Calcium Tetroxide, CaO₄

By heating the octahydrate at a low temperature a yellow powder is obtained. On raising the temperature still further it becomes white again. Riesenfeld and Nottebohm regarded the yellow as a labile form of calcium peroxide, and the white as the stable form. Traube and Schulze, however, showed that the yellow compound is probably calcium tetroxide, the composition of which, they considered, is best expressed by the formula CaO₂.O₂, comparable with the polysulphides of calcium. It may be obtained by simply filtering and drying the monoperoxyhydrate, or better still, by heating the peroxide octahydrate with 30 per cent, hydrogen peroxide on the water-bath until evolution of oxygen ceases. It can also be prepared by digesting finely powdered calcium carbonate with hydrogen peroxide, or by merely heating calcium peroxide octahydrate with water at 100° C. When the octahydrate is dehydrated in vacuo it takes on a slightly yellow colour, probably indicating the formation of a little tetroxide. The latter cannot be formed by treating anhydrous calcium peroxide with aqueous hydrogen peroxide, and this leads to the conclusion that the octahydrate is necessary for its production, because only the di-hydrate is obtained by the action of water on the anhydrous peroxide. When ultra-violet light acts upon the peroxide moistened with hydrogen peroxide the tetroxide is rapidly formed. It does not appear to be obtained by means of any other oxidising agent.

The tetroxide forms a yellow powder insoluble in water, alcohol, and ether. When dry it is a comparatively stable substance and can be heated to 130° C. without change. At 270°-290° C. the peroxide is formed. On treatment with acid there is effervescence, due to the evolution of half the total oxygen in an " inactive " form - that is, it has not the oxidising properties of nascent oxygen - and hydrogen peroxide remains in solution, hence the formula CaO₂.O₂, comparable with those of the alkali tetroxides.