5 a): To use a standard solution of hydrated ammonium iron (II) sulfate to standardise a solution of potassium manganate (VII) by titration

Theory

By titrating a provided potassium manganate (VII) (KMnO4) solution of unknown concentration against a standard solution of hydrated ammonium iron (II) sulfate ((NH4)2 SO4.FeSO4.6H2O ), the concentration of the potassium manganate (VII) can be determined

Equation

MnO4– + 8H+ + 5Fe2+ → Mn2+ + 5Fe3+ + 4H2O

Procedure

1) Filling the burette with the potassium manganate (VII) solution to be standardised

2) Making the hydrated ammonium iron (II) sulfate up into a standard solution

3) Transferring an exact volume of hydrated ammonium iron (II) sulfate solution into a conical flask

4) Carrying out the titration

Suitable indicator for this titration

No indicator required, potassium manganate acts as its own indicator (self-indicating)

Colour change observed

During titration – As the potassium manganate is added to the acidified hydrated ammonium iron (II) sulfate, its purple colour decolourises

At end point – A permanent pink colour is observed in the conical flask

Explaining the colour change

During titration – Mn7+ ions cause the purple colour in potassium manganate (VII)

As the potassium manganate (VII) is added to the acidified hydrated ammonium iron (II) sulfate, the Fe2+ ions lose electrons to the Mn7+ ions i.e. Fe2+ ions cause Mn7+ ions to be reduced

➢ Fe2+ ions are oxidised to Fe3+

➢ Mn7+ ions are reduced to Mn2+

As Mn2+ ions are formed, the purple colour decolourises

At end point – At the end point, there are no more Fe2+ ions left in the conical flask to reduce the Mn7+ ions - the last drop of potassium manganate (VII) added is not decolourised and will leave a permanent pink colour in the conical flask

Explain why a standard solution of potassium manganate (VII) CANNOT be directly made up i.e. why must potassium manganate (VII) be standardised by titration?

Potassium manganate (VII) is not a primary standard – it cannot be obtained in a pure state

Explain why a standard solution can be directly made up from hydrated ammonium iron (II) sulfate

Hydrated ammonium iron (II) sulfate is a primary standard - it is pure, is stable in air, has a high molar mass and can dissolve easily in water to directly make up a solution of accurately known concentration (a standard solution)

Why is hydrated ammonium iron (II) sulfate used as a primary standard instead of iron (II) sulfate?

• Iron (II) sulfate is oxidised easily by oxygen in the air and is not suitable as a primary standard

• Ammonium iron (II) sulfate is highly pure and does not react with air when acidified - suitable for use as a primary standard

It is noted during the titration that the first few drops of KMnO4 are slow to decolourise, but subsequent drops decolourise rapidly. Explain.

Mn+2 ions act as a catalyst. As more Mn+2 is formed it speeds up the reaction and causes the purple colour to decolourise more rapidly – auto catalysis

At what two occasions is dilute sulfuric acid required to be added and explain why it is required on each occasion.

1) Sulfuric acid is added when making up and dissolving the hydrated ammonium iron (II) sulfate into solution

Reason: Prevents the Fe2+ ions being oxidised to Fe3+ ions BY OXYGEN IN THE AIR

2) Excess sulfuric acid is added to the hydrated ammonium (II) sulfate solution in the conical flask just before titrating against potassium manganate (VII)

Reason: Only in an acidic environment are Mn7+ ions fully reduced to Mn2+ ions and not to Mn4+ ions

Note: In a neutral or basic environment Mn7+ ions are only reduced to Mn+4 ions and a brown precipitate of MnO2 is formed

If a brown precipitate forms during the titrations, what conclusion can be drawn?

Not enough dilute sulfuric acid has been added to the hydrated ammonium iron (II) sulfate In the conical flask just before titrating against potassium manganate (VII) - Mn+7 ions have only reduced to Mn+4 ions and insoluble brown MnO2 precipitate has formed

(Why sulfric acid must be in excess)

How is the potassium manganate (VII) read in the burette during the titrations? Why?

The potassium manganate (VII) is read from the top of the meniscus eye level

The intense purple colour of potassium manganate (VII) makes it difficult to read from the bottom of the meniscus

Note: Also for this reason, dilute solutions of potassium manganate (VII) are used