Phosphoric Acid — Frequently Asked Questions

Wet process acid is made by digesting phosphate rock with sulphuric acid and carries impurities from the ore, including fluoride, iron, aluminium and heavy metals. Thermal acid is made by burning elemental phosphorus and is far purer. Wet process acid is used for fertiliser and general industrial work; thermal or purified acid is required for food, pharmaceutical and electronic grades.

It reacts with iron oxide to form iron phosphate, a dark, adherent, insoluble layer that is bonded to the metal rather than sitting loose on it. That converted layer is stable and provides a key for paint, which is why phosphoric acid is used both in rust converter products and in the phosphating pretreatment applied to steel before painting.

Because it can give up three protons in sequence, with pKa values of roughly 2.1, 7.2 and 12.3. Neutralising the first gives dihydrogen phosphates such as MSP and MKP, the second gives hydrogen phosphates such as DSP and DKP, and the third gives the fully neutralised trisodium and tricalcium salts. The whole phosphate family is made by stopping the same neutralisation at different points.

Most commonly 75 or 85 percent, expressed as H3PO4. Both are viscous syrupy liquids. The concentration should be stated at enquiry along with the grade, since the required purity differs enormously between fertiliser, technical, food and electronic applications even at the same strength.