Phosphoric Acid from Burned Phosphorus: the Thermal Process

Burn phosphorus in air and it becomes a white smoke of phosphorus pentoxide. That oxide takes up water and becomes phosphoric acid, giving off heat. This is the thermal process that makes high-purity acid. Burn weighed white phosphorus in a closed chamber, add the white powder to cold water a little at a time, and bottle the acid, usually sold as an 85 % solution. The picture: our diagram, phosphorus to smoke to acid.