Electrorefining of Metals. Concentration Polarization and the Limiting Current Density

Illustration 3.2 Electrorefining of Metals, Concentration Polarization and the Limiting Current Density 

When a potential is applied to the electrodes, two processes take place. At the anode, the metal, along with its impurities, dissolves as positive cations into the electrolytic bath. These ions migrate to the negative cathode where they are discharged as metal. The impurities, such as iron, usually require a higher potential to be deposited, and consequently remain in solution. 

Electrorefining and concentration polarization (a) sample electrode pair and electrolytic bath (b) concentration gradients (c) flux due to diffusion and electrostatic potential. 

The maximum flux occurs when the concentration at the cathode has dropped to zero. Noting that the concentration at the anode is then twice the initial electrolyte concentration, C° we can write 

Let us apply this equation to a particular condition. Suppose that the electrodes are spaced 1 cm apart and the initial concentration of copper sulfate is 0.1 molar. Using the diffusivity for Cu++ listed in Table 3.3C we then obtain 

---