# 1911 Encyclopædia Britannica/Ozone

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​OZONE, allotropic oxygen, O$_3$. The first recorded observations of the substance are due to Van Marum (1785), who found that oxygen gas through which a stream of electric sparks had been passed, tarnished mercury and emitted a peculiar smell. In 1840 C. F. Schonbein (Pogg. Ann. 50, p. 616) showed that this substance was also present in the oxygen liberated during the electrolysis of acidulated water, and gave it the name ozone (Gr. ὄζειν, to smell). Ozone mixed with an excess of oxygen is obtained by submitting dry oxygen to the silent electric discharge [at the temperature of liquid air, E. Briner and E. Durand (Comptes rendus, 1907, 145, p. 1272) obtained a 90% yield]; by the action of fluorine on water at 0° C. (H. Moissan, Comptes rendus, 1899, 129, p. 570); by the action of concentrated sulphuric acid or barium peroxide or on other peroxides and salts of peracids (A. v. Baeyer and V. Villiger, Ber. 1901, 34, p. 355); by passing oxygen over some heated metallic oxides, and by distilling potassium permanganate with concentrated sulphuric acid in vacuo. It is also formed during many processes of slow oxidation. For a description of the various forms of ozonizers used on the large scale see N. Otto, Rev. gén. de chemie pure et appliquée, 1900, ii. p. 405; W. Elworthy, Elekt. Zeits., 1904, ii. p. i), and H. Guilleminot (Comptes rendus, 1903, 136, p. 1653). Ozone is also produced by the action of cathode and ultra-violet rays on oxygen. These methods of preparation give an ozone diluted with a considerable amount of unaltered oxygen; A. Ladenburg (Ber. 1898, 31, pp. 2508, 2830) succeeded in liquefying ozonized oxygen with liquid air and then by fractional evaporation obtained a liquid containing between 80 and 90% of ozone.
Ozone is a colourless gas which possesses a characteristic smell. When strongly cooled it condenses to an indigo blue liquid which is extremely explosive (see Liquid Gases). In ozonizing oxygen the volume of the gas diminishes, but if the gas be heated to about 300° C, it returns to its original volume and is found to be nothing but oxygen. The same change of ozone into oxygen may be brought about by contact with platinum black and other substances. Ozone is only very slightly soluble in water. It is a most powerful oxidizing agent, which rapidly attacks organic matter (hence in preparing the gas, rubber connexions must not be used, since they are instantly destroyed), bleaches vegetable colouring matters and acts rapidly on most metals. It liberates iodine from solutions of potassium iodide, the reaction in neutral solution proceeding thus: O$_3$+2KI+H$_2$O＝O$_2$+I$_2$+2KHO, whilst in acid solution the decomposition takes the following course: 4O$_3$+10HI＝5I$_2$+H$_2$O$_2$+4H$_2$O+3O$_2$ (A. Ladenburg, Ber. 1901, 34, p. 1184). Ozone is decomposed by some metallic oxides, with regeneration of oxygen. It combines with many unsaturated carbon compounds to form ozonides (C. Harries, Ber. 1904, 37, pp. 839 et seq.).
The constitution of ozone has been determined by J. L. Soret (Ann. chim. phys., 1866 [4], 7, p. 113; 186S [4], 13, p. 257), who showed that the diminution in volume when ozone is absorbed from ozonized oxygen by means of oil of turpentine is twice as great as the increase in volume observed when ozone is reconverted into oxygen on heating. This points to the gas possessing the molecular formula O$_3$. Confirmation was obtained by comparing the rate of diffusion of ozone with that of chlorine, which gave 24·8 as the value for the density of ozone, consequently the molecular formula must be O$_3$ (cf. B. C. Brodie, Phil. Trans., 1872, pt. ii. p. 435). More recently A. Ladenburg (Ber. 1901, 34, p. 631) has obtained as a mean value for the molecular weight the number 47·78, which corresponds with the above molecular formula. Ozone is used largely for sterilizing water.