The maximum and minimum oxidation states of P are -3 and +5. But the oxidation state of P in `H_(3)PO_(3)` is +3. Therefore, it can increase its oxidation state to +5 in `H_(3)PO_(4)` and decrease its oxidation state to -3 in `PH_(3)`. Thus, `H_(3)PO_(3)` shows disproportionation reaction.
`underset("Orthophoshorus acid")overset(+3)(4H_(3)PO_(3))overset("Heat")tounderset("Phoshine")overset(-3)(PH_(3))+underset("Orthophosphoric acid")overset(+5)(3H_(3)PO_(4))`
In contrast, the oxidation state of P in `H_(3)PO_(4)" is "+5`, therefore, it cannot increase its oxidation state beyond +5 and hence it does not show disproportionation reaction.
