Phosphoric Acid
Properties
| State | Liquid (viscous, syrupy when concentrated); solid below 40°C |
| Color | Colorless |
| Solubility | Miscible with water |
| Melting Point | 40°C |
| Boiling Point | 158°C (decomposes) |
About Phosphoric Acid
Phosphoric acid is the acid of life and the acid of fertilizer — those two markets account for nearly everything the world makes of it, and they make a lot. About 50 million tonnes per year flow out of the wet process, where ground phosphate rock (mostly Florida and Moroccan apatite) is digested with sulfuric acid to give H3PO4 and gypsum byproduct. That stream goes almost entirely to diammonium phosphate (DAP) and monoammonium phosphate (MAP) fertilizers feeding global grain production. A smaller, much purer stream from the thermal process — burning elemental phosphorus to P4O10 then hydrating it — supplies food-grade and electronic-grade acid. H3PO4 is a triprotic weak acid with pKa1 = 2.15, pKa2 = 7.20, and pKa3 = 12.35, which is why phosphate buffer (the H2PO4⁻/HPO4²⁻ pair around pKa2) is the workhorse buffer for biology between pH 6.2 and 8.2. The taste-receptor angle is the surprise for most people: every cola on the shelf gets its sharp tang from 0.05-0.06% phosphoric acid, which gives Coca-Cola its measured pH around 2.5 — closer to lemon juice than to most other soft drinks. Inside cells, phosphoric acid esters are the chemistry of life: the phosphodiester backbone of DNA and RNA, the high-energy phosphoanhydride bonds in ATP, the head groups of phospholipids, the regulatory phosphorylations on every signaling protein.
Where you'll encounter it
If you've ever bought naval jelly to take rust off a tool, you've spread roughly 25% phosphoric acid on iron oxide and watched it convert Fe2O3 to a stable iron(III) phosphate film — a chemical conversion coating that primes the surface for paint better than bare metal. Dentists do the equivalent on tooth enamel: 37% phosphoric acid etchant gel sits on the enamel for 15-30 seconds, dissolves the surface hydroxyapatite to a roughened topography, and the dental adhesive bonds mechanically into the etched microstructure. In a biochemistry lab, phosphate-buffered saline (PBS) at pH 7.4 — 137 mM NaCl, 2.7 mM KCl, 10 mM phosphate — is mixed by titrating dibasic Na2HPO4 with monobasic NaH2PO4 around the pKa2 = 7.20 buffer point, and the ratio you calculate from Henderson-Hasselbalch is the recipe every cell-culture lab follows. The same buffer at higher concentration shows up in the column-loading buffers for IMAC nickel affinity purification of His-tagged proteins.
Common Uses
- Wet-process feedstock for DAP and MAP phosphate fertilizers (50 Mt/year globally)
- 0.05-0.06% acidulant in Coca-Cola, Pepsi, and other cola beverages (E338)
- 37% etchant gel for tooth enamel preparation in dental composite bonding
- 25% rust-conversion coating in naval jelly for ferrous metal preparation
- Phosphate-buffered saline (PBS) preparation at pH 7.4 for cell culture and protein purification
Safety Information
Concentrated H3PO4 (≥75%) is a Category 1B skin and eye corrosive. OSHA PEL 1 mg/m³ TWA, ACGIH TLV 1 mg/m³ TWA with a 3 mg/m³ STEL. GHS H314 (causes severe skin burns and eye damage), H290 (may be corrosive to metals). FDA-recognized food additive E338 at concentrations below 0.1% in beverages and processed foods. Dental and naval-jelly formulations are diluted to safe handling concentrations but still require gloves and eye protection. Spills on metal floors etch concrete and corrode mild steel; neutralize with sodium carbonate or lime before cleanup.
This safety summary is for educational reference only and may not be complete. It is not a substitute for Safety Data Sheets (SDS), medical advice, or professional chemical safety guidance. Always consult appropriate SDS and qualified professionals before handling chemicals.