Acid-Base Titrations
Section: 12. Experimental Techniques & Chemical Analysis | Syllabus: Cambridge IGCSE Chemistry (0620)
What is a Titration?
A titration is a technique used to find the volume of one solution that exactly reacts with a measured volume of another - for example, to find how much acid is needed to exactly neutralise a measured volume of alkali.
Acid + Base → Salt + Water
Example: HCl + NaOH → NaCl + H₂O
A titration uses three key pieces of apparatus:
- A burette, to deliver a variable, accurately-measured volume of one solution
- A volumetric pipette, to measure out a fixed volume of the other solution into a conical flask
- A suitable indicator, added to the flask so that the exact point of neutralisation can be seen
Indicators and the End Point
The end point of a titration is the point at which the indicator changes colour, showing that the reaction is just complete.
| Indicator | Colour in Acid | Colour in Alkali |
|---|---|---|
| Methyl orange | Red | Yellow |
| Phenolphthalein | Colourless | Pink |
The end point is identified as the point where the indicator's colour just permanently changes as the last drop of solution is added.
Carrying out a Titration
- Rinse the pipette with the solution to be measured, then use it to transfer a fixed volume (e.g. 25.0 cm³) into a clean conical flask
- Add a few drops of a suitable indicator to the flask
- Rinse the burette with the solution to be used in it, fill above the 0.00 cm³ mark, and record the initial reading
- Place the flask on a white tile beneath the burette (the white background makes the colour change easier to see)
- Add solution from the burette while continuously swirling the flask, adding it dropwise as the end point approaches
- Record the final burette reading once the indicator has just permanently changed colour
- Titre (volume added) = final reading − initial reading
- Repeat the titration until concordant results are obtained (titres within 0.10 cm³ of each other), and average these results
Worked Example
- Question: why is a conical flask, rather than a beaker, normally used to hold the solution during a titration?
- Answer: its narrow neck makes it easy to swirl the contents thoroughly without splashing or spilling solution out, so the reaction mixes fully as the titration proceeds.
Common Mistakes
- ❌ Rinsing the pipette or burette with water only, and using it while still wet
- ✓ leftover water dilutes the solution added, giving an inaccurate titre - rinse with the solution that will actually be used
- ❌ Adding solution quickly all the way up to the end point
- ✓ overshooting the end point gives a titre that is too large - slow down and add dropwise as the colour change approaches
- ❌ Averaging every titre, including ones that are not concordant
- ✓ only titres within 0.10 cm³ of each other should be averaged; a rough/outlying titre should be discarded
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