Required Practical: Titration with a Strong Acid and a Strong Alkali

Aims

Foundation + Higher – To determine the reacting volumes of a strong acid and a strong alkali through titration.

Higher – To determine the concentration of one of the solutions in mol/dm³ and g/dm³, using both the reacting volumes and known concentrations of other solutions.

Apparatus

  • Burette
  • 25 cm³ volumetric pipette
  • Pipette filler
  • 250 cm³ conical flask
  • Plastic funnel – Remove the funnel after filling the burette because it could drip into the burette, which can lead to a false initial reading
  • Clamp stand, clamp and white tile
On the left: a labelled titration apparatus where droplets of alkaline solution are added. On the right: a titration apparatus showing a colour change due to the indicator.

Reagents

  • Phenolphthalein indicator
  • Hydrochloric acid (concentration unknown)
  • Sodium hydroxide solution (concentration known)

Method

1. Use the pipette and pipette filler to add 25 cm³ of sodium hydroxide to a clean conical flask.

2. Place the conical flask on a white tile, and insert the tip of the burette into the flask.

3. Add a few drops of phenolphthalein indicator to the flask.

4. Fill the burette with hydrochloric acid and record the initial reading.

5. Slowly open the burette tap (stopcock) and add the acid to the flask, then swirl the flask to mix the solution.

6. Stop adding the acid when the endpoint is reached, which is when the colour permanently changes from pink to colourless. Then record the final burette reading.

7. Repeat steps 1–6 until you have concordant titles (results that are within 0.1 cm³ of each other).

  • Near the endpoint, add the acid drop by drop to increase your accuracy.

Results

Record your results in an appropriate table, as shown below. Remember to record readings to two decimal places, ending in 0 or 5.

RunFinal reading (cm³)Initial reading (cm³)Titre (cm³)
Rough25.550.0025.55
126.550.5026.05 ✓
226.350.2026.15 ✓
Mean titre26.1

The “Rough” titre is the first result obtained and is typically not as accurate as the following titres. It is also recommended to repeat the experiment until concordant titres are obtained, as shown in the table above.

For students doing higher papers, you can use the mean titre to calculate the concentration of the unidentified solution based on the known concentration of the counterpart solution.

You’ve used 10 of your 10 free revision notes for the month

Sign up to get unlimited access to revision notes, quizzes, audio lessons and more

Sign up