Sodium

Sodium [[[symbol]] Na, from Lat. natrium; atomic weight 23.00 (O= r6)], a chemical element belonging to the group of alkali metals. It is abundantly and widely diffused in nature, but always in combination. Sodium chloride, or common salt, is exceedingly common, being the chief salt present in sea-water, besides occurring in extensive stratified deposits. Sodium carbonates are also widely dispersed in nature, forming constituents of many mineral waters, and occurring as principal saline components in natron or trona lakes, as efflorescences in Lower Egypt, Persia and China, and as urao in Mexico, Colombia and Venezuela. The solid crusts found at the bottom of the salt lakes of the Araxes plain in Armenia contain about 16% of carbonate and 80 of sulphate. In Colombia there occurs a double salt, Na 2 CO 3 CaCO 3.5H 2 O, known as gay-lussite. In Wyoming, California and Nevada enormous deposits of carbonates, mixed in some cases with sulphate and with chloride, occur. About Szegedin in Hungary and all over the vast pusztas (steppes) between the Theiss and the Danube, and from the Theiss up to and beyond Debreczin, the soil contains sodium carbonate, which frequently assumes the form of crude alkaline crusts, called "szekso," and of small saline ponds. A purified specimen of such Debreczin soda was found to contain as much as 90% of real carbonate, NaCO 3, and 4 of common salt. Natural sulphate occurs in an anhydrous condition as thenardite, Na2SO 4, at Tarapaca, Chile, and in the rock-salt deposits at Espartinas near Aranjuez, Spain. Hydrated sulphates occur at several localities in the province of Madrid and in other provinces of Spain, and at Miihlingen in Aargau, and copious deposits of glauberite, the double sulphate of sodium and calcium, are met with in the salt-mines of Villarrubia in Spain, at Stassfurt, and in the province of Tarapaca, Chile, &c. A native nitrate of soda is obtained in great abundance in the district of Atacama and the province of Tarapaca, and is imported into Europe in enormous quantities as cubic nitre for the preparation of saltpetre. Cryolite, a fluoride of aluminium and sodium, is extensively mined in Greenland and elsewhere for industrial purposes. These form the principal natural sources of sodium compounds - the chloride as rock salt and in sea-water being of, such predominating importance as quite to outweigh all the others. But it is questionable whether, taken altogether, the mass of sodium they represent is as much as that disseminated throughout the rocky crust in the form of soda felspar (i.e. as silicate of soda) and in other soda-containing rocks. From this source all soils contain small proportions of sodium in soluble forms, hence the ashes of plants, although they preferably imbibe potassium salts, contain traces and sometimes notable quantities of sodium salts. Sodium salts also form essential ingredients in all animal juices.

Although many sodium compounds have been known from very remote times, the element was not isolated until 1807, when Sir H. Davy obtained it by electrolysing caustic soda. This method was followed by that proposed by Gay-Lussac and Thenard, who decomposed molten caustic soda with red-hot iron; and this in turn was succeeded by Brunner's process of igniting sodium carbonate with charcoal. Deville made many improvements, but the method remained wasteful and uneconomical, and in 1872 the metal cost 4s. a pound. In 1886, however, Castner replaced the carbonate by caustic soda, and materially cheapened the cost of production; but this method was discarded for an electrolytic one, patented by Castner in 1890. Electrolytic processes had, in fact, been considered since 1851, when Charles Watt patented his method for the production of sodium and potassium from fused chlorides. Among the difficulties here to be contended with are the destructive action of fused chlorides and of the reduced alkali metals upon most non-metallic substances available for the containing vessel and its partition, and also of the anode chlorine upon metals; also the low fusing-point (95° C. for sodium, and 62° C. for potassium) and the low specific gravity of the metals, so that the separated metal floats as a fused layer upon the top of the melted salt. Again, pure sodium chloride melts at about 775° C., while sodium boils at 877° C., so that the margin of safety is but small if loss by vaporization is to be prevented. Borchers endeavoured to contend against the first difficulty by employing an iron cathode vessel and a chamotte (fire-clay) anode chamber united by a specially constructed water-cooled joint. The other difficulty is to some extent met by using mixed chlorides (e.g. sodium, potassium and strontium chlorides for sodium extraction), as these melt at a lower temperature than the pure chloride. In Castner's process (as employed at Oldbury and Niagara Falls and in Germany) fused caustic soda is electrolysed. The apparatus described in the patent specification is an iron cylinder heated by gas rings below, with a narrower cylinder beneath, through which passes upwards a stout iron cathode rod cemented in place by caustic soda solidified in the narrower vessel. Iron anodes are suspended around the cathode, and between the two is a cylinder of iron gauze at the bottom with a sheet-iron continuation above, the latter being provided with a movable cover. During electrolysis, oxygen is evolved at the anode and escapes from the outer vessel, while the sodium deposited in globules on the cathode floats upwards into the iron cylinder, within which it accumulates, and from which it may be removed at intervals by means of a perforated iron ladle, the fused salt, but not the metal, being able to pass freely through the perforations. The sodium is then cast into moulds. Sodium hydroxide has certain advantages compared with chloride, although it is more costly; its fusing-point is only 320° C., and no anode chlorine is produced, so that both containing vessel and anode may be of iron, and no porous partition is necessary.

