H₃PO₂ and H₃PO₃ act as good reducing agent while H₃PO₄ does not. 

Dear Student,

Please find below the solution to the asked query:

Reducing agents are species that are oxidised in a redox reaction.
The oxidation state of P in H₃PO₂, H₃PO₃ and H₃PO₄ is +1, +3 and +5. H₃PO₂ and H₃PO₃ can be oxidised to higher oxidation state and thus they are able to act as a reducing agent.
H₃PO₄ being in the highest possible oxidation state for P cannot be oxidised and hence does not act as a reducing agent. 

Hope this information will clear your doubts about H₃PO₄ does not act as a reducing agent..

If you have any more doubts just ask here on the forum and our experts will try to help you out as soon as possible.

Regards