Part 5: Titration curves, indicators and required practical 9

Choose the correct calculation region, explain all four acid–base curve types and obtain a reliable pH curve.

Equivalence and endpoint are different

The equivalence point is where the reacting amounts match the equation’s stoichiometric ratio. The endpoint is the observed indicator change used to estimate it. Equivalence does not always mean pH 7, and it does not always occur at equal volumes: concentrations and coefficients matter.

A pH curve plots pH against cumulative volume of titrant added. With base added to acid it rises; with acid added to base it falls. Mark the initial pH, equivalence volume, pH near equivalence and the excess-titrant region. Exact steep-region limits depend on concentrations and strengths, so do not memorise one universal set of pH values.

Explain all four combinations

At the same concentration a weak acid starts at a higher pH than a strong acid. Excess strong base can eventually take either acid titration to a similar high-pH region. A weak base such as ammonia produces a lower final pH than an equally concentrated strong hydroxide under comparable conditions.

Choose the equation for the part of the curve

For a weak acid titrated with strong base, use the acid-only Ka calculation initially. Before equivalence, use the buffer ratio after the neutralisation mole balance. At half-equivalence, the acid and conjugate base amounts are approximately equal, so pH ≈ pKa. At equivalence the salt’s hydrolysis makes the solution alkaline; the buffer equation cannot be used with zero HA. Beyond equivalence, excess OH⁻ commonly dominates.

Consider 25.0 cm³ of 0.100 mol dm⁻³ HA titrated with 0.100 mol dm⁻³ NaOH, with Ka = 2.00 × 10⁻⁵. Equivalence is at 25.0 cm³. Initially pH ≈ 2.85. At 12.5 cm³, pH ≈ pKa = 4.70. At 20.0 cm³, HA = 0.000500 mol and A⁻ = 0.00200 mol; [H⁺] = 5.00 × 10⁻⁶ and pH = 5.30.

At 30.0 cm³, excess OH⁻ = 0.000500 mol in 0.0550 dm³, so [OH⁻] = 0.00909 mol dm⁻³. Using Kw = 1.00 × 10⁻¹⁴ gives pH = 11.96. This last calculation treats the excess strong base as dominant. At exactly 25.0 cm³, explain hydrolysis qualitatively unless the question provides the extra information needed for a numerical salt calculation.

Put the transition range inside the steep region

An indicator’s protonated and deprotonated forms have different colours. Added H⁺ favours HIn; removing H⁺ favours In⁻. The colour changes over a range rather than at one infinitely precise pH. Choose an indicator whose full useful transition interval lies inside the steep part of the actual titration curve.

Typical reference ranges are methyl orange about pH 3.1–4.4 (red to yellow) and phenolphthalein about pH 8.2–10.0 (colourless to pink). Use the ranges supplied in a question if they differ. Phenolphthalein commonly suits ethanoic acid with NaOH; methyl orange commonly suits HCl with ammonia at ordinary laboratory concentrations.

Use only a few drops so the indicator’s own acid–base reaction has negligible effect. A weak/weak titration usually needs another endpoint method; forcing an indicator onto a gradual curve gives a poorly defined volume.

Required practical 9 covers two pairings

Investigate a weak acid with a strong base and a strong acid with a weak base. Calibrate the pH meter according to its manufacturer’s method using appropriate buffer standards, ideally spanning the measurement range. Rinse between standards and samples with deionised water and gently blot where instructed; do not carry one solution into the next. Keep temperature controlled.

Measure a known acid volume into a suitable vessel, place the probe in the solution and record its initial pH. Add measured titrant increments from a rinsed, filled burette, mix thoroughly, wait for the reading to stabilise and record cumulative volume and pH. Use much smaller increments near the expected steep change, then continue beyond equivalence. Repeat with the second acid/base pairing.

Keep the probe immersed, protect it from the stirrer and follow its storage instructions after use. Record readings to the instrument’s resolution rather than inventing decimal places. Eye protection and the school’s acid/alkali handling instructions apply.

Plot a smooth best-fit curve and identify equivalence from the rapid-change region where present. For the weak-acid/strong-base curve, read pH at half the equivalence volume to estimate pKa and then calculate Ka. Poor calibration biases pH-derived Ka even if the equivalence volume looks sensible.

Use the titre to find an unknown concentration

For a monoprotic acid, 20.0 cm³ of an unknown sample requires 18.60 cm³ of 0.125 mol dm⁻³ NaOH. Base moles = 0.01860 × 0.125 = 0.002325 mol; the acid sample contains the same amount. Acid concentration = 0.002325/0.0200 = 0.116 mol dm⁻³ (3 s.f.).

For a conventional indicator titration, use concordant titres according to the stated method rather than averaging a rough run indiscriminately. Rinse the pipette with the sample and the burette with titrant, remove the filling funnel and eliminate tip bubbles. A little deionised water in the receiving flask does not change sample moles, though extra dilution can alter a pH curve’s detailed shape.

Quick checks

Original Finesse questions. Reveal the indicative worked solutions after attempting each question; these are not official AQA mark allocations.

Sources