RP9: Investigate how pH changes when a weak acid reacts with a strong base and when a strong acid reacts with a weak base

Describe how you would investigate how pH changes during a titration. (6 marks)

1. Pipette a known volume of the acid into a beaker and place it on a magnetic stirrer.

2. Calibrate a pH meter using buffer solutions of known pH, then place the probe in the acid.

3. Record the initial pH.

4. Add the base from a burette in small measured portions, stirring after each.

5. Record the pH after each addition, adding in smaller portions near the equivalence point.

6. Continue well past the equivalence point, then plot pH against volume of base added.

Explain why a pH meter must be calibrated before use. (2 marks)

• Readings drift over time, so the meter may give a systematic error.

• Calibrating against buffers of known pH corrects this.

Describe how a pH meter is calibrated. (3 marks)

1. Rinse the probe with distilled water and blot it dry.

2. Place it in a buffer of known pH, such as pH 4, and adjust the meter to read that value.

3. Repeat with a second and often a third buffer, such as pH 7 and pH 10, rinsing between each.

Explain why smaller volumes are added near the equivalence point. (2 marks)

• The pH changes very rapidly over a small volume there.

• Adding in small portions gives enough points to define the vertical section accurately.

State two reasons why a pH meter is used rather than an indicator. (2 marks)

• It gives a numerical pH value, so the whole curve can be plotted rather than just the end point.

• It works for a weak acid and weak base titration, where no indicator gives a sharp colour change.