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 ...

, the Randles–Ševčík equation describes the effect of scan rate on the peak current ''i_p''. For simple redox events such as the

ferrocene Ferrocene is an organometallic compound with the formula . The molecule is a complex consisting of two cyclopentadienyl rings bound to a central iron atom. It is an orange solid with a camphor-like odor, that sublimes above room temperature, ...

ferrocenium Ferrocenium tetrafluoroborate is an organometallic compound with the formula e(C5H5)2F4. This salt is composed of the cation e(C5H5)2sup>+ and the tetrafluoroborate anion (). The related hexafluorophosphate is also a popular reagent with simil ...

couple, ''i_p'' depends not only on the concentration and diffusional properties of the electroactive species but also on scan rate. :

i_p = 0.4463 \ nFAC \left(\frac\right)^

Or if the solution is at 25 °C: Allen J. Bard and Larry R. Faulkner, "Electrochemical Methods: Fundamentals and Applications" (2nd ed.) John Wiley & Sons 2001. :

i_p = 2.69\times  10^ \ n^ AD^ Cv^

*''i_p'' = current maximum in amps *''n'' = number of electrons transferred in the redox event (usually 1) *''A'' = electrode area in cm² *''F'' =

Faraday Constant In physical chemistry, the Faraday constant, denoted by the symbol and sometimes stylized as ℱ, is the electric charge per mole of elementary charges. It is named after the English scientist Michael Faraday. Since the 2019 redefinition of S ...

in C mol⁻¹ *''D'' = diffusion coefficient in cm²/s *''C'' = concentration in mol/cm³ *''ν'' = scan rate in V/s *''R'' =

Gas constant The molar gas constant (also known as the gas constant, universal gas constant, or ideal gas constant) is denoted by the symbol or . It is the molar equivalent to the Boltzmann constant, expressed in units of energy per temperature increment p ...

in J K⁻¹ mol⁻¹ *''T'' = temperature in K * The constant with a value of 2.69x10⁵ has units of C mol⁻¹ V^−1/2 For novices in electrochemistry, the predictions of this equation appear counter-intuitive, i.e. that ''i_p'' increases at faster voltage scan rates. It is important to remember that current, i, is charge (or electrons passed) per unit time. In cyclic voltammetry, the current passing through the electrode is limited by the diffusion of species to the electrode surface. This diffusion flux is influenced by the concentration gradient near the electrode. The concentration gradient, in turn, is affected by the concentration of species at the electrode, and how fast the species can diffuse through solution. By changing the cell voltage, the concentration of the species at the electrode surface is also changed, as set by the

Nernst equation In electrochemistry, the Nernst equation is a chemical thermodynamical relationship that permits the calculation of the reduction potential of a reaction ( half-cell or full cell reaction) from the standard electrode potential, absolute tempera ...

. Therefore, a faster voltage sweep causes a larger concentration gradient near the electrode, resulting in a higher current.

Uses

Using the relationships defined by this equation, 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 ...

of the electroactive species can be determined. Linear plots of ''i_p'' vs. ''ν''^1/2 provide evidence for a chemically reversible redox process vs the cases where redox causes major structural change in the analyte. For species where the diffusion coefficient is known (or can be estimated), the slope of the plot of ''i_p'' vs. ''ν''^1/2 provides information into the stoichiometry of the redox process.

Uses

References

See also