What is Abnormal Transport Number?
Abnormal transport number means the transport number of an ion in an electrolyte is either greater or less than its theoretical value. In an aqueous solution of CdI2, the transport number of Cd+2 ion can be negative at high concentration due to the formation of CdI4−2 complex ion. The negative transport number of Cd+2 ion indicates that it is moving in the opposite direction to the net migration of other cations in the solution.
CdI2 ⟷ Cd+2 + 2I−
When CdI2 dissolves in water, it dissociates into Cd+2 and I− ions. These ions move towards their respective electrodes in the presence of an electric field. However, when the concentration of iodide ions exceeds a certain value, the Cd+2 ions begin to form complex ions with the excess iodide ions. These complex ions have a higher mobility than the Cd+2 ions and move faster towards the anode. As a result, the transport number of Cd+2 ion becomes negative.
| Concentration | 0.01 | 0.02 | 0.05 | 0.10 | 0.20 | 0.50 |
| tCd+2 | 0.444 | 0.442 | 0.396 | 0.296 | 0.127 | 0.003 |
The experimental data shows that the Cd+2 is moving towards anode in large amounts than towards the cathode and this is the reason for the negative value of transport number.