What is the role of acidity in the decrease of formal potential of $\ce{Fe^{3+}$/$Fe^{2+}}$ in a sulfuric acid and phosphoric acid mixture?
I've observed that the formal potential of the $\ce{Fe^{3+}$/$Fe^{2+}}$ electrode in a mixture of sulfuric acid and phosphoric acid, $E_0 = \pu{+0.61V}$ is much lower than the standard electrode potential, $E_0 = \pu{+0.76V}$.
I came up with a plausible explanation that phosphoric acid would cause $\ce{Fe^{3+}}$ to get precipitated as $\ce{FePO4}$ and the presence of excess acid will help in the formation of a complex $\ce{[FeHPO4]+}$. Am I correct in my observation? How would the situation change if the medium was basic?