A modification of this process has lately been introduced, consisting in the treatment of tank waste liquor, or red liquors, with sulphurous acid obtained by the combustion of pyrites. The sulphurous gas is passed up a wrought-iron tower packed with coke, down which the liquors are run. This process yields a cheaper product, and is therefore upon the whole to be preferred. The older method has been abandoned on account of its high cost—caused principally by the heavy repairs of pans, &c., and the limited demand for the salt. About 10 tons per week are being made at the present time iu Liverpool, by the sulphurous acid process.
A third process that has never been worked upon any large scale, consists in reducing sulphate of soda to sulphide by calcination with coal, lixiviating the product and passing sulphurous acid gas into the settled solution. The sodic hyposulphite is then crystallized out by concentrating and cooling.
Iodide of Sodium. NaL—This interesting salt is contained in the mother liquors of kelp. Its preparation is similar to that of the corresponding potassium salt. It crystallizes from aqueous solutions at 20° iu anhydrous cakes. At lower temperatures, the crystals present the appearance of rhombic prisms, and contain two equivalents of water of crystallization. When heated, the hydrated crystals melt in their water of crystallization and become anhydrous. At a high temperature the salt volatilizes, giving off a portion of its iodine. The crystals are deliquescent, and readily soluble in water, 100 parts at 14° dissolving 173 parts of salt. The following table gives the different densities of various solutions at 19.5°.
Iodide of sodium forms a source of iodine. It is occasionally used in dyeing and plinting, but the colours formed are all more or less unstable.
Nitrate of Sodium. (Fn., Nitrate de Soude ; Grit, Wiirfelsalpeter.) Formula, NaNO„. Synonyms, nitre, cubic saltpetre, Chili, or Chili saltpetre.—Nitrate of sodium occurs very largely as a natural product, ehiefly in South America. Large beds of it, many feet in thickness, associated with gypsum, common salt, sulphate of sodium, &e., aro met with, extending 40 leagues at one stretch. Tho richer deposits lic about 2 ft. below the surface of the ground, and are covered with "costra," a hard rocky formation of somewhat similar constitution to the crude nitrate of soda below, and utilized to some extent in the manufacture. The nature of the salts, and remains of old shells,
point to a time when the land was submerged in the ocean, or some huge inland salt lake. From the method of deposition, it will bo readily understood that the constitution of the crude salt— " caliche "—varies widely. It has been found in the natural state as pure as the following :— 94.3 per cent. of nitrate of sodium; 2 per cent. of chloride; 0.2 sulphate of potassium ; 0.4 nitrate of potassium; 0.9 nitrate of magnesium ; 2 water, and 0.2 insoluble matter. About an average sample will test as follows :— Nitmte of sodium .. 43-00 Chloride of sodium .. 16.00Sulphate of sodium .. 26.00 Iodide of sodium .. 0.50 „ calcium .. 2.00 Insoluble .. •• • . • . 7'00 „ magnesium trent) Water 5.50 Nitrate of sodium crystallizes in obtuse rhombohedra, whence its name of cubic saltpetre. Its specific gravity is 2.24. It is hygroscopic, aud readily soluble in water ; 1.25 parts at 0° C., in 1.136 parts at 18-75°. According to Schiff the composition of solutions of various densities and at 20.2° is as follows :— Tho salt is also soluble in alcohol, the solubility decreasing rapidly witb an increase in the strength of the solvent. Thus spirit containing 10 parts of alcohol in 100 dissolves 39.5 parts of nitrate, but with 80 parts of alcohol only 2.7 parts of nitrate are found in 100 parts of spirit. At about 310°, nitrate of sodium fuses, solidifying upon cooling to a white MOSS. At a strung red heat, oxygen gas is given off and then free nitrogen, the salt being reduced first to nitrite. and finally to a mixture of protoxide and peroxide. It defiagmtes with charcoal aud other combustible bodies, but somewhat slowly.
Commercial nitrate of sodium is obtained from the caliche in the manner already described when treating of the potassiura salt—by lixiviation in boiling water, concentration, and crystallization. As usually brought into this market, it contains from 92 to 97 per cent. of nitrate, with from I to 2.k per cent. of sodium chloride, a little sulphate of soda—occasionally iodide and iodate—lime, magnesia, and water. To obtain really pure crystals, the commercial salt is crushed and heated with nitric acid. The chlorides present are thereby destroyed, and by dissolving and reerystalltziug perfectly pure nitrate separates out.