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Titration curves and indicatorsEdexcel International A Level Chemistry: Revision notes

Section 1

Strong acid with strong base

A titration curve plots pH against volume of alkali added. For a strong acid with a strong base (for example 0.100 mol dm⁻³ HCl with NaOH):

  • The pH starts low (about 1) and rises slowly
  • There is a vertical section (very steep rise) of about pH 3 to 11 around the equivalence point
  • The equivalence point is at pH 7
  • The pH then levels off close to the pH of the alkali

The equivalence point is where the amount of base added exactly neutralises the acid.

Key termsequivalence pointvertical section
Exam tip

Name the pH where each curve starts, the pH range of the vertical section and the pH at equivalence: examiners credit these three numbers.

Section 2

Weak acid with strong base

For ethanoic acid titrated with NaOH:

  • The starting pH is higher than for a strong acid (about 3 for 0.1 mol dm⁻³)
  • The pH rises gradually through a buffer region, where ethanoic acid and ethanoate coexist; at half-neutralisation pH = pKa
  • The vertical section is shorter, from about pH 7–8 to 11
  • The equivalence point is above 7 (about 8.7) because the ethanoate ion is a weak base that accepts protons from water

Worked check: at equivalence, [CH₃COO⁻] ≈ 0.050 mol dm⁻³, [OH⁻] = √(Kw/Ka × 0.050) = 5.4 × 10⁻⁶, so pH ≈ 8.7.

Key termsbuffer regionhalf-neutralisation
Common mistake

Saying the equivalence point is at pH 7 for every titration. It is only 7 for a strong acid with a strong base.

Section 3

Strong acid with weak base, and weak with weak

For HCl titrated with ammonia solution:

  • The curve starts at a low pH like any strong acid
  • The vertical section is shorter and lies on the acidic side, about pH 3 to 6
  • The equivalence point is below 7 (about 5.3) because NH₄⁺ is a weak acid
  • After equivalence, the pH rises slowly as excess ammonia forms a buffer

When a weak acid is titrated with a weak base there is no vertical section at all, only a gradual S-shaped rise, so no indicator gives a sharp end point.

Key termsweak base
Exam tip

Match the equivalence pH to the salt: strong/strong gives 7, weak acid/strong base gives above 7, strong acid/weak base gives below 7.

Section 4

Diprotic acids

A diprotic acid such as ethanedioic acid, H₂C₂O₄, loses its two protons in turn: H₂A → HA⁻ → A²⁻. If pKa₁ and pKa₂ are far enough apart, the curve has two vertical sections and two equivalence points. The second equivalence point is at twice the volume of the first, because the same amount of alkali removes the second proton.

For 25.0 cm³ of 0.0500 mol dm⁻³ H₂C₂O₄ titrated with 0.100 mol dm⁻³ NaOH, the equivalence points are at 12.5 cm³ and 25.0 cm³. A buffer region lies before each one. For a strong diprotic acid (H₂SO₄) both protons are removed effectively together, giving one steep rise.

Key termsdiprotic acid
Common mistake

Using the same volume of alkali for both protons. The second equivalence point is at double the first volume.

Section 5

Choosing an indicator

An indicator is a weak acid whose conjugate forms have different colours. It changes colour over a pH range, which must lie within the vertical section of the curve, so that one drop of alkali changes the colour sharply.

  • Strong acid, strong base: methyl orange (3.1–4.4) or phenolphthalein (8.3–10.0)
  • Weak acid, strong base: phenolphthalein (8.3–10.0) only
  • Strong acid, weak base: methyl orange (3.1–4.4) or methyl red (4.2–6.3)
  • Weak acid, weak base: none is suitable

Use the data given for the curve and the indicator range, and state the pH of the equivalence point to justify the choice.

Key termsindicatorpH range
Common mistake

Choosing an indicator by its colour change alone. Always compare its pH range with the vertical section of the curve.

That's the notes covered.

Carry on to the next subtopic.

Exam questions on Titration curves and indicators

  1. A student titrates 25.0 cm³ of 0.100 mol dm⁻³ ethanoic acid with 0.100 mol dm⁻³ sodium hydroxide solution. The Ka of ethanoic acid is 1.74 × 10⁻⁵ mol dm⁻³. Phenolphthalein changes colour over the pH range 8.3–10.0 and methyl orange over the range 3.1–4.4.
    Explain why methyl orange would give an unreliable end point in this titration.2 marks
  2. A technician titrates 25.0 cm³ of 0.100 mol dm⁻³ hydrochloric acid with 0.100 mol dm⁻³ sodium hydroxide in one experiment and with 0.100 mol dm⁻³ ammonia solution in a second experiment. The pH at the equivalence point of the ammonia titration is 5.3. The pH ranges over which some indicators change colour are: methyl red 4.2–6.3; phenolphthalein 8.3–10.0; thymolphthalein 9.3–10.5; alizarin yellow R 10.1–12.0.
    Explain why the pH at the equivalence point of the ammonia titration is below 7.2 marks
  3. A student titrates 25.0 cm³ of 0.0500 mol dm⁻³ ethanedioic acid, H₂C₂O₄, a diprotic acid with pKa₁ = 1.25 and pKa₂ = 4.27, with 0.100 mol dm⁻³ sodium hydroxide solution. Methyl orange changes colour over the pH range 3.1–4.4 and phenolphthalein over the range 8.3–10.0.
    Describe the key features of the titration curve, including the volumes of sodium hydroxide at the equivalence points.3 marks
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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).