All subjects · Chemistry · WCH15 (Unit 5)

Redox titrations with manganate(VII) (Core Practical 13a)

Potassium manganate(VII) is a powerful oxidising agent that acts as its own indicator. Core Practical 13a uses it to find the amount of iron(II) in a sample.

The reaction (spec 16.15)

MnO4-(aq) + 8H+(aq) + 5Fe2+(aq) -> Mn2+(aq) + 4H2O(l) + 5Fe3+(aq)

Mole ratio MnO4- : Fe2+ = 1 : 5. With ethanedioate ions (warm to about 60 °C):

2MnO4- + 16H+ + 5C2O42- -> 2Mn2+ + 8H2O + 10CO2, ratio 2 : 5.

Core Practical 13a (spec 16.17 i)

Put the manganate(VII) solution in the burette and pipette the iron(II) solution, acidified with dilute sulfuric acid, into the conical flask. Purple MnO4- is decolourised as it reacts; the end point is the first permanent pale pink colour. Read the top of the meniscus, because the dark solution makes the bottom hard to see.

Use sulfuric acid: hydrochloric acid would be oxidised to chlorine, and ethanoic acid is too weak to supply enough H+. Make up iron(II) ammonium sulfate standards in previously boiled, cooled dilute sulfuric acid, which limits oxidation of Fe2+ by air.

Typical calculation

1. Moles of MnO4- = concentration x titre (in dm3).

2. Moles of Fe2+ = 5 x moles of MnO4-.

3. Scale up for the whole solution (for example x 10 for 25.0 cm3 from 250 cm3).

4. Convert to mass, percentage or formula as asked.

Uncertainty (spec 16.16)

Each burette reading has an uncertainty of ±0.05 cm3, so a titre (two readings) has ±0.10 cm3. Percentage uncertainty = (uncertainty / value) x 100. Larger titres and masses give smaller percentage uncertainties. Concordant titres (within 0.10 cm3) reduce random error.

Worked example

An iron tablet is dissolved in dilute sulfuric acid and made up to 250 cm3. 25.0 cm3 needs 12.40 cm3 of 0.0100 mol dm-3 KMnO4. Find the mass of iron in the tablet.

  1. n(MnO4-) = 0.01240 x 0.0100 = 1.24 x 10-4 mol
  2. n(Fe2+) in 25.0 cm3 = 5 x 1.24 x 10-4 = 6.20 x 10-4 mol
  3. In 250 cm3: 6.20 x 10-3 mol; mass = 6.20 x 10-3 x 55.8

Answer: 0.346 g of iron

Key idea: MnO4- : Fe2+ = 1 : 5; MnO4- : C2O42- = 2 : 5. Acidify with dilute H2SO4. End point: first permanent pale pink. Titre uncertainty ±0.10 cm3.

Check yourself

  1. 25.0 cm3 of an iron(II) solution needs 22.50 cm3 of 0.0200 mol dm-3 KMnO4. Calculate the concentration of Fe2+.
    Show answer

    0.0900 mol dm-3. n(MnO4-) = 4.50 x 10-4; n(Fe2+) = 2.25 x 10-3; c = 2.25 x 10-3 / 0.0250 = 0.090 mol dm-3.

  2. An iron tablet of mass 1.20 g contains the amount of iron found in the worked example (6.20 x 10-3 mol Fe). Calculate the percentage by mass of iron in the tablet. (Fe = 55.8)
    Show answer

    28.8 %. mass Fe = 6.20 x 10-3 x 55.8 = 0.346 g; 0.346 / 1.20 x 100 = 28.8%.

  3. 3.00 g of FeSO4.xH2O is dissolved and made up to 250 cm3. 25.0 cm3 needs 21.60 cm3 of 0.0100 mol dm-3 KMnO4. Find x. (Fe = 55.8, S = 32.1, O = 16.0, H = 1.0)
    Show answer

    7. n(Fe2+) in 250 cm3 = 5 x 2.16 x 10-4 x 10 = 0.0108 mol; M = 3.00 / 0.0108 = 277.8; water = 277.8 - 151.9 = 125.9 g mol-1; x = 125.9 / 18.0 = 6.99, so x = 7.

  4. 25.0 cm3 of 0.0500 mol dm-3 ethanedioic acid is warmed and titrated with 0.0200 mol dm-3 KMnO4. What titre is expected?
    Show answer

    25.0 cm3. n(C2O42-) = 1.25 x 10-3; n(MnO4-) = 2/5 x 1.25 x 10-3 = 5.00 x 10-4; V = 5.00 x 10-4 / 0.0200 = 0.0250 dm3 = 25.0 cm3.

  5. What is seen at the end point of a titration of iron(II) with potassium manganate(VII)?
    • A sudden change from orange to green
    • A blue-black colour
    • The first permanent pale pink colour
    • The purple colour disappearing
    Show answer

    The first permanent pale pink colour. Once all Fe2+ has reacted, the next drop of MnO4- is not decolourised.

  6. Why is dilute sulfuric acid used rather than ethanoic acid in manganate(VII) titrations?
    • Ethanoic acid is a weak acid and cannot supply enough H+
    • Ethanoic acid is oxidised to chlorine
    • Sulfuric acid is the indicator
    • Ethanoic acid reacts with Fe2+
    Show answer

    Ethanoic acid is a weak acid and cannot supply enough H+. 8H+ are needed for each MnO4-; a weak acid gives too little, and brown MnO2 may form.

  7. A titre of 24.50 cm3 is found from two burette readings, each ±0.05 cm3. Calculate the percentage uncertainty in the titre.
    Show answer

    0.41 %. Total uncertainty = ±0.10 cm3; 0.10 / 24.50 x 100 = 0.41%.

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