Pulse Voltammetry

The basis of all pulse techniques is the difference in therate of the decay of the charging and the faradaiccurrents following a potential step (or "pulse"). Thecharging current decays exponentially, whereas thefaradaic current (for a diffusion-controlled current)decays as a function of 1/(time)½; that is, the rate ofdecay of the charging current is considerably faster thanthe decay of the faradaic current.

K. Aoiki et al. J. Electroanal. Chem. Interfacial Electrochem. 1980, 110, 1-18.

Normal Pulse Voltammetry

Figure 4. Potential wave form for normal pulse voltammetry.

Consider a reduction. If the Initial Potential is well positive of the redox potential, the application of smallamplitude pulses does not cause any faradaic reactions, hence there is no current response. When the pulseamplitude is sufficiently large that the pulse potential is close to the redox potential, there is a faradaic reactionin response to the potential pulse (assuming moderately fast electron transfer kinetics), and the magnitude ofthis current may depend on both the rate of diffusion and the rate of electron transfer. When the pulsedpotentials are sufficiently negative of the redox potential that the electron transfer reaction occurs rapidly, thefaradaic current depends only on the rate of diffusion; that is, a limiting current has been attained

BASi

Differential Pulse Voltammetry

Consider a reduction (Ox + e- = Red). At potentials well positive of the redox potential, there is no faradaicreaction in response to the pulse, so the difference current is zero. At potential around the redox potential,the difference current reaches a maximum, and decreases to zero as the current becomes diffusion-controlled. The current response is therefore a symmetric peak.

Figure 9. A typical differential pulse voltammogram.Figure 7. Potential wave form for differential pulse voltammetry.

Square Wave Voltammetry

This difference current (if - ir) is displayed as a function of the applied potential.

If we consider a reduction, then at potential well positive of the redox potential, both the forward andreverse currents are zero, so the difference current is also zero. At potentials well negative of the redoxpotential, the current is diffusion-controlled, and the potential pulse has no effect; hence, the forward andreverse currents are equal, and the difference current is again zero. The largest difference between theforward and reverse currents (and hence the largest current response) is at the redox potential.

Advantages: difference current better discriminates against charging current and gives symmetric peak.

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Rotating Disc Voltammetry

Rotating the electrode leads to controlled mass flux to the electrode surface. Transport is by both convection anddiffusion through a stagnant layer on the electrode surface.

Rotating Disc Voltammetry

X, distance perpendicular to discr, distance from center of disc in plane of discΘ, angle around the disc

Rotating Disc Voltammetry

6/1

2/13/2

61.1

Dkm

Dkm

Rotating Disc Voltammetry

CnFD

x

CnFDj

x

l

0

Rotating Disc Voltammetry

2/1

6/1

3/2

62.0

bl CnFAD

i

A = geometric area (cm2)D = diffusion coefficient (cm2/s)ν = kinematic viscosity ( H2O, 0.01 cm2/s)ω = rotation velocity (radians/s)Cb = bulk solution concentration (mol/cm3)

Levich Equation

Rotating Disk Voltammetry

Slope related to n and D2/3

Fe(CN)6-3 + e- = Fe(CN)6

-4

Rotating Disc Voltammetry

Rotating Disc Voltammetry

2/13/2

6/1 161.111

OOc CnFDCnFkj

Plot 1/j vs. 1/ω1/2

Y intercept = 1/ω1/2 = 0

(Reflects kinetics of ET)

kc or ka (both f(E)) can be determined from the slope and intercept.

Rotating Disc Voltammetry