4.1.2 Required Practical 1 - Performing an Acid-Base Titration

Aim

To determine the concentration of an unknown acid (or alkali) solution using a standard solution in a titration. The endpoint is identified by a colour change of an indicator, signifying neutralization.

Equipment

Chemicals

• Acid solution of unknown concentration (e.g., hydrochloric acid, $HCl$)
• Standard solution of alkali (e.g., sodium hydroxide, $NaOH$)
• Indicator (e.g., phenolphthalein or methyl orange)

Apparatus

• Burette (50 cm³)
• Conical flask (250 cm³)
• Pipette (25 cm³) and pipette filler
• White tile (to observe colour change)
• Clamp stand and burette clamp
• Distilled water for rinsing
• Funnel for filling burette
• 2 dp balance (if preparing a standard solution)
• Volumetric flask (if preparing a standard solution)

Risk Assessment

• Acids and alkalis are corrosive:
  • Wear safety goggles, gloves, and a lab coat.
  • Immediately rinse skin with water if contact occurs.
• Handle glass equipment with care to prevent breakage.
• Ensure the burette tap is tightly closed before filling to avoid spills.

Method

1. Prepare the equipment:

• Rinse the burette with the solution it will contain.
• Rinse the pipette with the solution it will transfer.
• Rinse the conical flask with distilled water.

2. Fill the burette:

• Using a funnel, fill the burette with the standard solution (e.g., $NaOH$), ensuring the bottom of the meniscus is on the 0.00 cm³ mark.
• Remove the funnel to avoid drips.

3. Pipette the acid solution:

• Use a 25 cm³ pipette to transfer the acid solution into a conical flask.
• Add a few drops of the indicator (e.g., phenolphthalein turns from colourless to pink at the endpoint).

4. Perform the titration:

• Place the conical flask on a white tile.
• Add the alkali from the burette while swirling the flask gently.
• Slow down the addition near the endpoint until a permanent colour change is observed.

5. Record the titre:

• Note the initial and final volume readings on the burette.
• Repeat the titration until you have at least two concordant results (within 0.10 cm³ of each other).

Results

• Record all titre volumes to 2 decimal places (0.05 cm³ accuracy).
• Use only the concordant titres to calculate the average titre volume.

Errors and Improvements

• Uncertainty in burette readings is typically ±0.10 cm³ (two readings of ±0.05 cm³ each).
• Add another ±0.05 cm³ to account for the endpoint judgement.
• Reduce uncertainty by:
  • Using larger volumes in the conical flask.
  • Employing pipettes or burettes for measurements instead of measuring cylinders.
• Avoid leaving $NaOH$ in the burette as it can cause damage, leading to inaccuracies.