전기화학9

전기화학9

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1 2. Potential Step Methods § In this chapter, we will consider systems in which the mass transport of electroactive species occurs only by diffusion. § Also, we will restrict our view to methods involving only step-functional changes in the working electrode potential. § This potential potential step step technique technique contains some of the most powerful experimental approaches available to electrochemistry. § In the methods covered in this chapter, the electrode area (A) is small enough and the solution volume (V) is large enough that the passage of current does not alter the bulk concentration of electroactive species. Such circumstances are known as small A/V conditions. Introduction
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2 § Figure 5.1.1 is a picture of the basic experimental system. An instrument known as a potentiostat adjusts the potential difference between the working and reference electrode in accord with the program defined by a function generator. § Since the current is closely related with electrode potential, the current is measured as a function of time or potential. § Current is the flow of electrons needed to support the active electrochemical processes at rates consistent with the potential. Thus the response from the potentiostat (current or charge) actually is experimentally observable.
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3 § Figure 5.1.2 is a diagram of the waveform applied in basic potential step experiment. § Let us consider its effect on the interface between a solid electrode and an unstirred solution containing an electroactive species. § As an example, take anthracene in deoxygenated dimethylformamide (DMF). We know that there generally is a potential region where faradaic processes (An + e ® An × - ) do not occur ; let E 1 be in this region. § On the other hand, we can also find a more negative potential at which the kinetics for reduction of anthracene be come so rapid that no anthracene can coexist with the electrode, and its surface concentration goes nearly to zero. § Consider E 2 to be in this “mass-transfer-limited” region. If so, what is the response of the system to the step perturbation?
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4 § First, the electrode must be reduced the nearby anthracene to the stable anion radical, An + e ® An × - § It requires a very large current, because it occurs instantly. Current flows subsequently to maintain the fully reduced condition at the electrode surface. § The
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This note was uploaded on 04/27/2011 for the course CHEM 101 taught by Professor Suh during the Spring '11 term at University of Toronto- Toronto.

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전기화학9

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