Second Dissociation Constant of Phosphoric Acid

This is the biologically important one. With K_a2 = 6.32 × 10⁻⁸ the dihydrogen/monohydrogen phosphate pair buffers around pH 7.2, essentially on top of physiological pH, which is why phosphate-buffered saline is the default medium in cell culture and why phosphate is the dominant intracellular buffer. Mixing H₂PO₄⁻ and HPO₄²⁻ in roughly a 2:3 ratio gives a solution that holds pH 7.4 against metabolic acid production.

Its one drawback is that phosphate precipitates divalent cations — calcium and magnesium in particular — so experiments that need free Ca²⁺ use HEPES or Tris instead. In boiler water treatment the same reaction is deliberate: phosphate is dosed so that any calcium leaking past the softener precipitates as a soft, mobile sludge that blows down rather than as hard adherent scale on a tube.