GEN, FIXATION OF.) A triatomic form of hydrogen, H,, has been described by G. L.
Wendt and R. S. Landauer by electric discharge in the dry gas at low pressures; this is only stable for one minute and can only be obtained to the extent of 0.02%; it is very reactive chemically, reducing potassium permanganate solution or sulphur instantly, and condenses to a liquid when cooled by liquid oxygen. The existence of this active hydrogen is disputed by F. Panetti (1927).
Hydrogen combines with oxygen to produce water (q.v.) in the ratio of 2.00288 volumes to 1 (at o° C and 760° mm.) or 2.0154 parts to 16 by weight. A second oxide, hydrogen peroxide, is well known, and a third, ozonic acid, H204, has been described by A. Baeyer and V. Villiger (Berichte, 1902, 35, p. Hydrogen peroxide was discovered by L. J. Thenard in 1818; it occurs in small quantities in the atmosphere, probably owing to the decomposition of resins, turpentine, etc., in damp air and sunlight, for it is known to be formed in many chemical reactions involving very slow oxidation processes. (It seems pos sible that much of the so-called "ozone" in air is really hydrogen peroxide.) It is also formed in traces when metals undergo slow oxidation in air and water. When the oxy-hydrogen flame is directed on a block of ice or on solid carbon dioxide, the instan taneous cooling of the products of the high-temperature reaction allows no time for the decomposition of the hydrogen peroxide which is formed, and it may be detected in the products ; more over, it is possible that the peroxide is the primary product and that water is only a secondary product Hydrogen peroxide may be prepared, ad mixed with persulphuric acids (see below), by the electrolysis of 75% sulphuric acid; it is more usually prepared from barium peroxide or its hydrate, by adding them to a solution of hydrochloric, hydrofluosilicic, phosphoric, or, best, sulphuric acid, or to ice-cold water through which carbon dioxide is being bubbled; after filtration, the solution can be concentrated by freezing or by careful evaporation under reduced pressure.
Peroxide of 99.93% purity was obtained by 0. Maass and W. H. Hatcher by concentrating a 3% solution of ordinary purity to 30% in a special apparatus, then distilling it at 65° C under omm. pressure, whereby the content reached 85%, again concentrating in a special apparatus to 90%, and finally separating it by frac tional freezing. The pure product is not very explosive, but methods involving the extraction of the peroxide from its aqueous solutions by ether give rise to traces of organic peroxides which render the product highly explosive. The pure material boils at 69° C under 26mm. pressure, and freezes at —1.7° C, but it can be greatly supercooled without freezing ; the liquid has a specific gravity of 1.4633 at o° C, is very slightly more viscous than water, and is apparently associated to about the same extent as water, i.e., it may be or (H,02)3 (see ASSOCIATION), but in solution it has a normal molecular weight for The solid has a specific gravity of 1.644. The aqueous solution is practically neutral in its reaction, and readily decomposes on being warmed, giving off oxygen violently if heated too quickly ; it is sold as "10 volume," 20 volume, 4o volume, or as "perhydrol," the first three being about 3,6, or 12% solutions (and so named because they give up 10, 20 or 4o times their volume of oxygen), and the last 30% or Ioo volume. Pure aqueous solutions have a slight metallic taste, and, being slowly decomposed by traces of alkali from glass vessels, are better kept in waxed glass, or they may be stabilized by the addition of a mere trace of sulphuric acid or of certain organic substances, such as acetanilide. A stable solid compound with urea, has been patented ; it behaves exactly like hydrogen peroxide when dissolved in water.
Hydrogen peroxide frequently behaves as a powerful oxidizing agent; thus lead sulphide (black) is oxidized to the sulphate (white), and hence oil paintings, in which the "white lead" has become blackened by conversion to sulphide, are cleaned by treatment with dilute solutions of the peroxide. It converts hydroxides of the alkaline-earth metals to sparingly soluble per oxides of the type Ba02.8H2O, and with caustic potash or soda it gives perhydroxides, (these reactions may be re garded as evidence of its feebly acidic character) . It liberates iodine somewhat slowly from potassium iodide, converts ferrous salts to ferric, and oxidizes many sulphur compounds to sulphates.
It has been applied to some interesting oxidations in organic chem istry by H. J. H. Fenton (1900), who used it in the presence of ferrous sulphate as a catalyst, and also to the oxidation of cer tain sugars. On the other hand, some oxidizing agents undergo mutual reduction with hydrogen peroxide, giving free oxygen; thus, silver oxide gives the metal, and potassium permanganate is rapidly reduced in acid solution, A reaction of interest, in that it is neither an oxidation nor a reduc tion, is B. Radziszevski's hydrolysis of nitriles to amides by means of hydrogen peroxide, e.g., benzonitrile, gives benzamide, With concentrated sulphuric acid hydrogen peroxide sets up an equilibrium involving permonosul phuric acid (Caro's acid, see SULPHUR) : H2O2-{-H,SO4 ±H,SOd-H2O.
Hydrogen peroxide is catalytically decomposed by a great num ber of substances ; thus, platinum black or colloidal platinum readily effects decomposition, and many similar cases have been studied by G. Bredig and others. Its constitution may be repre sented as either HO.OH or 0 the latter being more probable (J. W. Briihl) ; from its production in numerous cases of slow oxidation, it would appear that the oxygen molecule is first absorbed as a whole and not as atoms. and that the unstable per oxides subsequently decompose to give hydrogen peroxide ; in fact, it is known that on exposure to air benzaldehyde gives perbenzoic acid as a primary product, and many such cases are recorded in organic chemistry, so a similar hypothesis is extended to inorganic chemistry. For further in formation, reference should be made to the works of J. H. Kastle and A. S. Loevenhart, Amer. Chem. J., pp. 397, 517; C. F. Schonbein, 1858-68; M. Traube, 1882-89; S. Tanatar, Berichte, 1903, 36, p. 1893 ; and H. E. Armstrong, Proc. Roy. Soc. (1886 et seq.).
The per-acids and their salts (e.g., perborates, percarbonates, persulphates) are discussed under the relevant headings (see BORAX, CARBONATES, SULPHUR, etc.), and hydrogen peroxide is readily obtainable from some of these. It may be mentioned that in a rational classification of super-oxides, those which give rise to hydrogen peroxide would be styled peroxides, and all others dioxides, t troxides, etc.
Hydrogen peroxide is used as a bleaching agent, where it is especially valuable because it leaves no harmful products, as an "antichlor" for removing excess of chlorine (or of sulphur diox ide) left after bleaching, as an antiseptic wash, and as a preserva tive; it also finds extensive use in analytical chemistry. It may be estimated by titration with potassium permanganate in acid solution (see above), or with ferricyanide in alkaline solution: or by addition to excess of arsenious acid in alkaline solution and titra tion of the excess by iodine. It may be recognized by the blue coloration it gives when added to a very dilute acidic solution of potassium dichromate; this is more marked if the solution is shaken with a little ether which extracts the coloured perchromic acid, HCr0,, or, more probably, HCr05. An orange-red colour, due to pertitanic acid, is similarly given with a solution of titanium dioxide in concentrated sulphuric acid. (A. D. M.)