Schbnbein showed that the active principle of grass bleaching is ozonised oxygen, but the investigations of Schtfne, Houzeau, and Goppelsroder prove that ozone is not formed in the air during the bleaching process; moreover, all reactions hitherto attributed to ozone are due to hydrogen peroxide. Although grass bleaching is effective, it is very slow, involving much time and expense. It is therefore of great importance to use a more economical process. All chemical substances hitherto used as substitutes for atmospheric oxygen injure the vegetable tissue to be bleached, some more than others. Hydrogen peroxide, however, is perfectly inactive in this respect. It has found many applications in the arts. In 1870 it was brought into commerce as a bleach for hair, under the name of " golden hair water," and simultaneously attention was drawn to its medicinal properties. The difficulty of preparing it on a large scale has hindered its general utility. Gradually, however, this has been overcome, and hydrogen peroxide is at the present time a commercial product easily obtainable and at reasonable prices. It is used on a large scale for bleaching substances of animal origin. To effect this, all fat and adhering impurities must be removed.
The bleaching liquid is neutralised with ammonia, and during the operation all light should be excluded, and the temperature not raised above 77° F. (25° C). The bleaching of hair is effected by digesting in ammonium carbonate, washing, soaping, and again immersing in solution of ammonium carbonate to remove all fat. The hair is steeped in a bath of hydrogen peroxide neutralised with ammonia. As a bleach for feathers, hydrogen peroxide is said to supersede all other substances. This applies also to the bleaching of silk. It has further been used with great advantage for bleaching ivory and bone. All fatty substances must first be removed. As to the application of hydrogen peroxide in medicine, it is maintained that all fermentation due to an organised ferment is immediately and definitely arrested by hydrogen peroxide, the ferment is killed, and even after the removal of the hydrogen peroxide by one of the substances which destroy it most rapidly, the fermentation does not recommence. Brewers' yeast is in this manner killed instantly, although it possesses it-self the property of decomposing hydrogen peroxide.
Specimens of wine, urine, and milk, each containing a few drops of hydrogen peroxide, have been exposed for several months in open vessels without exhibiting the least sign of alteration, while other specimens of the same identical liquid, without the addition of hydrogen peroxide, placed beside them, were in a state of complete decomposition. Soluble ferments do not seem to be affected by hydrogen peroxide, as saliva, diastase, the gastric and panpreatic fluids continue to act in solutions containing it. For surgical application as an antiseptic dressing, in injections, vaporizations, and internally, it may well replace carbolic acid preparations; but for such purposes it must be entirely neutral, whereas that ordinarily in commerce frequently contains a notable quantity of sulphuric acid. As a disinfectant, hydrogen peroxide has the advantage of being free from odour, giving up oxygen, leaving pure water as a residue, and having no injurious action on the organism. Sauer has used it in dentistry for bleaching discoloured and carious teeth.
Hydrogen peroxide may be prepared (f) by the action of dilate hydrochloric acid on barium, dioxide, thus -
BaO2+2 HCl=H2O2+BaCl2, or (2) by passing a current of carbonic acid through water, and gradually adding barium dioxide in very small quantities, thus -
Ba02+C02+H20=H202+BaC08; or (3), which is the most generally adopted, by decomposing pure barium dioxide with dilute sulphuric acid, thus: -
Ba02+H,S04 = H202+BaS04. Watts gives the following account of this manufacture, taken from Thlnard: "Pure baryta is prepared by igniting, in a porcelain retort, nitrate of barium free from iron and manganese. The baryta, broken into pieces about the size of a nut, is then put into a coated glass tube and heated to low redness, while a current of oxygen gas, free from carbonic acid and dried by means of quicklime, is passed over it. For the first 8 minutes the gas is eagerly absorbed by the baryta. After it has begun to escape from the farther end of the tube (to which a gas delivery-tube passing under water is fitted), the stream is still kept up for the space of 10 or 15 minutes. The peroxide of barium obtained by this process is, after cooling, preserved in a bottle. In the next place 200 grm. of water are mixed with as much hydrochloric acid as will neutralize 15 grm. baryta. Into this liquid, contained in a cylinder, or, better, in a dish of silver or platinum kept cool by surrounding it with ice, 12 grm. peroxide of barium, slightly moistened and rubbed up in an agate mortar, are. introduced by. means of a wooden spatula: on agitating or stirring the liquid with the pestle, the whole, dissolves completely and without effervescence.
