Sodium (sym. Na; equiv. 22.29; sp. grav. 0.973) is one of the metals of the alkalis, its oxide being soda. Its properties closely resemble those of the allied metal potassium. It is of a bluish-white colour, is somewhat more volatile than potassium, and further differs from that metal in having a higher fusing-point—about 208° (97° C.), a greater specific gravity, and in not catching fire when dropped in water (unless the water is heated), although, like potassium under similar conditions, it partially decomposes it and liberates hydrogen, and at the same time communicates a strong alkaline reaction to the solution. If, however, a piece of unsized paper is placed on the surface of cold water, and the sodium is placed on the paper, the metal takes fire and burns with a deep yellow flame. Strictly speaking, it is the liberated hydrogen rather than the metal which burns; but a little sodium, volatilised by the heat, burns with the hydrogen. When heated in the air it burns with its characteristic yellow flame, and is converted into soda. When exposed in vacuo to a red heat it assumes the form of vapour, and admits of distillation. Like potassium, it must be kept immersed in naphtha, so as to exclude the oxidising action of the air. As a reducing agent it is little inferior to potassium; and as its combining power is lower, and it is obtained much more cheaply, it may usually be advantageously substituted for potassium in reducing operations. Sodium does not occur in the metallic form in nature, but its compounds are very widely distributed. It is found by far the most abundantly in the form of chloride of sodium (or common salt), but it likewise occurs as albite or soda-felspar, cryolite (the double fluoride of sodium and aluminium), borax (the bitorate of soda), trona (the sesquicarbonate of soda), and Chili saltpetre (nitrate of soda). Dulhamel in 1736 discovered that potash and soda (now known to be the oxides of potassium and sodium) were distinct bodies. Sir H. Davy first obtained the metal Sodium in 1807. The symbol of this metal, Na, is the abbreviation of Natrium, which is derived from Natron, one of the old names of native carbonate of soda.
The methods of obtaining sodium are similar to those already described for obtaining potassium. Intimately mix 30 parts of common soda-ash with 13 parts of small coal and 3 parts of chalk, knead them into a stiff paste with oil, heat them in a covered iron pot till the oil is decomposed, and finally distil them in an iron retort with the precautions which are noticed in describing the preparation of potassium. The object of adding the chalk is to prevent the separation of the charcoal from the carbonate of soda when the latter fuses. This mixture ought to yield nearly one-seventh of its weight of sodium.
Sodium combines with all the elementary gaseous bodies, and two of these combinations, those with oxygen and chlorine, are of extreme importance and value. With oxygen sodium forms two compounds—an oxide, Na2O, and a peroxide, Na2O2. The latter is of no practical value. The oxide (soda) was formerly known as fossil or mineral alkali, to distinguish it from potash, which, from the source from which it was procured, was termed vegetable alkali. Anhydrous soda, Na2O, is procured by burning the metal in dry air; it is of a yellowish-white colour, powerfully attracts moisture, and retains the water so firmly that it cannot be expelled by heat. Hydrated or caustic soda, NaHO, closely resembles, both in its properties and in the mode of procuring it, the corresponding potash compound. It is, however, not so fusible as the latter, and is gradually converted, by exposure to the air, into carbonate of soda, which is also an infusible salt in its anhydrous state. Solution of hydrate of soda (or soda lye) is largely employed in the arts. It is prepared by boiling a tolerably strong solution of carbonate of soda in milk of lime until a portion of the filtrate ceases to effervesce on the addition of an acid. The solid hydrate has a specific gravity of 2.13, and the quantity of anhydrous soda in any solution may be closely approximated to by determining the specific gravity of the fluid and referring to a table indicating the strength corresponding to the specific gravity.
Many of the combinations of the oxide of sodium (soda) with acids—constituting soda-salts—are of great importance. Carbonic acid forms three salts with soda—a normal carbonate, a sesquicarbonate, and a bicarbonate of soda.
Carbonate of Soda, Na2CO3 + 10H2O, the Soda of commerce, is a colourless, inodorous salt, with a nauseous alkaline taste. It crystallises in large transparent rhomboidal prisms, which contain nearly 63 per cent. of water, but it readily parts with all this water on the application of heat. The crystals also lose the greater part of their water on mere exposure to the air, when they effloresce, and fall to powder. Water at 60° (15° C.) dissolves half its weight of the crystals, and boiling water considerably more, the solution acting like an alkali on vegetable colours. This salt, the natron of commerce, occurs native in the natron-lakes of Hungary, Armenia, &c., in association with sulphate of soda and chloride of sodium. In other regions it appears in an efflorescent form on the surface of the earth. It is now, however, almost entirely manufactured from sea-salt. For its manufacture, see SODA.
Sesquicarbonate of Soda, Na2CO3 + 2NaHCO3 + 3H2O, occurs native in the form of large, hard, non-efflorescent prisms, in Hungary, Egypt, Mexico, &c., under the name of Trona. When strongly heated it loses one-third of its carbonic acid, and becomes converted into the preceding salt.
Bicarbonate of Soda, NaHCO3, may be formed by passing a current of carbonic acid through a strong solution of carbonate of soda, till saturation takes place, and allowing the mixture to crystallise; or it may be produced on a large scale by exposing crystals of carbonate of soda to a prolonged current of carbonic acid. Most of the bicarbonate in commerce is now, however, prepared by the ammonia-soda process (see SODA). In this a current of carbonic acid gas is passed through a solution of salt in aqueous ammonia, when chloride of ammonium and bicarbonate of soda are produced. The bicarbonate crystallises in four-sided prisms, which require 10 parts of water at an ordinary temperature for their solution. This salt is used largely in medicine. See AERATED WATERS.
