Basic Experiment with DC Current Flow

With no current through the electrolytic cell, it does not matter whether the electrodes are large or small the equilibrium potentials are the same. But with current flow, the current density and therefore the voltage drop and the polarization, will he much higher at the small electrode. An increased potential drop will occur in the constrictional current path near the small electrode, and in general the properties of the small electrode will dominate the results. The small electrode will be the electrode studied, often called the working electrode. It is a monopolar system, meaning that the effect is determined hy one electrode. The other electrode becomes the indifferent or neutral electrode. Note that this division is not true in potentiometry, electrode area is unimportant under no-current conditions. 

Polarography is voltammetry, preferably with the dripping mercury electrode and with a diffusion-controlled current in a monopolar system. 

R is small, even at zero DC current. The system is nonlinear, and care must be taken with regard to the use of linear models. 

With a sufficiently large voltage of any polarity, DC faradaic current flows and we have an electrode reaction at the platinum surface. R falls to smaller values. Evidently large activation energy is necessary to obtain electron transfer and an electrode reaction. 

Suppose a small AC signal is superimposed on the DC voltage. At equilibrium, DC voltage with no DC current, the AC current will flow in both directions because it is a reversible redox electrode process at the AgCl surface. With a DC current, the small AC current will be superimposed on the larger DC current, and thus just change the reaction rate a small amount. 

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