(or transference number), a quantity used in electrochemical calculations. It is the ratio of the current carried by a given ionic species through a cross section of an electrolytic solution to the total current passing through the cross section. The transport number is equal to the ratio of the velocity, or mobility, of a given ion to the sum of the velocities, or mobilities, of the cation and anion. It is a characteristic dependent on the mobilities of all the ions in the electrolytic solution, on the concentrations of the ions, and on the temperature of the solution.

The transport number is usually determined by the Hittorf method—that is, by the change in the concentrations of the ions near the electrodes. During electrolysis, ions, owing to solvation, transport not only electric charge but also the solvent in their solvation sheaths. For this reason, transport numbers determined by the Hittorf method are called apparent transport numbers—in contrast to true transport numbers, which take into account only the velocities of the ions.