Third pKₐ of Phosphoric Acid

At pH 12.35 hydrogen phosphate and orthophosphate are equal, and only above that does PO₄³⁻ take over. The three pK_a values — 2.15, 7.20 and 12.35 — are spaced almost exactly five units apart, which is close to the theoretical minimum for a polyprotic acid and the reason phosphate diagrams look so tidy. That spacing also means a phosphate titration curve shows three separate inflections, though the third is usually lost in the alkaline drift of the electrode.

The practical consequence is that phosphate rock, apatite, must be attacked with strong acid rather than base to release its phosphorus, and that phosphate removal in wastewater is achieved by precipitation with iron or aluminium rather than by pH manipulation alone. The value is quoted more loosely than the others because measurements above pH 12 are difficult and the published figures scatter over about a tenth of a unit.