Ion transport number, also called the transference number, is the fraction of the total current carried in an electrolyte by a given ion. 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 cations and more than half is carried by the negatively charged chloride anions because the chloride ions are able to move faster, i.e., chloride ions have higher mobility than sodium cations. The sum of the transport numbers for all of the ions in solution equals unity.

The concept of transport number or transference number was introduced by Johann Wilhelm Hittorf in the year 1853.[1] Liquid Junction Potential can arise due to ions in a solution having different ion transport numbers.

The Hittorf method is an experimental technique for the determination of transport numbers.[2] For the simplest case of a solution of a single salt of univalent ions, the transport numbers are defined as the mobility of the ion divided by the sum of mobilities of the two ions. If there are more than one solutes present (e.g., an acidified sodium chloride solution or a mixture of sodium chloride and potassium bromide), every ion will have its own transport number with the sum of them being unity. In these cases, the concentrations of the ions must also be taken into account in the calculation of the transport numbers, and in the case of polyvalent ions, the charges of the ions must also be accounted for.
See also

Debye length
Electrochemical kinetics


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