Performing a Titration
- Titrations are a method of analysing the concentration of solutions.
- Acid-base titrations are one of the most important kinds of titrations.
- They can determine exactly how much alkali is needed to neutralise a quantity of acid – and vice versa.
- You may be asked to calculate the moles present in a given amount, the concentration or volume required to neutralise an acid or a base.
- Titrations can also be used to prepare salts or other precipitates and in redox reactions.
- Single indicators are used as a sharp colour change is needed.
- Wide range indicators such as Universal Indicator change colour gradually from dark blue in alkaline conditions to bright red in acidic conditions.
- Single indicators change abruptly at the end-point which corresponds to when the neutralisation has occurred.
- Some of the most common indicators with their corresponding colours are shown below.
Required Practical 2: Acid-Base Neutralisation
To determine the reacting volumes of a strong acid and a strong alkali by titration.
The titration method can be used to calculate the concentration of an acid.
- 25 cm3 volumetric pipette
- pipette filler
- 50 cm3 burette
- 250 cm3 conical flask
- small funnel
- 0.1 mol/dm3 sodium hydroxide solution
- sulfuric acid – concentration unknown
- phenolphthalein indicator.
- clamp stand, clamp & white tile
Diagram showing the apparatus needed to prepare a salt by titration
Make sure you remove the funnel after filling the burette as if left it can drop solution into the burette, leading to error.
- Use the pipette and pipette filler and place exactly 25 cm3 sodium hydroxide solution into the conical flask.
- Place the conical flask on a white tile so the tip of the burette is inside the flask.
- Add a few drops of a suitable indicator to the solution in the conical flask.
- Perform a rough titration by taking the burette reading and running in the solution in 1 – 3 cm3 portions, while swirling the flask vigorously.
- Quickly close the tap when the end-point is reached (sharp colour change) and record the volume, placing your eye level with the meniscus.
- Now repeat the titration with a fresh batch of alkaline.
- As the rough end-point volume is approached, add solution from the burette one drop at a time until the indicator changes colour.
- Record the volume to the correct number of decimal places (to 0.1cm3).
- Repeat until you achieve two concordant results (two results that are within 0.1cm3 of each other) to increase accuracy.
Results: Record your results in a suitable table, e.g:
Concordant results should be achieved to calculate a mean titre.
The mean titre is calculated and used to calculate the concentration of the acid in mol/dm3, ignoring any anomalous results.
- The concentration of a solution can be expressed either in moles per dm3 or in grams per dm3.
- To go from g dm-3 to mol dm-3:
- Divide by the molar mass in grams.
- To go from mol dm-3 to g dm-3:
- Multiply by the molar mass in grams.
- Once a titration is completed and the average titre has been calculated, you can now proceed to calculate the unknown variable using the formula triangle as shown below.
- It can be used to derive equations for concentration and volume.
- From the triangle:
Concentration = moles ÷ volume
Volume = moles ÷ concentration
Formula triangle showing the relationship between concentration, number of moles and volume of liquid
AQA GCSE Chemistry Notes
Want to aim for a Level 9?
See if you’ve got what it takes. Test yourself with our topic questions.
More AQA GCSE (9-1) Chemistry Revision Resources
Morgan’s passion for the Periodic Table begun on his 10th birthday when he received his first Chemistry set. After studying the subject at university he went on to become a fully fledged Chemistry teacher, and now works in an international school in Madrid! In his spare time he helps create our fantastic resources to help you ace your exams.