Transition elements
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Please help me with the reactions concerning the preparation of ammonium iron(III) sulphate dodecahydrate
Ulex says
The reactions are going to depend on the method chosen. Starting from iron(II) sulphate you can oxidise it to iron(III) sulphate using concentrated nitric acid.
2FeSO4 + 2HNO3 + H2SO4 -> Fe2 (SO4)3 + 2NO2 + 2H2O
We are not qualified to give safety advice on React and you must do your own risk assessment. However, note that concentrated nitric acid is corrosive and nitrogen dioxide is toxic. Work in a fume cupboard.
The resulting solution is mixed with the calculated quantity of ammonium sulphate solution
(NH4)2SO4 + Fe2(SO4)3 +24 H2O -> (NH4)2SO4 . Fe2(SO4)3 . 24H2O
Here is how to do it.
Warm a mixture of 20 cm3 of pure water and 5 cm3 of dilute sulphuric acid in a boiling tube, add 1/100 mol of hydrated iron(II) sulphate and shake to dissolve. Stand the boiling tube in a rack and place in a fume cupboard.
When the solution is cool, oxidize the Fe 2+ ions to Fe 3+ . Do this by adding concentrated nitric acid (TAKE CARE) one drop at a time. The solution will go deep brown, and then lighten to yellow-brown when the oxidation is complete. Avoid breathing the brown gas nitrogen dioxide which is given off.
Dissolve your calculated amount of ammonium sulphate in 10 cm3 of water, add to your iron(III) sulphate solution and transfer to a small beaker.
Boil the mixture until it is dark brown, then label and leave to crystallize in a warm dry place, protected from dust. This will take quite a while.
What shape and colour are the crystals you obtain? Leave some crystals exposed to see whether they are stable in the air. What ions are present in a solution of iron alum? Which compounds have been oxidized and which reduced in this preparation?
Always carry out a risk assessment and check with your teacher before starting any practical work.
Risk assessment
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updated: 19 March 2006
