What you'll learn
Titration is a precise technique for finding out exactly how much acid is needed to neutralise a given amount of alkali (or vice versa). For AQA GCSE Chemistry you need to know how to carry out a titration accurately, the equipment used, how to identify the end point with an indicator, and how to obtain reliable, concordant results. This guide covers the apparatus, the method step by step, the choice of indicator, and how to process the results. By the end you should be able to describe the titration practical precisely and explain how accuracy and reliability are achieved. (The calculations from titration results are covered separately.)
Key terms and definitions
Titration — A technique to find the volume of one solution that exactly reacts with a known volume of another.
Burette — A long graduated tube with a tap, used to add a solution a little at a time and measure the volume added.
Pipette — Apparatus used to measure a fixed, accurate volume of solution.
Indicator — A substance that changes colour at the end point of the reaction.
End point — The point at which the reaction is just complete, shown by the indicator changing colour.
Neutralisation — The reaction between an acid and an alkali to form a salt and water.
Concordant results — Repeat titration values that are very close together (within 0.10 cm³).
Titre — The volume of solution added from the burette to reach the end point.
Core concepts
The purpose of a titration
A titration is used to find out exactly what volume of one solution reacts with a known volume of another — most often, how much acid neutralises a known volume of alkali. Because it is done carefully with accurate apparatus, it gives a precise result that can then be used to calculate an unknown concentration.
The apparatus
Titration uses specific accurate equipment:
- A pipette measures a fixed, accurate volume of one solution (for example 25.0 cm³ of alkali) into a conical flask.
- A burette holds the other solution and lets you add it a little at a time, measuring the volume delivered.
- A conical flask holds the measured solution and indicator, and its shape allows swirling without spilling.
- A white tile placed under the flask makes the colour change easier to see.
Using a pipette and burette rather than a measuring cylinder is essential, because they are far more accurate.
The method step by step
To carry out a titration of an acid against an alkali:
- Use the pipette (with a pipette filler) to measure an accurate volume of alkali, for example 25.0 cm³, into a clean conical flask.
- Add a few drops of a suitable indicator to the flask.
- Fill the burette with the acid, recording the starting reading at eye level (reading the bottom of the meniscus).
- Place the flask on a white tile under the burette.
- Add the acid from the burette, swirling the flask constantly, until the indicator just changes colour — this is the end point.
- Record the final burette reading and work out the volume added (the titre).
- This first titration is a rough one. Repeat, adding the acid drop by drop near the end point, until you get concordant results.
Choosing an indicator
A titration uses a single indicator that changes colour sharply at the end point, so you can see exactly when the reaction is complete. Suitable indicators are those that give a clear, sudden colour change, such as:
- Phenolphthalein — pink in alkali, colourless in acid.
- Methyl orange — yellow in alkali, red in acid.
Universal indicator is not used for titrations, because it changes through a range of colours gradually rather than giving a single sharp end point.
Getting accurate and reliable results
Accuracy and reliability come from good technique:
- Read the burette at eye level to the bottom of the meniscus, to 0.05 cm³.
- Add the solution dropwise near the end point so you do not overshoot.
- Swirl the flask throughout to mix the solutions fully.
- Repeat the titration until you get concordant titres (within 0.10 cm³ of each other), then take a mean of the concordant results only.
The first "rough" titration is not included in the mean; only the close, concordant results are averaged.
Processing the results
The titres are recorded in a results table, showing the start reading, end reading and titre for each run. You identify the concordant results, ignore the rough run and any anomalies, and calculate the mean titre from the concordant values. This mean titre is the accurate volume used, which can then be used in calculations to find an unknown concentration.
