Stationary or steady state techniques

After perturbation, the surface concentrations of the reactants, intermediates, and products attain new values. In a genuine stationary technique, these new values are maintained by providing a constant rate of mass transport (hydrodynamic techniques). The surface concentration, c[ of a species i is related to its flux, J, by 

Equation (11) is also applicable as a good, or reasonably good, approximation to a number of techniques classified as d.c. voltammetry , in which the response to a perturbation is measured after a fixed time interval, tm. The diffusion layer thickness, 5/, will be a function of D, and tm and the nature of this function has to be deduced from the rigorous solution of the diffusion problem in combination with the appropriate initial and boundary conditions [21—23]. The best known example is d.c. polarography [11], where the d.c. current is measured at the dropping mercury electrode at a fixed time, tm, after the birth of a new drop as a function of the applied d.c. potential. The expressions for 5 pertaining to this and some other techniques are given in Table 1. 

It will be obvious that eqn. (11) has to be combined with the proper rate equation that describes the interfacial redox process (or processes) that originated the concentration gradient. For example, the stationary rate of the reaction O + n e = R obeys, in the most simple case, the equation 

Expressions for the thickness of the diffusion layer, being approximations of rigorous theories 

Semi-infinite to expanding sphere 5 = (3tt tmDj/7)l/2[l- 1.03(fmDI)1/2/ro] 1 

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