Mohr's salt titration: a 96% figure that hides two procedural mistakes
0.980 g of Mohr's salt, FeSO4·(NH4)2SO4·6H2O (M = 392 g mol−1), is dissolved in dilute H2SO4 and made up to 100.0 mL; part of the Fe2+ has been air-oxidised to Fe3+. Three 25.0 mL portions are titrated with 0.0200 mol L−1 KMnO4, giving concordant burette readings of 6.00 mL. A second student, working on the same sample, acidifies with dilute HCl instead of H2SO4 and titrates cold without warming; the pink colour appears, then fades again after about 20 s, and the titre keeps drifting upward. The demonstrator asks for a percentage purity figure, an explanation of the two procedural choices (acid and temperature), and an assessment of which careless mistakes would actually change the reported value - water left in the conical flask, or water left in the burette.
1. Calculate the percentage purity of the sample from the titration data, showing the stoichiometry.
Read the worked answer
Half-reactions: MnO4− + 8H+ + 5e− → Mn2+ + 4H2O (reduction) Fe2+ → Fe3+ + e− (oxidation) So 1 mol MnO4− ≡ 5 mol Fe2+.
Moles of KMnO4 used per titration: 0.0200 mol L−1 × 6.00 × 10−3 L = 1.20 × 10−4 mol.
Moles of Fe2+ in 25.0 mL: 5 × 1.20 × 10−4 = 6.00 × 10−4 mol.
Moles in 100.0 mL: 4 × 6.00 × 10−4 = 2.40 × 10−3 mol.
Mass of Mohr's salt equivalent: 2.40 × 10−3 × 392 = 0.941 g.
Percentage purity: (0.941/0.980) × 100 = 96.0%.
Interpretation. For a sample containing no impurity at all, the titre would be 6.25 mL (0.980/392 = 2.50 × 10−3 mol Fe2+ in 100 mL). The 0.25 mL shortfall corresponds to about 4% of the iron already present as Fe3+, which KMnO4 does not titrate. The figure of 96.0% is therefore the Fe2+ content; total iron is higher.
2. Why must dilute H2SO4, not HCl, be used, and why is warming to 60–70 °C needed? Write the relevant equations.
Read the worked answer
Acid choice. In acidic solution permanganate is a strong oxidant and will oxidise chloride to chlorine: 2MnO4− + 10Cl− + 16H+ → 2Mn2+ + 5Cl2 + 8H2O. If HCl is used, some titrant is consumed by chloride rather than by Fe2+, the titre is too high, and the endpoint is unstable: the pink colour appears, then fades as the remaining chloride reduces the excess permanganate - exactly the drift observed. Sulphate is not oxidised under these conditions, so dilute H2SO4 is used. HCl may also contain reducing impurities.
Temperature. The MnO4−/Fe2+ reaction is slow at room temperature because it proceeds through intermediate manganese species; Mn2+ formed acts as an autocatalyst, but the first few drops react slowly, so the colour lingers locally and the endpoint is premature. Warming to 60–70 °C accelerates the reaction so that each addition decolourises promptly. The solution should not be boiled: hot acid can air-oxidise Fe2+ and decompose permanganate, both of which lower the titre. The endpoint is the first pale pink that persists for about 30 s.
3. Which errors change the reported purity - water left in the conical flask or water left in the burette? Quantify the direction of any effect.
Read the worked answer
Water in the conical flask: no effect on the result. The aliquot contains a fixed number of moles of Fe2+; added water changes only the concentration and the volume, not the moles. The same amount of permanganate is required (the endpoint is judged by colour, and dilution only makes the pink slightly harder to see, not chemically different).
Water left in the burette: a systematic error that inflates the answer. If the burette was rinsed with water and not with the KMnO4 solution, the standard is diluted, say by 2%. The titre then rises by about 2% (6.12 mL instead of 6.00 mL), and the calculated Fe2+ - and hence the purity - is over-reported by ~2% (about 98% instead of 96%). Burettes should be rinsed with a little of the titrant.
Other real errors: an air bubble in the tip that disappears during titration (titre too high, purity over-reported); a wet pipette used for the sample (aliquot diluted, titre too low); and any Fe3+ in the solid (purity under-reported, as here). Only errors in measured volume or in the amount of Fe2+ delivered to the flask matter.