Written by the Fiveable Content Team • Last updated August 2025
Written by the Fiveable Content Team • Last updated August 2025
Definition
The Cottrell equation describes the change in current over time during the reduction or oxidation of a species at an electrode surface in an unstirred solution. It is key for understanding electrochemical processes involving diffusion control.
The Cottrell equation is given by $I(t) = \frac{nFAc_{0}D^{1/2}}{\pi^{1/2}t^{1/2}}$, where $I$ is the current, $n$ is the number of electrons, $F$ is the Faraday constant, $A$ is the electrode area, $c_0$ is the initial concentration, and $D$ is the diffusion coefficient.
It illustrates that current decreases with time as $t^{-1/2}$ during a controlled potential experiment.
The Cottrell equation assumes semi-infinite linear diffusion to the electrode surface.
Often used in chronoamperometry experiments to analyze reaction kinetics and mass transport properties.
In practical applications, deviations from ideal behavior can occur due to convection or finite diffusion layers.
Related terms
Chronoamperometry: An electrochemical technique where a constant potential is applied to an electrode and the resulting current is measured over time.
Diffusion Coefficient: A parameter that quantifies how fast a substance diffuses through a medium.
Faraday Constant: $F = 96485 \text{C/mol}$, representing the charge per mole of electrons.