A.J. Bard, L.R. Faulkner - Electrochemical methods - Fundamentals and Applications (794273), страница 68
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Potential Sweep Methodsо-0.4-0.6-0.8-1.0-1.2Potential (V vs. SCE)-1.4Figure 6.10.2 Cyclic voltammogramsof 1.18 mM FePc in Me2SO/imidazolemixtures containing 0.1 M TEAP. Scan rate0.100 V/s. Imidazole concentrations: (a)0.00 M; (b) 0.01 M; (c) 0.95 M. [Reprintedwith permission from К. М. Kadish, L. A.Bottomley, and J. S. Cheng, /. Am. С hem.Soc, 100, 2731 (1978). Copyright 1978,American Chemical Society.]reached with reduction near —0.4 V vs.
SCE. The polarogram under these limiting conditions isapproximately twice as high as it was in methanol-free solution, and the wave slope is 78 mV.(a) Identify the reduction product in methanol-free solution.(b) Identify the reduction product under limiting conditions in methanol-containing solution.(c) Comment on the charge-transfer kinetics in methanol-free solution.(d) Explain the voltammetric responses.6.9 Cyclic voltammetry was studied for a DMF solution containing 0.68 mM azotoluene and 0.10 M TBAPat 25°C. The working electrode was a Pt disk with A = 1.54 mm 2 , and the reference electrode was anSCE. A typical cyclic voltammogram is shown in Figure 6.10.4, and other data are tabulated below.AzotolueneCoulometry shows that the first reduction step involves one electron. Work up these data and discuss what information is obtained about the reversibility of the reactions, stability of products, dif-CathodicAnodic-0.6-0.8-1.0-1.2Figure 6.10.3 Cyclic voltammogram at anHMDE of oxygen in pyridine with 0.2 MTBAP.
Frequency 0.1 Hz. [Reprinted withpermission from M. E. Peover and B. S.White, Electrochim. Ada, 11, 1061 (1966).Copyright 1966, Pergamon Press PLC]6.10 Problems *« 259-0.8-1.0-1.2-1.4 -1.6 -1.8£/(V vs. SCE)-2.0 -2.2Figure 6.10.4Cyclic voltammogramof azotoluene inAW-dimethylformamide.[See J. L. Sadler andA. J.
Bard, / . Am. Chem.Soc, 90, 1979 (1968).]-2.4fusion coefficients, etc. (This is a set of actual data, so don't expect numbers to conform exactly totheoretical treatments.)First WaveaScan rate(mV/sec)ipc(M)ipa(MA)43029820391738.06.75.23.43.08.06.75.23.42.9Second Wave£pc ~£pa(Vvs. SCE)1.421.421.421.421.421.361.361.361.361.36QiA )-£Pc-Л/2(Vw?. SCE)7.06.54.73.02.82.102.092.082.072.062.002.002.001.991.98aFor scan reversed 100 mV past Epc.6.10 R. W. Johnson described the electrochemical behavior of 1,3,5-tri-teTt-butylpentalene (I).Solutions of I in CH 3 CN with 0.1M tetra-n-butylammonium perchlorate (ТВ АР) were subjected topolarographic and cyclic voltammetric examination.
The results were as follows:Polar-ography.9 one wave at Ец2 — -1.46 V vs. SCE. Wave slope of 59 mV.Cyclic voltammetry. Illustrated in Figure 6.10.5; the scan starts at 0.0 V vs. SCE and movesfirst in a positive direction.In addition, bulk electrolysis at +1.0 V produced a green solution giving a well-resolved ESR spectrum, and bulk electrolysis at -1.6 V gave a magenta solution that also produced a well-resolvedESR spectrum. Both bulk transformations were carried out in CH2C12.(a) Describe the chemistry of the system.(b) Account for the shape of the cyclic voltammetric curve. Identify all peaks.(c) Interpret the polarogram and relate the cyclic voltammogram to it.(d) What would you expect the diffusion current constant for the polarogram to be (in conventionalunits)? TakeDj = 2 X 10~5 cm2/s.9Polarography is sampled-current voltammetry at a dropping mercury electrode. The time scale is normally \-\s.
See Sections 7.1 and 7.2.260 • Chapter 6. Potential Sweep Methods+0.80.0-0.8-1.6-2.4Figure 6.10.5 Cyclic voltammogram of I inCH3CN with 0.1 M TBAP at a Pt electrode vs.SCE and at a scan rate of 500 mV/s. [Reprintedwith permission from R. W. Johnson, / . Am.Chem. Soc, 99, 1461 (1977). Copyright 1977,American Chemical Society.](e) Make sketches showing the expected variations with v of forward peak current and A£ p for thecouple responsible for the green solution. Do the same for the couple responsible for the magenta solution.6.11.