Metallic sodium possesses a silvery lustre, but on exposure to moist air the surface is rapidly dulled by a layer of the hydroxide. It may be obtained crystallized in the quadratic system by melting in a sealed tube containing hydrogen, allowed to cool partially, and then pouring off the still liquid portion by inverting the tube. The specific gravity is 0.9735 at 13.5° (Baumhauer). At ordinary temperatures the metal has the consistency of wax and can be readily cut; on cooling it hardens. On heating it melts at 95.6° (Bunsen) to a liquid resembling mercury, and boils at 877.5° (Ruff and Johannsen, Ber., 1905, 38, p. 3601), yielding a vapour, colourless in thin layers but a peculiar purple, with a greenish fluorescence, when viewed through thick layers. (For the optics of sodium vapour see R. W. Wood, Physical Optics.) According to A. Matthiessen, sodium ranks fourth to silver, copper and gold as a conductor of electricity and heat, and according to Bunsen it is the most electropositive metal with the exception of caesium, rubidium and potassium.

The metal is very reactive chemically. Exposed to moist air it rapidly oxidizes to the hydroxide; and it burns on heating in air with a yellow flame, yielding the monoxide and dioxide. A fragment thrown on the surface of water rapidly disengages hydrogen, which gas, however, does not inflame, as happens with potassium; but inflammation occurs if hot water be used, or if the metal be dropped on moist filter paper. Sodium also combines directly, sometimes very energetically, with most non-metallic elements. It also combines with dry ammonia at 300-400° to form sodamide, NaNH 2, a white waxy mass when pure, which melts at 155°. Heated in a current of carbon dioxide sodamide yields caustic soda and cyanamide, and with nitrous oxide it gives sodium azoimide; it deflagrates with lead or silver nitrate and explodes with potassium chlorate. Sodamide was introduced by Claisen (Ber., 1905, 3 8, p. 6 93) as a condensing agent in organic chemistry, and has since been applied in many directions. Sodium is largely employed in the manufacture of cyanides and in reduction processes leading to the isolation of such elements as magnesium, silicon, boron, aluminium (formerly), &c.; it also finds application in organic chemistry. With potassium it forms a liquid alloy resembling mercury, which has been employed in high temperature thermometers (see Thermometry).

Compounds. In its chemical combinations sodium is usually monovalent; its salts are generally soluble in water, the least soluble being the metantimonate.

Sodium hydride, NaH, is a crystalline substance obtained directly from sodium and hydrogen at about 400°. It burns when heated in dry air, and ignites in moist air; it is decomposed by water, giving caustic soda and hydrogen. Dry carbon dioxide is decomposed by it, free carbon being produced; moist carbon dioxide, on the other hand, gives sodium formate.

