Polyprotic species involve the movement of 3 or more acidic hydrogens.
Example #1: Polyprotic acids can donate three or more acidic hydrogens and as a result will possess multiple Ka values.
Example #2: Polyprotic bases can accept 3 or more acidic hydrogens and as a result possess multiple Kb values.
Concept #1: With even more forms than diprotic species, polyprotic species have even more Ka and Kb values involved.
Example #3: Calculate the equilibrium concentrations of H3PO4, H2PO4–, HPO42-, PO43-, and H3O+ for 0.35 M H3PO4. Ka1 = 7.2 x 10-3, Ka2 = 6.3 x 10-8, and Ka3 = 4.2 x 10-13.
Example #4: Determine the pH of 0.250 M sodium hydrogen phosphate, Na2HPO4. Phosphoric acid, H3PO4, contains Ka1 = 7.5 x 10-3, Ka2 = 6.2 x 10-8 and Ka3 = 4.2 x 10-13.
Example #5: Determine the pH of 0.150 M citric acid, H3C6H5O7. It possesses Ka1 = 7.4 x 10-4, Ka2 = 1.7 x 10-5 and Ka3 = 4.0 x 10-7.