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A titration is a careful, accurate way of finding out exactly how much of one solution reacts with a measured volume of another. It is one of the key required practicals of Topic C5 of OCR Gateway Science A, and it is the technique behind the titration calculations of the next lesson. In a titration an acid is added bit by bit to a measured volume of alkali (or the other way round) until they have just reacted completely, using an indicator to spot that moment. This lesson sets out the apparatus, the method, and how to obtain concordant results so the answer can be trusted.
By the end of this lesson you should be able to name and use the apparatus for a titration, describe the method step by step, explain the use of an indicator and the end point, and calculate a mean titre from concordant results.
A titration is a skill as much as a calculation: the marks come from describing the careful, repeatable technique that produces a trustworthy result. Get the method right and the numbers that follow are straightforward.
A titration measures the volume of one solution needed to react exactly with a known volume of another. The most common school titration is between a strong acid (such as hydrochloric or sulfuric acid) and a strong alkali (such as sodium hydroxide), and its purpose is usually to find the concentration of one of them when the concentration of the other is known.
To get a reliable answer the volumes must be measured very precisely, so a titration uses two specialised pieces of apparatus — a pipette and a burette — together with an indicator to show the exact moment the reaction is complete.
Exam Tip: A titration finds the volume of one solution that exactly reacts with a measured volume of another. Its whole point is accuracy, so the marks are in the careful use of the apparatus and in obtaining concordant (closely agreeing) results.
| Apparatus | What it does |
|---|---|
| Pipette | Measures an accurate, fixed volume (e.g. 25.0 cm3) of one solution into the conical flask |
| Pipette filler | Draws liquid safely into the pipette (never use the mouth) |
| Burette | Delivers a variable, measured volume of the other solution, read to 0.05 cm3 |
| Conical flask | Holds the measured solution; its shape lets you swirl without spilling |
| Indicator | Changes colour at the end point to show the reaction is just complete |
| White tile | Placed under the flask so the colour change is easy to see |
Exam Tip: Match each piece of apparatus to its job: the pipette measures the fixed volume into the flask; the burette delivers the variable volume and is the one you read at the start and end. Naming the wrong one (or saying "measuring cylinder") loses the mark — a measuring cylinder is not accurate enough for a titration.
A titration needs an indicator that gives a sharp, single colour change at the point of exact neutralisation — called the end point. Universal indicator is not used, because its gradual range of colours makes the exact end point hard to judge. The two common choices are:
| Indicator | Colour in acid | Colour in alkali | At the end point |
|---|---|---|---|
| Phenolphthalein | Colourless | Pink | Pink → colourless (acid into alkali) |
| Methyl orange | Red | Yellow | Red → yellow (acid into alkali) |
For example, with phenolphthalein, if the flask contains alkali the indicator is pink; as acid is added from the burette, the moment just enough acid has been added to neutralise the alkali the solution turns colourless — that is the end point.
Exam Tip: Use a single-change indicator such as phenolphthalein or methyl orange, not universal indicator. Quote the actual colour change (e.g. "pink to colourless") rather than just naming the indicator.
The standard method for a strong acid–strong alkali titration is:
The first rough titration is done quickly just to find roughly where the end point lies; its result is not used in the final mean. The repeats are then done slowly and carefully so the end point is caught precisely.
Exam Tip: Two method marks examiners look for are swirling the flask throughout and adding the acid drop by drop near the end point. Doing a rough titration first so you know roughly where the end point is, then repeating carefully, is the technique that gives precise results.
A burette is read to a precision of 0.05 cm3 — that is, to the nearest half-division. Two rules matter:
Exam Tip: A burette is numbered 0 at the top, so the scale increases downwards and the titre is final − initial. Always read the bottom of the meniscus at eye level, to 0.05 cm3. Reading "from the bottom up" is a classic error.
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