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For redox titrations of $\ce{Fe^2+}$ versus $\ce{KMnO_4}$ or versus $\ce{K_2Cr_2O_7}$ , role of phosphoric acid is quite unclear to me. In dichromometric titration, $\ce{H_3PO_4}$ complexes $\ce{Fe^3+}$ and thus lowers the potential of the $\ce{Fe^3+/Fe^2+}$ system, but if it is a permanganometric titration, $\ce{H_3PO_4}$ strictly complexes to mask the yellow colour of $\ce{FeCl_3}$ but plays no part in lowering the potential of the $\ce{Fe^3+/Fe^2+}$ system. (role of ZR solution if $\ce{Cl-}$is present in the system )

My professor strictly mentions this as a key point of difference between roles of phosphoric acid in these two types of titrations and we cannot write the lowering potential point for the latter but that makes no sense to me. I do not understand why $\ce{H_3PO_4}$ in titrations versus $\ce{KMnO_4}$ will not reduce the potential of $\ce{Fe^3+/Fe^2+}$ system.

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  • $\begingroup$ I agree with you. I don't see why $\ce{H3PO4}$ should react with $\ce{Fe^3+}$ ions after the oxidation of $\ce{Fe^2+}$ by $\ce{MnO4-}$ and should not do it after the oxidation of the same ion by $\ce{Cr2O7^2-}$ $\endgroup$
    – Maurice
    Dec 17, 2019 at 21:04

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I don't know why your professor is unnecessarily creating an example which has no real value! No one would do potentiometric titrations of iron in the presence of phosphate.

(i) Potentiometric titrations have nothing to do with color or need masking. (ii) One would need to do a preliminary separation of iron via ion exchange resin.

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