Phosphoric acid species in phosphoric acid
Phosphoric acid, its unique nature, in solution, there are many species.

Phosphoric acid is a weak electrolyte, partially ionized in water. The first is the phosphoric acid molecule ($H_3PO_3 $), which is an unionized state and accounts for a certain proportion in the solution.

Its ionization process proceeds in steps. In the first step, the phosphoric acid molecule ionizes a hydrogen ion ($H ^ + $) and a hydrogen phosphite ion ($H_2PO_3 ^ - $). The ionization equilibrium constant of this step is referred to as $K_ {a1} $. The generated hydrogen phosphite ion can be further ionized. This is the second step. The ionization equilibrium constant of a hydrogen ion and phosphite ion ($HPO_3 ^ {2 - }$）， is $K_ {a2} $. Since the second step ionization is inhibited by the hydrogen ion generated by the first step ionization, the degree of ionization in the second step is weaker than that in the first step.

In addition to the above ions, there are still hydrogen ions and hydroxide ions ($OH ^ - $) generated by the weak ionization of water in the solution. Because the ionization constant of water is extremely small, at room temperature $K_w = c (H ^ + ) \ c dot c (OH ^ - ) = 1.0\ times10 ^ {-14} $, and the ionization of hydrogen ions from phosphorus will inhibit the ionization of water, resulting in extremely low concentrations of hydroxide ions.

In summary, the species contained in the phosphite solution are phosphite molecules ($H_3PO_3 $), hydrogen ions ($H ^ + $), hydrogen phosphite ions ($H_2PO_3 ^ - $), phosphite ions ($HPO_3 ^ {2 -} $) and hydroxide ions ($OH ^ - $).