Polyproticity

Polyproticity refers to the ability of a molecule or ion to donate more than one proton (H+) during an acid-base reaction. Substances that can donate only one proton are called monoprotic. Acids that can donate two protons are diprotic, and those that can donate three are triprotic. Acids capable of donating more than three protons are called polyprotic acids.

The dissociation of polyprotic acids occurs in a stepwise manner. Each step involves the donation of one proton and is characterized by its own acid dissociation constant, Ka. For a diprotic acid H2A, the first dissociation is H2A <=> H+ + HA-, with a dissociation constant Ka1. The second dissociation is HA- <=> H+ + A2-, with a dissociation constant Ka2. Typically, Ka1 is significantly larger than Ka2, meaning the first proton is easier to remove than the subsequent ones. This is because the negative charge on the conjugate base of the first dissociation (e.g., HA-) makes it more difficult to remove a positively charged proton.

Examples of polyprotic acids include sulfuric acid (H2SO4), which is diprotic, and phosphoric acid (H3PO4), which is triprotic. Carbonic acid (H2CO3) in dissolved carbon dioxide is another common example of a diprotic acid. The concept of polyproticity is important in understanding the behavior of acids in solution, particularly in buffer systems and titration curves. The pH of a solution containing a polyprotic acid depends on the relative concentrations of all the species involved in the stepwise dissociations.