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_∞ = l_c + l_a.

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
Λ_∞KCl - Λ_∞KNO₃ = 3.7
Λ_∞KCl - Λ_∞NaCl = 21.1
Λ_∞NaCl - Λ_∞NaNO₃ = 3.7
Λ_∞KNO₃ - Λ_∞NaNO₃ = 21.1

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

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