# Role of phosphoric acid in redox titrations

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.

• 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 $\ce{Cr2O7^2-}$ Dec 17 '19 at 21:04