Question

Ferric alum gives red colour with ${\text{N}}{{\text{H}}_{\text{4}}}{\text{SCN}}$ due to formation of:
A. ${\text{Al}}{\left( {{\text{SCN}}} \right)_{\text{3}}}$
B. ${\text{FeS}}{{\text{O}}_{\text{4}}}$
C. ${\text{Fe}}{\left( {{\text{SCN}}} \right)_{\text{3}}}$
D. ${\text{Fe}}{\left( {{\text{SCN}}} \right)_{\text{2}}}$

Answer 1

Ferric alum will react with thiocyanate ion to form a red color complex. In this complex, there is no change in the oxidation state of iron as this is not a redox reaction. It is a complex formation reaction.

Complete answer:
Ferric alum contains ferric $\left( {{\text{F}}{{\text{e}}^{3 + }}} \right)$ ions. Ferric alum is a double salt containing ammonium sulphate and ferric sulphate. The chemical formula of ferric alum is ${\left( {{\text{N}}{{\text{H}}_{\text{4}}}} \right)_2}{\text{S}}{{\text{O}}_4} \cdot {\text{F}}{{\text{e}}_2}{\left( {{\text{S}}{{\text{O}}_4}} \right)_3} \cdot 24{{\text{H}}_2}{\text{O}}$

Ferric alum is used as an indicator in the Volhard method. We can use the Volhard method for the titration of silver chloride with ammonium thiocyanate. In an acidic medium, ${\text{A}}{{\text{g}}^ + }$ ions react with ammonium thiocyanate to form a precipitate of silver thiocyanate. After complete precipitation of ${\text{A}}{{\text{g}}^ + }$ ions, the excess titrant (ammonium thiocyanate) reacts with ferric ions from ferric alum to form soluble red complex. Here ferric alum is used as an indicator. The red coloured complex is due to the formation of ferric thiocyanate ${\text{Fe}}{\left( {{\text{SCN}}} \right)_{\text{3}}}$
${\text{F}}{{\text{e}}^{3 + }}{\text{ + 3SC}}{{\text{N}}^ - } \to {\text{Fe}}{\left( {{\text{SCN}}} \right)_{\text{3}}}$

Thus, one mole of ferric ions reacts with three moles of thiocyanate ions to form one mole of ferric thiocyanate complex. Ferric ions are from ferric alum and thiocyanate ions are from ammonium thiocyanate.

Additional information:
The oxidation number of iron in ferric alum is +3. The oxidation number of iron in red coloured complex ferric thiocyanate is also +3. Thus, during complex formation reactions, there is no change in the oxidation number.

Hence the correct answer is option ‘C’.
Note: We can use Volhard method for the determination of chloride, bromide and iodide ions in acidic medium. We cannot use basic medium as ferric ions will hydrolyze in basic medium.