All subjects · Chemistry · WCH14 (Unit 4)

pH titration curves and indicators (Core Practical 11)

A pH curve shows how pH changes as an acid is neutralised. Its shape tells you which indicator to use, and for a weak acid the curve gives Ka directly, which is Core Practical 11.

Shapes of curves (spec 14.15)

Adding 0.1 mol dm-3 base to 25 cm3 of 0.1 mol dm-3 acid:

A diprotic acid (or a base such as CO32- accepting two protons) shows two steep sections, one for each proton.

Choosing an indicator (spec 14.16)

An indicator changes colour over about 2 pH units around its pKa. Choose one whose range lies entirely within the vertical section.

Core Practical 11: Ka of a weak acid (spec 14.21, 14.23)

Titrate a weak acid such as propanoic acid with sodium hydroxide, recording pH after each addition (smaller additions near the end point). Plot pH against volume and find the equivalence volume. At half that volume, half the acid has been converted to its salt, so [HA] = [A-] and Ka = [H+]: pH = pKa. Read off pKa and calculate Ka = 10-pKa. Calibrate the pH meter with buffer solutions first.

Alternatively, titrate to find the equivalence volume with an indicator, add that volume of the same acid to the neutralised mixture, and measure the pH: it equals pKa.

Worked example

In CP11 the equivalence volume is 24.0 cm3. At 12.0 cm3 the pH is 4.87. Find Ka.

  1. At half-equivalence [HA] = [A-], so pH = pKa = 4.87
  2. Ka = 10-4.87

Answer: Ka = 1.35 x 10-5 mol dm-3

Key idea: Strong acid-strong base: equivalence pH 7. Weak acid-strong base: above 7, phenolphthalein. Strong acid-weak base: below 7, methyl orange. At half-neutralisation of a weak acid, pH = pKa.

Check yourself

  1. Which indicator should be used to titrate ethanoic acid with sodium hydroxide?
    • Phenolphthalein
    • Methyl orange
    • Universal indicator
    • Starch
    Show answer

    Phenolphthalein. The vertical section is about pH 7 to 11, which includes phenolphthalein's range but not methyl orange's.

  2. Which indicator is suitable for titrating aqueous ammonia with hydrochloric acid?
    • Phenolphthalein
    • Litmus
    • Methyl orange
    • No indicator works
    Show answer

    Methyl orange. The vertical section is about pH 3 to 7, which includes methyl orange's range.

  3. Why can no indicator give a sharp end point for ethanoic acid titrated with ammonia?
    • There is no steep vertical section on the pH curve
    • The equivalence point is at pH 7
    • The two solutions are colourless
    • The reaction does not go to completion
    Show answer

    There is no steep vertical section on the pH curve. Without a vertical section, colour changes gradually over many cm3.

  4. In Core Practical 11, the pH at half the equivalence volume is 4.87. Calculate Ka.
    Show answer

    1.35 x 10-5 mol dm-3. pH = pKa, so Ka = 10-4.87 = 1.35 x 10-5 mol dm-3.

  5. 25.0 cm3 of 0.100 mol dm-3 ethanoic acid is titrated with 0.125 mol dm-3 sodium hydroxide. What volume of alkali is needed to reach the equivalence point?
    Show answer

    20.0 cm3. n(acid) = 2.50 x 10-3 mol; V = 2.50 x 10-3 / 0.125 = 0.0200 dm3 = 20.0 cm3.

  6. Why is the equivalence point above pH 7 when a weak acid is titrated with a strong base?
    • Excess acid remains
    • The weak acid is fully dissociated
    • Water is not formed
    • The salt formed contains the conjugate base of the weak acid, which makes the solution alkaline
    Show answer

    The salt formed contains the conjugate base of the weak acid, which makes the solution alkaline. For example, ethanoate ions accept protons from water, giving OH-.

  7. How many steep vertical sections appear when sodium carbonate solution is titrated with hydrochloric acid?
    Show answer

    2. CO32- accepts two protons in turn (to HCO3-, then to H2CO3), giving two equivalence points.

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Last updated 7 November 2026. Spotted a mistake? Tell us.