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Theory

Preparation of ammonium iron(II) sulfate

Iron(II) sulfate was made from the reaction between iron metal and dilute sulfuric acid. In this reaction the iron is oxidized to iron(II) sulfate:

Fe(s) + H2SO4(aq) ⇒ FeSO4(aq) + H2(g)

27.5 cm3 of dilute sulfuric acid was just brought to the boil in a 250 cm3 conical flask. 1/20 of a mole of iron was added carefully. Care must be taken as hydrogen/air mixtures are explosive. A cotton wool plug was used to prevent loss of acid spray caused by the effervescence. The reaction was allowed to continue whilst the ammonium sulfate was made. When this was done any unreacted iron was removed by filtration. Whilst we used a 10% excess of sulfuric acid there is always some unreacted iron left even after about an hour.

Ammonium sulfate was made by neutralizing ammonia solution with dilute sulfuric acid:

2NH3(aq) + H2SO4(aq) ⇒ (NH4)2SO4(aq)

Both the ammonia and sulfuric acid had the same concentrations (2 M). So we needed to use twice the volume of ammonia solution to have twice as many moles, as required by the above equation. This should be 50 cm3, but don't be surprised if you need to use quite a bit more. Ammonia gas easily escapes from the solution, and so ammonia solutions are often less concentrated than they claim to be! Excess ammonia was added (this was checked using red litmus paper), and the excess ammonia was removed by boiling the solution which drives off the ammonia gas.

The solutions of ammonium sulfate and iron(II) sulfate were now mixed and heated to remove some of the water. On cooling green crystals of ammonium iron(II) sulfate formed.

Unlike the original crystals of iron(II) sulfate, the crystals of ammonium iron(II) sulfate are resistant to oxidation by the air. No change is seen on leaving the Mohr's salt for some time. The pale green crystals of iron(II) sulfate are readily oxidized by the air, and samples of these crystals are quickly contaminated by yellow iron (III) compounds.

Sample of green iron(II) sulphate showing surface oxidation to yellow iron(III)

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