Second Dissociation Constant of Carbonic Acid

The second proton comes off bicarbonate far more reluctantly than the first — a factor of about ten thousand harder, since pulling a positive charge away from an already-negative ion costs energy. K_a2 = 4.69 × 10⁻¹¹ means that at pH 8 only about 0.5 % of the carbonate system exists as CO₃²⁻, yet that tiny fraction is what determines whether calcium carbonate precipitates, because it is carbonate and not bicarbonate that appears in the solubility product.

This is why a small pH rise causes a disproportionate scaling problem: raise pH from 7.5 to 8.5 and the carbonate concentration goes up tenfold, and the ion product Ca²⁺ × CO₃²⁻ follows. Heating does the same thing by driving off CO₂ and pushing the equilibria to the right, which is the mechanism behind kettle scale, and the reason that softening, degassing or acid dosing are all really just ways of manipulating this one constant's consequences.