Relationship Between Rate and Potential Under Stationary Diffusion

1 Relationship Between Rate and Potential Under Stationary Diffusion 

Consider the following situation. The oxidized form of a reactant consists of metal ions in solution, and the electrode material (metal) is the reduced form. Such electrochemical system is called class 1 electrode. An example may be a copper plate immersed into a solution of copper sulfate. If some external source of current is used to create a potential on such an electrode and the potential is more negative that the equilibrium one, then copper ions will be reduced on the cathode. Ions near the electrode will be reduced first. This will lead to the decrease in concentration of the discharging particles and new portions of the oxidized form will diffuse from the bulk of the electrode. 

Consider a situation when this stage is rate determining. 

Assume the diffusion is stationary, i.e. the reagent concentration does not change over time and we can put dC/dt = 0 into the Pick s law. Then, obviously, d Cjdx = OanAdC/dx = const. 

The concentration of the reductant near surface,, is smaller than that in the bulk, C . Assume the concentration linearly changes inside some specific layer 8 (called diffusion layer)  

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