Acid-Base Titrations (Cambridge International IGCSE Chemistry)

Spec point — 12.1.3 · Describe an acid–base titration to include the use of a: burette volumetric pipette suitable indicator

1. Describe how to identify the end-point of a titration using an indicator

Acid-base titrations

• Titrations are a method of analysing the concentration of solutions
• They can determine exactly how much alkali is needed to neutralise a quantity of acid – and vice versa
• You may be asked to perform titration calculations to determine the moles present in a given amount or the concentration /volume required to neutralise an acid or a base
• Titrations can also be used to prepare salts

Apparatus

• 25 cm³ volumetric pipette
• Pipette filler
• 50 cm³ burette
• 250 cm³ conical flask
• Small funnel
• 0.1 mol / dm³ sodium hydroxide solution
• Sulfuric acid of unknown concentration
• A suitable indicator
• Clamp stand, clamp & white tile

The steps in performing a titration

Method

1. Use the pipette and pipette filler and place exactly 25 cm³ sodium hydroxide solution into the conical flask
2. Using the funnel, fill the burette with hydrochloric acid placing an empty beaker underneath the tap. Run a small portion of acid through the burette to remove any air bubbles
3. Record the starting point on the burette to the nearest 0.05 cm³
4. Place the conical flask on a white tile so the tip of the burette is inside the flask
5. Add a few drops of a suitable indicator to the solution in the conical flask
6. Perform a rough titration by taking the burette reading and running in the solution in 1 – 3 cm³ portions, while swirling the flask vigorously
7. Quickly close the tap when the end-point is reached

• The endpoint is when one drop causes a sharp colour change

1. Record the volume of hydrochloric acid added, in a suitable results table as shown below

• Make sure your eye is level with the meniscus

1. Repeat the titration with a fresh batch of sodium hydroxide
2. As the rough end-point volume is approached, add the solution from the burette one drop at a time until the indicator just changes colour
3. Record the volume to the nearest 0.05 cm³
4. Repeat until you achieve two concordant results (two results that are within 0.1 cm³ of each other) to increase accuracy

Exam Hint Common errors during a titration include:

• Not removing the funnel from the burette

  • This can lead to some liquid dripping into the burette and cause false / high readings

• Not filling the jet space of the burette

  • The jet space is the part of the burette after the tap
  • Not filling this space can lead to false readings

• Reading the volume from the burette incorrectly

  • Readings should be taken from the bottom of the meniscus
  • Careful: The scale on the burette has 0.0 cm³ at the top and 50 cm³ (typically) at the bottom

Indicators

• Indicators are used to show the endpoint in a titration

• Wide range indicators such as litmus are not suitable for titration as they do not give a sharp colour change at the endpoint

  • However, methyl orange and phenolphthalein are very suitable

• Some of the most common indicators with their corresponding colours are shown below:

Common acid-base indicators