PAG11: pH Measurement, Curves and Buffers

This group develops pH measurement. A centre may use unknown acid/base solutions, titration curves or buffer investigations as suitable contexts. The examples compare weak acid–strong base and strong acid–weak base curves, while the essential method is a justified pH measurement with controlled conditions. pH is the measured response; an indicator colour alone is not a full numerical curve.

Calibrate the probe with suitable known-pH buffers following its instructions. Rinse between solutions, remove excess rinse water as instructed, immerse the sensing part properly and stir gently. Wait for a stable reading after each addition. Keep a consistent temperature because both electrode response and chemical equilibria depend on it.

Use an initial trial or stoichiometric estimate to locate equivalence, then use smaller additions near the rapid pH change. Record the actual cumulative burette volume, not just a sequence number. Keep the probe clear of the stirrer and do not scrape the bulb against glass. Dense sampling near equivalence improves volume estimation more effectively than collecting many extra points on a flat part.

For weak acid plus strong base, a buffer region precedes equivalence and the equivalence solution is alkaline because the conjugate base hydrolyses. For strong acid plus weak base, equivalence is acidic because the conjugate acid of the weak base releases H⁺. At 298 K, neither equivalence is automatically pH 7. The direction of the plotted pH change also depends on which reagent is added.

Scroll the diagram to see the full view.

At half-neutralisation of a weak monoprotic acid by strong base, the remaining acid and formed conjugate base have equal amounts, so pH ≈ pKa under the buffer assumptions. Read this point using half the equivalence volume, not half the final recorded volume. For a weak base titrated by strong acid, the corresponding half-equivalence pH concerns its conjugate acid’s pKa.

Choose an indicator range lying inside the steep portion for that specific acid–base pair. A weak–weak combination has no similarly sharp ordinary indicator transition and is not a replacement for either required pairing. Evaluate probe calibration, response delay, addition size and mixing separately: repeated volume readings do not remove a systematic pH calibration offset.

A tenfold dilution of an ideal dilute strong monoprotic acid raises pH by about one while the acid dominates water ionisation. The same rule does not generally apply to a weak acid or to extremely dilute solutions. A buffer containing appreciable weak acid and conjugate base resists a small acid/base addition; record both the added amount and the final volume.

To compare buffer capacity fairly, state whether the mixtures have the same starting pH, total buffer concentration and sample volume. A buffer that starts at a different pH or receives a different amount cannot be judged solely from its final pH. Check the probe calibration and temperature before interpreting a deviation as a failure of the chemical model.

Attempt these problems on paper before opening the worked reasoning. Explain your steps as well as giving a final answer.