Equivalent Weight of Phosphoric Acid
| Value | 0.097994 kg/mol |
| Status | Measured: ± 0.000004 kg/mol (0.000041 relative) |
| Source | IUPAC |
| Categories | Chemistryequivalent-weightwater-treatment |
| gram per mole | 97.994 g/mol |
| kilogram per kilomole | 97.994 kg/kmol |
| milligram per millimole | 97.994 mg/mmol |
| pound per pound-mole | 97.994 lb/lb-mol |
| dalton (as molar mass) | 97.994 Da |
| kilogram per mole | 0.097994 kg/mol |
| kilodalton (as molar mass) | 0.097994 kDa |
Learning zone
Phosphoric acid has three protons and, for alkalinity purposes, delivers one. The value here follows the convention that matters at the feed pump: alkalinity is titrated to the methyl-orange endpoint at pH 4.5, and of phosphoric acid's three dissociations — pKa 2.15, 7.20, 12.35 — only the first has happened by then. The second proton does not come off until pH 7.2, far above the endpoint, so against alkalinity H₃PO₄ behaves as a monoprotic acid and its working equivalent weight is the whole molar mass: 3 × 1.008 + 30.974 + 4 × 15.999 = 97.994 g/eq. Dividing by three, as a bench-chemistry table aimed at complete neutralisation would, understates the required feed by a factor of three.
That makes phosphoric a heavy, expensive way to move pH — twice sulfuric's mass per equivalent — and nobody feeds it primarily for alkalinity reduction. It appears in treatment programs for what the anion does afterwards: orthophosphate is a corrosion inhibitor at the pipe wall and, in boiler drums, the precipitating agent that turns calcium leakage into mobile sludge instead of adherent scale. The acid form simply brings its own pH correction along with the inhibitor.