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Kohlraush’s Law

الكلية كلية العلوم للبنات     القسم قسم الكيمياء     المرحلة 3
أستاذ المادة حازم يحيى محمد علي الجبوري       23/11/2019 05:00:21
F .W. Kohlraush (1875) determined the values of equivalent conductivity at
infinite dilution for a large number of electrolytes and observed that the
difference between the equivalent conductivity at infinite dilution of pairs of salts,
having either a cation or anion in common, is constant at constant temperature.
For example, the difference between at 25°C for K and Na salts having a
common anion is always found to be and that for
chloride and bromide having a common cation is always .
It is clear from the following table:From the study of above table, it may be pointed out that at infinite dilution, each
ion makes a definite contribution towards the equivalent conductivity of
electrolyte, irrespective of the nature of the other ion with which it is associated
in solution. This important generalisation, called the law of
independent migration of ions(also called (Kohlraush’s law, after the
discoverer) is strictly true at infinite dilution only where there is no mutual
interaction between the different ions.
It may be stated as:
“The equivalent conductivity of an electrolyte at infinite dilution is the sum of two
values, one depending only on the cation and the other only on the anion”.
Mathematically,
where ka and kc are the ionic mobilities or ionic conductivities of the anion and
cation respectively. By the term ionic mobility of an ion we mean, ‘the
conductivity of one equivalent of the ion when the solution containing that
quantity of the ion is placed between two electrodes 1 cm apart.’

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