylether) containing acrown ether to facilitate specific ion transport (33).6.9 REFERENCES1. J. E. B. Randies, Trans. Faraday Soc, 44, 327(1948).2.
A Sevcik, Coll. Czech. Chem. Commun., 13, 349(1948).3. R. S. Nicholson and I. Shain, Anal. Chem., 36,706 (1964).4. W. H. Reinmuth, /. Am. Chem. Soc, 79, 6358(1957).5. H. Matsuda and Y. Ayabe, Z. Electrochem., 59,494 (1955).6. Y. P. Gokhshtein, Dokl. Akad. Nauk SSSR, 126,598 (1959).256Chapter 6. Potential Sweep Methods7. J. C. Myland and K. B. Oldham, /. Electroanal.C/H?/W.,153,43(1983).19. P. T. Kissinger, J. B. Hart, and R. N. Adams,Brain Res., 55, 20(1913).8. A. C. Ramamurthy and S. K. Rangarajan, Electrochim.
Acta, 26, 111 (1981).20. J. C. Imbeaux and J.-M. Saveant, /. Electroanal.Chem., 44, 1969 (1973).9. D. O. Wipf and R. M. Wightman, Anal. Chem.,60,2460(1988).21. К. В. Oldham and J. Spanier, /. Electroanal.Chem., 26, 331 (1970).10. С P. Andrieux, D. Garreau, P. Hapiot, J. Pinson,and J.-M. Saveant, /. Electroanal.
Chem., 243,321 (1988).11. C. Amatore in "Physical Electrochemistry—Principles, Methods, and Applications," I. Rubinstein, Ed., Marcel Dekker, New York, 1995,p. 191.12. J.-M. Saveant, Electrochim. Acta, 12, 999(1967).13. W. M. Schwarz and I. Shain, /. Phys. Chem., 70,845 (1966).14. R. S. Nicholson, Anal.
Chem., 37, 1351 (1965).15. Y. P. Gokhshtein and A. Y. Gokhshtein, in "Advances in Polarography," I. S. Longmuir, Ed.,Vol. 2, Pergamon Press, New York, 1960, p.465; Dokl. Akad. Nauk SSSR, 128, 985 (1959).16. D. S. Polcyn and I. Shain, Anal. Chem., 38, 370(1966).17. F. Ammar and J.-M. Saveant, /. Electroanal.Chem., 47, 215 (1973).18. J. B. Flanagan, S. Margel, A. J. Bard, and F. СAnson, /. Am. Chem.
Soc, 100, 4248 (1978).22. К. В. Oldham, Anal. Chem., 44, 196 (1972).23. К. В. Oldham, Anal. Chem., 45, 39 (1973).24. R. J. Lawson and J. T. Maloy, Anal. Chem., 46,559 (1974).25. P. E. Whitson, H. W. Vanden Born, and D. H.Evans, Anal. Chem., 45, 1298 (1973).26. J. H. Carney and H. С Miller, Anal. Chem., 45,2175 (1973).27.
J.-M. Saveant and D. Tessier, /. Electroanal.Chem., 65, 57 (1975).28. L. Nadjo, J.-M. Saveant, and D. Tessier, /. Electroanal. Chem., 52, 403 (1974).29. H. H. J. Girault and D. J. Schiffrin, Electroanal.Chem., 15, 1 (1989).30. C. Gavach and F.
Henry, Compt. Rend. Acad.Sci., C274, 1545 (1972).31. Z. Samec, V. Marecek, J. Koryta, and M. W.Khalil, /. Electroanal. Chem., 83, 393 (1977).32. P. Vany sek, Electrochim. Acta, 40, 2841 (1995).33. S. Sawada, T. Osakai, and M. Senda, Anal. Sci.,11, 733 (1995).6.10 PROBLEMS6.1 Derive (6.2.6) from (6.2.4) and (6.2.5).6.2 Show that equations (6.2.8) and (6.2.9) lead directly to (5.4.26).l26.3 From the data in Table 6.3.1 plot the linear potential sweep voltammograms, that is, Tr ' x(bt) vs.potential for a one-step, one-electron process with several values of k°, given a = 0.5, T = 25°C,v = 100 mV/s, and Do = 10~5 cm2/s. Compare these results with those for a nernstian reactionshown in Figure 6.2.1.6.4 T. R.
