Question

Which of the following statement(s) is (are) correct with reference to the ferrous and ferric ions

• A. $Fe ^{3+}$ gives brown colour with potassium ferricyanide
• B. $Fe ^{2+}$ gives blue precipitate with potassium ferricyanide
• C. $Fe ^{3+}$ gives red colour with potassium thiocyanate
• D. $Fe ^{2+}$ gives brown colour with ammonium thiocyanate

Answer 1

Answer: (B), (C)

Ferrous salts react with potassium ferricyanide to give blue colouration due to the formation of Turnbull’s blue in this reaction, first ferrous salt is oxidised to ferric salt by the ferricyanide ion which itself is reduced to ferrocyanide.

$Fe ^{+2}+\underset{\text{Ferricyanide}}{{[ Fe ( CN )_{6}]^{3-}}} \to Fe ^{+3}+\underset{\text{Fenocyanide}}{{[Fe(CN)_{6}]^{4-}}}$

$Fe^{+3}+[Fe(CN)_{6}]^{4-} \to \left\{ Fe \left[ Fe ( CN )_{6}\right]\right\}^{-}$

$Fe^{+3}+[Fe(CN)_{6}]^{4-}+K^{+} \to \underset{\text{Pot. ferric feno cyanide or Turnbull's blue}}{{K \left\{ Fe \left[ Fe ( CN )_{6}\right]\right\}}}$

Ferric ions react with potassium thiocyanate to give blood red colouration due to the formation of ferric thiocyanate

$FeCl_{3} +3KCNS \to \underset{\text{Ferric thiocyanate (Blood red)}}{{Fe(CNS)_{3}+3KCl}}$