Question

The formal potential of $F{e}^{3+}/F{e}^{2+}$ in a sulphuric acid and phosphoric acid mixture $(E^{\circ }=+0.61V)$ is much lower than the standard potential $(E^{\circ }=+0.77V)$. This is due to
(i) formation of the species $[FeHP{O}_4{]}^{+}$
(ii) lowering of potential upon complexation
(iii) formation of the species $[FeS{O}_4{]}^{+}$
(iv) high acidity of the medium.

• A. (i) and (ii) only
• B. (i), (ii) and (iv) only
• C. (iii) only
• D. all of these

Answer 1

Answer: (A)

In the aqueous solution, if $H_{2}S{O}_4$ is present then iron will form hydrated iron sulphate rather than forming $[FeS{O}_4{]}^{+}$ solution. Moreover, tendency of formation of $FeS{O}_4$ or $F{e}_2(S{O}_4{)}_3$ is even more. Formation of $[FeHP{O}_4{]}^{+}$ is reasonably fine. Due to the formation of complex, $[F{e}^{3+}]$ decreases and accordingly, potential decreases.
$E=E^{\circ }-\frac{0.0591}{n}lo{g}_{10}\frac{\left[F{e}^{2+}\right]}{\left[F{e}^{3+}\right]}$