A level Chemistry · Edexcel 9CH0 · Topic 12C
Buffers and titration curves
Key points
- A buffer minimises pH change when small amounts of acid or alkali are added. An acidic buffer is a weak acid plus its salt (e.g. ethanoic acid and sodium ethanoate).
- Added H⁺ reacts with the large reservoir of A⁻; added OH⁻ reacts with HA, so the ratio [HA]/[A⁻] barely changes.
- Buffer pH: , or . At half-neutralisation, pH = pKₐ.
- Blood is buffered mainly by the carbonic acid–hydrogencarbonate system.
- Titration curves: strong acid–strong base has a vertical section from about pH 3 to 11; weak acid–strong base has a buffer region and an equivalence point above 7.
- Choose an indicator whose colour change range lies within the vertical section; for weak acid–weak base titrations no indicator works well.
- Measure pH with a calibrated pH meter to plot curves accurately.
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A buffer keeps pH almost steady, and a titration curve shows how pH changes as acid and alkali react. Both rest on the same equilibrium: a weak acid and its conjugate base present together in large amounts.
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