Phosphoric Acid (H₃PO₄) — Properties, Uses & Safety

Overview

Phosphoric acid (H3PO4) is a triprotic mineral acid with a molar mass of 97.99 g/mol. It appears as a colorless, viscous liquid or white crystalline solid and is highly soluble in water. As a triprotic acid, phosphoric acid can donate three protons in sequential ionization steps, with pKa values of 2.15, 7.20, and 12.35, producing dihydrogen phosphate (H2PO4−), hydrogen phosphate (HPO42−), and phosphate (PO43−) ions respectively. This stepwise dissociation makes phosphoric acid one of the best examples for teaching polyprotic acid equilibria and buffer systems — the H2PO4−/HPO42− pair is a key component of biological buffer systems that maintain blood pH near 7.4. Phosphoric acid is the world's third most important industrial acid by production volume. Its largest use is in the manufacture of phosphate fertilizers (superphosphate and triple superphosphate), which are essential for global agriculture. In the food industry, phosphoric acid (E338) is used to acidify cola drinks, giving them their characteristic tangy flavor, and it appears in jams, cereals, and processed cheeses. It is also used as a rust converter and remover, converting iron oxide to iron phosphate, and in dental cements and etching solutions. GHS hazard codes include H290 (may be corrosive to metals) and H314 (causes severe skin burns and eye damage), with the corrosion pictogram. For chemistry students, phosphoric acid connects acid-base chemistry, equilibrium constants, buffer calculations, and titration curves. Its industrial applications bridge chemistry with agriculture, food science, and materials engineering. The phosphate ion itself is central to biochemistry, forming the backbone of DNA and RNA and serving as the energy currency in ATP.

IUPAC Name

phosphoric acid

CAS Number

7664-38-2

PubChem CID

1004

Molecular Formula

H₃PO₄

Molar Mass

97.994 g/mol

State at STP

liquid

Appearance

Colorless viscous liquid or white solid