Sodium chloride, NaCl

Large quantities of sodium chloride are found in nature, the average amount present in the water of the ocean being about 2.7 per cent., while certain inland lakes contain a much higher percentage. The Great Salt Lake in Utah has up to 30 per cent, of the chloride in solution, and the Dead Sea 22 to 23 per cent. The natural product known as " rock-salt " is the residue left on evaporation of inland seas, such deposits being found in Cheshire, at Stassfurt and Berchtesgaden in Germany, at Vic in France, and at Wieliczka in Galicia. Rock-salt is usually associated with calcium sulphate, alumina, and sandstone. At Stassfurt the layer of rock-salt is covered by another layer of readily soluble salts, technically known as "Abraum" salts; that is, salts which must be removed before the rock-salt is reached.

Rock-salt can be obtained from the salt-deposits either by mining or by making borings through which the salt can be extracted by water. The colour of the mined product varies very much; it may be colourless, yellow, red, grey, or green. Iron is a frequent impurity, and other foreign substances often present are magnesium salts, calcium sulphate, alumina, silica, and so on. The presence of a small proportion of magnesium chloride renders the salt hygroscopic; it must be purified by crystallization.

When the salt is extracted from the deposits by the solution-method, two concentric copper tubes are introduced through a boring, the outer tube serving to admit the water, and the inner as a conduit for pumping off the salt-solution. The dissolved salt is obtained by evaporation.

The salt found at Droitwich is in natural solution, and is not contaminated with iron. The liquor is pumped to the surface and evaporated.

The concentration of the water of certain mineral springs and of the ocean also affords a means of isolating salt. Less soluble constituents, such as calcium sulphate, separate first. Admixture of the salt with more soluble compounds, such as magnesium chloride, is obviated by not carrying the concentration too far. Shipper states that the elimination of potassium chloride can be effected by repeated crystallization of the salt from water.

For laboratory use pure sodium chloride is obtained by dissolving the commercial product in water, and reprecipitating by saturating the solution with hydrogen chloride. The pure substance can also be prepared by the action of hydrochloric acid on sodium carbonate or hydroxide.

Sodium chloride crystallizes in transparent or opaque cubes, precipitation with hydrogen chloride also yielding octahedra. In deposits of rock-salt a blue variety of the compound is often found, the colour being probably an optical phenomenon and independent of the presence of any colouring matter, since in solution the blue salt is colourless, and organic solvents fail to extract any coloured substance. Wittjen and Precht attribute the blue colour to reflection of the light from hollow spaces in the crystals, bounded with parallel walls and filled with gas. Their view accords with the fact that heating decolorizes the crystals, probably owing to the expulsion of the gas. Siedentopf considers the probable cause of the colour to be the presence of metallic sodium in the fissures of the crystals. This phenomenon, and the development of a blue colour in sodium chloride exposed to cathode-rays or heated with sodium-vapour, have occasioned much discussion.

There are remarkable discrepancies between the numerous observations recorded for the melting-point of sodium chloride, the observed temperatures lying between 733° C. and 820° C. Carnelley gives 772° C. and 776° C.; Victor Meyer, Riddle, and Lamb, 815.4° C.; Le Chatelier, 780° C.; McCrae, 811° C. and 814.5° C.; Ramsay and Eumorfopoulos, 733° C.; Ruff and Plato, 820° C.; Huttner and Tammann, 810° C.; Arndt, 805° C,; Schemtschushny and Rambach, 819° C.; Schaefer, 802° C.; Haigh, and Foote and Dana, 801° C.; Wolters, 797° C.; Korreng, 800° C.; and Schemtschushny, 816° C. For the freezing-point Plato gives 804.3° C. The latent heat of fusion per gram is 0.097 Cal. Even at its melting-point sodium chloride is somewhat volatile, evolving heavy white fumes of the vapour. It has the simple molecular formula NaCl, as is proved by the vapour-density method, by the ebullioscopic method with bismuth trichloride as solvent, and by the cryoscopic method with fused mercuric chloride as solvent.

The fused chloride solidifies in cubes. The question of the possibility of the formation of mixed crystals with the chlorides of potassium, lithium, and ammonium has been much debated, but Schaefer demonstrated that sodium chloride forms a complete series of mixed crystals with lithium chloride. It appears from the work of Kurnakoff and Schemtschushny that the chlorides of sodium and potassium are completely miscible at high temperatures, although the individual crystals begin to separate about 400° C.

