Thought Experiment II Curve B

Let us now add some soluble redox couple, such as a mixture of Fe2+ and Fe3+ in approximately 10-3 M concentrations and record the current-voltage curve again while stirring (Curve B). This redox couple is defined by the reaction shown in (5.6). 

The reason why Curve B looks different from Curve A is that the added redox couple allows the electrons to jump across the interface (capacitor) at much lower energy (voltage) than before. As a result, it shunts (depolarizes) the capacitor. Thus, the energy stored in the capacitor has been used to oxidize or reduce the added depolarizer in the solution. We can make four observations from Thought Experiment II. 

Current-voltage Curve B is even more nonlinear than Curve A. It intercepts the zero current line, or open cell potential line, at a single point, at which i = 0. That point is the equilibrium potential Eeq of the above redox couple. 

Upon making the potential of the working electrode slightly more negative with respect to Eeq, the current increases due to the reduction of Fe3+ ions (5.6). Similarly, upon increase of the potential in the positive direction from Eeq, ferrous ions (Fe2+) are oxidized, according to (5.6). 

A further increase of the applied voltage in either direction causes the current to reach the limiting current plateau. Within this window, reduction and oxidation of ferric and ferrous ions, respectively, are taking place at a constant rate. 

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