Acetate of Sodium.—Sce Acetic Acid and salts.
Arsenate of Sodium.—This salt is used to a considerable extent in calico-printing, as a " dung" substitute. Formerly the dunging, or fixing of the mordants, was accomplished by passing the cloth, after being dried, through a bath of cow-dung, the alkaline quality of which neutralized the acid of the mordant, and promoted its fixation. The exceedingly variable quality of cow-dung, however, led to various attempts after substitutes, and those now most commonly used are the arsenate, arsenite, silicate, and phosphate of sodium, or a mixture of these salts. Arsenate of sodium is prepared by fusing white arsenic with sodium nitrate. The arsenious acid is first dissolved in caustic soda, and the arsenite formed. Nitrate of soda is then added, and the mixture heated in a reverberatory furnace to dryness, with evolution of ammonia and nitric oxide.
These dung substitutes are better in every way than the old material. They not only "cleanse " the cloth, but throw down the metallic base of the mordant, neutralizing the acid. -The arsenite enters into the composition of certain "ageing" liquors.
Messrs. Roberts, Dale, and Co. prepare alkaline arsenates by adding a solution of arsenic acid to nitrate of potassium or sodium.
Borates of Sodium.—See Borax.
Chloride of Sodium—See Salt.
Hyposulphite of Sodium.—This salt is an article in somewhat limited demand for paper-making, photographic and printing purposes. In paper-making it is used to discharge the bleach from the pulp, going by the name of " antichlor." In printing, it forms a red mordant with rnuriate of alumina, hyposulphite of alumina being produced. When the mordanted cloth is heated, the alumina is precipitated with liberation of sulphur and sulphurous acid.
There are several ways of procuring hyposulphite of soda. Very fine crystals may be obtained by passing sulphurous acid gas, well washed, into a strong solution of sodium carbonate, forming neutral sulphite of soda, and then digesting the solution with sulphur at a gentle heat. Upon a fairly large scale, the usual method of manufacture is the following :—Tank waste is exposed in small heaps to the influence of the air for about a fortnight, the heaps being turned over occasionally.
The sulphide of calcium is thereby converted into hyposulphite. The heaps are then thrown into small wooden tanks and treated with water. The dissolved hyposulphite of calcium is drawn off and agitated with a quantity of impure carbonate of soda—refuse soda ash, or the sweepings of the alkali houses, &c. The hyposulphite of calcium is thereby converted into hyposulphite of sodium, calcium carbonate being precipitated. The hypo-solutiou is pumped up into a wrought-iron pan about 15 ft. long, 3 ft. deep, and 5 ft. wide, heated by an underneath fire. Here it is concentrated and run into deep iron pans to cool. An impure sodium hyposulphite crystallizes out, which is removed and dissolved in the mother liquors of the succeeding crystallization. This solution is concentrated and again crystallized. The hyposulphite now separates out in clearer and almost colourless crystals. They are, however, redissolved, the solution concentrated in a small open pan, filtered through cloths, and run into small circular glass basins ranged upon long wooden benches. Here pure, colourless, transparent crystals form. The basins are then inclined so that the mother liquor falls into a spout running below the bench, and after being left for a while to drain, the crystals are removed to the drying house, heated by a hot air flue running underneath a series of sloping benches. Here they are dried at a gentle heat, and packed into casks holding from 1 to 5 cwt. If the final crystals dry with an opaque yellowish appearance, they are returned to the second dis solving stage. For paper-making purposes, a sufficiently good article may be produced from the second crystallization, in iron vessels. In place of the glass basins described, square wooden boxes lined with sheet indiarubber may be used, or small wrought-iron basins. Upon the whole, however, the glass crystallizers are the best.