Phosphoric acid ($H_ {3} PO_ {3} $) is a binary weak acid, which is ionized in water in steps. The ionization equation is as follows:

The first step of ionization: $H_ {3} PO_ {3}\ rightleftharpoons H ^ {+} + H_ {2} PO_ {3 }^{-}$

The second step of ionization: $H_ {2} PO_ {3} ^ {-}\ rightleftharpoons H ^ {+} + HPO_ {3} ^ {2 -} $

It should be noted that there is a hydrogen atom directly connected to the phosphorus atom in the molecular structure of phosphoric acid, which cannot ionize hydrogen ions in aqueous solution, so phosphoric acid is a dibasic acid. The first step of ionization is relatively easier, and the second step of ionization is relatively difficult to occur due to the inhibition of hydrogen ions generated by the first step of ionization.