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most advantageous for the purpose. This salt, made into a paste with water, is gradually added to the diluted and cooled acid, until the acid is nearly but not quite neutralised. The slight excess of acid is removed by the addition of the exact quantity of barium hydroxide (baryta-water) necessary to neutralise it, and the insoluble barium sulphate is removed by filtration. On a large scale silicofuoric acid or phosphoric acid is usually employed, preferably the latter, as it is found that small quantities of free phosphoric acid in hydrogen peroxide greatly retard its decomposition.

(4.) Hydrogen peroxide is also readily obtained by decomposing potassium peroxide by means of tartaric acid. The potassium peroxide is added to a cooled strong aqueous solution of tartaric acid, when potassium tartrate separates out, and an aqueous solution of hydrogen peroxide is obtained.

(5.) When small quantities of hydrogen peroxide are required for the purpose of illustrating its properties, it is most conveniently obtained by adding sodium peroxide to dilute and well-cooled hydrochloric acid, whereby sodium chloride and hydrogen peroxide are formed, both of which remain in solution

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(6.) Hydrogen peroxide is formed in considerable quantity when ozone is passed through ether floating upon water. Probably a peroxidised compound of ether is first produced, which is then decomposed by the water. This production of hydrogen peroxide may readily be demonstrated by placing a small quantity of water and ether in a beaker, and suspending into the vapour a spiral of platinumn wire which has been gently heated. The combustion of the ether vapour upon the wire, whereby the latter is maintained at a red heat, is attended with the formation of ozone, and this acting upon the ether, as already described, results in the production of hydrogen peroxide, which may be detected in solution in the water.

(7.) In small quantities, hydrogen peroxide is produced when moist ether is exposed to the action of oxygen, under the prolonged influence of sunlight.

Properties.-The dilute aqueous solution of hydrogen peroxide, obtained by the foregoing methods, is concentrated by evaporation over sulphuric acid in vacuo. In the pure condition it is a colourless and odourless, syrupy liquid, having an extremely bitter and

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metallic taste. The specific gravity of the liquid is 1.4532. The substance is extremely unstable, giving up some of its oxygen even at temperatures as low as – 20°, and decomposing with explosive violence when heated to 100°. Hydrogen peroxide bleaches organic colours, but less rapidly than chlorine. When placed upon the skin it destroys the colour, and gives rise to an irritating blister. When diluted with water, and especially if rendered acid, the compound is far more stable, and in this condition may be preserved at the ordinary temperature for a considerable length of time. When such an aqueous solution is strongly cooled, it deposits ice, and in this way, by the removal of the frozen water, the solution may be concentrated ; hydrogen peroxide itself remains liquid at – 30°. When heated the solution is decomposed into water and oxygen

H,0=H,0+0.

Owing to the readiness with which hydrogen peroxide gives up the half of its oxygen and is converted into water, its properties are generally those of a powerful oxidising agent. It liberates iodine from potassium iodide ; it converts sulphurous acid inte sulphuric acid, and oxidises lead sulphide into lead sulphate, ils action upon lead sulphide is made use of in restoring something of the original brilliancy to oil paintings that have become discoloured. The “white-lead” used in oil paints is gradually converted into lead sulphide when such paintings are exposed o air, especially the air of towns, which is liable to contain small quantities of sulphuretted hydrogen. Lead sulphide being black, the picture slowly assumes a uniformly dark colour, until it is finally quite black. When such a discoloured picture is washed over with dilute hydrogen peroxide, the black sulphide is oxidised into the white lead sulphate

PbS+4H,02=4H,0+ PbSO4 This compound is employed for bleaching articles that would suffer injury by the use of other bleaching agents, such as irons, feathers, and even the teeth.

Hydrogen peroxide is also capable of oxidising hydrogen when that element in the nascent condition is brought in contact with this compound. Thus, if a dilute acidulated solution of hydrogen peroxide be subjected to electrolysis, oxygen will be evolved from the positive electrode, but no gas will be disengaged at the

negative pole. The hydrogen, in the presence of the hydrogen peroxide, is oxidised into water

H,02+2H=2H,0. Hydrogen peroxide, in many of its reactions, appears to act as a deoxidising agent ; thus, manganese dioxide in contact with this substance is reduced to manganous oxide

MnO,+H,0,= MnO+O, +,0. Similarly, silver oxide is reduced to metallic silver with the evolution of oxygen

