A diprotic acid is an acid that can donate two hydronium ions (H+).
Since a diprotic acid has two acidic hydrogens it would have 2 equilibrium equations.
Concept #1: Diprotic Acids
The diprotic bases would also have 2 equilibrium equations.
Concept #2: Diprotic Acids
Concept #3: As a result of these equations for diprotic acids and bases the relationship between Ka and Kb can be established.
Example #1: Sulfurous acid, H2SO3, represents a diprotic acid with a Ka1 = 1.6 x 10-2 and Ka2 = 4.6 x 10-5. Calculate the pH and concentrations of H2SO3, HSO3– and SO32– when given 0.250 M H2SO3.
Example #2: Determine the pH of 0.115 M Na2S. Hydrosulfuric acid, H2S, contains Ka1 = 1.0 x 10-7 and Ka2 = 9.1 x 10-8.