A Discrepant Redox Titration
A student standardises a potassium permanganate solution against 0.0500 mol/L iron(II) sulfate. In acid, MnO4⁻ + 8H⁺ + 5Fe²⁺ → Mn²⁺ + 5Fe³⁺ + 4H2O. Three 25.00 mL aliquots of Fe²⁺ require 24.80, 24.85 and 24.78 mL of KMnO4. The student then titrates 25.00 mL of an unknown Fe²⁺ solution and obtains 31.20 mL, but the solution had been left open to air for two days. A second, freshly prepared sample requires 29.90 mL. The student reports the first value as the true concentration because it is 'the direct measurement'.
1. Calculate the permanganate concentration from the standardisation and the Fe²⁺ concentration in the fresh sample.
Read the worked answer
Mean titre = (24.80 + 24.85 + 24.78)/3 = 24.81 mL. Moles Fe²⁺ in aliquot = 0.0500 mol/L × 0.02500 L = 1.250 × 10⁻³ mol. From the 5:1 stoichiometry, moles MnO4⁻ = 1.250 × 10⁻³ / 5 = 2.500 × 10⁻⁴ mol. So [MnO4⁻] = 2.500 × 10⁻⁴ mol / 0.02481 L = 1.008 × 10⁻² mol/L. For the fresh sample: moles MnO4⁻ = 0.01008 × 0.02990 = 3.014 × 10⁻⁴ mol; moles Fe²⁺ = 5 × 3.014 × 10⁻⁴ = 1.507 × 10⁻³ mol; [Fe²⁺] = 1.507 × 10⁻³ / 0.02500 = 0.0603 mol/L.
2. Explain why the two titres differ and evaluate the student's choice of the first value.
Read the worked answer
Iron(II) is readily oxidised to iron(III) by dissolved oxygen in air, especially over two days: 4Fe²⁺ + O2 + 4H⁺ → 4Fe³⁺ + 2H2O. The aged solution therefore contains less Fe²⁺ than when prepared, so it needs less permanganate - yet the first titre (31.20 mL) is larger, not smaller. That is the opposite of the expected direction, so the discrepancy is not simply air oxidation; it may reflect evaporation concentrating the solution, contamination by other reducing substances, or an error in the first titration (e.g. endpoint overshoot or a dirty burette). Either way, the student's reasoning is flawed: 'direct measurement' is not a criterion for reliability, and a result from a degraded sample is less trustworthy, not more. The fresh sample is the better estimate, but the anomaly should be investigated rather than dismissed.