To prepare a standard solution of ammonium iron sulfate and use this to standardise a solution of potassium manganate by titration

Describe the appearance of the ammonium iron(II) sulfate crystals.

a green crystalline material.

Why do you use ammonium iron(II) sulfate rather than just use iron(II) sulfate?

1. It can be obtained in a high degree of purity.

2. It is not affected by the air, i.e. the Fe2+ ions in ammonium iron(II) sulfate are not oxidised to Fe3+ ions by the oxygen in the air.

FeSO4 is not a primary standard because (a) the crystals are oxidised slightly by the air and (b) the crystals lose their water of crystallisation when exposed to the air (efflorescence).

What colour change is observed during this titration?

colourless to pink.

Why was it not necessary to weigh out exactly 9.8 g of ammonium iron(II) sulfate?

In order to calculate the concentration of the ammonium iron(II) sulfate solution we need to know the exact amount that has dissolved in the dilute acid. This amount need not be exactly 9.8 g. It can be any amount that has been accurately measured. When calculating molarity all we need to know is the precise amount of salt in the solution.

Describe how you would take the reading in the burette when using KMnO4?

from the top rather than the bottom of the meniscus.

Dilute sulfuric acid is added on two occasions during this experiment. Give the reason for its addition on each occasion.

1. when making up the solution of ammonium iron(II) sulfate. It is used to prevent the Fe2+ ions being oxidised to Fe3+ ions by oxygen in the air or oxygen dissolved in the water;

2. to the conical flask at the beginning of the titration to supply the H+ ions in order for the reaction to occur

When carrying out the titration another student observed that the first few drops of KMnO4added to the conical flask were decolourised slowly, but subsequent drops were decolourised rapidly. Explain this observation.

This observation is due to autocatalysis, i.e. the reaction is catalysed by the Mn2+ ions formed in the reaction: MnO4- + 8H+ + 5e-Æ Mn2+ + 4H2O

As soon as the Mn2+ ions are formed the rate of the reaction increases.

When performing this experiment a student noticed a dark brown colour being formed in the conical flask. What conclusion would you draw from this observation?

The student has probably forgotten to add the dilute sulfuric acid at the beginning of the titration. This omission causes a dark brown precipitate of MnO2 to be formed.

Primary standard

A substance that can be obtained in a stable, pure and soluble solid form so that it can be weighed out and dissolved in water to give a solution of accurately known concentration

Standard solution

a solution of known concentration

How did you prepare the standard solution?

1.Weigh out 9.8g ammonium iron sulfate on a clock glass

2. Add 20 cm cubed dilute sulphuric acid to a beaker containg 100cm cubed of water

3. Transfer the crystals to the beaker containing the sulphuric acid, ensuring all washing are included

4. Using a funnel, transfer the solution to a 250 cm cubed volumetric flask and make up with deionised water to the mark

5. Stopper and invert 20 times

Why was sulphuric acid added?

To prevent the Fe2+ ion in ammonium iron sulfate from being oxidised to Fe3+ by oxygen in the air

Identify the primary standard

Ammonium iron sulfate

Why does the potassium manganate have to be freshly prepared?

It is unstable and easily decomposes in light

What is the oxidising agent?

Potassium manganate

How was the end point detected?

Faint permanent pink colour remains

What would be observed if sulphuric acid was not added to the potassium manganate?

A brown colour would be obbserved as the Mn7+ is reduced partially to Mn4+

Why do you take reading from the top of the meniscus when using the burette?

Potassium manganate has such a deep purple colour, that you cannot see a meniscus

Indicator used

Potassium manganate acts as its own indicator

What colour change is observed when potassium manganate is added to the iron solution?

Purple to colourless

Why is the decolourising of the potassium manganate slow at the beginning of the reaction and then speeds up?

The first few Mn2+ ions are produced, they act as a catalyst for the reaction, this is an example of autocatalysis

Why is sulphuric acid added to potassium manganate?

In order to get the Mn7+ ions in it to reduce fully to Mn2+

Why can't hydrochloric acid be used?

The Cl- ions in it would get oxidised.

Why can't nitric acid be used?

It acts acts as an oxidising agent

How was pipette rinsed?

- Rinse with deionised water

- Rinse with the iron solution

How was pipette used?

- Fill using a pipette filler

- Fill pipette until bottom of the meniscus is at the mark

- Read meniscus at eye level

- Place 25 cm cubed of iron solution into conical flask using pipette

- Touch the pipette gently against the inside of the conical flask to get out the last drop

How was the burette rinsed?

- Rinse with deionised water

- Rinse with potassium manganate solution

How was the burette used?

- Using a funnel, fill the burette with potassium manganate solution

- Read from the top of the meniscus as the dark colour of potassium manganate makes it difficult to read the bottom of the meniscus

- Clamp vertically

- Ensure the part below the tap has been filled

How was the conical flask rinsed?

Rinse with deionised water only

How was the conical flask used?

- Place on a white tile

- Swirl throughout to ensure thorough mixing of the chemicals

- Rinse down the sides of the conical flask during the titration as necessary with deionised water