Potassium tetraperoxochromate(V) - K3Cr(O2)4
Potassium tetraperoxochromate (V) is one of the few compounds of chromium, in which the chromium is in the +5 oxidation state. Almost all chromium compounds either have chromium in the +3 oxidation state or the +6 oxidation state.
This chemical cannot be purchased commercially, it was synthesized from an ice cold concentrated solution of potassium chromate in excess potassium hydroxide, to which carefully an ice cold solution of hydrogen peroxide (30% by weight) was added. Crystals of K3Cr(O2)4, sometimes written as K3CrO8, slowly separate from the ice cold liquid.
This solid is sparingly soluble in water, giving yellow/brown solutions, which slowly decompose, giving oxygen. When the solution is acidified, then the ion [Cr(O2)4]3- quickly disproportionates, giving chromium (III) and deep blue/indigo CrO5, which contains chromium in oxidation state +6. This in turn decomposes, giving chromium (III) and oxygen.
Although solutions of this compound are not very stable, the solid can be stored indefinitely, as long as it is kept dry. The solid is not hygroscopic at all and hence keeps very well.