electrochemistry Electrochemistry is the branch of physical chemistry concerned with the relationship between electrical potential difference, as a measurable and quantitative phenomenon, and identifiable chemical change, with the potential difference as an outco ...

, the diffusion layer, according to

IUPAC The International Union of Pure and Applied Chemistry (IUPAC ) is an international federation of National Adhering Organizations working for the advancement of the chemical sciences, especially by developing nomenclature and terminology. It is ...

, is defined as the "region in the vicinity of an electrode where the concentrations are different from their value in the bulk solution. The definition of the thickness of the diffusion layer is arbitrary because the concentration approaches

asymptotically In analytic geometry, an asymptote () of a curve is a line such that the distance between the curve and the line approaches zero as one or both of the ''x'' or ''y'' coordinates tends to infinity. In projective geometry and related contexts, ...

the value in the bulk solution". The diffusion layer thus depends on the

diffusion coefficient Diffusivity, mass diffusivity or diffusion coefficient is a proportionality constant between the molar flux due to molecular diffusion and the gradient in the concentration of the species (or the driving force for diffusion). Diffusivity is enco ...

(D) of the analyte and for voltammetric measurements on the scan rate (V/s). It is usually considered to be some multiple of (Dt)^1/2 (where 1/t = scan rate). The value is physically relevant since the concentration of solute varies according to the expression derived from Fick's Laws:

\tfrac=erf(\tfrac)

When

x=\sqrt

, the concentration is approximately 52% of the bulk concentration:

erf(1/2)=0.520499878...

At slow scan rates, the diffusion layer is large, on the order of

micrometers The micrometre (American and British English spelling differences#-re, -er, international spelling as used by the International Bureau of Weights and Measures; SI symbol: μm) or micrometer (American and British English spelling differences# ...

, whereas at fast scan rates the diffusion layer is nanometers in thickness. The relationship is described in part by the

Cottrell equation In electrochemistry, the Cottrell equation describes the change in electric current with respect to time in a controlled potential experiment, such as chronoamperometry. Specifically it describes the current response when the potential is a step f ...

.Bard, A. J.; Faulkner, L. R. “Electrochemical Methods. Fundamentals and Applications” 2nd Ed. Wiley, New York. 2001. {{ISBN, 0-471-04372-9 Relevant to

cyclic voltammetry Cyclic voltammetry (CV) is a type of potentiodynamic electrochemical measurement. In a cyclic voltammetry experiment, the working electrode potential is ramped linearly versus time. Unlike in linear sweep voltammetry, after the set potential is re ...

, the diffusion layer has negligible volume compared the volume of the bulk solution. For this reason, cyclic voltammetry experiments have an inexhaustible supply of fresh analyte.

References

Diffusion