3.6 - Investigating neutralisation and pH

Why adding the base changes pH

Hydrochloric acid supplies H⁺ ions. Calcium hydroxide and calcium oxide are bases that react with the acid to form calcium chloride and water:

Ca(OH)₂(s) + 2HCl(aq) → CaCl₂(aq) + 2H₂O(l)

CaO(s) + 2HCl(aq) → CaCl₂(aq) + H₂O(l)

During neutralisation, hydrogen ions are used up as water forms. With hydroxide, the essential ionic change is H⁺ + OH⁻ → H₂O. Calcium oxide also removes H⁺ as its oxide reacts to form water. The falling H⁺ concentration makes pH rise. After the acid is consumed, dissolved excess base makes the mixture alkaline.

Choose one solid and use it throughout a run. Equal masses of calcium oxide and calcium hydroxide do not neutralise equal amounts of acid, because their formula masses differ.

Planning the measurements

The independent variable is cumulative mass of powdered solid added, in grams. The dependent variable is the mixture's pH. Use a measuring cylinder to measure a fixed acid volume into a beaker, a balance and weighing boat for the powder, a stirring rod, and a calibrated pH meter or probe.

Keep the starting acid volume and concentration, solid identity and temperature consistent between repeat runs. Record actual masses; identical heaped spatulas are not equal measured masses. The size of the additions is a sampling choice: smaller additions near neutral give more detail.

Universal indicator paper and its colour chart can give approximate pH readings. A calibrated pH meter usually resolves smaller changes and avoids subjective colour matching. Select a balance whose resolution is suitable for the small portions being weighed.

Collecting stable readings

1. Wear eye protection. Measure a fixed volume of dilute hydrochloric acid, for example 50 cm³, into a beaker.
2. Calibrate the pH probe as instructed. Support it with the sensing bulb immersed and record the initial pH at zero added mass.
3. Weigh a small portion of the chosen powder. Select the portion size for the acid concentration; use smaller portions if the neutral point could be missed.
4. Add the powder carefully and stir with a separate rod. Wait for the reaction and pH reading to settle before recording.
5. Record the cumulative mass (the sum of all portions so far) and pH. Continue through the steep change into a clearly alkaline region as instructed.
6. Repeat the run with fresh acid of the same starting volume and concentration. Near neutral, collect readings at closer mass intervals.

Keep lumps away from the probe bulb. Rinse with distilled water and gently blot as instructed when transferring the probe between samples; do not use it as a stirring rod.

Reading the neutralisation curve

This idealised example uses calcium hydroxide added to 50 cm³ of 0.100 mol/dm³ hydrochloric acid at room temperature. The extra readings close to neutral show why small increments matter. These are model data for interpretation, not promised experimental results.