Question

$F{e}^{3+}$ ions in an aqueous solution gives deep red colouration with both sodium acetate (in excess) and ammonium thiocyanate (slightly acidic). The deep red colouration obtained with ammonium thiocyanate gets decolourised on addition of solution containing $F^{-}$ ions. Excess of $C{H}_{3}COONa$ is added in the reaction with $F{e}^{3+}$ because :

• A. it acts as a buffer to control pH of the reaction otherwise reaction become reversible
• B. it makes the solution strongly alkaline which destroys acidic effect if any
• C. it makes the solution acidic which destroys alkaline effect if any
• D. none of the above

Answer 1

The correct option is A it acts as a buffer to control pH of the reaction otherwise reaction become reversible

Excess of $C{H}_{3}COONa$ is added in the reaction with $F{e}^{3+}$ because it acts as a buffer to control pH of the reaction otherwise reaction becomes reversible.

The deep red colouration is due to the formation of $\left[F{e}_3\right(OH{)}_2(C{H}_{3}COO{)}_6{]}^{+}$ and with ammonium thiocyanide, a undissociated iron (III) thiocyanate complex is formed.