Depending on the pH of the solution different forms of protic product can exist. 

Principal Species

Concept #1: By comparing the pH to the pKa value you can determine which form will predominate within a given solution. 

Example #1: What is the predominant form of the diprotic acid, methionine, at a pH equal to 4.18? Ka1 = 6.6 x 10-3 and Ka2 = 8.3 x 10-10.

Example #2: What is the predominant form of histidine at a pH equal to 8.00? pKa1 = 1.6, pKa2 = 5.97 and pKa3 = 9.28. 

Example #3: What is the second most predominant form in the previous question?