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.
More to read
14.6 Buffers
A buffer is a weak acid mixed with its own salt, so the solution holds big reserves of both HA and A-. Pour in some acid and the A- reserve gobbles it up, turning it into more HA. Pour in alkali and the HA reserve neutralises that instead, turning into more A-. Either way the ratio barely moves, so the pH holds steady. Blood runs on exactly this trick with carbonic acid and hydrogencarbonate. When muscles dump lactic acid into the blood, the hydrogencarbonate converts it to CO2, which the lungs breathe out. That is why blood pH sits near 7.4 through a sprint and a rest alike.
14.6: Buffers
Buffer pH comes from Ka and the ratio of the two reserves. The working rearranges the Ka expression into pH = pKa + log([salt]/[acid]), so equal amounts of acid and salt give pH equal to pKa. Double the salt relative to the acid and the pH lands a little above pKa, which is how chemists tune a buffer to the value they need. This also explains buffer range. A buffer copes while both reserves last, roughly within one pH unit of pKa. Flood it with more acid or alkali than the smaller reserve can eat, and the pH crashes or soars like any unbuffered solution.
14.7 Acid-Base Titrations
Titrate and plot pH against volume and each acid base pair draws its own S shaped curve. All four combinations share the slow start, the steep vertical run at equivalence, and the flat landing, but the run sits at different heights. A weak acid with a strong base climbs through equivalence above pH 7, while a weak base with a strong acid falls through it below 7. The flat zone before equivalence on a weak titration is the buffer region, where the forming salt buffers the leftover acid. Indicators are chosen off the vertical run: the colour change range must sit inside the steep part. That is why phenolphthalein fits a weak acid titration but methyl orange is needed for the ammonia with acid case.
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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.