Perchromic Acid, H₃CrO₈

Perchromic Acid, H₃CrO₈.2H₂O, may be prepared by the interaction at -30° C. of chromium trioxide and 97 per cent, hydrogen peroxide in methyl-ether solution. The reaction proceeds according to the equation

2CrO₃ + 7H₂O₂ = 2H₃CrO₈ + 4H₂O.

The blue ethereal solution is poured off from excess of either reagent, dried with phosphorus pentoxide and evaporated in vacuo at -30° C. The pure acid remains as a dark blue crystalline mass, which decomposes at a few degrees above -30° C.

The red perchromates, belonging to the first type, are formed by the action of hydrogen peroxide on well-cooled alkaline, aqueous solutions of chromates. Red ammonium perchromate, (NH₄)₃CrO₈, for example, may be prepared as a reddish-brown, well-crystallised salt by the interaction at a low temperature of 30 per cent, hydrogen peroxide and an ammoniacal solution of ammonium chromate. The corresponding sodium salt forms reddish-yellow, doubly refractive, glistening plates, while the potassium salt is darker in colour than the ammonium salt, which it resembles in other respects. These salts are readily reduced, even by moderately dilute alcohol, at ordinary temperatures. They are unstable in the dry state, and on heating sometimes evolve oxygen explosively.

Blue perchromates, which are referred to the second type, R^•H₂CrO₇ or R^•H₂CrO₅.H₂O₂, are formed by the interaction at a low temperature of 30 per cent, hydrogen peroxide with acid solutions of chromates; a modified method of preparation consists in the neutralisation of a cooled blue ethereal solution of perchromic acid by an alcoholic solution of the alkali. Similar to the red salts, the crystalline blue perchromates are unstable; in the moist condition they are even more so than the red variety, but both forms may best be preserved by keeping in an atmosphere saturated with aqueous vapour.