Titrations (chemistry only) — AQA GCSE Chemistry
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Titrations (chemistry only) explained
Titrations (chemistry only) covers 2 specification points, including The volumes of acid and alkali solutions that… and (chemistry only) determination of the reacting….
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Open a point below for its explanation and revision guidance.
The volumes of acid and alkali solutions that react with each other can be measured by titration using a suitable indicator.
A titration is a practical method used to measure the volume of one solution that reacts exactly with a known volume of another solution. In an acid-alkali titration, a suitable indicator is added to the alkali in a conical flask. A burette is filled with the acid, and the acid is added gradually while the flask is swirled. The indicator changes colour at the end point, showing that the acid and alkali have reacted completely. The volume of acid added is read from the burette before and after the titration, and the difference gives the volume used. Repeating the titration and taking a concordant result improves reliability. A suitable indicator changes colour sharply at the end point, so the volume can be measured accurately.
Required practical 2: (chemistry only) determination of the reacting volumes of solutions of a strong acid and a strong alkali by titration.
In this required practical you find the volume of one solution that exactly reacts with a known volume of another. A strong acid, such as hydrochloric acid, is placed in a burette and a measured volume of strong alkali, such as sodium hydroxide solution, is pipetted into a conical flask with a few drops of indicator. You record the initial burette reading, then add acid steadily while swirling, slowing near the end point as the indicator colour changes. Repeating until concordant titres agree improves reliability. You must use accurate procedural techniques, such as rinsing the burette with the acid and reading the bottom of the meniscus at eye level, to ensure precise volume measurements.
Your focus
- Describe how to carry out an acid-alkali titration using a suitable indicator.
- Calculate the volume of acid or alkali used from burette readings.
- Explain why repeating a titration improves the reliability of the result.
Show all 6 objectives
- Carry out a titration accurately to obtain concordant titres.
- Identify the end point using a suitable indicator.
- Apply precise procedural techniques to measure reacting volumes.
Titrations (chemistry only) exam tips
Marking Points
- States that titration measures the volume of acid that reacts exactly with a known volume of alkali, or vice versa.
- Describes using a burette to add one solution gradually to a measured volume of the other in a conical flask.
- Explains that a suitable indicator is used to show the end point by a sharp colour change.
- Records the initial and final burette readings and calculates the volume used by subtraction.
- Repeats the titration to obtain concordant results and improve reliability.
- Records initial and final burette readings to two decimal places and calculates each titre by subtraction.
- Uses a pipette and filler to deliver an accurate fixed volume of alkali into a clean conical flask.
- Adds indicator in small amounts and swirls the flask during the titration to mix the solutions.
- Repeats the titration until concordant titres are obtained and uses a mean of concordant values.
- Applies accurate procedural techniques, such as reading the meniscus at eye level, to ensure precise measurements.
Examiner Tips
- 💡When describing a titration, include the key apparatus: burette, pipette, conical flask and a suitable indicator.
- 💡Calculate the volume used by subtracting the initial burette reading from the final burette reading, and show your working clearly.
- 💡Explain that repeating the titration and using concordant results reduces the effect of random errors.
- 💡State the colour change at the end point for the indicator used, for example phenolphthalein changing from pink to colourless when acid is added to alkali.
- 💡Ensure you subtract the initial volume from the final volume to find the titre.
- 💡Give burette readings to two decimal places and quote the mean titre with the same precision.
Common Mistakes
- Reading the burette scale from the bottom up instead of from the top down; correct this by remembering the zero mark is at the top, so you must read downwards to the bottom of the meniscus at eye level.
- Forgetting to record the initial burette reading before starting; correct this by always recording both initial and final readings so the volume used can be calculated.
- Choosing an indicator that changes colour gradually over a wide pH range; correct this by selecting a suitable indicator that gives a sharp colour change at the end point.
- Reading the top of the meniscus instead of the bottom; correct by aligning eye level with the bottom of the curved surface.
- Forgetting to record the initial burette reading; correct by always noting it before adding any acid.
- Using the first rough titre in the mean; correct by repeating and averaging only concordant accurate titres.