14 Faraday Formal potential

Here, i is the faradaic current, n is the number of electrons transferred per molecule, F is the Faraday constant, A is the electrode surface area, k is the rate constant, and Cr is the bulk concentration of the reactant in units of mol cm-3. In general, the rate constant depends on the applied potential, and an important parameter is ke, the standard rate constant (more typically designated as k°), which is the forward rate constant when the applied potential equals the formal potential. Since there is zero driving force at the formal potential, the standard rate constant is analogous to the self-exchange rate constant of a homogeneous electron-transfer reaction. 

An electron transfer reaction, Equation 6.6, is characterised thermodynamically by the standard potential, °, i.e. the value of the potential at which the activities of the oxidised form (O) and the reduced form (R) of the redox couple are equal. Thus, the second term in the Nernst equation, Equation 6.7, vanishes. Here and throughout this chapter n is the number of electrons (for organic compounds, typically, n = 1), II is the gas constant, T is the absolute temperature and F is the Faraday constant. Parentheses, ( ), are used for activities and brackets, [ ], for concentrations /Q and /R are the activity coefficients of O and R, respectively. However, what may be measured directly is the formal potential E° defined in Equation 6.8, and it follows that the relationship between E° and E° is given by Equation 6.9. Usually, it maybe assumed that the activity coefficients are unity in dilute solution and, therefore, that E° = E°. 

Ef is the formal potential, c0x>buik> cRed,buik> c0x> =o and cRediX=0 are the bulk and surface concentrations of Ox and Red, F is Faraday s constant, R is the gas constant, T is the temperature (K), D0x and DRed are the diffusion coefficients of Ox and Red, and p = 1/2 for semi-infinite linear diffusion in the absence of hydrodynamics, p = 1 for steady-state at small electrodes in the absence of hydrodynamics, and p = 2/3 for steady-state at a rotating disk electrode (see - rotating disk elec-... 

Faraday constant formal potential galvanic cell indicator electrode Nemst equation oxidant oxidation oxidizing agent... 

As = surface area of a semiconductor contact [A ] = concentration of the reduced form of a redox couple in solution [A] = concentration of the oxidized form of a redox couple in solution A" = effective Richardson constant (A/A ) = electrochemical potential of a solution cb = energy of the conduction band edge Ep = Fermi level EF,m = Fermi level of a metal f,sc = Fermi level of a semiconductor SjA/A") = redox potential of a solution ° (A/A ) = formal redox potential of a solution = electric field max = maximum electric field at a semiconductor interface e = number of electrons transferred per molecule oxidized or reduced F = Faraday constant / = current /o = exchange current k = Boltzmann constant = intrinsic rate constant for electron transfer at a semiconductor/liquid interface k = forward electron transfer rate constant = reverse electron transfer rate constant = concentration of donor atoms in an n-type semiconductor NHE = normal hydrogen electrode n = electron concentration b = electron concentration in the bulk of a semiconductor ... 

Nemst s law (Eq. II. 1.7, which is simply the Nernst equation written in the exponential form) defines the surface concentrations of the oxidised, [A]jc=o. and die reduced form, [B] =o. of the redox reagents for a reduction process A + n e B as a function of E t) and, the applied and the formal reversible potential [4], respectively, where t is time, n is the number of electrons transferred per molecule of A reacting at the electrode surface, F, the Faraday constant, R, the constant for an ideal gas, and T, the absolute temperature. Pick s second law of diffusion (Eq. II. 1.6) governs the mass transport process towards the electrode where D is the diffusion coefficient. The parameter x denotes the distance from the electrode surface. 

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