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?

  • $\begingroup$ Mamun, you must point out where youre stuck. Suppose, I was wondering which α-carbon of a ketone is more acidic. I would ask the question, and also add how I try to find out. I would also include something like "I considered both resonance and inductive effects, but the former increases stability, while the latter decreases it". $\endgroup$ – Pritt says Reinstate Monica Apr 21 '17 at 15:15
  • $\begingroup$ For your situation, you wonder how acidity affects the electrode potential. Try to solve your situation. Add some points as to why acidity might affect the potential, or some points saying why acidity doesnt affect. $\endgroup$ – Pritt says Reinstate Monica Apr 21 '17 at 15:17
  • $\begingroup$ what i know is phosphoric acid precipitates Fe3+ as FePo4 ,,then excess phosphoric acid complexes with it to form FeHPO4+ in 'acidic medium' now i dont understand what is the problem if this complex is in basic medium.? $\endgroup$ – positron Apr 21 '17 at 15:37
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    $\begingroup$ omg! how did my question change by itself!! $\endgroup$ – positron Apr 22 '17 at 1:41
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    $\begingroup$ @Mamun We (myself and berryholmes) changed it for you. $\endgroup$ – Pritt says Reinstate Monica Apr 22 '17 at 6:14

There are numerous reactions that involve iron and many involve either $\ce{H+}$ and $\ce{OH-}$. Take a look at this table and search for "$\ce{Fe}$". The non-standard potential for each one depends on the concentration of the species involved, and for the ones with hydrogen or hydroxide ions this means it includes the pH. This is described by the Nernst Equation. The potential varies as the natural log of the concentration, so you would add or subtract the pH or pOH, depending on the half reaction in question.

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