The cation of the indicator electrolyte should not move faster than the cation whose transport number is to be determined, and it should have same anion as the principle electrolyte. Besides the principal electrolyte (e.g., HCl) is kept light so that it floats on indicator electrolyte. CdCl 2 serves best because Cd is less mobile than H and Cl is common to both CdCl 2 and the principal electrolyte HCl.