5 b): To determine the amount of iron in an iron tablet using previously standardised potassium maganate (VII) solution

Theory

• Iron tablets taken to treat anaemia contains iron and other ingredients

• A number of iron tablets can be made up into a solution and titrated against a previously standardised solution of potassium manganate (VII) (KMnO4) in order to determine the exact concentration of iron in the iron tablets

Equation

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

Procedure

1) Filling the burette with the standard solution of potassium manganate (VII)

2) Making the iron tablets up into a solution

3) Transferring an exact volume of iron tablet solution into a conical flask

4) Carrying out the titration

Suitable indicator for this titration

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

Colour change observed

During titration – As the potassium manganate (VII) is added to the acidified iron tablet solution its purple colour decolourises

At end point – A permanent pink colour is observed

Explaining the colour change

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

As the potassium manganate is added to the iron tablet solution, 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 added will leave a permanent pink colour in the conical flask

What is the active compound in iron tablets?

Iron (II) sulfate Fe2+

Why are iron tablets sometimes medically prescribed?

• Iron is part of the molecule haemoglobin in our red blood cells

– which carry oxygen around the body providing energy

• A person suffering from anaemia may be prescribed iron tablets to treat tiredness and fatigue

How was it possible to have a standard solution of potassium manganate (VII) to use in this titration despite the fact it is not a primary standard?

The potassium manganate was previously standardised by titrating it against a standard solution of hydrated ammonium iron (II) sulfate – a primary standard (Titration 5 a))

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 iron tablets 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 iron tablet 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 irons tablets solution 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 sulfuric 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

Why should the potassium manganate (VII) be used immediately after being standardised

Potassium manganate (VII) is unstable and decomposes in the presence of light and heat