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Analytical Chemistry Techniques

Redox titration of iron tablets and evaluating titrations

  • 3 things to know
  • A2 9

Overview

Manganate(VII) titration of iron tablets and its self-indicating end point, the iron mass calculation, judging accuracy and reliability, percentage uncertainty, improvements and instrumental alternatives.

What you need to know

Answer each one before you reveal it.

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Key term

Why is potassium manganate(VII) used to find the iron content of iron tablets?

Key term

Write the ionic equation for manganate(VII) reacting with iron(II) ions.

Key term

How is the end point recognised in a manganate(VII) titration?

Questions from this subtopic

Each one with the model answer underneath.

  1. 1

    Why is potassium manganate(VII) used to find the iron content of iron tablets?

    Model answer

    Manganate(VII) ions oxidise iron(II) ions to iron(III) ions in a known ratio. The purple colour of manganate(VII) also acts as its own indicator.

  2. 2

    Write the ionic equation for manganate(VII) reacting with iron(II) ions.

    Model answer

    MnO₄⁻ + 5Fe²⁺ + 8H⁺ → Mn²⁺ + 5Fe³⁺ + 4H₂O. One mole of manganate(VII) reacts with five moles of iron(II).

  3. 3

    How is the end point recognised in a manganate(VII) titration?

    Model answer

    Manganate(VII) is decolourised while iron(II) remains. The end point is the first permanent pale pink colour, showing a slight excess of manganate(VII).

  4. 4

    Why is the iron solution acidified with dilute sulfuric acid rather than hydrochloric acid?

    Model answer

    Hydrochloric acid would itself be oxidised by manganate(VII) to chlorine. The titre would then be too large and the iron content too high.

  5. 5

    An iron tablet dissolved in acid needs 11.60 cm³ of 0.0200 mol/dm³ KMnO₄. What mass of iron does it contain?

    Model answer

    The tablet contains about 64.7 mg of iron. The working is 0.01160 × 0.0200 = 0.000232 mol MnO₄⁻, × 5 = 0.00116 mol Fe, × 55.8 = 0.0647 g.

  6. 6

    Why is the burette reading taken from the top of the meniscus in a manganate(VII) titration?

    Model answer

    Manganate(VII) solution is so deeply coloured that the bottom of the meniscus cannot be seen. The top is read consistently instead.

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