Several oxides are known. A suboxide, Na 3 0, appears to be formed as a grey mass when a clean surface of the metal is exposed to air, or when pure air is passed through the metal just above its melting point (De Forcrand, Compt. rend., 1898, 12 7, pp. 3 6 4, 514). The monoxide, Na 2 0, is obtained by heating the metal above 180° in a limited amount of slightly moist oxygen (Holt and Sims, Journ. Chem. Soc., 1894, i. 442); it may also be prepared by heating the nitrate or nitrite with metallic sodium, free nitrogen being eliminated (German patent, 142467, 1902). It forms a grey mass, which melts at a red heat and violently combines with water to give the hydroxide. The hydroxide or caustic soda, NaOH, is usually manufactured from the carbonate or by electrolysis of salt solution (see Alkali Manufacture). When anhydrous it is a colourless opaque solid which melts at 310 °, and decomposes at about 110o°. It is very soluble in water, yielding a strongly alkaline solution; it also dissolves in alcohol. It absorbs moisture and carbon dioxide from the atmosphere. Several hydrates are known: 2NaOH 7H 2 O is obtained as large monoclinic crystals by' cooling a solution of specific gravity 1.365 to -8'; Pickering (Journ. Chem. Soc., 1893, 65, p. 890) obtained NaOH H 2 O from hot concentrated solutions and NaOH2H 2 O from a solution of the hydroxide in 96.8% alcohol. (See also De Forcrand, Compt. rend., 1901, 133, p. 223.) Sodium dioxide, Na202, is formed when the metal is heated in an excess of air or oxygen. In practice the metal is placed on aluminium trays traversing an iron tube heated to 300 0, through which a current of air, freed from moisture and carbon dioxide, is passed; the process is made continuous, and the product contains about 93% Na202. When pure, sodium dioxide has a faint yellowish tinge, but on exposure it whitens (W. R. Bousfield and T. M. Lowry, Phil. Trans., 1905, A. 204, p. 2 53). When dissolved in water it yields some NaOH and H202; on crystallizing a cold 'solution Na202.8H20 separates as large tabular hexagonal crystals, which on drying over sulphuric acid give Na 2 0 2.2H 2 0; the former is also obtained by precipitating a mixture of caustic soda and hydrogen peroxide solutions with alcohol. Acids yield a sodium salt and free oxygen or hydrogen peroxide; with carbon dioxide it gives sodium carbonate and free oxygen; carbon monoxide gives the carbonate; whilst nitrous and nitric oxides give the nitrate. A solution in hydrochloric acid, consisting of the chloride and hydrogen peroxide, is used for bleaching straw under the name of soda-bleach; with calcium or magnesium chlorides this solution gives a solid product which, when dissolved in water, is used for the same purpose (Castner, Journ. Soc. Chem. Ind., 1893, p. 603). Sodium dioxide is chiefly employed as an oxidizing agent, being used in mineral analysis and in various organic preparations; it readily burns paper, wood, &c., but does not evolve oxygen unless heated to a high temperature. Sodyl hydroxide, NaHO 2, exists in two forms: one, NaO.OH, obtained from hydrogen peroxide and sodium ethylate; the other, 0 :NaOH, from absolute alcohol and sodium peroxide at 0 °. They are strong oxidizing agents and yield alkaline solutions which readily evolve oxygen on heating. Sodium trioxide, Na 2 O 31 is said to be formed from an excess of oxygen and a solution of sodammonium in liquid ammonia. Water decomposes it, giving oxygen and the dioxide.

Generally speaking, sodium salts closely resemble the corresponding potassium salts, and their methods of preparation are usually the same. For sodium salts not mentioned below reference should be made to articles wherein the acid is treated, unless otherwise indicated.