For the density of pure sodium chloride Clarke gives 2.135; Retgers, 2.167 at 17° C.; Krickmeyer, 2.174 at 20° C.; and Haigh, 2.170 at 20° C. Brunner has investigated the density of the fused salt between its melting-point and 1000° C.

The mean value of several investigations of the index of refraction of rock-salt at 18° C. for the D-line is 1.54432. For the electric conductivity of fused sodium chloride at 960° C. Braun gives 0-9206 reciprocal ohms, and at 750° C. Poincare found 3.339 reciprocal ohms. For the specific heat of the fused chloride between 13° and 46° C. Kopp gives 0.213, and from 15° to 98° C. Regnault gives 0.2140. For rock-salt at 0° C. Weber gives 0.2146, and from 13° to 45° C. Kopp gives 0.219.

As indicated in the table, the solubility of sodium chloride in water is only slightly augmented by rise of temperature:

Temperature °C	0	10	20	30	40	50	60	70	80	90	100	118	140	160	180
Grams NaCl in 100 grams water	35.63	35.69	35.82	36.03	36.32	36.67	37.06	37.51	380	38.52	39.12	39.8	42.1	43.6	44.9

According to Gerlach, a saturated solution contains 40.7 grams per 100 grams of water, and in contact with the solid boils at 108.8° C. The Earl of Berkeley and Applebey state the boiling-point to be 108.668° C. at 760 mm. pressure. The heat of solution is given by Thomsen as -1.2 Cal.

At 25° C., 100 grams of ethyl alcohol dissolve 0.065 gram of sodium chloride.

Several hydrates of sodium chloride have been described, but only the dihydrate, NaCl,2H₂O, seems to have been definitely isolated.

For the heat of formation of sodium chloride from its elements, Thomsen gives 97.69 Cal., and from the interaction of aqueous solutions of sodium hydroxide and hydrogen chloride 13.745 Cal.

Matignon has determined the freezing-point of solutions of sodium chloride of various concentrations. Some of his results are given in the table:

Percentage of Sodium Chloride	Freezing-point, °C.
11	-6.6
15	-9.25
20	-12.7
25	-16.66

Matignon's work indicates that a stable liquid mixture of water and sodium chloride does not exist below –21.3° C.

References are appended to investigations of the properties of dilute aqueous solutions of sodium chloride, including the molecular depression of the freezing-point and elevation of the boiling-point, vapour-pressure, density, viscosity, refractive index, specific heat, diffusion, electric conductivity, and the effect of other dissolved substances on the solubility - also to work on the solubility in non-aqueous solvents, and on the compressibility.

The electrolysis of sodium chloride is an important technical process, producing the hydroxide and carbonate, as well as chlorine and from it bleaching-powder. In the formation of sodium hydroxide, precautions to prevent conversion of the product into hypochlorite and chlorate are necessary.

In one process the cathodic and anodic chambers are separated by a diaphragm of porous clay, the cathode being a rod of iron to resist the action of the caustic alkali, and the anode being of carbon to withstand the corrosive action of the chlorine. Hargreaves and Bird employ a cathode of iron-gauze.

In the mercury process the bottom of the electrolytic cell is covered with a layer of mercury into which a non-porous diaphragm dips so that the mercury forms a partition between the anodic and cathodic chamber. The anode is made of carbon, and is immersed in sodium-chloride solution; the cathode is made of iron, and is dipped into water. The mercury acts as cathode, taking up the liberated sodium to form an amalgam, which reacts with the water to produce sodium hydroxide. Various modifications of the process have been devised, one being the substitution of fused sodium chloride for the solution, and of fused lead or tin for mercury, the alloy produced being subsequently decomposed by water.

The so-called " bell process " gives a better yield than the diaphragm process. The anode is contained in a bell-shaped vessel open at the bottom, the cathode being outside. The sodium-hydroxide solution formed at the cathode floats on the surface of the sodium-chloride solution, and is thus kept from contact with the chlorine evolved at the anode. The gas is removed at the top of the vessel.

At high temperatures in presence of acids such as silicic and boric, or of alumina, the chlorine of sodium chloride can be displaced by oxygen. At 400° C. under pressure the chlorine can be partially replaced by bromine.

Sodium chloride is employed as a glaze for pottery, since at red heat in presence of moisture it combines with the alumina and silica to form a transparent glaze of sodium aluminium silicate. Other technical applications are the manufacture of sodium hydroxide and carbonate, and the isolation of silver and copper from their ores. It also has a dietetic value, and is employed as a preservative for meat.

Schreinemakers and de Baat have described double salts of sodium chloride with cupric chloride and barium chloride.