Intermediate Topic 651 of 1102 Inorganic Chemistry
Phosphoric Acid
A triprotic acid produced from phosphate rock for fertilizers.
In-depth explanation of Phosphoric Acid
Phosphoric acid, H3PO4, is a weak, nonvolatile triprotic acid with pKa values of about 2.1, 7.2 and 12.4. Most of it is made by the wet process, in which phosphate rock reacts with sulfuric acid to give phosphoric acid and insoluble calcium sulfate. The thermal process burns phosphorus to the pentoxide and dissolves it in water, giving a purer product for food use. It is the main raw material for phosphate fertilizers such as superphosphate. It is also used as an acidulant in cola drinks and in rust removers.
Going deeper: Phosphoric Acid
Key ideas: H3PO4 is triprotic with pKa values 2.15, 7.20 and 12.35, so it forms three series of salts: dihydrogen, hydrogen and normal phosphates. It is made by the wet process: Ca3(PO4)2 + 3H2SO4 gives 2H3PO4 + 3CaSO4, or by burning P4 to P4O10 and hydrating. Phosphate buffers work near pH 7.2 using H2PO4- and HPO4 2-. Unlike HNO3, it is nonoxidizing.
Common mistakes and connections: Students treat all three protons as equally acidic; successive Ka drops by about five orders of magnitude. Phosphorous acid H3PO3 is dibasic, because one H is bonded directly to P, a classic exam trap. Used in fertilizers, soft drinks and ATP chemistry. Links to buffers, polyprotic acids and fertilizers.
Visualization of Phosphoric Acid
Species distribution of a weak acid with pKa near 7.2 (the second ionization of H3PO4): the acid and conjugate base are equal at pH = pKa. The interactive lesson shows this as a live animation or chart.
Worked example and use case
Phosphate rock and sulfuric acid give phosphoric acid and gypsum: Ca3(PO4)2 + 3H2SO4 giving 2H3PO4 + 3CaSO4.
Frequently asked questions about Phosphoric Acid
What is Phosphoric Acid?
A triprotic acid produced from phosphate rock for fertilizers.
Is Phosphoric Acid a beginner, intermediate or advanced topic?
Phosphoric Acid is a intermediate topic in Inorganic Chemistry. It builds on the vocabulary of beginner topics in this area.
What should I learn before Phosphoric Acid?
Study these earlier topics in Inorganic Chemistry first: Oxides of Nitrogen, Nitric Acid, Phosphorus Allotropes.
What comes after Phosphoric Acid?
The next topics in Inorganic Chemistry are Fertilizers and Oxygen Family.
Can I practise Phosphoric Acid with quizzes?
Yes. The interactive lesson includes a self-check question on Phosphoric Acid, and the Inorganic Chemistry quiz draws on every topic in this area.
Where Phosphoric Acid fits in Inorganic Chemistry
Phosphoric Acid is topic 36 of 113 in Inorganic Chemistry.