4.5 - A Potassium Manganate (VII) / Ammonium Iron (II) Sulphate Titration

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Experiment Summary

In this experiment, a solution of potassium manganate (VII) ( $KMnO₄$ ) is standardised by titration against a standard solution of ammonium iron(II) sulfate.

The potassium manganate (VII) acts as an oxidising agent, and no indicator is required because the reaction itself provides a colour change.
Under acidic conditions, the $MnO₄⁻$ ions are reduced from purple to colourless, with the endpoint being detected when a permanent pale pink colour remains.

Materials and Apparatus Required

Chemicals

Potassium manganate (VII) solution
Ammonium iron (II) sulfate solution (0.1 M)
1.5 M sulfuric acid
Deionised water

Apparatus

Pipette (25 cm³) and pipette filler
Burette (50 cm³) with retort stand and clamp
Conical flask (250 cm³)
White tile
Filter funnel
Wash bottle
Beakers (250 cm³)
Graduated cylinder (100 cm³)
Safety glasses

Safety Precautions

Wear safety glasses at all times.
Potassium manganate (VII) is a powerful oxidising agent and can irritate the skin. Avoid contact and inhalation.
Sulfuric acid is corrosive to eyes and skin; handle with care and always add acid to water when diluting.

Method

Rinse the pipette, burette, and conical flask with deionised water.
Rinse the burette with potassium manganate (VII) solution and the pipette with iron (II) sulfate solution.
Use the pipette to transfer 25 cm³ of iron (II) sulfate solution into a conical flask.
Add about 10 cm³ of dilute sulfuric acid to the flask.
Fill the burette with potassium manganate (VII) solution, ensuring the part below the tap is filled.
Take readings from the top of the meniscus due to the intense colour of the solution.
Begin the titration by adding potassium manganate (VII) solution from the burette to the conical flask while swirling.
Use a white tile to observe the colour change.
Continue adding the solution until a pale pink colour persists, indicating the endpoint.
Perform additional titrations until two readings agree to within 0.1 cm³.
Calculate the concentration of the potassium manganate (VII) solution using the titration results.

Results

Measurement	Value
Rough titre	22.8 cm³
Second titre	22.4 cm³
Third titre	22.5 cm³
Average of accurate titres	22.45 cm³
Volume of iron (II) solution used	25.0 cm³
Concentration of iron (II) solution	0.1 M

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Sample Calculation

Using the formula:

\text{Moles of Fe(II)} = \frac{25.0 \times 0.1}{1000} = :highlight[0.0025] \, \text{moles}

From the balanced equation:

\text{MnO}_4⁻ + 5\text{Fe}^{2+} + 8\text{H}^+ \rightarrow \text{Mn}^{2+} + 5\text{Fe}^{3+} + 4\text{H}_2\text{O}

Moles of $MnO₄⁻$ reacting:

:highlight[0.0025 / 5] = :highlight[0.0005] \, \text{moles}

Concentration of potassium manganate (VII) solution:

\frac{0.0005}{22.45 / 1000} = :highlight[0.022] \, \text{M}

Example Questions with Answers

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Q1: Why is ammonium iron (II) sulfate suitable as a primary standard?

It is stable, highly pure, and does not oxidise easily in air.

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Q2: Why is sulfuric acid added to the iron (II) solution before titration?

Acidic conditions are required for the reduction of $MnO₄⁻$ to $Mn²⁺$

Hydrochloric acid cannot be used because it reacts with $KMnO₄$ , and nitric acid is an oxidising agent that would interfere with the reaction.

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Q3: Why are the pipette and burette rinsed with deionised water followed by their respective solutions?

To remove any residual solution, rinsing with the solution to be used prevents dilution of the titration solution.

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Q4: One product of this reaction acts as a catalyst. Which product is it?

$Mn²⁺$ ions act as a catalyst.

This can be demonstrated by adding $Mn²⁺$ to the flask before starting the titration, which speeds up the decolourisation of $KMnO₄$ .

- A Potassium Manganate (VII) / Ammonium Iron (II) Sulphate Titration Simplified Revision Notes for Leaving Cert Chemistry

4.5 - A Potassium Manganate (VII) / Ammonium Iron (II) Sulphate Titration

Experiment Summary

Materials and Apparatus Required

Chemicals

Apparatus

Safety Precautions

Method

Results

Sample Calculation

Example Questions with Answers

Q1: Why is ammonium iron (II) sulfate suitable as a primary standard?

Q2: Why is sulfuric acid added to the iron (II) solution before titration?

Q3: Why are the pipette and burette rinsed with deionised water followed by their respective solutions?

Q4: One product of this reaction acts as a catalyst. Which product is it?

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