Diprotic species involve the movement of two acidic hydrogens.
Concept #1: Diprotic acids can donate two acidic hydrogens and as a result possess two Ka values.
Concept #2: Diprotic bases can accept two acidic hydrogens and as a result possess two Kb values.
Concept #3: With the presence of different forms it becomes essential to know which Ka or Kb value to use.
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 0.200 M H2SO3.
Example #2: Determine the pH of 0.080 M Na2S. Hydrosulfuric acid, H2S, contains Ka1 = 1.0 x 10-7 and Ka2 = 9.1 x 10-8.
Example #3: If Ka1 = 4.46 x 10-7 and Ka2 = 4.69 x 10-11 for H2CO3 what is the pH for a 0.15 M solution of NaHCO3?
Practice: An unknown diprotic acid has an initial concentration of 0.025 M. What is the pH of the solution if pka1 is 3.25 and pKa2 is 6.82?