Choosing a Titration Indicator for a Weak Acid
A student must determine the concentration of a household vinegar sample. They dilute 10.00 cm³ of vinegar to 100.0 cm³ and titrate 25.00 cm³ portions with 0.1000 mol dm⁻³ NaOH. The available indicators are methyl orange (pH 3.1–4.4), bromothymol blue (pH 6.0–7.6) and phenolphthalein (pH 8.3–10.0). The student uses methyl orange and obtains an average titre of 12.40 cm³. A second student uses phenolphthalein and gets 24.90 cm³.
1. Identify which result is trustworthy and calculate the concentration of the original vinegar.
Read the worked answer
Vinegar is mainly acetic acid, a weak acid, so the equivalence point is basic (pH ≈ 8–9) because the conjugate base acetate hydrolyses. Phenolphthalein is the correct indicator; methyl orange changes colour well before the equivalence point, so the 12.40 cm³ result is too low. Using the phenolphthalein data: n(NaOH) = 0.1000 × 0.02490 = 2.490 × 10⁻³ mol. This equals n(CH3COOH) in the 25.00 cm³ aliquot. In the 100.0 cm³ flask: 2.490 × 10⁻³ × (100.0/25.00) = 9.960 × 10⁻³ mol. This came from 10.00 cm³ of vinegar, so c = 9.960 × 10⁻³ / 0.01000 = 0.996 mol dm⁻³, i.e. about 1.00 mol dm⁻³.
2. The first student argues that 'the colour changed, so the endpoint was reached'. Explain why this is a flawed judgement.
Read the worked answer
A colour change only shows that the indicator's transition range has been crossed, not that the stoichiometric equivalence point has been reached. With methyl orange, the solution becomes yellow at about pH 4.4, while the true equivalence point is near pH 8.5; the acid is still partly unneutralised. The endpoint must be matched to the equivalence point by choosing an indicator whose range brackets the pH at equivalence. The correct judgement is to check the shape of the titration curve (or the acid/base strengths) before trusting a colour change.
3. A third student suggests using a pH meter instead. State one advantage and one practical caution.
Read the worked answer
Advantage: the pH meter lets you plot the titration curve and locate the equivalence point from the steepest part of the curve, removing indicator-choice error. Caution: the electrode must be calibrated with buffer solutions before use and rinsed between measurements, and it should be stirred gently to reach a stable reading; an uncalibrated electrode can introduce a systematic offset of several tenths of a pH unit.