H_3PO_3 is reducing, dibasic and has no P-O-P bond. H_4P_2O_7 is tetrabasic, has one P-O-P bond and is reducing. H_3PO_4 has no P-O-P bond, is tribasic and non-reducing. H_4P_2O_5 is dibasic, has P-O-P bond and is reducing.