The resultant titration curve, when plotted as pH versus milliliters of NaOH added, would be similar to that shown in Figure 22.1. At the point at which one-half of the protons in the first dissociation step of H3PO4 have been titrated with NaOH, the H3PO4 concentration is equal to the H2PO4- concentration. Substituting [H3PO4] = [H2PO4-] into Equation [1] yields Ka1 = [H3O+], or pH = pKa1 at this point