`H_(3)PO_(3)` on heating undergoes self-oxidation reduction, (i.e., disproportionation) to form `PH_(3)` in which P is reduced and `H_(3)PO_(4)` in which P is oxidised.
`underset("Orthophosphorous acid")overset(+3)(4H_(3)PO_(3))overset(Delta)tounderset("Phosphine")overset(-3)(PH_(3))+underset("Orthophosphoric acid")overset(+5)(3H_(3)PO_(4))`
