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Iron(III) complex colour

A crystal of iron(III) alum showing violet colour with brown hydrolysis spots

The hexaquo iron(III) ion, [Fe(H2O)6]3+ is actually violet in colour. This can be seen in iron(III) alum crystals,
NH4Fe(SO4)2.12H2O.

It is the deprotonation reaction:

[Fe(H2O)6]3+(aq) [Fe(H2O)5OH]2+(aq) + H+(aq)

that produces other complex ions which give the characteristic yellow colour of iron(III) in aqueous solution.