Here's why:

* Redox titrations involve the transfer of electrons. Phosphoric acid is a weak acid and does not readily participate in electron transfer reactions. It primarily acts as a proton donor (acid) and not as an oxidizing or reducing agent.

* Phosphoric acid is often used in acid-base titrations, where its role is to provide a suitable acidic environment for the reaction to occur. It can also be used to adjust the pH of solutions.

Instead of phosphoric acid, redox titrations typically employ strong oxidizing or reducing agents:

* Oxidizing agents: Potassium permanganate (KMnO₄), potassium dichromate (K₂Cr₂O₇), cerium(IV) sulfate (Ce(SO₄)₂)

* Reducing agents: Sodium thiosulfate (Na₂S₂O₃), iron(II) sulfate (FeSO₄)

Therefore, phosphoric acid does not have a specific purpose in redox titrations. If you are using phosphoric acid in a titration, it is likely part of an acid-base titration, not a redox titration.