Gay Lussac investigated chloric acid; Stadion discovered perchloric acid, since more fully studied by G.
It can be prepared by the action of iodine on perchloric acid, or by boiling normal silver periodate with water: 2AgIO 4 +4H 2 O = Ag 2 H 3 10 6 +H10 4.2H 2 0.
The halogen of lower atomic weight can displace one of higher atomic weight from its hydrogen compound, or from the salt derived from such hydrogen compound, while, on the other hand, the halogen of higher atomic weight can displace that of lower atomic weight, from the halogen oxy-acids and their salts; thus iodine will liberate chlorine from potassium chlorate and also from perchloric acid.
Several oxy-acids of chlorine are known, namely, hypochlorous acid, HC10, chlorous acid, HC10 2 (in the form of its salts), chloric acid, HC10 3, and perchloric acid, HC10 4.
Further concentration leads to decomposition, with evolution of oxygen and formation of perchloric acid.
Perchloric acid is best prepared by distilling potassium perchlorate with concentrated sulphuric acid.
Roscoe, pure perchlo: is acid distils over at first, but if the distillation be continued a white crystalline mass of hydrated perchloric acid, HC104 H20, passes over; this is due to the decomposition of some of the acid into water and lower oxides of chlorine, the water produced then combining with the pure acid to produce the hydrated form.