Sketch the distribution diagrams for the water/1,2-DCE system (concentration of ion in water/totalconcentration of that ion vs. E) for the four ions (Li + , С Г , TPAs + , TPB~) using the followingA G ^ | r л values (kJ/mol): Li + , 48.2; С Г , 46.4; TPAs\ -35.1; TPB~, -35.1. Use this plot to predict the current-potential behavior, such as that shown in Figure 6.8.ЗА6.12 (a) A study of the heterogeneous electron-transfer rate for the oxidation of ferrocene in acetonitrile(0.5 M TBABF4) produced the following [M. V.
Mirkin, Т. С Richards, and A. J. Bard, /.Phys. Chem., 97, 7672 (1993)]: k° = 3.7 cm/s and DR= 1.70 X 10~5 cm2/s. Calculate ф andA£ p for the cyclic voltammetric oxidation of ferro cene at 25°C, assuming DR — £>0, at scanrates of 3, 30, 100, 200, 300, and 600 V/s.(b) The results tabulated below were reported for AEp as a function of v for the oxidation of 2 mMferrocene in acetonitrile (0.1 M ТВABF4) at а 25-/Ш1 diameter Au electrode [I. Noviandri et.al., /. Phys. Chem., 103, 6713 (1999)]. How do you account for these results based on the calculations in part (a)?u(V/s)A£p(mV)3.277329410296204120297134320158640300CHAPTER7POLAROGRAPHY ANDPULSE VOLTAMMETRYIn Chapter 5, we laid a foundation for understanding controlled-potential methods generally and potential step methods in particular.
The focus there was on broadly applicable concepts, so we restricted our view of voltammetry to the basic sampled-currentidea. Building on that development of fundamentals, we followed in Chapter 6 with afull treatment of potential sweep methods, including cyclic voltammetry, which has become so important in practice. Now we return to voltammetry based on potential stepwaveforms. Originating historically with dc polarography (the simplest form ofvoltammetry at the dropping mercury electrode), this group of methods has becomequite diverse as more complex schemes have been devised for applying potential stepsand sampling currents. The name pulse voltammetry is often used to encompass thegroup aside from dc polarography.
We have already encountered normal pulse voltammetry (NPV) as the most straightforward version of sampled-current voltammetry. NPVis often carried out with a dropping mercury electrode, in which case it is called normalpulse polarography (NPP).Because these methods are so deeply rooted in the polarographic tradition and evennow are frequently used with polarographic electrodes, we begin with a discussion of phenomena at dropping mercury electrodes and then develop the subject through conventional polarography and into various forms of pulse voltammetry.7.1 BEHAVIOR AT POLAROGRAPHIC ELECTRODES7.1.1 The Dropping Mercury and Static Drop ElectrodesAn instrument of enormous importance to the history of electroanalytical chemistry is thedropping mercury electrode (DME), which was invented by Heyrovsky (1) for measurements of surface tension (Section 13.2.1).
Using the DME, he discovered a form ofvoltammetry, which he named "polarography" and which became the foundation for mostof the methods discussed in this book. Heyrovsky was recognized with the Nobel Prizein Chemistry for his achievement. The term polarography has since become a generalname for voltammetry at a dropping mercury electrode. In this book, we refer to the historic form as dc polarography or conventional polarography.Figure 7.1.1 depicts a classical dropping mercury electrode. Several excellent discussions of the construction and operation of the electrode are available (2-6).
A capillary with an internal diameter of ~ 5 X 10~3 cm is fed by a head of mercury 20 to 100cm high. Mercury issues through the capillary to form a nearly spherical drop, whichgrows until its weight can no longer be supported by the surface tension. A mature droptypically has a diameter on the order of 1 mm. If electrolysis occurs during the drop's261262 • Chapter 7. Polarography and Pulse VoltamrnetryContactLevelling bulbVertical support with scale - ^LStand tubeHgFlexible tubingCapillaryFigure 7.1.1 A dropping mercury electrode.growth, the current has a time dependence that reflects both the expansion of the spherical electrode and the depletion effects of electrolysis.
Upon falling, the drop stirs the solution and largely (but not completely1) erases the depletion effects, so that each drop isborn into fresh solution. If the potential does not change much during the lifetime of adrop (2-6 s), the experiment is indistinguishable from a step experiment in which the potential transition coincides with the birth of a new drop. Each drop's lifetime is itself anew experiment.The classical DME has two principal disadvantages. First, it has a constantly changing area, which complicates the treatment of diffusion and creates a continuous background current from double-layer charging. Second, its time scale is controlled by thelifetime of the drop, which cannot be varied conveniently outside the range of 0.5-10 s.By 1980, Princeton Applied Research Corporation, later followed by others, hadcommercialized a replacement for the classical DME that does not suffer these drawbacks(7, 8).
The static mercury drop electrode (SMDE, Figure 7.1.2) is an automated device inwhich the mercury flow is controlled by a valve. A head of only about 10 cm drives mercury through a wide-bore capillary when the valve is opened in response to an electricalsignal. A drop is extruded in less than 100 ms, then growth is stopped by closure of thevalve.
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