Crystal growing Inorganic Synthesis

Iron (III) ammonium alum preparation


The compound Iron (III) potassium alum is a lesser-known member of the alum family, which is described here. These compounds are often very colorful, and capable of growing very large crystals making the alums popular among crystal growing enthusiasts. Many compounds in the alums group also find great use in industry for purposes varying from use as astringent and in photography[1]. However little research has been done on Iron (III) potassium alum and it’s use in industry. This may pursue further research in  the field of double salts. If lacking the necessary iron (III) sulfate for this reaction, it is possible to start with metallic iron and sulfuric acid, however the product of this reaction is simply iron (II) sulfate. Upon addition of stoichiometric amount of ammonia into iron(II) sulfate solution it is possible to crystallize hexahydrate of double salt with composition Fe(NH4)2(SO4)2.6H2O (also called Mohr´s salt). This can be used as a precursor for iron(III) potassium alum.

If an additional powerful oxidizing agent is added to the the solution of Mohr´s salt it is possible to oxidize iron atoms from +II state to +III state according to the equation below:

Fe(NH4)2(SO4)2 + FeSO4 + 2HNO3+ H2SO4→ Fe(NH4)2(SO4)2 + 2NO2 + 2H2O

In this process, NO2 is evolved when nitric acid is reduced by ferrous ions, so the experiment must be performed with caution using fuming hood!

4Fe2+ + 4HNO3 → 4NO2 + 4Fe3+ + 4e + 2H2O + O2


Iron(II) sulfate heptahydrate (7.09 g, 0.0255 mol) was dissolved in 5 cm3 of hot distilled water followed by dissolution of ammonium sulfate (3.37 g, 0.0255 mol) in the same amount of distilled water. The solutions were then thoroughly mixed together and cooled down in an ice-bath. Crystals of Mohr´s salt were formed after several minutes. These were filtered off and dried in the air for further use. 

Stoichiometric amount of Mohr´s salt (3.68 g, 0.01 mol) and iron(II) sulfate heptahydrate (2.61 g, 0.01 mol) were dissolved in 10 cm3 of distilled water on heating. The mixture was ten transferred into 250 cm3 Erlenmayer´s flask. Concentrated sulfuric acid (0.5 cm3) was pour into the mixture. To oxidize all iron atoms excess of concentrated nitric acid (approximately 2 cm3) was added to the mixture under stirring. Vigorous reaction took place and dark red NO2 was evolved. The final light brown solution was let freely to crystallize. After several days pinkish crystals of the product appeared.

ammonium iron(III) sulfate dodecahydrate (6)

Crystals of ammonium iron(III) sulfate dodecahydrate prepared by the above procedure [Photo by Michal Hegedus]

ammonium iron(III) sulfate dodecahydrate (4)

Crystal of ammonium iron(III) sulfate dodecahydrate prepared by recrystallization of the material [Photo by Michal Hegedus]


  1. Handa, Parvesh (1982). Herbal beauty care. New Delhi: Orient Paperbacks. p. 12. ISBN 9788122200249.


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