The baryta is next precipitated by sulphuric acid, added drop by drop till slightly, in excess: the presence of an excess of the acid may be known by the sulphate of barium falling down more quickly than before. 12 grm. more of the peroxide are then dissolved in the same liquid, and likewise precipitated by .sulphuric acid. The liquid, which now, contains hydrochloric acid, sulphuric acid, a large quantity of water, and a small quantity of peroxide of hydrogen, is next separated from the sulphate of barium by filtration, the precipitate washed with a little water, and the last wash-water retained for future washings. The filtrate is again mixed as above, twice with peroxide of barium, and twice with sulphuric acid. The filtration is then repeated', and the process continued in the same way, till 90 or 100 grm. of the peroxide are consumed. The liquid thus obtained would, on decomposition, yield 25 to 30 measures of oxygen gas. To separate silica, alumina, sesquioxide of iron, sesquioxide of manganese, etc, which proceed from the porcelain retort in which the nitrate of baryta was ignited, the liquid is mixed with concentrated solution of phosphoric acid (2 or 3 parts of phosphoric acid to 100 parts peroxide of barium), then surrounded with ice, and supersaturated, with pounded peroxide of barium: silica, and the phosphates of iron, manganese, and aluminium, then separate rapidly in flakes, and must be removed from the liquid by filtration through linen, and if necessary through paper.
The presence of a large quantity of sulphate of barium renders the nitration, difficult (if no phosphoric acid were present, the sesquioxides of iron and manganese would fall down by themselves, and give rise to a rapid evolu-lution of oxygen gas; but when they are mixed with phosphoric acid, they do not produce this effect). Should the liquid contain portions of these oxides,, they must be separated by the addition of a slight excess of baryta water; whereupon the liquid must be immediately and rapidly filtered through, several filters at once, and the filters squeezed between linen to get all out. The whole of the baryta must then be: separated by carefully adding sulphuric acid in very slight excess, and filtering. The filtrate now contains nothing but. water, peroxide of hydrogen, hydros-chloric acid, and a very little sulphuric acid. To separate the hydrochloric, acid, the liquid is surrounded with ice,, and mixed with sulphate of silver. In the first place, sulphate of silver, ob tained by heating nitrate of silver in contact with oil of vitriol in a platinum: crucible, is introduced in the form of powder into the liquid - the whole being, constantly stirred till the liquid becomes clear, a sign that the hydrochloric acid is wholly or nearly precipitated.: Any hydrochloric acid that may still remain must be separated by cautiously, adding more sulphate of silver.
If the latter has been added in excess, it must be precipitated by carefully dropping in a dilute solution of chloride of barium. The liquid should contain neither hydrochloric acid nor silver, and should therefore give no precipitate, either with solution of silver or with hydrochloric acid. The chloride of silver is separated by filtration and pressure, any portion of liquid which comes through turbid being filtered over again. To remove the sulphuric acid • also, and obtain a pure mixture of water and peroxide of hydrogen, the-liquid is placed in a glass mortar surrounded with ice, and rubbed up with slaked baryta previously pounded and diffused through water: the baryta is added till the sulphuric acid is very nearly saturated. The liquid is then filtered, the filter pressed between linen, and baryta water added in slight excess.
This often occasions the precipitation of oxide of iron and oxide of manganese, as well as sulphate of barium; hence the filtration must be rapidly performed. The excess of baryta is removed by cautiously adding dilute sulphuric acid, so that there may be rather a very slight excess of the acid than of the baryta. The whole of the sulphuric acid may likewise be removed by means of carbonate of barium obtained in a finely-divided state by precipitation, instead of by slaked baryta and baryta water. Finally, to separate the whole of the water, the vessel containing the liquid is placed in a dish containing oil of vitriol, and the whole is put under the receiver of an air-pump; the water then evaporates before the peroxide of hydrogen. The fluid is agitated from time to time. If it should deposit flakes of silica, which give rise to the escape of oxygen gas, it must be decanted off from them by means of a siphon; if it should evolve oxygen - which it will do as soon as it is so far concentrated as to contain about 250 times its volume of oxygen - 2 or 3 drops of sulphuric acid must be added to it.
The concentration must be stopped after a few days, when the liquid is brought to such a state that when decomposed it would evolve 475 volumes of oxygen gas; for this residue, if left longer in vacuo, would evaporate as a whole. The peroxide of hydrogen must be kept in long glass tubes closed with stoppers and surrounded with ice; but even under these circumstances it decomposes slowly and evolves oxygen gas."