First pKₐ of Phosphoric Acid

A phosphate buffer near pH 2.1 is a workhorse of reversed-phase HPLC, where a low pH keeps silanol groups protonated and acidic analytes neutral, sharpening peaks that would otherwise tail. It is also close to the pH of gastric acid, though the stomach's acidity comes from hydrochloric acid rather than phosphate.

Because 2.148 is below the pH of nearly all natural waters, the fully protonated H₃PO₄ species is essentially absent in the environment; environmental phosphate chemistry lives entirely in the H₂PO₄⁻ / HPO₄²⁻ region between pK_a1 and pK_a2. That happens to place the phosphate system's buffering capacity right where biology needs it, which is one reason phosphate is the intracellular buffer of choice across the tree of life.