Neutralisation

Determination of the reacting volumes of solutions of a strong acid and a strong alkali by titration.

The required practical activity can be adapted for students of higher and lower tiers of entry. Students are assessed on their ability to use appropriate laboratory apparatus to make accurate measurements. Students will use a burette and a colour-changing indicator to investigate what volume of Sulfuric Acid will neutralise 25mls of Sodium Hydroxide.

Equipment (per participant)

• Safety goggles
• 250ml conical flask
• 25ml volumetric pipette
• Pipette filler
• Burette funnel
• Burette holder

• Retort stand
• White tile
• Sodium Hydroxide (0.1M)
• Sulfuric Acid (0.08M)
• Phenolphthalein Indicator

Method

The necessary equipment is collected by the students and a burette is set up using the burette holder and retort stand, making sure the burette tap is closed.
Using a graduated glass pipette and pipette filler, students should measure precisely 25mls of Sodium Hydroxide and deposit this into a 250ml conical flask.
The flask containing Sodium Hydroxide is placed onto a white tile.
Sulfuric Acid is dispensed into the 100ml beaker. Then, using a burette funnel, students should carefully fill the burette with acid so that the level is just above the maximum volume line and remove the funnel. The 100ml beaker is then placed underneath the burette to catch excess acid.
Students should then slowly open the burette tap and alter the flow so that single drops are dispensed. They must watch the fill line on the burette and close the tap
completely when the meniscus line meets the maximum volume mark. The 100ml beaker can be discarded.
Students then need to add 5-10 drops of Phenolphthalein indicator into the conical flask containing Sodium Hydroxide and swirl the flask to ensure it is thoroughly mixed. The Sodium Hydroxide in the flask should now be pink in colour. The conical flask and white tile should be positioned underneath the burette.
Using the burette tap, whilst constantly swirling the conical flask, students should carefully dispense 10mls of Sulfuric Acid into the flask. Then they must adjust the tap so that the acid is added by the drop. Students should immediately close the tap as soon as the pink colour disappears permanently from the solution.
Students then read the burette scale and record the volume of acid used to two decimal places. Their results should be recorded in a suitable table.
They should then repeat the process, so they have completed three titrations and are able to take a mean average of their results.

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Technician tips

• Always consult the CLEAPSS method for diluting concentrated acids. 0.08M Sulfuric Acid is made up wearing splash-proof goggles or face shield and chemical resistant gloves. 4.32mls of concentrated sulfuric acid is slowly and carefully added to 200mls of iced water on a magnetic stirrer. Once the ice has melted, the solution is then made up to 1 Litre.
• Always consult the CLEAPSS method for making a solution of Sodium Hydroxide. Wearing appropriate PPE, 4g of Sodium Hydroxide pellets are dissolved in 1 Litre of water to make a 0.1M solution