Transport number

Ion transport number, also called the transference number, is the fraction of the total electrical current carried in an electrolyte by a given ionic species ${\displaystyle i}$,

Differences in transport number arise from differences in electrical mobility. For example, in a solution of sodium chloride, less than half of the current is carried by the positively charged sodium ions (cations) and more than half is carried by the negatively charged chloride ions (anions) because the chloride ions are able to move faster, i.e., chloride ions have higher mobility than sodium ions. The sum of the transport numbers for all of the ions in solution always equals unity.

The concept and measurement of transport number were introduced by Johann Wilhelm Hittorf in the year 1853.Liquid Junction Potential can arise from ions in a solution having different ion transport numbers.

At zero concentration, the limiting ion transport numbers may be expressed in terms of the limiting molar conductivities of the cation (${\displaystyle \lambda _{0}^{+}}$), anion (${\displaystyle \lambda _{0}^{-}}$), and electrolyte (${\displaystyle \Lambda _{0}}$):

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