Mueller and R. N. Adams (see R. N. Adams, "Electrochemistry at Solid Electrodes," MarcelDekker, New York, 1969, p. 128) suggested that by measurement of ip/vl/2 for a nernstian linear potential sweep voltammetric curve, and by carrying out a potential step experiment in the same solution at the same electrode to obtain the limiting value of itl/2, the n value of an electrode reactioncan be determined without the need to know A, C*, or Do. Demonstrate that this is the case. Whywould this method be unsuitable for irreversible reactions?6.5 The oxidation of o-dianisidine (o-DIA) occurs in a nernstian 2e reaction.
For a 2.27 mM solution ofo-DIA in 2 M H 2 SO 4 at a carbon paste electrode of area 2.73 mm 2 with a scan rate of 0.500 V/min,/p = 8.19 fiA. Calculate the D value for o-DIA. What /p is expected for v = 100 mV/s? What /p willbe obtained for v = 50 mV/s and 8.2 mM o-DIA?6.6 Figure 6.10.1 shows a cyclic voltammogram taken for a solution containing benzophenone (BP)and tri-/?-tolylamine (TPTA), both at 1 mM in acetonitrile. Benzophenone can be reduced insidethe working range of acetonitrile and TPTA can be oxidized.
However, benzophenone cannot be6.10 Problems25720 r-15/, цА 101.51.0/U5•c-1.5-10Figure 6.10.1Cyclic voltammogramof benzophenone andtri-/?-tolylamine inacetonitrile. [From P. R.Michael, PhD Thesis,University of Illinois atUrbana-Champaign, 1977,with permission.]-2.0-2.5£(V vs. QRE)0.1 MTBABF4-15oxidized, and TPTA cannot be reduced. The scan shown here begins at 0.0 V vs. QRE and firstmoves toward positive potentials. Account for the shape of the voltammogram.(a) Assign the voltammetric features between +0.5 and 1.0 V and between —1.5 and -2.0 V toappropriate electrode reactions. Comment on the heterogeneous and homogeneous kinetics pertaining to these electrode reactions.(b) Why does the falloff in current appear between 0.7 and 1.0 V vs.
QRE?(c) What constitutes the anodic and cathodic currents seen at - 1 . 0 V vs. QRE?6.7 K. M. Kadish, L. A. Bottomley, and J. S. Cheng presented results of a study of the interactions between Fe(II) phthalocyanine (FePc) and various nitrogen bases, such as imidazole (Im).Iron phthalocyanine, FePcImThe work was carried out in dimethylsulfoxide (DMSO) containing 0.1M tetraethylammonium perchlorate (TEAP).
Some results are shown in Figure 6.10.2. In (a), couples I and II both show peakpotentials and current functions that are invariant with scan rate. Interpret the voltammetric properties of the system before and after addition of imidazole.6.8 Consider the electrochemical reduction of molecular oxygen in an aprotic solvent such as pyridineor acetonitrile. In general, a cyclic voltammogram like that in Figure 6.10.3 is obtained. A sampled-current voltammogram on a 4-s timescale at a mercury electrode (i.e., a polarogram, Section7.2) gives a linear plot of E vs.
log [(/d — /)//] with a slope of 63 mV. The reduction product at— 1.0 V vs. SCE gives an ESR signal. If methanol is added in small quantities, the cyclic voltammogram shifts toward positive potentials, the forward peak rises in magnitude, and the reversepeak disappears. These trends continue with increasing methanol concentration until a limit is258Chapter 6.
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