oxoacids of phosphorus (phosphoric acid)

Phosphorus, like nitrogen, builds acids around itself with oxygen, but its oxoacids tell a different story. The headline one, phosphoric acid (H3PO4, phosphorus in the +5 state), is the tangy acid in cola drinks, a workhorse of the fertiliser and food industries, and the chemical backbone idea behind the phosphates that run the energy economy of every living cell. Alongside it sit phosphorous acid (H3PO3, phosphorus +3) and the curious hypophosphorous acid (H3PO2, phosphorus +1).

The structures hide a trap that the formulas alone do not reveal. In phosphoric acid, H3PO4, the central phosphorus is bonded to four oxygens; three carry O-H groups and one is a P=O, and all three hydrogens are ionisable, so it is a triprotic acid. But phosphorous acid, H3PO3, despite having three hydrogens written, is only diprotic: one of those hydrogens is bonded directly to phosphorus (a P-H bond) and is not acidic. Hypophosphorous acid, H3PO2, has two P-H bonds and is only monoprotic. The rule is simple once you see it: only hydrogens attached to oxygen (O-H) can be given up as protons; hydrogens bonded straight to phosphorus cannot. The lower oxoacids, with their P-H bonds, are also good reducing agents.

Phosphoric acid and its salts are everywhere. The acid is mass-produced for fertilisers (the phosphate that crops need), in soft drinks for tartness, in rust-conversion treatments, and in detergents. Its real biological significance comes through the phosphate linkages of DNA, RNA, and the energy currency ATP. A nice teaching point lives here too: counting acidic protons by looking at the structure, not the molecular formula, is exactly the kind of thinking inorganic chemistry rewards — and it punishes anyone who assumes every hydrogen is acidic.