Polyprotic Acids
Most of the acids like HCl, HNO3, CH3COOH are monoprotic because they only produce 1 H+ per molecule. Sulphuric acid, H2SO4, and phthalate acid, C6H4(COOH)2 are diprotic acids. Phosphoric acid, H3PO4, and citric acid are triprotic acids. There are two commonly encountered acids that have never been isolated as pure species. These are carbonic acid, H2CO3, and sulphurous acid, H2SO3.
 
There is a separate ionization constant for the ionization of each hydrogen ion from a polyprotic acid.
              H2CO3(aq) <-------->  H+(aq) + HCO3-(aq)
              carbonic                                       bicarbonate
                 acid                                                 ion
 
Ka1 = [H+][HCO3-] = 4.5 x 10-7 pKa1 = 6.35
             [H2CO3]
 
The second acid ionization Ka2 for this equilibrium:
 
               HCO3-(aq)   <------->   H+(aq)    +    CO32-(aq)
               bicarbonate                                         carbonate
                    ion                                                       ion
 
Ka2 = [H+][CO32-] = 4.7 x 10-11 pKa2 = 10.33
             [HCO3-]
 
In general for any diprotic acid the value of Ka1 > Ka2 since it is easier to pull a H+ from a neutral molecule. Ka1 is usually 104 to 105 times larger than Ka2.
 
Because the first acid ionization constant, Ka1, is so much greater than the second, Ka2, the pH of a dilute solution of a weak diprotic or triprotic acid can be calculated just from the value of Ka1. The contributions to [H+] from the 2nd or 3rd dissociation are so small that they can be ignored.