HYDROGEN DIOXIDE, PEROXIDE OF HY DROGEN, or OXYGENATED WATER, A colorless compound of hydrogen and oxygen containing, for the same amount of hydrogen, twice as much oxygen as water. It was discovered in 1818 by the French chemist Th(Mard. Minute quantities of it occur in the air, in rain-water, and in snow, and, according to some, in the juices of certain plants. lt, may be prepared by the action of cold diluted hydrochloric or sulphuric acid on hydrated barium peroxide. It can also he pre pared by the action of hydrofluoric, hydrofluo silicie, phosphoric, or compressed carbonic acid on barium peroxide; and it has been prepared on a somewhat large scale by the action of acids on the peroxide of sodium. By any of these methods the peroxide is obtained in the form of a dilute aqueous solution. This may be con centrated by allowing the water to freeze out, or by passing a current,of dry air through the solu tion kept at ordinary temperatures, or by allow ing it to evaporate in a vacuum over sulphuric acid. The anhydrous peroxide thus obtained is a colorless, odorless, oily liquid with a harsh, bitter, metallic taste. It remains unfrozen at —30° C.,and when heated to the boiling-point of water it decomposes with violence into oxygen and water. When brought in contact with the skin it causes a white blister, which after a time produces an irritating, itching sensation. It is a powerful bleaching agent and is largely used to remove color; it is particularly valuable for bleaching ostrich-feathers, bones, ivory, silver, wood, silk, cotton, etc. Dilute solutions of it under special trade names are employed to pro duce a light color in hair. It has also been used
to restore paintings that have become darkened by age. The peroxide is very useful in medicine and surgery as an antiseptic, rapidly destroying pus and similar discharges, in consequence of which it has been employed in diphtheria. Hydrogen dioxide has the power of freeing water from micro-organisms, which has led to its use in brewing. It also destroys the acid and mold ferments in the wort. if taken internally, it has the effect of improving digestion, and has there fore been used in certain forms of dyspepsia. Ordinarily the peroxide acts as a strong oxidiz ing agent, owing to the ease with which it parts with half of its oxygen. In certain eases, how ever. the same property has the effect of causing reduction. This namely. whenever the peroxide is brought into contact with substances like oxide of mercury, or ozone. which readily undergo decomposition. The reduction of such substances is accompanied by the evolution of much heat, great volumes of oxygen being given off by the peroxide itself as well as by the sub stances undergoing reduction. The peroxide itself is readily decomposed into water and free oxygen by mere contact with certain substances, e.g. gold. without these undergoing any change. Among the substances that have this effect on the peroxide are most alkalies, and hence the peroxide can only be preserved in slightly acid solution. Before using such a solution, a few drops of ammonia may be employed to neutralize the acid.