Abnormal Transport Number

Abnormal transport number refers to a situation where the transport number of an ion in an electrolyte is either greater or less than its theoretical value. The theoretical transport number is determined based on the charge of the ion and its mobility relative to the other ions in the solution.

CdI2 Aqueous Solution

In an aqueous solution of CdI2, the transport number of Cd2+ ion can be negative under certain conditions. This occurs when the concentration of iodide ions exceeds a certain value, leading to the formation of CdI4 2- complex ions. The negative transport number of Cd2+ ion indicates that it is moving in the opposite direction to the net migration of other cations in the solution.

Explanation of the Phenomenon

The negative transport number of Cd2+ ion in an aqueous solution of CdI2 can be explained by the formation of complex ions. When CdI2 dissolves in water, it dissociates into Cd2+ 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 Cd2+ ions begin to form complex ions with the excess iodide ions.

Cd2+ + 4I- → CdI4 2-

These complex ions have a higher mobility than the Cd2+ ions and move faster towards the anode. As a result, the transport number of Cd2+ ion becomes negative, indicating that it is moving in the opposite direction to the other cations in the solution.

Conclusion

In conclusion, abnormal transport number refers to a situation where 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 Cd2+ ion can be negative when the concentration of iodide ions exceeds a certain value, leading to the formation of complex ions. This phenomenon can be explained by the higher mobility of the complex ions relative to the Cd2+ ions.