Oxygen

OXYGEN, a colourless, odourless, tasteless gas, somewhat heavier than air (sp.gr. 1.10523) ; it is slightly soluble in water. and this dissolved oxygen plays an essential part in the respiration of aquatic animals. (Symbol 0, atomic number 8, atomic weight 16.000.) Oxygen combines with nearly all the elements under suit able conditions, but these reactions often do not take place if the substances are perfectly dry. With the exception of the "noble" metals, gold and the platinum group (q.v.), the metallic elements all oxidize when heated in oxygen, although molten silver dissolves 22 volumes of the gas without combining chemically with it, the absorbed oxygen escaping again when the metal cools.

Oxygen is by far the most abundant element, being nearly equal in amount to all the others put together. It forms 21% by volume of the atmosphere, eight-ninths by weight of water, and nearly one-half by weight of all the rocks composing the half mile crust of the earth. Growing plants absorb carbon dioxide (q.v.), assimilating carbon and rejecting oxygen. It is the only gas able to support respiration.

Preparation and Properties.

It is readily prepared on a small scale by heating a mixture of potassium chlorate with about one-third of its weight of manganese dioxide, which facilitates the decomposition. From 1886, for about 20 years, oxygen was pro duced commercially by the Brin or barium oxide process. Barium monoxide was heated to 1,000° F and absorbed oxygen from the air to form the dioxide. At 1,600° the oxygen thus absorbed was again set free, the monoxide being regenerated. In later methods the temperature was maintained constant at about 1,200° F, and change of pressure was relied upon for determining the respective phases of oxidation and deoxidation. The purity obtainable, how ever, was not more than 94%.

In 1902 Linde applied the process of rectification to liquid air and was able to extract oxygen of 98.5% purity in economical yield. Air is freed of its carbon dioxide, compressed, cooled and then expanded, either in an engine doing external work or merely through a valve to perform internal molecular work; in both cases heat is lost and the temperature further lowered. This cool

ing effect is made cumulative by means of heat interchangers, tu bular metal devices by which the cold gases after expansion are made to travel in indirect contact with and in counter-current to the incoming uncooled compressed air, so that the latter be comes continually colder at the point of expansion, until a limit is set by liquefaction. The liquid so produced is made to flow down a rectification column, fitted with numerous plates fashioned to ensure its intimate contact with ascending gas, which is produced at the base of the column by evaporating the liquid as it arrives there. When equilibrium is attained, almost pure oxygen can be drawn from the bottom of the column in gaseous or liquid form. Improvements in construction introduced in 1926 enable the separation of the nitrogen and oxygen of air to be effected almost completely, the oxygen purity being 99.5%, and the nitrogen over 99%.

Oxygen is also a by-product in hydrogen manufacture by the electrolysis of water. The much greater power consumption, however, prevents this process competing with the liquid-air process for oxygen production only. The gas is supplied in steel cylinders at a pressure of 120-150 atmospheres. During 1926 the world's production of oxygen, in cylinders, was about 5,000 million cubic feet. The estimation of oxygen in a gas is most conve niently made by means of an alkaline solution of pyrogallol, by metallic copper in conjunction with an ammoniacal solution of ammonium carbonate, by sticks of phosphorus, or by a weakly alkaline solution of sodium hydrosulphite.

Oxygen condenses to a pale blue magnetic liquid, boiling at —182.9° C. By rapid evaporation and consequent cooling, or by cooling in liquid hydrogen, oxygen forms a bluish-white solid, melting at —259° C. When gaseous oxygen is acted on by the silent electric discharge, an unstable modification, ozone (q.v.) is produced.