A polyprotic acid possesses more than 2 hydronium ions (H+).
Polyprotic Acids
Our understanding of diprotic acids and bases can be used to understand polyprotic acids and bases.
Concept #1: The most common type of polyprotic acid is the triprotic acid, which contains 3 acidic hydronium ions.
Concept #2: Triprotic bases contain 3 Kb values, so they would have 3 equilibrium equations.
Concept #3: As a result of these equations for polyprotic acids and bases the relationship between Ka and Kb can be established.
Example #1: Determine the pH of 0.300 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 #2: Determine the pH of 0.300 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.