Diprotic buffers involve the 3 dominant forms of a diprotic acid and its 2 Ka Values.
Concept #1: Monoprotic buffers involve only the weak acid and its conjugate base and therefore only 1 Ka value.
Concept #2: A diprotic buffer can be approached in a way similar to monoprotic buffers. The key difference is that a diprotic acid has 2 pKa values.
Example #1: What is the pH of a solution consisting of 2.5 M potassium dihydrogen phosphite (KH2PO3) and 2.75 M phosphorus acid (H3PO3)? Ka1 = 3.0 x 10-2 and Ka2 = 1.66 x 10-7.
Example #2: Sulfurous acid, H2SO3, is a major component in the creation of commercial fertilizers. What is the buffer component concentration ratio of a buffer that has a pH of 1.15? Ka1 = 1.39 x 10-2 and Ka2 = 6.73 x 10-8.
Practice: Calculate the pH of a solution made by mixing 8.627 g of sodium butanoate in enough 0.452 M butanoic acid, HC4H7O2, to make 250.0 mL of solution. Ka = 1.5 x 10-5.