Phosphoric acid

Lore & Background

Phosphoric acid is produced industrially by two routes: wet processes and dry processes. In the wet process, phosphate-containing minerals such as calcium hydroxyapatite or fluorapatite are treated with sulfuric acid, producing phosphoric acid along with by-products like calcium sulfate (gypsum) and hydrogen fluoride. The resulting phosphoric acid solution typically contains 23–33% P2O5 and can be concentrated to merchant-grade (54–62% P2O5) or superphosphoric acid (above 70% P2O5). The dry, or thermal, process involves reducing phosphate ore with coke in an electric arc furnace to produce elemental phosphorus, which is then burned to form phosphorus pentoxide and dissolved in water to yield high-purity phosphoric acid.

Reader's Guide

Phosphoric acid is a foundational industrial chemical, with approximately 90% of its production dedicated to fertilizers. Its significance extends to food and beverage applications as additive E338, where it acidifies colas and jams and acts as a preservative. The compound also serves in anti-rust treatment, fuel cells, semiconductor etching, and as a pH adjuster in cosmetics. The wet process dominates global production due to lower cost, though the thermal process yields higher purity for food-grade acid. Purification methods such as liquid–liquid extraction, nanofiltration, and fractional crystallization enable production of food-grade and semiconductor-grade phosphoric acid. Safety considerations note that while not a strong acid, moderate concentrations require handling precautions.

Did You Know?

• Phosphoric acid is a triprotic acid, with successive pKa values of 2.14, 7.20, and 12.37.
• The wet process produces gypsum (calcium sulfate) as a by-product, which may contain trace radioactive elements and is called phosphogypsum.
• Aqueous solutions up to 62.5% H3PO4 are eutectic, with freezing points as low as −85 °C.
• Phosphoric acid in soft drinks has the potential to cause dental erosion and contribute to kidney stone formation.