What a learner can do afterwards
- Explains how a buffer absorbs added acid and added alkali, naming the species that does each job
- Calculates the pH of a buffer from Ka and the ratio of acid to salt
- Sketches the four titration curves and marks the equivalence point and the buffer region on each
- Chooses an indicator by matching its range to the vertical section of the curve
1 · Read
A buffer is a weak acid mixed with its salt, such as acetic acid with sodium acetate. Extra acid is eaten by the conjugate base turning into HA, and extra alkali is eaten by the weak acid turning into A-, so the H+ level barely moves. The shield has a limit: each spare H+ uses up one conjugate base particle, so a large dose empties the store and the pH falls.
Buffer pH comes from pH = pKa + log of salt over acid, so equal amounts give pH = pKa and extra salt pushes it higher. Your blood does this with carbonic acid and hydrogencarbonate: spare H+ ends up as CO2, which you breathe out.
On titration curves the steep run marks equivalence: around pH 7 for a strong acid with a strong base, and above 7 for a weak acid with a strong base. The gentle early stretch is the buffer region, where the mixture resists change.
Pick the indicator whose range sits fully inside the steep run, so one drop flips the colour. For ammonia titrated with hydrochloric acid that is methyl orange, whose range near 3 to 4 sits on the steep fall.
Buffers eat spare acid and alkali, salt ratios set the pH, curves mark equivalence steeply, and indicators must match the run.
2 · Watch
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Where it sits
8 questions wait behind this lesson, each with its answer explained. Every answer feeds the sky: stars light as they are learned, and dim when it is time to come back.