Monoprotic and Polyprotic Acids
Chemistry (Year 12) - Acids and Bases
You may have noticed that the number of hydrogen cations that an acid can donate will vary. Acids such as hydrochloric acid and nitric acid are limited to donating one hydrogen ion, while acids such as sulfuric acid and carbonic acid are able to donate two hydrogen ions. Acids such as phosphoric acid are even able to donate three hydrogen ions.
When we compare the donation capabilities of different acids, we can classify them as either monoprotic or polyprotic acids. Each of these definitions is based on the number of hydrogen ions they are capable of donating.
Monoprotic acids are those acids that are only capable of donating one proton per molecule (e.g. hydrochloric acid, nitric acid, ethanoic acid, ammonium, hydroflouric acid).
Polyprotic acids are those acids that are capable of donating two or more protons per molecule (e.g. sulfuric acid, carbonic acid, phosphoric acid).
When an acid is polyprotic, it will undergo a different reaction for each hydrogen ion it can donate. An acid that can donate two hydrogen ions is 'diprotic' and will undergo two reactions, and an acid that can donate three hydrogen ions is 'triprotic' and will undergo three reactions. In each of these reactions, they will be donating one hydrogen ion each time.
For instance, consider the ionisation of sulfuric acid. For this diprotic acid, we would show two ionisations: first, the ionisation of sulfuric acid into hydrogen sulfate and second, the ionisation of the hydrogen sulfate into a sulfate ion.
But do all of these ionisations actually occur for these polyprotic acids?
As a polyprotic acid undergoes each succcessive ionisation, the tendency for it to undergo another ionisation and donate an additional hydrogen ion significantly decreases. If it remained high after each ionisation, then the reverse reactions would all occur to a large extent and counteract the ionisations.
Therefore, with a strong diprotic acid like sulfuric acid, we see a complete reaction in its first ionisation and a moderate reaction in its second ionisation.
However, what if the polyprotic acid is not strong? Take the ionisation of phosphoric acid:
When a weak triprotic acid like phosphoric acid undergoes its first ionisation to form dihydrogen phosphate, this ionisation only occurs to a partial extent (moderate acidity constant).
As a result, the second ionisation of dihydrogen phosphate into hydrogen phosphate occurs to a near negligible extent (small acidity constant).
By the time we reach the third ionisation of hydrogen phosphate into phosphate, it is occurring to such a negligible extent (extremely small acidity constant) that the hydrogen phosphate will actually prefer to act as a base and convert back into dihydrogen phosphate.