Third Dissociation Constant of Phosphoric Acid

K_{\mathrm{a3}}(\mathrm{H_3PO_4}) = 4.5 \times 10^{-13}

Value	4.5e-13
Status	Measured: ± 5.00e-14 (0.11 relative)
Source	CRC Handbook
Categories	Chemistry equilibrium acid-base

Learning zone

The last proton on phosphoric acid barely comes off at all: K_a3 = 4.5 × 10⁻¹³ puts the crossover at pH 12.35, so free orthophosphate PO₄³⁻ is essentially absent from any solution that is not deliberately made strongly alkaline. Trisodium phosphate, the cleaning agent, is the salt of this third dissociation, which is why a TSP solution is caustic enough to strip paint and degrease masonry.

The measured value is the least certain of the three, because at pH 12 and above the hydroxide concentration swamps the equilibrium and the ionic-strength corrections become large; different sources quote anything from 4.2 to 4.8 × 10⁻¹³. For every practical calculation below pH 11 the third dissociation can simply be ignored, and the phosphate system treated as diprotic.

Used by these solvers

Henderson–Hasselbalch Equation (Weak Acid Buffer)

\mathrm{pH} = \mathrm{p}K_a + \log_{10}\!\frac{[\mathrm{A^-}]}{[\mathrm{HA}]}