Phosphoric acid

Phosphoric acid (also called orthophosphoric acid, monophosphoric acid, or phosphoric(V) acid) is a colorless, odorless solid inorganic compound with the formula H3PO4. It is most often seen as an 85% water solution, which is a non-volatile, syrupy liquid that is also colorless and odorless. This acid is an important industrial chemical, especially as an ingredient in many fertilizers.

As a triprotic acid, it can lose up to three hydrogen ions. Removing all three yields the phosphate ion (PO₄³⁻). Losing one or two protons gives the dihydrogen phosphate ion (H₂PO₄⁻) or the hydrogen phosphate ion (HPO₄²⁻), respectively. Phosphoric acid also forms esters known as organophosphates. The name "orthophosphoric acid" distinguishes it from other phosphoric acids like pyrophosphoric acid, but in common and current IUPAC usage, "phosphoric acid" usually refers to this specific compound.

**Production**

Industrially, phosphoric acid is made through two main methods: the wet process and the dry process.

In the wet process, phosphate minerals such as calcium hydroxyapatite or fluorapatite are reacted with sulfuric acid. The reactions produce phosphoric acid along with calcium sulfate (gypsum) and hydrogen fluoride as byproducts. The hydrogen fluoride gas can be captured in a water scrubber to make hydrofluoric acid. The calcium sulfate, called phosphogypsum, often contains trace radioactive elements like radium, making it unsuitable for commercial construction use, so it is usually stored indefinitely. The resulting phosphoric acid solution typically contains 23–33% P₂O₅ (32–46% H₃PO₄). It can be concentrated to commercial-grade acid (54–62% P₂O₅, or 75–85% H₃PO₄), and further water removal yields superphosphoric acid with over 70% P₂O₅ (nearly 100% H₃PO₄). Both processes may include additional purification to remove arsenic and other toxic impurities.

The dry process, also called the thermal or electric furnace process, is used to produce food-grade phosphoric acid. Phosphate ore is reduced with coke in an electric arc furnace, with silica added to create calcium silicate slag. Elemental phosphorus is distilled out, burned in air to form high-purity phosphorus pentoxide, and then dissolved in water to make phosphoric acid. This method yields acid with a very high P₂O₅ concentration (about 85%) and low impurity levels. However, it is more expensive and energy-intensive than the wet process, which produces acid with lower P₂O₅ (26–52%) and more impurities. The wet process is the most common method for fertilizer-grade acid; even in China, where cheap coal makes the thermal process more common than elsewhere, over seven-eighths of phosphoric acid is made via the wet process.

**Purification**

Phosphoric acid from either production route often needs purification. A common method is liquid–liquid extraction, where organic solvents like tributyl phosphate, methyl isobutyl ketone, or n-octanol separate the acid from water and impurities. Nanofiltration uses a membrane modified with a high-molecular-weight polycationic polymer (polyethyleneimines) to significantly reduce levels of cadmium, aluminum, iron, and rare earth elements, and has been shown at lab and pilot scale to produce food-grade acid. For even higher purity, such as for semiconductor use, fractional crystallization is employed. A static crystallizer uses vertical plates suspended in the molten feed, which are alternately cooled and heated. Slow cooling causes crystals to grow on the plates, rejecting impurities into the remaining melt. After the desired crystal fraction forms, the impure melt is drained, the plates are heated to melt the crystals, and the purified acid is collected. The cycle then repeats with fresh feed.

**Properties**

In water, phosphoric acid behaves as a triprotic acid with three dissociation steps: H₃PO₄ loses a proton to form H₂PO₄⁻ (pKa₁ = 2.14), then HPO₄²⁻ (pKa₂ = 7.20), and finally PO₄³⁻ (pKa₃ = 12.37). The gaps between these pKa values are large enough that salts of either H₂PO₄⁻ or HPO₄²⁻ can be prepared by adjusting the pH to a point midway between the corresponding pKa values.

Aqueous solutions of phosphoric acid up to 62.5% H₃PO₄ are eutectic, with freezing points as low as -85°C. Above 62.5%, the freezing point rises, reaching 21°C at 85% H₃PO₄ (the monohydrate). Beyond this concentration, the phase diagram becomes complex with significant local maxima and minima, which is why phosphoric acid is rarely sold at concentrations above 85%.