Page 75 - Analytical Electrochemistry 2d Ed - Jospeh Wang

Analytical Electrochemistry, Second Edition. Joseph Wang
Copyright # 2000 Wiley-VCH
ISBNs: 0-471-28272-3 (Hardback); 0-471-22823-0 (Electronic)

CHAPTER 3

CONTROLLED-POTENTIAL
TECHNIQUES

The basis of all controlled-potential techniques is the measurement of the current
response to an applied potential. There exist a multitude of potential excitations,
including a ramp, potential steps, pulse trains, a sine wave, and various combinations
thereof. The present chapter reviews those techniques that are widely used.

3-1 CHRONOAMPEROMETRY

Chronoamperometry involves stepping the potential of the working electrode from a
value at which no faradaic reaction occurs to a potential at which the surface
concentration of the electroactive species is effectively zero (Figure 3-1a). A
stationary working electrode and unstirred solution are used. The resulting
current±time dependence is monitored. As mass transport under these conditions
is solely by diffusion, the current±time curve re¯ects the change in the concentration
gradient in the vicinity of the surface (recall Section 1.2). This involves a gradual
expansion of the diffusion layer associated with the depletion of the reactant, and
hence decreased slope of the concentration pro®le as time progresses (see Figure
3-1b). Accordingly, the current (at a planar electrode) decays with time (Figure
3-1c), as given by the Cottrell equation:

nFACD 1=2 1=2
i t†ˆ ˆ kt 3-1†
p 1=2 1=2
t
Such an it 1=2 constancy is often termed ``Cottrell behavior.'' Deviations from such
behavior occur at long times (usually over 100 s) as a result of natural convection

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