1. Tardigrade
  2. Question
  3. Chemistry
  4. The formal potential of Fe3+/Fe2+ in a sulphuric acid and phosphoric acid mixture (E° = +0.61 V) is much lower than the standard potential (E° = +0.77 V). This is due to (i) formation of the species [FeHPO4]+ (ii) lowering of potential upon complexation (iii) formation of the species [FeSO4]+ (iv) high acidity of the medium.

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Q. The formal potential of $Fe^{3+}/Fe^{2+}$ in a sulphuric acid and phosphoric acid mixture $(E^{\circ} = +0.61 \,V)$ is much lower than the standard potential $(E^{\circ} = +0.77 \,V)$. This is due to
(i) formation of the species $[FeHPO_4]^+$
(ii) lowering of potential upon complexation
(iii) formation of the species $[FeSO_4]^+$
(iv) high acidity of the medium.

Electrochemistry

Solution:

In the aqueous solution, if $H_2SO_4$ is present then iron will form hydrated iron sulphate rather than forming $[FeSO_4]^+$ solution. Moreover, tendency of formation of $FeSO_4$ or $Fe_2(SO_4)_3$ is even more. Formation of $[FeHPO_4]^+$ is reasonably fine. Due to the formation of complex, $[Fe^{3+}]$ decreases and accordingly, potential decreases.
$E=E^{\circ}-\frac{0.0591}{n} log_{10} \frac{\left[Fe^{2+}\right]}{\left[Fe^{3+}\right]}$