Returning to figure 16 and the oxidation/reduction process depicted by equation (23),
αA + ne- ⇔ βB
Let
us assume that at first there is no product (B)
formed. Then, by programming the potential on the working electrode from positive to negative, substance (A) will be reduced to
product (B). This will result in a cathodic current that is considered positive by convention.
Assuming
the reduction/oxidation process is reversible, then by programming the working
electrode potential from negative to positive, product (B) will be oxidized to substance (A). An anodic current that, by convention, is
considered negative will accompany this oxidation process. Anode and cathode
currents are always assumed to be opposite in sign. Figure 16 shows the curve
relating electrode current against applied potential, called a voltammogram and depicts this reduction/oxidation process.
It
is seen from figure 16 that as the potential of the working electrode is
programmed (either positively or negatively), As the reaction proceeds the
current increase and finally flattens out to a maximum (or minimum) and further
change of electrode potential has no effect on the current which means that
there is no change in the rate of reaction (oxidation or reduction). The reason for this is that as the reaction
takes place on the surface of the electrode, the reaction becomes limited by
the rate of transport of the reactant from the bulk solution to the electrode
surface.