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Appendix D – Electrochemical Equations 101
Chronoamperometry
The Cottrell Equation
When chronoamperometry is performed on an unstirred solution at a
planar electrode the faradaic current response is described by the
Cottrell equation
where
n = the number of electrons transferred to (or from) the substrate
molecule
F = Faraday's constant, 96485 C/mol
A = area of the exposed surface of the electrode (cm
2
)
D = diffusion coefficient of the substrate molecule (cm
2
/s).
C = concentration of the substrate molecule (mol/L)
t = time (s).
The factor of 1000 is to bring both cm and dm (1 dm
3
= 1000 cm
3
=
1 L) in the equation to common units.
Thus a plot of i versus 1/Ð t should produce a straight line.
In aqueous solution the coefficient of diffusion usually has a value of
between 10
–5
and 10
–6
cm
2
/s.
i
nFAD
1 2
⁄
C
1000
π
1 2
⁄
t
1 2
⁄
------------------------------------ =