Walden's Rule

Walden's Rule

This empirical rule was suggested by P. Walden (1863–1957). According to this rule, the molar conductance of an electrolyte at infinite dilution is inversely proportional to the viscosity of the medium.
i.e. λ^o_M ∝ 1/η
or, λ^o_M x η = Constant
Where, λ^o_M is the molar conductance at infinite dilution and η is the coefficient of viscosity of the solvent.
Thus, Walden's rule states that the product of coefficient of viscosity of a solvent and the equivalent conductance of an electrolyte at infinite dilution is approximately constant and independent of the nature of the solvent.

Solvents	CH3OH	CH3OCH3	CH3CN	C2H4Cl2	CH3NO2	C6H5NO2	C6H5OH
λoM	0.63	0.66	0.64	0.60	0.69	0.67	0.63

Walden's rule holds only for large ions such as tetra methyl ammonium ion (CH₃)₄N⁺ and the picrate ion C₆H₂(NO₂)₃O^⁻. However, if we exclude water, the smaller ions also obey Walden's rule, though not very satisfactorily because the smaller ions are more solvated than larger ions.