Sodium combines directly with the halogens to form salts which are soluble in water and crystallize in the cubic system. The fluoride, NaF, is sparingly soluble in water (I part in 25). For the chloride see Salt. The bromide and iodide crystallize from hot solutions in anhydrous cubes; from solutions at ordinary temperatures in monoclinic prisms with 2H 2 O; and at low temperatures with 5H 2 O. According to M. Loeb (Journ. Amer. Chem. Soc., 1905, 27, p. 1019) the iodide differs from the other haloid salts in separating from solution in alcohols with "alcohol of crystallization." Sodium sulphide, Na 2 S, obtained by saturating a caustic soda solution with sulphuretted hydrogen and adding an equivalent of alkali, is employed in the manufacture of soluble soda glass. Sodium sulphite, Na2S03, which is employed as an antichlor, is prepared (with 7H20) by saturating a solution of sodium carbonate with sulphur dioxide, adding another equivalent of carbonate and crystallizing. The anhydrous salt may be prepared by heating a saturated solution of the hydrated salt. H. Hartley and W. H. Barrett (Journ. Chem. Soc., 1909, 95, p. 1184) failed to obtain a decahydrate which had been previously described. The acid sulphite, NaHSO 3, obtained by saturating a cold solution of the carbonate with sulphur dioxide and precipitating by alcohol, is employed for sterilizing beer casks. Sodium sulphate, Na2S04, known in the hydrated condition (with ioH 2 O) as Glauber's salt, is manufactured in large quantities for conversion into the carbonate or soda (see Alkali Manufacture). It has long been doubted whether sodium yielded an alum; this was settled by N. I. Surgunoff in 1909 (Abst. Journ. Chem. Soc. ii. Iooi), who obtained cubic crystals from a supersaturated solution of sodium and aluminium sulphates below 20°, higher temperatures giving monoclinic crystals. The acid sulphate, NaHSO 4, also known as bisulphate of soda, is obtained as large asymmetric prisms by crystallizing a solution of equivalent quantities of the normal sulphate and sulphuric acid above 50°. The acid salts Na 3 H (S04)2 and Na 3 H (S04) 2 4120 are obtained from the normal sulphate and sulphuric acid (J. D'Ans, Ber., 1906, 39, p. 1534).

The manufacture of sodium carbonate, commonly called soda, is treated under Alkali Manufacture. The anhydrous salt is a colourless powder or porous mass, having an alkaline taste and reaction. It melts at io08°. On solution in water, heat is evolved and hydrates formed. Common washing soda or soda-crystals is the decahydrate, Na2C03IoH 2 O, which appears as large clear monoclinic crystals. On exposure, it loses water and gives the monohydrate, Na2C03H20, a white powder sold as "crystal carbonate"; this substance, which is also formed on heating the decahydrate to 34°, crystallizes in the rhombic system. Both these hydrates occur in the mineral kingdom, the former as natron and the latter as thermonatrite. The heptahydrate, Na2C03.7H20, is obtained by crystallizing a warm saturated solution in a vacuum; it appears to be dimorphous. The acid carbonate or bicarbonate of soda, NaHCO 3, is produced in the ammonia-soda process for alkali manufacture. Another acid carbonate, Na2C03.2NaHC03.3H20, is the mineral trona or urao. We may here notice the "percarbonates" obtained by Wolffenstein and Peltner (Ber., 1908, 41, pp. 2 75, 280) on acting with gaseous or solid carbon dioxide on Na202, Na203 and NaHO 2 at low temperatures; the same authors obtained a perborate by adding sodium metaborate solution to a 50% solution of sodium peroxide previously saturated with carbon dioxide. For sodium nitrite see Nitrogen; for sodium nitrate see Saltpetre; for the cyanide see Prussic Acid; and for the borate see Borax.

Of the sodium silicates the most important is the mixture known as soluble soda glass formed by calcining a mixture of white sand, soda-ash and charcoal, or by dissolving silica in hot caustic soda under pressure. It is a colourless transparent glass mass, which dissolves in boiling water to form a thick liquid. It is employed in certain printing processes, as a cement for artificial stone and for mending glass, porcelain, &c., and also for making the so-called silicated soaps (see SoAP).

