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The formal potential of $Fe^{3+}/Fe^{2+}$ in a sulphuric acid and phosphoric acid mixture $\left(E^{\circ}=+0.61 \mathrm{V}\right)$ is much lower than the standard potential $\left(E^{\circ}=+0.77 \mathrm{V}\right)$. This is due to
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formation of the species $\left[\mathrm{FeHPO}_{4}\right]^{+}$, lowering of potential upon complexation, high acidity of the medium
Since, $\mathrm{H}_{2} \mathrm{SO}_{4}$ and $\mathrm{H}_{3} \mathrm{PO}_{4}$, both are strong acids so their presence makes the medium more acidic. The $\mathrm{Fe}^{3+}$ ions form complex, IFeHPO $\left._{4}\right]^{+}$ with these acid and hence, the concentration of $\mathrm{Fe}^{3+}$ ion reduces which results in lowering in the formal reduction potential.
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