Polyprotic acid

A polyprotic acid is one that has more than one proton to dissociate. Typical examples are malonic acid, which has two ionizable protons, and phosphoric acid, which has three. The constant for dissociation of the first proton may be denoted as pKa1 and the constants for dissociation of successive protons as pKa2, etc.

It is generally true that successive pK values increase (Pauling's first rule).[1] For example, for a diprotic acid, H2A, the two equilibria are

H2A <--> HA- + H+
HA- <--> A- + H+

It can be seen that the second proton is removed from a negatively charged species. Since the proton carries a positive charge extra work is needed to remove it. Therefore pKa2 > pKa1. There are a few exceptions to this rule which occur when there is a major structural change such as in the sequence