Question

Prussian Blue is an iron-containing pigment which is commonly used to formulate blue paints. Give a possible formula for Prussian Blue if you know that it contains Fe3+ and Fe2+ in a 4:3 ratio and cyanide ion (CN-).

Answer 1

Both Fe²⁺ and Fe³⁺ ions give octahedral cyano anionic complex ions with cyanide ions.

[Fe(H₂O)₆]²⁺_(aq) + 6CN^–_(aq) ==> [Fe(CN)₆]^4–_(aq) + 6H₂O_(l)

Fe²⁺ gives the hexacyanoferrate(II) ion.

This complex ion can be crystallised as potassium hexacyanoferrate(II), K₄[(CN)₆]. If a solution of potassium hexacyanoferrate(III) is added to a solution of iron(II) ions a dark blue precipitate of what was known as Turnbull's Blue is formed.Turnbull's blue is identical to Prussian blue whose formation is explained as below,

Turnbull's blue formation,

Fe²⁺_(aq) + [Fe^III(CN)₆]^3–_(aq) ==> Fe³⁺_(aq) + [Fe^II(CN)₆]^4–_(aq); The iron atoms exchange oxidation states.

K⁺_(aq) + Fe³⁺_(aq) + [Fe^II(CN)₆]^4–_(aq) ==> K⁺Fe³⁺[Fe^II(CN)₆]^4–_(s)

Purssian blue formation,

[Fe(H₂O)₆]³⁺_(aq) + 6CN^–_(aq) ==> [Fe(CN)₆]^3–_(aq) + 6H₂O_(l)

K⁺_(aq) + Fe³⁺_(aq) + [Fe^II(CN)₆]^4–_(aq) ==> K⁺Fe³⁺[Fe^II(CN)₆]^4–_(s)

Fe³⁺ gives the hexacyanoferrate(III)

This complex can be crystallised as potassium hexacyanoferrate(III), K₃[(CN)₆].

If a solution of potassium hexacyanoferrate(II) is added to a solution of iron(III) ions the dark blue precipitate of Prussian blue is formed.The formation of the dark blue precipitate from potassium hexacyanoferrate(II) can be used as a test for iron(III) ion Fe³⁺_(aq).There is no exchange of the iron oxidation states in this reaction.

This complex compound the formula can also be written as Fe₄[Fe(CN)₆]₃ · xH2O.