Apparatus Required:

• 25.0 cm³ volumetric pipette
• Burette (50.0 cm³)
• Conical flask
• Distilled water
• Phenolphthalein indicator
• White tile
• Stirring rod

Procedure:

1. Carefully pipette 25.0 cm³ of the HCl solution into a clean conical flask. Rinse the pipette thoroughly with distilled water to ensure all the solution is transferred.
2. Add 2-3 drops of phenolphthalein indicator to the conical flask.
3. Fill the burette with distilled water, ensuring no air bubbles are present at the top. Record the initial reading of the burette to the nearest 0.05 cm³.
4. Slowly add the HCl from the burette to the conical flask while swirling the flask continuously. As the titration proceeds, the solution in the conical flask will gradually change from colorless to a faint pink colour around the endpoint.
5. Continue adding the HCl dropwise until a faint pink colour persists for at least 30 seconds when swirling the solution. This indicates the endpoint of the titration.
6. Record the final reading of the burette to the nearest 0.05 cm³.
7. Repeat the titration at least three times to obtain concordant results (results within 0.2 cm³).

Calculations:

The volume of HCl used in the titration is calculated by subtracting the initial burette reading from the final burette reading. The concentration of the HCl solution can then be calculated using the following equation:

Concentration of HCl (mol/dm³) = (Volume of HCl used in the titration (dm³) x Concentration of HCl (mol/dm³)) / Volume of solution in the conical flask (dm³)

Where the volume of solution in the conical flask is 0.025 dm³.

Ferroin is a suitable indicator for the titration of potassium thiocyanate with iodine because it exhibits a distinct colour change over the pH range of the reaction. The reaction between iodine and thiocyanate results in the formation of a complex ion, which is a relatively weak acid. The pH of the solution changes as the reaction proceeds.

Ferroin is a diprotic indicator, meaning it has two distinct colour forms depending on the pH. It typically appears colourless in acidic solutions (pH 6.2). The reaction between iodine and thiocyanate produces HI, which is a strong acid. As the reaction proceeds, the pH of the solution decreases. The end-point is reached when the pH falls below 2.3, causing the ferroin to change from colourless to a deep blue colour. This sharp colour change signals the complete consumption of iodine by the thiocyanate.

Apparatus Required:

• Burette (50.0 cm³)
• 25.0 cm³ volumetric pipette
• Conical flask
• Distilled water
• Indicator (e.g., methyl orange, bromothymol blue)
• White tile
• Stirring rod

Procedure:

1. Carefully pipette 25.0 cm³ of the strong acid solution into a clean conical flask. Rinse the pipette thoroughly with distilled water.
2. Add 2-3 drops of the chosen indicator to the conical flask.
3. Fill the burette with distilled water, ensuring no air bubbles are present. Record the initial burette reading.
4. Slowly add the strong acid from the burette to the conical flask while swirling the flask continuously.
5. Continue adding the acid dropwise until a permanent colour change occurs, indicating the endpoint of the titration.
6. Record the final burette reading.
7. Repeat the titration at least three times to obtain concordant results.

Importance of Multiple Titrations:

Performing multiple titrations is crucial for ensuring accuracy because it helps to reduce the impact of random errors. Each titration may have a slightly different result due to minor variations in the experimental conditions (e.g., slight variations in the amount of indicator added, slight variations in the precision of reading the burette). By taking multiple measurements and averaging them, these random errors are minimised, and a more reliable and accurate result is obtained. The more titrations performed, the more reliable the average concentration will be.