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

\(F{{e}^{+2}}+\underset{\text{Ferricyanide}}{\mathop{{{[Fe{{(CN)}_{6}}]}^{3-}}}}\,\to F{{e}^{+3}}+\underset{\text{Ferrocyanide}}{\mathop{{{[Fe{{(CN)}_{6}}]}^{4-}}}}\,\)

\(F{{e}^{+3}}+{{[Fe{{(CN)}_{6}}]}^{4-}}\to {{\{Fe[Fe{{(CN)}_{6}}]\}}^{-}}\)

\(F{{e}^{+3}}+{{[Fe{{(CN)}_{6}}]}^{4-}}+{{K}^{+}}\to \underset{\begin{smallmatrix} 
 \text{Pot}\text{. ferric ferro cyanide} \\ 
 \text{or Tumbull }\!\!'\!\!\text{ s blue } 
\end{smallmatrix}}{\mathop{K\{Fe[Fe{{(CN)}_{6}}]\}}}\,\)Ferric ions react with potassium thiocyanate to give blood red colouration due to the formation of ferric thiocyanate

\(FeC{{l}_{3}}+3KCNS\to \underset{\begin{smallmatrix} 
 \text{Ferric}\,\text{thiocyanate} \\ 
 \,\,\,\,\,\,\,\text{(Blood}\,\text{red)} 
\end{smallmatrix}}{\mathop{Fe{{(CNS)}_{3}}+3KCl}}\,\)