Sources of error and how to reduce them
Even a careful titration has possible sources of error, and being able to discuss them is important. Overshooting the end point makes the titre too large, so adding the solution dropwise near the end point is essential. Misreading the burette — for example reading the top of the meniscus instead of the bottom, or not reading at eye level — introduces errors, so consistent, careful reading matters. Air bubbles in the burette tip can also cause errors: a bubble that escapes during the titration adds to the apparent volume, so the tip should be filled with no bubble before starting. Repeating until results are concordant and taking a mean reduces the effect of random errors, giving a more reliable final value.
Washing and preparing the apparatus
Correct preparation of the apparatus is part of good technique. The burette should be rinsed with the solution it will contain (the acid), and the pipette with the solution it will measure (the alkali), so that any water left inside does not dilute the solutions and change the concentrations. The conical flask, however, should be rinsed only with water, not the solution, because it is the fixed number of moles pipetted into it that matters, not the volume of liquid in the flask — a little water does not change the amount of alkali present. Understanding why each piece is treated differently shows a good grasp of the practical.
Worked examples
Example 1: Choosing the right apparatus
Why is a pipette used to measure the alkali rather than a measuring cylinder? A pipette measures a fixed volume much more accurately than a measuring cylinder. Since the whole calculation depends on knowing the exact volume of alkali, this accuracy is essential for a reliable result.
Example 2: Identifying the end point
How do you know when the end point has been reached in a titration using phenolphthalein? When acid is added to alkali containing phenolphthalein, the end point is reached when the pink colour just disappears and the solution becomes colourless, showing the alkali has been exactly neutralised.
Example 3: Concordant results
A student obtains titres of 24.10, 23.55, 23.50 and 23.60 cm³. Which should be used to find the mean? The concordant results are 23.55, 23.50 and 23.60 cm³, which are all within 0.10 cm³ of each other. The 24.10 cm³ result is the rough run and is ignored. The mean is taken from the three concordant values.
Example 4: Improving accuracy near the end point
Explain why the acid is added drop by drop near the end point. Near the end point, a single drop can cause the colour to change. Adding drop by drop and swirling lets you stop exactly when the indicator changes, so you do not overshoot and add too much acid, which would make the titre too large.
Common mistakes and how to avoid them
A common error is using universal indicator for a titration. It gives a gradual range of colours, not a sharp end point, so a single indicator such as phenolphthalein or methyl orange must be used.
Students often forget to read the burette correctly. Read it at eye level to the bottom of the meniscus, to 0.05 cm³, both before and after, to get an accurate titre.
Another mistake is including the rough titration in the mean. Only the concordant results (within 0.10 cm³) are averaged; the first rough run and any anomalies are ignored.
When describing the method, do not forget to swirl the flask and add the acid dropwise near the end point — overshooting the end point is a major source of error.
Finally, remember to use a pipette for the fixed volume and a burette for the variable volume; using a measuring cylinder instead would make the result far less accurate.
Exam technique for "Required practical: titration"
Method questions expect precise, ordered detail: pipette an accurate volume of alkali into a flask, add a single indicator, fill the burette with acid, add it while swirling until the indicator changes, record the titre, and repeat for concordant results. Include the white tile and reading at eye level.
Be ready to explain why each step matters — the pipette and burette for accuracy, a single indicator for a sharp end point, dropwise addition to avoid overshooting, and repeats for reliability.
For results questions, know how to identify concordant results, ignore the rough run and anomalies, and calculate a mean titre. Use precise terms — titre, end point, concordant, meniscus — and quote volumes to 0.05 cm³.
Quick revision summary
- A titration finds the volume of one solution that exactly reacts with a known volume of another, using a pipette (fixed volume) and a burette (variable volume).
- Add a single indicator (phenolphthalein or methyl orange) for a sharp end point; never use universal indicator.
- Add the solution while swirling, going dropwise near the end point to avoid overshooting.
- Read the burette at eye level to the bottom of the meniscus, to 0.05 cm³.
- Repeat until titres are concordant (within 0.10 cm³), then take the mean of the concordant results only, ignoring the rough run.
- The mean titre is used to calculate an unknown concentration.