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Electrolysis : Kohlrausch's Law



The independent movement of the ions of a salt assumed in the theory of electrolytic dissociation provides a simple explanation of a result discovered experimentally by in 1875 viz., that the equivalent conductivity of a salt at dilution is the sum of two parts, one depending only on the cation and the other only on the anion:

Λ = lc + la.

This is known as Kohlrausch's law. For example:

KCl Λ = 130.0
KNO3 Λ = 126.3
NaCl Λ = 108.9
NaNO3 Λ = 105.2
ΛKCl - ΛKNO3 = 3.7
ΛKCl - ΛNaCl = 21.1
ΛNaCl - ΛNaNO3 = 3.7
ΛKNO3 - ΛNaNO3 = 21.1

The value 3.7 is the difference in la values for Cl and NO3, and the value 21.1 is the difference in lc values for K and Na: la and lc are called the mobilities of the anion and cation, respectively, expressed in conductivity units.


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