What Does Concentration Polarization Mean?
Concentration polarization occurs in the portion of the polarization of a cell produced by concentration changes resulting from passage of current through the electrolyte.
In electrochemistry, concentration polarization is the polarization of an electrolytic cell resulting from changes in the electrolyte concentration due to the passage of current through the electrode/solution interface. Concentration polarization commonly occurs during the cathodic processes, depending on the reduction of dissolved oxygen, since it is usually in low concentration.
Concentration polarization also refers to the concentration gradient of salts on the high-pressure side of the reverse-osmosis membrane surface created by the less-than-immediate redilution of salts left behind as water permeates through a membrane.
Corrosionpedia Explains Concentration Polarization
In concentration polarization the electrode is polarized due to a concentration effect that occurs at the electrode surface. The cause of the changes in concentration (emergence of concentration gradients in the solution adjacent to the electrode surface) is the difference in the rate of electrochemical reaction at the electrode and the rate of ion migration in the solution to or from the surface.
For cathodic processes, the most important concentration effect is the diffusion of dissolved oxygen to the metal surface and the subsequent reduction of oxygen by the half-cell reaction:
O2(g) + 2H2O(l) + 4e- = 4OH-(aq)
For anodic processes, concentration polarization occurs due to the slow diffusion away from the metal surface of the metal cations, which are produced by anodic dissolution. For example, the anodic reaction:
Fe(s) = Fe2+(aq) + 2e-
will produce a concentration effect if Fe2+ ions are slow to diffuse away from the metal surface.
Concentration polarization effects can also occur in the corrosion of metals in acid solutions. When a chemical species participating in a corrosion process is in short supply, the mass transport of that species to the corroding surface can become rate controlling in order to maintain the balance of consumption and delivery of that species.
Concentration polarization strongly affects the performance of the separation process. Lower rate of separation under the same external driving force means increased power consumption.
Concentration polarization is inherent to all types of membrane separation processes. It can be used in:
- Gas separations
- Membrane distillation
- Reverse osmosis
- Microfiltration separations
This technique results in better mixing of the solution and in reducing the thickness of the diffusion boundary layer.
Concentration polarization usually predominates when the concentration of the active species is low.