Polyprotic Acid
Definition and meaning of Polyprotic Acid in chemistry.
A polyprotic acid is an acid capable of donating more than one proton (H+) per molecule during ionization. Common examples include phosphoric acid (H3PO4), sulfuric acid (H2SO4), and carbonic acid (H2CO3).
In more detail
Each proton in a polyprotic acid has its own ionization constant (Ka), and successive ionizations become progressively weaker because removing a proton from an increasingly negatively charged conjugate base requires overcoming greater electrostatic attraction between the departing proton and the remaining negative charge. Polyprotic acids are classified by the number of ionizable hydrogens: diprotic acids have two, and triprotic acids have three. Understanding their stepwise ionization is essential for accurate pH calculations and buffer chemistry applications.
Key facts
| Field | Inorganic Chemistry |
|---|---|
| Examples | H2SO4, H3PO4, H2CO3 |
| Classification | Diprotic (2 protons), triprotic (3 protons), tetraprotic (4 protons) |
| Characteristic | Each successive proton removed has a progressively smaller Ka value |
Phosphoric acid (H3PO4) is a triprotic acid with three distinct ionization steps, each with a progressively smaller Ka value, making it useful in buffer preparations and as a food additive.
Frequently asked questions
How many protons can a polyprotic acid donate?
A polyprotic acid donates more than one proton. Diprotic acids donate two, triprotic acids donate three, and some acids have four or more ionizable protons.
Why does ionization strength decrease with each step?
As negative charge accumulates on the conjugate base after each deprotonation, electrostatic attraction between that conjugate base and the remaining proton increases, making it increasingly difficult to remove the next proton.