Ag2O+H,O,=Ag +0, + H,0. In like manner, when ozone is acted upon by hydrogen peroxide, a reaction takes place exactly analogous to that with silver oxide, which will be the more obvious if the formula for ozone be written 0.,O instead of 03, thus

0,0+H,02=0, +0, +H,O. Although, in a sense, these reactions may be regarded as reducing, or deoxidising, actions, in essence they are not different from those which have been given as illustrative of the oxidising power of hydrogen peroxide. It will be seen that they all depend upon the readiness with which the compound parts with an atom of oxygen, but that in these latter cases the oxygen that is so given up is engaged in oxidising another atom of oxygen, contained in the other compound. Thus, in the case of silver oxide, its atom of oxygen is oxidised by the liberated oxygen from the hydrogen peroxide, and converted into the complete molecule of oxygen. By these reactions Brodie first demonstrated the dual, or diatomic, character of the molecule of oxygen.

When hydrogen peroxide is added to a dilute acidulated solution of potassium dichromate, a deep azure-blue solution is obtained (see Chromium), which affords a delicate . test for this compound. To apply the test, the dilute hydrogen peroxide is shaken up with ether, and being soluble in this liquid, the ethereal layer which rises to the surface will contain nearly the whole of the peroxide ; a few drops of acidulated potassium dichromate are then added, and the mixture again shaken, when the ethereal liquid will separate as a blue layer. In this way, the presence of 0.00025 grarnmes of hydrogen peroxide in 20 c.c. of water can be detected.

Hydrogen peroxide is decomposed by contact with many substances which themselves do not combine with the oxygen ; thus charcoal, finely divided palladium, platinum, mercury, and notably silver, when brought into hydrogen peroxide, determine its decomposition into water and oxygen, the rapidity of the action being increased if the liquid be made alkaline. The action is doubtless catalytic, although in all cases the exact modus operandi is not clearly understood. In the case of silver it is believed that silver oxide (perhaps peroxide) is first formed, and then decomposed, thus

Ag,+H.0,=H,O+ Ag,0

Ag2O + H20,=H2O + O2 + Agz. When hydrogen peroxide is added to solutions of the hydroxides of barium, strontium, or calcium, the peroxide of the metal is precipitated

| Ba(HO), + H2O =2H2O+ BaO,. The compound is deposited in crystals having the composition Ba0,8H2O.

With the hydroxides of the alkali metals, the peroxide (which is soluble in water) may be precipitated by the addition of alcohol: when in the case of sodium peroxide, crystals are obtained of Na.O.,,8H,O.

Hydrogen peroxide is a useful antiseptic ; it possesses the ad. vantages of being free from smell, without poisonous or injurious action upon the system, and of leaving as a residue, after having furnished its available oxygen, only water.

CHAPTER IV

NITROGEN Symbol, N. Atomic weight = 14.04. Molecular weight = 28.08. History.--Nitrogen was discovered by Rutherford in 1772. He showed that when an animal is placed in a confined volume of air for some time, and the air afterwards treated with caustic potash, to absorb from it the carbon dioxide (" fixed air”), there still remained a gas which was incapable of supporting either respiration or combustion. He called the gas mephitic air. Scheele was the first to recognise that this gas was a constituent of the air. Lavoisier applied the name azote to the gas, to denote its inability to support life. The name nitrogen, signifying the nitre-producer, was suggested by Chaptal, from the fact that the gas was a constituent of nitre.

Occurrence.- In the free state nitrogen is present in the atmosphere, of which it forms about four-fifths. Certain nebulæ have been shown by spectroscopic observation to contain nitrogen in the uncombined condition. In combination, nitrogen is found in ammonia, in nitre (potassium nitrate), and in a great number of animal and vegetable compounds.

Modes of Formation.-(1.) Nitrogen is very readily obtained from the atmosphere by the abstraction of the oxygen with which it is there mixed.* This is conveniently done by burning a piece of phosphorus in air, confined over water. The phosphorus in burning combines with the oxygen, forming dense white fumes of phosphorus pentoxide, which gradually dissolve in the water, and nitrogen remains in the vessel. The nitrogen obtained in this way is never quite pure, for the phosphorus becomes extinguished before the oxygen is entirely removed ; and also the gas will contain atmospheric carbon dioxide. (2.) Nitrogen in a purer state can be prepared from the atmos

* Experiments 254, 255, " Chemical Lecture Experiments," new ed.

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