Calcium Peroxide, CaO₂

Calcium peroxide, CaO₂, was first obtained in the hydrated form, CaO₂.8H₂O, by the action of hydrogen peroxide on lime-water. The anhydrous compound may be prepared by gently heating this octahydrate, or by drying over phosphorus pentoxide in a desiccator. Dissociation begins, however, before dehydration is complete. By precipitation from very concentrated solutions near 0° C., and even from very dilute solutions above 40° C., the anhydrous peroxide may be obtained without the intermediate formation of the hydrate.

Jaubert took out patents for the production of calcium peroxide, or " Bicalzit," on a commercial scale by compressing a mixture of 78 parts of sodium peroxide with 74 parts of dry hydrated lime into cylinders of 500 grams each. These, by subsequent treatment with iced water, gave a precipitate of calcium peroxide which was washed and dried, first at ordinary temperatures, and finally at 100° C.

According to Vaubel, when dry calcium hydroxide is exposed to the air for some days, a small amount of peroxide is formed. To this he ascribed the corrosive action of lime mortar on lead, and the disinfecting power of slaked lime. Ditz contradicts this.

The molecular heat of formation of calcium peroxide from the oxide and oxygen is 5.43 Cal., or from its elements, 157.37 Cal. When dry it may be heated up to 200° C. without decomposition. Riesenfeld and Nottebohm studied the variation of dissociation pressure with temperature. They observed no appreciable decomposition below 220° C. Up to 273° C. the decomposition was slow, but above that temperature it became rapid. At 255° C. the pressure was more than 190 atmospheres. They concluded that the production of calcium peroxide by the direct action of oxygen on lime would require pressures unattainable in practice.

Calcium peroxide is very little soluble in water, which decomposes it, and insoluble in alcohol and ether. Complete decomposition in aqueous solution is only effected after boiling for several hours.

Commercial calcium peroxide without water of crystallisation contains 60 per cent, peroxide, the rest being lime.

The production of Calcium, Peroxide Octahydrate, CaO₂.8H₂O, by Thenard has already been mentioned. It is a white crystalline body, and may also be obtained by the action of sodium peroxide on a solution of a calcium salt, or by pouring an excess of lime-water into a solution of sodium peroxide slightly acidified with nitric acid. De Forcrand stated that the precipitated octahydrate always contains lime unless more than two molecules of hydrogen peroxide to one of lime are present. The molecular heat of formation of the octahydrate is 24.666 Cal.

De Forcrand obtained the Calcium Peroxide Dihydrate, CaO₂.2H₂O, by precipitation with hydrogen peroxide above 20° C., but considered that its formula should be Ca(OH)₂.H₂O₂. According to von Foregger and Philipp, the dihydrate is formed when commercial calcium peroxide is added to water, but it then undergoes intramolecular change,

CaO₂.2H₂O = Ca(OH)₂.H₂O₂.

De Forcrand gave 6.815 Cal. as the molecular heat of formation from the anhydrous peroxide.

Calcium peroxide may be employed with sulphuric acid for bleaching cotton-seed oil. Von Foregger and Philipp suggested that it might prove a useful sterilising agent for milk. Freyssing and Roche proposed the use of " Bicalzit " to sterilise drinking water. The action is not very rapid, but is complete after two or three hours. According to Hetscli, under certain conditions it acts more energetically than hydrogen peroxide, and 0.5 grm. per litre is fatal to typhoid bacilli.