Polyprotic Acid
An acid capable of donating more than one proton per molecule.
Formula / equation
H₂A ⇌ H⁺ + HA⁻ ⇌ 2H⁺ + A²⁻
Unit
mol/L
In depth
Each proton is lost in a separate step with its own dissociation constant, and each successive constant is smaller — typically by four to five orders of magnitude — because removing a proton from an increasingly negative species is progressively harder.
Examples in the real world
Sulfuric acid, carbonic acid, and phosphoric acid, whose three pKa values are 2.1, 7.2 and 12.3.