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OCR GCSE Chemistry A · J248
OCR J248 · C5.1b Check the specification (PDF) (opens in a new tab)
A titration is used to find the volume of one solution needed to react completely with an accurately measured volume of another. In an acid–alkali titration, an acid reacts with an alkali in a neutralisation reaction. If the concentration of one solution is known, the measured reacting volumes can later be used to determine the concentration of the other solution.
The point at which the correct reacting amount has been added cannot usually be seen directly. A few drops of a suitable indicator are therefore added. Its colour change marks the end-point. The aim is to identify the first permanent colour change without adding an unnecessary excess of solution.
A volumetric pipette transfers one accurate, fixed volume of solution into a conical flask. It is used with a pipette filler, never by mouth. A burette delivers a precisely measured but variable volume of the other solution. Its graduated scale allows the volume delivered to be found from two readings.
The conical flask can be swirled while reducing the risk of splashing. A white tile underneath makes a faint indicator colour change easier to see. The burette is held vertically and securely by a clamp stand.
The burette measures the variable volume delivered; the volumetric pipette transfers an accurate fixed volume to the conical flask.
Use a pipette filler to draw solution into the volumetric pipette. Adjust the bottom of the meniscus to the calibration line at eye level, then allow the pipette to drain into the conical flask under gravity. A volumetric pipette is calibrated to deliver its stated volume, so do not force out the small amount normally left in its tip.
Add only a few drops of a suitable indicator. Too much indicator is unnecessary because the indicator is itself another chemical added to the mixture.
Fill the burette with the other solution and place the conical flask on a white tile beneath its jet. Record the initial burette reading at eye level, using the bottom of the meniscus for a colourless solution.
Run solution from the burette into the flask while swirling. As the expected end-point approaches, add it more slowly until the first permanent colour change appears. Record the final burette reading. The volume delivered is
This first result is a rough titre. It shows approximately where the end-point occurs, so it is not normally included in the final mean.
Transfer a fresh fixed volume to the conical flask and repeat the titration. This time, run in the burette solution fairly quickly until about – short of the rough end-point. Then add it dropwise, swirling continuously, until the indicator just changes colour permanently.
If liquid splashes onto the inside wall of the flask, distilled water can be used to wash it into the mixture. This adds water but does not add more acid or alkali, so it does not alter the amount of burette solution needed for complete reaction.
Repeat accurate titrations until closely agreeing, or concordant, titres are obtained. Calculate a mean from concordant accurate results only. Their agreement suggests that the end-point has been judged reproducibly, although it does not prove that every measurement is free from systematic error.
An appropriate indicator gives a sharp, visible colour change close to the reaction end-point. Universal indicator changes gradually through several colours, so it does not show a sufficiently precise end-point for an accurate titration.
Read the burette at eye level to avoid parallax error, keep it vertical and record both readings carefully. Near the end-point, one extra drop can change the result noticeably, which is why the solution is added dropwise while the flask is swirled.
Acids and alkalis may irritate or damage skin and eyes. Wear eye protection, keep the burette securely clamped, use a pipette filler and follow the stated procedure for dealing with spills.
Titration finds the volume of one solution needed to react completely with a measured volume of another. In an acid–alkali titration, a suitable indicator shows the end-point by changing colour.
Read the bottom of a colourless meniscus at eye level, swirl continuously, use a white tile and do not force the remaining liquid from the pipette tip. Wear eye protection and never pipette by mouth.
When describing a titration, name the measuring apparatus: use a volumetric pipette for a fixed volume and a burette for the variable volume.
State how accuracy is improved: read the burette at eye level, use a white tile, swirl continuously and add solution dropwise near the end-point.
Do not include the rough titre when calculating the mean; use closely agreeing accurate titres.
A titre is the volume delivered from the burette, so calculate it as final burette reading minus initial burette reading.
Do not say that an indicator makes neutralisation happen. It changes colour to show when the end-point has been reached.
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Titration
An analytical technique in which measured volumes of two solutions are reacted to determine the volume needed to reach an end-point.
Standard solution
A solution whose concentration is accurately known.
End-point
The point in a titration at which the indicator changes colour.
Titre
The volume of solution delivered from a burette during a titration.
Concordant titres
Titre values that agree closely with one another.
Indicator
A dye that changes colour over a particular pH range and is used to detect the end-point of an acid–alkali titration.
Put your knowledge into practice — try past paper questions for Chemistry A
Titration
An analytical technique in which measured volumes of two solutions are reacted to determine the volume needed to reach an end-point.
Standard solution
A solution whose concentration is accurately known.
End-point
The point in a titration at which the indicator changes colour.
Titre
The volume of solution delivered from a burette during a titration.
Concordant titres
Titre values that agree closely with one another.
Indicator
A dye that changes colour over a particular pH range and is used to detect the end-point of an acid–alkali titration.