pH curves, titrations and indicatorsAQA A-Level Chemistry: Revision notes
Section 1
Titration calculations
In an acid–base titration, a solution of known concentration is added from a burette until the end point is reached. For a 1 : 1 reaction:
- moles of known solution = concentration × volume (dm³)
- use the equation to find moles of the other reagent
- concentration = moles / volume (dm³)
Worked example. 25.0 cm³ NaOH needs 21.60 cm³ of 0.0500 mol dm⁻³ HCl. n(HCl) = 0.0500 × 0.02160 = 1.08 × 10⁻³ mol = n(NaOH). c(NaOH) = 1.08 × 10⁻³ / 0.0250 = 0.0432 mol dm⁻³.
If the sample was diluted, multiply back by the dilution factor.
Forgetting to convert cm³ to dm³, or using the 1 : 1 ratio when the equation is not 1 : 1.
Section 2
The four pH curves
A pH curve plots pH against volume of base (or acid) added. Each has a steep vertical section at the equivalence volume. For 0.1 mol dm⁻³ solutions:
- Strong acid + strong base: starts at about pH 1, vertical section about pH 3 to 11, ends at about pH 13.
- Weak acid + strong base: starts at about pH 3, vertical section about pH 7 to 11, ends at about pH 13.
- Strong acid + weak base: starts at about pH 1, vertical section about pH 3 to 7, ends at about pH 10 to 11.
- Weak acid + weak base: starts at about pH 3, ends at about pH 10 to 11, with no vertical section.
Section 3
Explaining the shapes
The initial pH depends on acid strength: a strong acid is fully dissociated, so its pH is low, whereas a weak acid is only slightly dissociated, so its pH is higher.
At the end, the pH depends on the base: excess strong base gives a high pH, but excess weak base is only slightly ionised, so the curve levels off lower.
The equivalence pH depends on the salt formed. A salt of a weak acid (ethanoate) is basic, so the equivalence pH is above 7; a salt of a weak base (ammonium) is acidic, so it is below 7. When both reactants are weak, the pH changes gradually, so there is no sharp end point.
When explaining a curve, say what the starting pH, vertical section and final pH are, and link each to the strength of the acid or base.
Section 4
Indicators and choosing one
An indicator is a weak acid whose acid and conjugate base forms have different colours. It changes colour over a narrow pH range.
- Methyl orange: pH 3.1–4.4 (red to yellow)
- Phenolphthalein: pH 8.3–10.0 (colourless to pink)
Choose an indicator whose range lies within the vertical section of the curve.
- Strong acid + strong base: either indicator.
- Weak acid + strong base: phenolphthalein.
- Strong acid + weak base: methyl orange.
- Weak acid + weak base: neither, as there is no vertical section.
Choosing an indicator by the equivalence pH alone. Its entire colour-change range must lie within the vertical section.
Section 5
Required practical 9: pH changes
Add a solution from a burette to a measured volume in a flask, stirring, with a pH meter (calibrated with buffer solutions at pH 4 and 7 or 10) in the flask. Record the pH after each small addition, with smaller additions near the equivalence point where the pH changes rapidly, and plot a pH curve.
Do this for a weak acid with a strong base and for a strong acid with a weak base. Rinse the electrode between solutions and keep it in solution. Read the equivalence volume from the middle of the vertical section.
That's the notes covered.
Carry on to the next subtopic.
Exam questions on pH curves, titrations and indicators
- A student titrates 25.0 cm³ portions of sodium hydroxide solution of unknown concentration with 0.0500 mol dm⁻³ hydrochloric acid from a burette. The mean titre is 21.60 cm³.Explain why either methyl orange or phenolphthalein is suitable as the indicator for this titration.2 marks
- Four titrations are carried out. In each, 25.0 cm³ of a 0.100 mol dm⁻³ acid is in the flask and a 0.100 mol dm⁻³ base is added from the burette. Titration 1: hydrochloric acid with sodium hydroxide. Titration 2: ethanoic acid with sodium hydroxide. Titration 3: hydrochloric acid with ammonia solution. Titration 4: ethanoic acid with ammonia solution. The pH is recorded throughout with a pH meter. Data given: methyl orange changes colour over the pH range 3.1–4.4 and phenolphthalein over the range 8.3–10.0.Explain why no indicator would give a sharp end point in titration 4.2 marks
- A student titrates 25.0 cm³ of 0.100 mol dm⁻³ ethanoic acid (Ka = 1.74 × 10⁻⁵ mol dm⁻³) with 0.125 mol dm⁻³ sodium hydroxide solution from a burette. Methyl orange changes colour over the pH range 3.1–4.4 and phenolphthalein over the range 8.3–10.0.Describe the main features of the pH curve for this titration, giving approximate pH values.3 marks
Written by the Exaim team, led by Shaun Daswani (Head of Upper Secondary, Improve ME Institute; MSc Financial Mathematics, Imperial College London; BSc, UCL) and Jason Daswani (operational lead, Improve ME Institute; LSE).