Sodium is most distinctly recognized by the yellow coloration which volatile salts impart to a Bunsen flame, or, better, by its emission spectrum which has a line (double), the Fraunhofer D, line, in the yellow (the wave-lengths are 5896 and 5890). The atomic weight was determined by Stas to be 22.87 (H = i); T. W. Richards and R. C. Wells (Journ. Amer. Chem. Soc., 1905, 27, p. 459) obtained the value 23.006 (0 =16).

Medicine. Pharmacology. - The metal sodium is not used in medicine, but many of its salts are employed. Besides liquor sodii ethylatis the following salts and preparations are used in the British Pharmacopoeia. (i) Sodii carbonis, known as washing soda; this carbonate on heating yields sodii carbonis exsiccatus and sodii bicarbonas; from the latter is made trochiscus sodii bicarbonatis. (2) Sodii phosphas. From sodium phosphate are made sodii phosphas effervescens and sodii hypophosphis (see Phosphorus). (3) Sodii sulphas (Glauber's salt), with its sub-preparation sodii sulphas effervescens. (4) Soda tartarata (Rochelle salt), a tartrate of sodium and potassium, from which is made pelvis sodae tartaratae effervescens, known as Seidlitz powder. (5) Sodii citro-tartras effervescens, a mixture of sugar, sodium bicarbonate, citric and tartaric acids. (6) Sodii chloridum, common salt.- (7) Sodii sulphas. For sodii bromidum, iodidum and salicylatum see Bromine, Iodine and Salicylic Acid respectively. For sodii arsenas and cacodylate see Arsenic. Sapo durus (hard soap) is a compound of sodium with olive oil, and sago animalis (curd soap) is chiefly sodium stearate.

Toxicology

Poisoning by caustic soda is rare, but occasionally it takes place by swallowing soap lees (sodium carbonate), which may contain some impurities of caustic soda. The symptoms and treatment are the same as described under Potassium. The salts of sodium resemble potassium in their action on the alimentary tract, but they are much more slowly absorbed, and much less diffusible; therefore considerable amounts may reach the small intestine and there act as saline purgatives. They are slowly absorbed into the blood, and are a natural constituent of the blood plasma, which derives them from the food. Sodium is excreted by all the mucous surfaces and by the liver and kidneys. On the latter they act as diuretics, but less powerfully than potassium, increasing the flow of water and the output of urea and rendering the urine less acid. They are said to diminish the secretion of the bronchial mucous membrane.

Therapeutics: External Use

The liquor sodii ethylatis is a powerful caustic and is used to destroy small naevi and warts. A lotion of sodium bicarbonate is useful to allay itching. Solutions of sodium sulphite are used as mild antiparasitics. Internal use. - Sodium chloride is occasionally used in warm water as an emetic, and injections of it into the rectum as a treatment for thread worms. A o9% solution forms what is termed normal saline solution, which is frequently injected into the tissues in cases of collapse, haemorrhage and diarrhoea. It forms a valuable treatment in diabetic coma and eclampsia, acting by diluting the toxins in the blood. From this has developed the intramuscular injection of diluted sea-water in the treatment of gastro-enteritis, anaemia and various skin affections. Sodium chloride is an important constituent of the waters of Homburg, Wiesbaden, Nauheim and Kissingen. Sodium bicarbonate is one of our most useful gastric sedatives and antacids, relieving pain in hyperchloridia. It is the constituent of most stomachic mixtures. Effervescent soda water is a mild gastric sedative. Sodium phosphate and sulphate are cholagogue purgatives and are used in the treatment of gallstones. The sulphate is the chief constituent of Marienbad and Carlsbad waters. Large doses of these salts are used to remove fluid in dropsy. Soda tartarate is purgative and diuretic, as is the citro-tartarate. These purgative sodium salts are most useful in the treatment of chronic constipation, and of the constipation associated with gout and hepatic dyspepsia. They should be dissolved in warm water and taken in the morning, fasting. In visceral gout and chronic catarrhal conditions of the stomach a course of alkaline waters is distinctly beneficial. Sodium salts have not the depressant effect so marked in those of potassium.