The titration required practical is used to find the exact volume of acid needed to neutralise a known volume of alkali. A pipette delivers a precise volume of alkali into a conical flask, and a few drops of indicator are added. Acid is run slowly from a burette, drop by drop near the endpoint, until the indicator changes colour permanently. The volume of acid delivered — the titre — is calculated by subtracting the initial burette reading from the final reading. At least two concordant titres, within 0.10 cm³ of each other, are needed before a reliable mean can be calculated. The titration method is used in GCSE Chemistry Higher tier and requires accurate burette technique and careful observation of the endpoint colour change.
Aim
To determine the reacting volume of acid needed to neutralise a known volume of alkali.
Method students should know
- Use a pipette to measure alkali into a conical flask.
- Add a few drops of indicator.
- Fill the burette with acid and record the initial reading.
- Add acid while swirling until the indicator changes colour permanently.
- Record the final burette reading and calculate the titre.
Variables
- Independent variable
- Volume of acid added.
- Dependent variable
- Endpoint colour change.
- Control variables
- Volume of alkali, Indicator amount, Concentration of solutions
Safety and common mistakes
- Wear goggles.
- Clamp the burette securely.
- Read the meniscus at eye level.
- Forgetting to subtract initial from final burette reading.
- Adding too much indicator.
- Using rough titres in the mean.
Common questions students ask
- Why is a conical flask used in a titration instead of a beaker?
- A conical flask can be swirled easily without spilling the contents. Swirling mixes the acid and alkali thoroughly and helps the student spot the endpoint more accurately. The narrow neck also reduces evaporation and splashing during mixing.
- What does concordant mean in a titration?
- Concordant titres are titre values that are close enough to each other to be considered reliable — usually within 0.10 cm³. At least two concordant titres are needed before calculating the mean. The rough titre is always excluded because it is less precise. Using concordant titres makes the mean more accurate.
- Why is acid added dropwise near the endpoint of a titration?
- Near the endpoint, the next drop of acid could carry the solution past neutral. Adding acid one drop at a time allows the student to identify the exact volume needed to neutralise the alkali without overshooting. The sides of the flask should also be rinsed with distilled water to wash any acid droplets into the mixture.
- Why is a white tile placed under the conical flask?
- A white tile is placed under the conical flask to make the indicator colour change easier to see at the endpoint. The pale colour that appears at the endpoint — for example, a faint permanent pink with phenolphthalein — is much easier to detect against a white background than against a dark bench surface.
- How do you read a burette correctly for GCSE Chemistry?
- Read the burette at eye level to avoid parallax error. The scale on a burette increases downward, so the top of the liquid level is a lower number than the bottom. Read from the bottom of the meniscus (the curved surface of the liquid) and record the reading to two decimal places, for example 12.35 cm³.