|Most of the acids like HCl, HNO3, CH3COOH
monoprotic because they only produce 1 H+ per
Sulphuric acid, H2SO4, and phthalate acid, C6H4(COOH)2
are diprotic acids. Phosphoric acid, H3PO4, and
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
hydrogen ion from a polyprotic acid.
H2CO3(aq) <--------> H+(aq) + HCO3-(aq)
|Ka1 = [H+][HCO3-]
= 4.5 x 10-7 pKa1 = 6.35
|The second acid ionization Ka2 for this
|Ka2 = [H+][CO32-]
= 4.7 x 10-11 pKa2 = 10.33
|In general for any diprotic acid the value of Ka1
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.|