The transport number of an ion, denoted by "t", represents the fraction of total current carried by that ion during electrolysis. These numbers are crucial for understanding the movement of ions in an electrolytic solution and are determined using various methods, two of which are:
1. Hittorf's Method:
This method involves analyzing the changes in concentration of electrolyte around the electrodes after electrolysis. Here's a step-by-step breakdown:
a) Experimental Setup:
* An electrolytic cell with two compartments separated by a porous diaphragm is used.
* Electrodes are immersed in the electrolyte solution, and a direct current is passed through the cell for a specific duration.
* The electrolyte solution in both compartments is analyzed before and after electrolysis to determine the concentration changes.
b) Principle:
* The transport number of an ion is determined by analyzing the change in concentration of the electrolyte around the electrodes.
* The change in concentration reflects the migration of ions during electrolysis.
* The ratio of the change in concentration of the ion around one electrode to the total change in concentration gives the transport number of that ion.
c) Calculations:
* The transport number of the cation, t+, is calculated as:
t+ = (Change in concentration of cation in anode compartment)/(Total change in concentration of electrolyte)
* Similarly, the transport number of the anion, t-, is calculated as:
t- = (Change in concentration of anion in cathode compartment)/(Total change in concentration of electrolyte)
* Note that t+ + t- = 1, reflecting the total current carried by both ions.
2. Moving Boundary Method:
This method involves observing the movement of a boundary between two solutions during electrolysis. The boundary is typically established between a colored solution containing the ion of interest and a colorless solution containing a different ion.
a) Experimental Setup:
* A vertical tube with a non-conducting material (e.g., glass) is filled with two solutions: the solution of interest containing a colored ion at the bottom and a colorless solution containing a different ion at the top.
* Electrodes are placed in each solution, and a direct current is passed through the tube.
* The boundary between the two solutions moves as electrolysis proceeds, and its movement is observed and measured.
b) Principle:
* The movement of the boundary reflects the migration of the colored ion.
* The rate of movement of the boundary is proportional to the transport number of the colored ion.
* The transport number is calculated using the distance moved by the boundary and the current passed.
c) Calculations:
* The transport number of the colored ion is calculated as:
t = (Distance moved by boundary * Faraday constant) / (Current * Time * Concentration of the colored ion)
Advantages and Disadvantages:
| Method | Advantages | Disadvantages |
|---|---|---|
| Hittorf's Method | Simple setup, relatively inexpensive | Time-consuming, difficult to analyze concentration changes accurately |
| Moving Boundary Method | More accurate determination of transport numbers | Requires special setup, limited to certain electrolytes |
Both methods have their advantages and disadvantages, and the choice of method depends on the specific electrolyte being studied and the desired accuracy.
