Chemical Equilibrium of Phosphoric Acid at pH 12

What happens to phosphoric acid at pH 12?

At a pH of 12, what is the significance of the pKa₃ value of phosphoric acid?

Answer:

Phosphoric acid ionizes in three steps, and at pH 12, the third dissociation is in equilibrium, where HPO4^2- loses a proton to form PO4^3- and H+, with their concentrations being equal at the pKa3 value.

Phosphoric acid, H3PO4, is a triprotic acid meaning it can donate three protons (H+). Due to the high pH of 12, corresponding to the third dissociation step of phosphoric acid, we are concerned with the third ionization step in equilibrium:

HPO42- ⇌ PO43- + H+

At a pH of 12, the concentration of H+ ions and PO43- ions would be equal since it is the pKa3 of phosphoric acid. The ionization equilibrium can be represented by the equation:

Ka3 = [PO43-][H+]/[HPO42-]