Phosphoric Acid in Wet Process for Industrial Grade

Phosphoric Acid

Chemical Property

Phosphoric acid is a colorless, odorless phosphorus-containing solid, and inorganic compound with the chemical formula H3PO4. It is commonly encountered as an 85% aqueous solution, which is a colourless, odourless, and non-volatile syrupy liquid. In aqueous solution phosphoric acid behaves as a triprotic acid.

H3PO4 ⇌ H2PO−4 + H+, pKa1 = 2.14

H2PO−4 ⇌ HPO2−4 + H+, pKa2 = 7.20

HPO2−4 ⇌ PO3−4 + H+, pKa3 = 12.37

The difference between successive pKa values is sufficiently large so that salts of either monohydrogen phosphate, HPO2−4 or dihydrogen phosphate, H2PO−4, can be prepared from a solution of phosphoric acid by adjusting the pH to be mid-way between the respective pK values.

Wet Process

In the wet process, a phosphate-containing mineral such as calcium hydroxyapatite and fluorapatite are treated with sulfuric acid.

Ca5(PO4)3OH + 5 H2SO4 → 3 H3PO4 + 5 CaSO4 + H2O

Ca5(PO4)3F + 5 H2SO4 → 3 H3PO4 + 5 CaSO4 + HF

Calcium sulfate (gypsum, CaSO4) is a by-product, which is removed as phosphogypsum. The hydrogen fluoride (HF) gas is streamed into a wet (water) scrubber producing hydrofluoric acid. In both cases the phosphoric acid solution usually contains 23–33% P2O5 (32–46% H3PO4). It may be concentrated to produce commercial- or merchant-grade phosphoric acid, which contains about 54–62% P2O5 (75–85% H3PO4). Further removal of water yields superphosphoric acid with a P2O5 concentration above 70% (corresponding to nearly 100% H3PO4). The phosphoric acid from both processes may be further purified by removing compounds of arsenic and other potentially toxic impurities.

Application

Specific applications of phosphoric acid industrial grade include:

In anti-rust treatment by phosphate conversion coating or passivation;

To prevent iron oxidation by means of the Parkerization process;

As an external standard for phosphorus-31 nuclear magnetic resonance;

In phosphoric acid fuel cells;

In activated carbon production;

In compound semiconductor processing, to etch Indium gallium; Arsenide selectively with respect to indium phosphide;

In microfabrication to etch silicon nitride selectively with respect to silicon dioxide;

In microfabrication to etch aluminum;

As a pH adjuster in cosmetics and skin-care products;

As a sanitizing agent in the dairy, food, and brewing industries.