Oxalic acid is a diprotic acid, meaning it can donate two protons (H+ ions). The basicity of an acid is determined by the number of protons it can donate.

The first proton of oxalic acid dissociates easily, and the second proton dissociates less easily. The equilibrium constant for the first dissociation is much larger than the equilibrium constant for the second dissociation.

* H2C2O4 ⇌ HC2O4- + H+ (Ka = 5.9 × 10−2)

The second dissociation is:

* HC2O4- ⇌ C2O4^2- + H+ (Ka = 6.4 × 10−5)

The basicity of oxalic acid is 2.