Burette, pipette, indicator — the three named items
A burette is a graduated glass tube with a tap, clamped vertically, usually reading to in divisions; it delivers a variable, measured volume, read before and after. A volumetric pipette, used with a filler, delivers one fixed, precise volume (e.g. ) into the flask. A few drops of a suitable indicator mark the end-point, and a white tile under the flask makes the colour change easy to see.
End-point — one drop, and the colour stays
The end-point is the point at which just one more drop from the burette changes the indicator colour permanently — the new colour persists when the flask is swirled, rather than fading back. The two solutions have then just exactly reacted. Learn both indicators and both directions: phenolphthalein is pink in alkali and colourless in acid; methyl orange is yellow in alkali and red in acid.
Titre — final reading minus initial
The titre is the volume delivered from the burette: . A quick rough (trial) titration is done first to find the approximate volume. The titration is then repeated carefully, dropwise near the end-point, until two or more titres are concordant — agreeing with each other to within . Only the concordant titres are averaged; the rough titre is always excluded from the mean.
Drawn from real examiner reports.
A burette is not "a pipette"
Examiners want the specific instrument named. A burette delivers a variable volume through a tap and is read before and after; a volumetric pipette delivers one fixed, precise volume. Writing "pipette" where the answer is a burette, or dropping "volumetric" from the fixed-volume instrument, does not identify the apparatus and loses the mark.
Jun 2022 P11 Q14; Nov 2022 P31 Q2bi
Burette job vs pipette job
The burette is the instrument you add from gradually until the colour changes, and the only one that gives an initial and a final reading. The volumetric pipette measures one fixed volume into the flask at the very start and is then finished with. Saying a pipette is used to "add acid until the indicator changes" describes the burette, not the pipette.
A fading flash is not the end-point
A colour that appears where the drop lands and then fades on swirling has not reached the end-point — more solution is still needed. A mark-scheme-precise answer needs both halves: one further drop causes the change, AND the new colour persists on swirling. "The end-point is when the colour changes" is too vague to score.
Burette readings written without a unit
Every burette reading, every titre and the mean titre must carry its unit, . A bare number costs the accuracy mark even when the arithmetic is right. Because the burette is graduated in , quote readings to two decimal places — write , not .
Averaging the rough titre as well
The mean titre uses only the concordant titres — those agreeing within . The rough run is deliberately fast and usually overshoots, so including it drags the mean away from the true value. Decide which titres are concordant first, then average just those, and say in your working which run you excluded.
Reading the burette off eye level
The burette scale must be read with the meniscus at eye level. Reading it from above or below introduces a parallax error, so both the initial and the final reading are wrong and the titre is unreliable. Examiners look for this on labelled titration diagrams as well as in written method descriptions.
Titre subtracted the wrong way round
The titre is always final minus initial. Reversing the order gives a negative volume, which can never be a titre. Treat a negative answer as an automatic signal that the readings have been subtracted the wrong way round, and check the sign before writing the value down.
Answer "describe a titration" in order
Work one fixed sequence: pipette the fixed volume into the flask, add indicator, fill the burette and take the initial reading, add while swirling, slow to dropwise near the end-point, stop at the permanent colour change, take the final reading, repeat to concordance, average.
Rinse both instruments first
Rinse the pipette with the solution it will measure, and the burette with the solution it will hold. Water left inside would dilute those solutions and change the titre. It is a creditable step in a method description, and the one candidates most often leave out.
Give one point per mark
"Describe" wants an ordered method, not a general account. Count the tariff and supply that many distinct, creditable steps — a 5-mark titration description needs five. A statement that is scientifically true but does not answer the question earns nothing.
Name both colours and the direction
Never give a single colour. State the starting colour, the finishing colour and which way the change runs — for example phenolphthalein goes pink to colourless as acid is added to an alkali. One colour on its own is not a complete description of an indicator at the end-point.
Cambridge 0654 spec reference: Section C12 "Experimental techniques and chemical analysis", sub-topic C12.2 "Acid-base titrations". Core (both statements listed under C12.2): (1) describe an acid-base titration to include the use of a burette, a volumetric pipette and a suitable indicator; (2) describe how to identify the end-point of a titration using an indicator. There are no Supplement/Extended statements under C12.2, and no calculation statement -- this topic uses no mole concept.
| Term | Mark-scheme-precise meaning |
|---|---|
| Titration | A technique in which a solution of accurately known volume (measured with a volumetric pipette) is reacted with a solution added gradually from a burette, using an indicator to find the volume at which the two solutions have exactly reacted |
| Burette | A graduated glass tube with a tap at the bottom, clamped vertically, used to deliver a variable, precisely measured volume of a solution, read before and after each addition |
| Volumetric pipette | An instrument (used with a pipette filler) that measures out one single, fixed, precise volume of a solution (e.g. ) |
| Indicator | A substance added in small amount to the solution being titrated that changes colour to show when the end-point has been reached |
| End-point | The point in a titration at which one further drop of solution from the burette causes the indicator to change colour permanently (the new colour persists on swirling) |
| Titre | The volume of solution delivered from the burette to reach the end-point: |
| Concordant titres | Two or more titres that agree with each other to within ; only concordant titres are averaged |
Define "titration".
In a titration, a student's burette reads before adding the acid and after reaching the end-point.
Calculate the titre for this run. (2 marks)
