H3PO4 + H2O <--> H3O+ + H2PO4- Ka1= ([H3O+][H2PO4-])/[H3PO4])
H2PO4- + H2O <--> H3O+ + HPO4^2- Ka2= ([H3O+][HPO4^2-])/[H2PO4-])
HPO4^2- + H2O <--> H3O+ PO4^3- Ka3= ([H3O+][PO4^3-])/[HPO4^2-])
Ka1 >> Ka2 = almost all the H3O+ is formed in the first step.
Ka3 will only happen under very basic conditions.
