Moving boundary method for determination of transport number

Summary

The script explains the 'Moving Boundary Method' for calculating the transport number of ions in an electrolyte solution. The method involves two types of electrolytes: the principal electrolyte, whose transport number is being calculated, and the indicator electrolyte, which helps form the moving boundary. By passing electricity through the solution, the boundary between the two electrolytes moves, allowing measurements of concentration, length, and area to calculate the transport number. The process involves tracking the movement of ions and using specific formulas to determine their transport efficiency.

Takeaways

• 🔢 Moving boundary method is used to calculate the transport number of ions in a solution.
• 📊 Principal electrolyte and indicator electrolyte are required to form a boundary in the solution.
• 🔍 Indicator electrolyte must move slower than the principal electrolyte for the boundary to form.
• 🌡️ The transport number is calculated based on the movement of ions towards the cathode.
• 📏 The movement of the boundary is measured by its length and area, helping to calculate the volume of the solution.
• 💧 Density of the solutions plays a key role, with heavier solutions settling at the bottom and lighter ones on top.
• ⚡ As electricity passes through, the ions move towards the cathode, causing the boundary to shift.
• 🧮 The formula for calculating transport number involves the concentration and volume of the solution, as well as the total current carried by the ions.
• 🔋 The key to transport number calculation is the difference in speed between the principal and indicator electrolytes.
• 🔗 The method requires understanding the relationship between concentration, volume, and ion movement to accurately calculate the transport number.

Q & A

What is the purpose of the moving boundary method discussed in the script?

The moving boundary method is used to calculate the transport number of ions in an electrolyte solution by tracking the movement of a boundary formed between two different electrolyte solutions.

What are the two types of electrolytes mentioned in the script, and how do they differ?

The two types of electrolytes mentioned are the principal electrolyte, whose transport number is being calculated, and the indicator electrolyte, which is used to form the moving boundary. The indicator electrolyte should move more slowly compared to the principal electrolyte.

What are the two key conditions for selecting the indicator electrolyte?

The indicator electrolyte should move slower than the principal electrolyte and have the same ion as the principal electrolyte to ensure a smooth formation of the boundary.

How does the concentration of HCl and CaCl2 solutions affect the moving boundary?

The concentration of HCl and CaCl2 is adjusted so that the CaCl2 solution has a higher density and sinks to the bottom, while the HCl solution, being lighter, floats on top, forming a distinct boundary.

What happens when electricity is passed through the solution in the moving boundary method?

When electricity is passed, the ions in the solutions start moving towards the electrodes. The boundary between the two solutions starts moving upward, indicating the movement of the ions.

What role does the platinum electrode play in the experiment?

The platinum electrode acts as the cathode, attracting the positive ions from the solution during the movement of the boundary, allowing the calculation of transport numbers.

How is the movement of the boundary measured in the method?

The movement of the boundary is measured in terms of the length it moves through the solution, which is tracked in centimeters.

How do you calculate the total volume through which the boundary moves?

The total volume is calculated by multiplying the length the boundary moves by the cross-sectional area of the solution column, giving the volume in cubic centimeters.

What is the formula to calculate the transport number in this method?

The transport number is calculated by dividing the current carried by the positive ions by the total current in the system.

Why is it important to adjust the concentrations of the principal and indicator electrolytes in the moving boundary method?

It is important to adjust the concentrations so that the boundary is distinct and moves predictably, ensuring accurate measurement of the transport number of the principal electrolyte.