Pyrophosphorous acid \(\rightarrow H _{4} P _{2} O _{5}\).

Hypophosphoric acid \(\rightarrow H _{4} P _{2} O _{6}\) 

Phosphoric acid \(\rightarrow H _{3} PO _{4}\)

Pyrophosphoric acid \(\rightarrow H _{4} P _{2} O _{7}\).