Michael Faraday,(1791 – 1867) was a British scientist, chemist, physicist and philosopher who
greatly contributed to the fields of electromagnetism and electrochemistry. His main discoveries include that of the Magnetic
Field, Electromagnetic Induction, Diamagnetism and Electrolysis
In order to study electrolysis in detail, Michael Faraday conducted a number of experiments. He used different electrolytic solutions. Based on his observations, he put forth two laws of electrolysis.
Faraday's Law I:
The mass of a substance released or deposited at an electrode is
directly proportional to the quantity of charge (earlier called quantity of electricity) passed through the electrolyte.
Diagrammatic representation of first law of electrolysis
Mathematical Description of Law I
Let 'I' be the current passing through the electrolyte for a time 't' and let 'm' be the mass
liberated at either electrode during that time.
Then, I = q / t, or q = I t.
According to the first law, m ∝ q, or m ∝ I t.
Hence m = z q or m = z I t.
The constant of proportionality z is known as the electro–chemical equivalence of ion deposited. Thus, the electrochemical equivalence (ECE) of a substance is equal to the mass of the substance liberated by the passage of unit charge through the electrolyte. The unit of ECE is kg / C in the S.I. system. In CGS system it is in g / C (C = coulomb, the unit of charge).
Diagrammatic representation of second law of electrolysis
Faraday's Law II: When the same amount of charge is passed through different electrolytes, the masses of substances released or deposited at the electrodes are directly proportional to their chemical equivalents.
Mathematical description of Law II
The chemical equivalent of an element is the ratio of its atomic weight to its valency. For example, the chemical equivalent of
silver is 108 / 1 = 108 and for aluminum it is 27 / 3 = 9. Let an amount of charge 'q' be passed through two
electrolytes.
Let the corresponding masses of two substances liberated be 'm1' and 'm2', respectively. Let E1 and E2 be their chemical equivalents and let z1 and z2 be their electrochemical equivalents, respectively.
From Faraday's second law,
But, from Faraday's first law,
Hence,
Combining the two equations, we get
E1/E2 = z1/z2
Therefore, by knowing the electrochemical equivalent of one substance and using the above expression, we can calculate that of the other substance.
The amount of electric charge that will release one gram equivalent of mass of a substance is obtained by
putting m = E in Faraday's first law:
Q = E / z.
From Faraday's second law, z1 / z2 = E1 / E2.
Or
E1 /z1 = E2 / z2 = ... = E / z = Q.
Therefore, the amount of charge required to liberate one gram (chemical) equivalent of a substance is the same for all substances. This quantity is given the name Faraday. Therefore, 1 Faraday is defined as, the quantity of charge required to liberate one gram equivalent of any substance. Experimentally, one Faraday is found to be equal to 96500 C (C = coulomb, the unit of charge).