Two oxides of sodium are known. Sodium monoxide, Na.0, may be prepared in several ways, but it is doubtful if it has yet been obtained in a state of absolute purity. It is formed when metallic sodium is oxidized in dry air or dry oxygen, at ordinary temperatures; but it is said that some small trace of moisture must be present, in order for the oxidation to pro ceed. This monoxide is a grayish solid, having a powerful affinity for water, with which it com bines to form sodium hydroxide (NaOH), as in dicated by the equation I•la.0 + H.0= 2NaOH ; the combination being attended by the develop ment of a considerable quantity of heat. The monoxide may also be prepared by heating dry sodium hydroxide with metallic sodium, its form ation in this case being attended by the liberation of hydrogen. Sodium peroxide, Nat°. (also called the "dioxide"), is of considerable com mercial importance, owing to its increasing use as a bleaching agent. It is prepared by passing a stream of dry air slowly over melted sodium at a temperature of about 570° F., the sodium being thereby converted into a mixture of the monoxide and peroxide. When all the sodium has been oxidized, the air current is replaced by a stream of dry oxygen gas, which transforms the monoxide that is present into peroxide. Sodium peroxide is soluble in water, but the solution readily decomposes with the formation of sodium hydroxide and the liberation of oxygen, as indicated by the equation Nak). 11,0= 2NaOH + 0.
The bleaching effect of the peroxide is due to this liberation of oxygen from the aqueous solu tion.
Sodium carbonate, Na,CO3, commonly known simply as "soda," or, in the impure form, as "soda ash," is one of the most important chem ical substances known. It was formerly pre pared mainly from the ashes of sea-plants, in the same way that carbonate of potassium is obtained from the ashes of land plants; but at the present time it is prepared almost exclu sively by chemical means, from common salt. For many years the only method in use for its manufacture was that invented by Leblanc, dur ing the first French Revolution, in the latter part of the 18th century. In 1838 the "ammonia" process was patented in England, but although this promised to be superior to the method of Leblanc, certain practical difficulties were encountered which proved to he fatal to its success, until they were overcome by E. Solvay, who erected the first commercially suc cessful "ammonia process" plant near Brussels, in 1861, and for whom the process itself has since been named. A certain proportion of the output of sodium carbonate is also manufactured by electrolysis; but the greater part is manu factured by the Solvay process though the Leblanc method is still used to a considerable extent.
Leblanc Process.— In the Leblanc method for the manufacture of sodium carbonate there are two successive operations to be performed. The first of these, which is known as the "salt cake" process, has for its object the transforma tion of chloride of sodium into sulphate of sodium. For this purpose the salt (sodium
chloride) is heated with sulphuric acid in large covered iron pans; hydrochloric acid being liberated in the course of the reaction, as indi cated by the. equation 2NaC1 H,S0,=Na2SO4-1- 2HCI. The hydrochloric acid vapors which are thus generated are passed through a "scrubbing tow er," which contains coke or brick, over which a stream of water is kept running, the hydro chloric acid dissolving in the water, and con stituting an important by-product. The crude sodium sulphate, Na,SO4, is the "salt-cake" from which this part of the process takes its name. Pure crystallized sodium sulphate (con taining 10 molecules of water of crystallization) constitutes the familiar substance known as °Glauber's salt," which is used in medicine as a saline purgative. The salt-cake, as obtained by the first operation, is subsequently treated by the *black ash" process, which is so called on account of the color of the immediate product that it yields. Ten parts of the salt-cake are mixed with 10 of limestone (calcium carbonate) and seven and a half of coke, and the mixture is heated in a reverberatory furnace, technically known as a "balling furnace." The carbon of the coal reduces the crude sodium sulphate to the form of sulphide of sodium, Na,S, in ac cordance with the equation Na,S0, 2C = Na2S 2CO2; while the sulphide, thus produced, is converted into sodium carbonate and calcium sulphide (CaS), by the limestone (CaCO3), in accord ance with the equation Na,S CaCO, = Na,CO, CaS.
In practice these two reactions go on to gether, and the final mixture of calcium sul phide, sodium carbonate and such impurities as may be present is the so-called "black-ash," or crude soda. Calcium sulphide is insoluble in water in the presence of lime, and hence it is easy to separate the sodium carbonate from the mass by solution and subsequent crystallization. "Crystallized soda," or "washing crystals," con sists of sodium carbonate crystallized with 10 molecules of water.
The Ammonia (or Solvay) Process.— The ammonia process for the manufacture of sodium carbonate depends upon the fact that when bicarbonate of ammonia, is added to a solution of common salt (NaCI), bicarbonate of sodium (HNaCO,) and sal ammoniac (NH.CI) are formed, as indicated by the equation NaCI H ( NH.) CO, = HNaCO, NH,C1. In practice, this operation is carried out by satu rating a solution of common salt with ammonia gas (NH,), and then passing carbon dioxide gas into the solution, under pressure. Bicarbon ate of ammonium is formed in the solution, but is immediately broken up with the formation of bicarbonate of sodium, as indicated by the foregoing equation. We may, therefore, regard the reaction as taking place directly in accord indicated by the equation