Reduction of Solution Resistance

The jRs potential drop due the uncompensated solution resistance associated with different geometries was discussed in Section 3.2.3. For a spherical electrode, which is of interest here, we can write 

As usual, a numerical example might help to illustrate the advantage of ultramicroelectrodes, from the point of view of solution resistance. In Section 14.3.2 we obtained a limiting current density of 0.8 A cm for an electrode having a radius of 0.25 pm, in a lOmM solution of the reactant. If we assume a specific conductivity of K = 25mScm the solution resistance Rs according to Eq. (14.49), is 1 X 10 Q cm. We assumed here a solution of medium specific conductivity, and 

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We can ask how effects of the double layer on electrode kinetics can be minimized and if the necessity of correcting values of a and of rate constants can be avoided In order for this to be possible, we have to arrange for s, that is all the potential drop between electrode surface and bulk solution is confined to within the compact layer, for any value of applied potential. This can be achieved by addition of a large quantity of inert electrolyte (—1.0 m), the concentration of electroactive species being much lower (<5mM). As stated elsewhere, other advantages of inert electrolyte addition are reduction of solution resistance and minimization of migration effects given that the inert electrolyte conducts almost all the current. In the case of microelectrodes (Section 5.6) the addition of inert electrolyte is not necessary for many types of experiment as the currents are so small. 

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