Figure 4-1C shows the profiles that result when the applied potential is sufficiently negative that the concentration of O at the electrode surface is effectively zero, i e., Co/CK 0.001. In this case, essentially all of O at the electrode surface must be electrolyzed to R in order to satisfy the Nersnt equation. Consequently, O is converted to R as rapidly as it can diffuse to the electrode surface. Since this is a limiting condition, application of even more negative potentials causes only negligible change in the profiles. Although thee transition between stagnant and flowing solution is considered to be abrupt in this example, the transition is in reality gradual. Consequently, the profiles are rounded as shown by the dotted line in Fig.4-1C. However, the hypothetical situation of an abrupt transition is a useful approximation in mathematical treatments.