Titrations

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.

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What a Titration Is For

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.

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The Apparatus

Apparatus	What it does
Pipette	Measures an accurate, fixed volume (e.g. $25.0 \text{ cm}^3$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 \text{ cm}^3$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.

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Indicators and the End Point

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.

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The Titration Method

The standard method for a strong acid–strong alkali titration is:

1. Use a pipette (with a filler) to measure an accurate $25.0 \text{ cm}^3$25.0 cm3 of the alkali into a clean conical flask, and add a few drops of indicator.
2. Fill the burette with the acid, making sure the part below the tap is filled and there are no air bubbles. Read and record the initial burette reading at eye level.
3. Stand the flask on a white tile and add the acid from the burette, swirling the flask constantly. As you near the end point, add the acid drop by drop.
4. Stop the moment the indicator just changes colour permanently (the end point). Read and record the final burette reading.
5. The titre is the volume of acid added: $\text{titre} = \text{final reading} - \text{initial reading}$titre=final reading−initial reading.
6. The first run is a quick rough (trial) titration to find the approximate end point. Repeat carefully, adding acid drop by drop near the end, until you have concordant titres.

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.

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Reading the Burette

A burette is read to a precision of $0.05 \text{ cm}^3$0.05 cm3 — that is, to the nearest half-division. Two rules matter:

• Read from the top down. A burette is numbered with 0 at the top, so the volume delivered increases as you go down. Read the level of the bottom of the meniscus (the curved surface of the liquid).
• Read at eye level. Your eye must be level with the meniscus to avoid a parallax error that would make the reading too high or too low.

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 \text{ cm}^3$0.05 cm3. Reading "from the bottom up" is a classic error.