Question

The formal potential of $Fe ^{3+} / Fe ^{2+}$ in a sulphuric acid and phosphoric acid mixture $\left(E^{\circ}=+0.61 \,V \right)$ is much lower than the standard potential $\left(E^{\circ}=+0.77 \,V \right) .$ This is due to

• A. formation of the species $[FeHPO_4]^+$
• B. lowering of potential upon complexation
• C. formation of the species $[FeSO_4]^+$
• D. high acidity of the medium

Answer 1

Answer: (D)

high acidity of the medium

Answer 2

Since, $H _{2} SO _{4}$ and $H _{3} PO _{4}$ both are strong acids so their presence makes the medium more acidic. The $Fe ^{3+}$ ions form complex, $\left[ FeHPO _{4}\right]^{+}$with these acid and hence, the concentration of $Fe ^{3+}$ ion reduces which results in lowering in the formal reduction potential.