Sulphuric acid forms with soda a normal and an acid sulphate. The normal Sulphate of Soda, , has been already described under its synonym of Glauber's Salt (q.v.). The acid salt, or bisulphate of soda, , is of no special interest.
The Hyposulphite of Soda, , occurs in large colourless, striated, rhombic prisms, of a cooling and sweet taste. When strongly heated in the air it burns with a blue flame. It dissolves readily in water, depositing sulphur if the solution be kept in a closed vessel. It may be obtained by digesting a solution of sulphite of soda with powdered sulphur. The sulphur is gradually dissolved, and forms a colourless solution, which, on evaporation, yields crystals of hyposulphite of soda. This salt is largely employed in photography, and is occasionally prescribed medicinally. Sulphurous acid forms two salts with soda—viz. a sulphite and a bisulphite. The Sulphite of Soda, , is obtained by passing sulphurous acid over carbonate of soda, dissolving the resulting mass in water, and crystallising; when the salt is obtained in efflorescent oblique prisms, which are soluble in 4 parts of cold water, the solution having a slightly alkaline reaction and a sulphurous taste. This compound was at one time commercially known as Antichlorine, and was largely used in paper-manufactories for the purpose of removing the last trace of chlorine from the bleached rag-pulp. The term is now applied only to the hyposulphite, which is both cheaper and more efficacious. The Bisulphite is of no importance. Nitrate of Soda, , known also as Cubie Nitre or Chili Saltpetre, occurs as a natural product on the surface of the soil of certain South American districts. In most of its properties, excepting its crystalline form, and further in its being deliquescent, it resembles nitrate of potash. It is used as a Manure (q.v.). The Phosphates of Soda are comparatively numerous. Hypo-chlorite of Soda, , is at present known only in solution, in which it occurs as a yellowish-green fluid, evolving a smell of chlorine; it has strong bleaching power, and when boiled becomes de-colourised, and evolves chlorine freely. It is formed by passing a stream of chlorine gas through a solution of carbonate of soda, the resulting solution containing the hypochlorite, together with undecomposed carbonate of soda and chloride of sodium. This solution is useful as a bleaching agent, as an oxidising agent in analytical chemistry, and as a disinfectant agent. There are two Borates of Soda, of which the only important one, the Biborate, is already described under its ordinary name of Borax (q.v.). Various Silicates of Soda have been formed (see SILICON, GLASS, SLAGS).
The Haloid Salts of sodium resemble, in their general characters, the corresponding salts of potash. Of these by far the most important is Chloride of Sodium or Common Salt, formerly known as Muriate of Soda, . It occurs naturally in far greater quantity than any other soluble salt, and is fully described at SALT. The other haloid salts—the iodide, bromide, and fluoride of sodium—require no notice.
Sodium has been recently found to enter into various groups of organic bodies—the sodium-alcohols for example. When sodium or potassium is gradually added to anhydrous alcohol the temperature rapidly rises, the metal is dissolved, hydrogen is evolved, and a fusible deliquescent compound is formed, which has received the name of Sodium-alcohol (or potassium-alcohol), or of ethylate of soda (or potash), its composition being such that it may be regarded as alcohol in which one atom of hydrogen is replaced by one of the metal.
The tests for the salts of sodium are not very satisfactory, because the metal forms scarcely any insoluble compounds. A salt of sodium is usually concluded to be present when, the absence of all other bases having been proved, a saline residue remains, which, with bichloride of platinum, yields yellow striated prisms by spontaneous evaporation. Before the blowpipe the salts of sodium are known by the intense yellow which they communicate to the outer flame, and if a weak alcoholic solution of one of the salts is burned a similar yellow tint is communicated to the flame. Spectrum analysis is too delicate to be of much practical use.
The medicinal uses of the sodium compounds may be considered alphabetically. Acetate of Soda is a mild diuretic, similar in operation to acetate of potash, for which it may be substituted. Arsenate of Soda is serviceable in periodic affections, chronic skin diseases, and the cases in which arsenic is generally employed in medicine. Paper impregnated with a solution of arsenate of soda sweetened with sugar is sold as a poison for flies. Biborate of Soda, or Borax, is employed principally as a topical astringent, and is used with advantage in aphthous eruptions of the mouth and throat. Bicarbonate of Soda is a most popular remedy in cases of dyspepsia, but its use is highly injurious when there are phosphatic deposits in the urine. Carbonate of Soda is not employed as an antacid so frequently as the bicarbonate, in consequence of its disagreeable taste; but in the dried state, when deprived by heat of its water of crystallisation, it is much used as an alternative. In dyspepsia attended with acidity a combination of the dried carbonate with blue pill and rhubarb pill is often extremely useful. As it has a very acrid taste, it should be combined, if given in powder, with some bland substance, such as Compound Tragacanth Powder. A solution of Chlorinated Soda is preferable to hypochlorite of lime in destroying noxious effluvia, as the salt which is left does not deliquesce. Phosphate of Soda, known also as Tasteless Purging Salt, is a mild saline purgative, with a far less unpleasant taste than sulphate of magnesia. Sulphate of Soda and Tartrate of Soda and Potash have been already described under their ordinary names of Glauber's Salt (q.v.) and Rochelle Salt (q.v.).