RP 9: pH curves with a calibrated probe

Hazards

Method

1. Calibrate the pH probeRinse the probe with distilled water and calibrate in standard buffers (pH 4, 7 and 10), rinsing between each. An uncalibrated probe reads every point wrong by the same offset — a textbook systematic error.
2. Set up the titrationPipette 25.0 cm³ of the weak acid (e.g. ethanoic) into a beaker on a magnetic stirrer, probe held clear of the stir bar; fill the burette with the strong base.
3. Add base in planned incrementsAdd in 1–2 cm³ steps, stirring, recording the settled pH each time — then switch to 0.2 cm³ dropwise steps within ~2 cm³ of the expected equivalence, so the vertical jump is actually captured.
4. Plot the curvePlot pH against volume added. Label the buffer region, the vertical section, and the equivalence volume (the mid-point of the vertical section).
5. Read pKa at half-equivalenceAt half the equivalence volume, [HA] = [A⁻], so pH = pKa. Read it straight off the curve — a routine two-mark question.
6. Swap: strong acid into weak baseRepeat with the strong acid in the burette and the weak base in the beaker: the curve mirrors — it starts high, the vertical section spans roughly pH 7 down to 3, and equivalence sits below 7.

Mistakes examiners see every year

What to expect

Ethanoic acid vs NaOH (both 0.1 mol dm⁻³): starts ~pH 2.9, buffer region, sharp jump ~pH 7–11, equivalence ≈ 8.7; pH at half-equivalence ≈ 4.76 = pKa (indicative values — compare with your own curve from the titration simulation).