Transport Number

The transport number is the fraction of the total current carried by cations or anions in an electrolytic solution.

Hittorf’s method explains transport numbers by examining ion migration in a cell divided into anodic, central, and cathodic compartments.

When a potential difference is applied, cations move toward the cathode and anions toward the anode, causing concentration changes near each electrode. The central compartment serves as a reference region that is less affected by electrode reactions.

The cation-to-anion mobility ratio equals the fall in concentration around the anode to that around the cathode.

The cation transport number is defined as the concentration fall around the anode divided by the total fall around both electrodes, while the anion transport number equals one minus the cation transport number.

An alternative approach is the moving boundary method, in which an electric current shifts the boundary between the principal electrolyte HCl and the indicator electrolyte CdCl₂.

If an applied potential Q moves the boundary by l cm, displacing lA cm³ containing c gram equivalents of HCl, the transport number of H⁺ ions can be calculated mathematically.