Electrochemical Equivalent (z)

According to Faraday’s first law of electrolysis,

m = z q

or, z = m / q,

where q = 1C, then

z = m

Hence, the electrochemical equivalent is the mass of ions deposited or liberated on an electrode during the electrolysis, where 1C of charge is passed through it.

Chemical Equivalent (E)

Chemical equivalent is defined as the atomic weight expressed in gram per unit valency.

E = atomic weight / Valency

It’s also known as gram equivalent weight of the substance.

Relation between E and z

Let us consider ‘q’ be the amount of charge which is passed through the 2 electrolytes whose chemical equivalence are E₁ and E₂, and their respective electrochemical equivalence are z₁ and z₁. If m₁ and m₂ are the masses of ions deposited or liberated on an electrode in 2 voltameters, then from first law of Faraday,

m₁ = z₁ q and m₂ = z₂ q

or, m₁ / m₂ = z₁ / z₂ ——–(i)

And, from Faraday’s second law,

m₁ / E₁ = m₂ / E₂

or, m₁ / m₂ = E₁ / E₂ ——–(ii)

Now, from equation (i) and (ii), We get,

E₁ / E₂ = z₁ / z₂

i.e. E₁ / z₁ = E2 / z₂

i.e. E / z = constant

i.e. E / z = F

i.e. E = F z,

where ‘F’ is a constant called Faraday’s constant.

Faraday’s Constant

According to first law of electrolysis,

m = z q

z = m / q

But, F = E / z = E * q /m

If E = m, then F = q

Therefore, Faraday’s constant can be defined as the amount of charge required to liberate a gram equivalent of a substance during the electrolysis.

Example,

For copper,

Chemical equivalent (E) = 31.5 gm

Electrochemical equivalent, e.c.e. (z) = 0.000329 gm/C

We know that,

F = E / z

or, F = 31.5 / 0.000329

∴ F = 96500C