The simple current approximation

In Chapt. 2, Eq. 2.9, it was shown that the current is proportional to the concentration gradient at the electrode (taking, for the moment, the simple case of only one redox couple taking part in the electrochemical 

These are the slope of the straight line joining Cq with c and, clearly, for the concentration profile in Fig. 4.1, are underestimates of g. The expressions are called two-point approximations for obvious reasons and this suggests the possibility of improving the approximations by using more points. To avoid confusion, this will be deferred until Sect. 4.7 for the moment, we simply stick to the 2-point formulae. 

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