Electrode polarization. Overvoltage.

When conducting electrolysis, the phenomenon is often observed of electrical current to slow down slowly, even to zero, even though the voltage at the electrolyzer terminals remains practically unchanged. This is due to the so-called electrode polarization. There are several types. But the essential ones are:

Chemical Polarization. 

It consists of a Galvanic Element of the cathode, which serves as a support on which molecular hydrogen is adsorbed together with the hydrogen ions, form a hydrogen electrode (substrate) -H2 + 2H +. It acts as an Anode of an Internal Galvanic Element with a current opposite to the working one. This is cathodic polarization. On the anode, analogous polarization occurs in the formation of the electrode (pad) - 1 / 2O2 / 2OH-, which acts as a Cathode of an Internal Galvanic Element with the current, contrary to the working current. 

The polarization resulting from the polarization oscillates from several millivolts to 1.5-2 volts
Cases of such polarization are observed in the electrolysis of aqueous solutions of HCl, H2SO4, NaOH and the like. 
Polarization refers to the more general case of adsorption polarization, since it is based on the adsorption of products obtained on the electrodes.
There is also the so-called chemosorption polarization induced by the chemical processes between the product and the adsorbed product and the material from which the electrodes are made. For example, this is the case when anoxide or chlorine is formed on the anode and the oxide is first formed in the second chloride and the like. It is complicated if the product is insoluble.

Concentration Polarization. 

Its occurrence is based on an uneven change in the concentration of electrochemically active substances in the thin layer directly adjacent to the electrodes. 
In this case, a Galvanic Concentration Circuit is formed with an electric current opposite the working current.
Polarization is an undesirable phenomenon in electrochemical practice, especially because it is supposed to work at an increased voltage at the terminals of the electrolyzer. 
For its elimination there are developed methods based on depolarizers - substances that chemically interact with the substances adsorbed on the electrodes and cause polarization or work at low current density. 
Agitation of the electrolyte and the like is also used.

The smallest voltage value, in which the electrolysis process was disrupted, is called the potential of dissection. It is determined in an experimental way. But its value under the same conditions depends on the nature of the electrodes. For acids and bases at a concentration of one gram-mole per liter and platinum electrodes amounts to 1.7 volts. The sulfuric acid at the Pb-cathode and the Pt-anode drops to 1.6 volts and at the Pt-cathode and Pb-anode increases to 2.25 volts and the like. Such deviations are observed in all cases, especially when gas is released. 
The difference between the experimentally determined strain and the theoretically next decomposition potential is called Overvoltage
It may in some cases reach significant values. But there are also positive aspects about electrochemical practice. On its basis, it is possible to use lead as an anode in the chromium electrochemical production of the Mn, Sn, Fe, Ni and other metals from the aqueous solutions of their salts, despite the presence of hydrogen cations, and that they are present in the electro-affinity line before hydrogen.

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