Stepwise Ionisation
The successive loss of protons from a polyprotic acid, each governed by its own equilibrium constant.
Formula / equation
Ka₁ ≫ Ka₂ ≫ Ka₃
Unit
mol/L
In depth
Because each constant is far smaller than the last, the pH of a polyprotic acid solution is usually determined almost entirely by the first ionisation, which greatly simplifies calculations. The successive steps also generate the buffer regions seen along a titration curve.
Examples in the real world
Phosphoric acid's constants of 7.5 × 10⁻³, 6.2 × 10⁻⁸ and 4.8 × 10⁻¹³.