Barium Chloride, BaCl₂

Barium Chloride, BaCl₂, was first described by Scheele. It may be formed from the metal by direct combination with chlorine; from barium oxide by the action of chlorine at red heat, or of hydrochloric acid; from barium sulphate by heating to redness in hydrochloric acid gas, or by fusing with excess of ammonium chloride; and from barium hydroxide, carbonate, or sulphide by the action of aqueous hydrochloric acid, evaporation to dryness, and fusion in an atmosphere of hydrochloric acid gas to remove the last traces of moisture.

On a commercial scale it may be prepared from the sulphate, barytes, by fusion with calcium chloride alone, or with carbon or iron, or a mixture of manganese chloride and carbon; by the action of hydrochloric acid on a mixture of barium sulphate and carbon at red heat; or, finally, by reduction to sulphide and fusion with magnesium chloride under pressure. If witherite form the starting-point, it may be fused with ammonium chloride or calcium chloride.

Barium chloride may be purified by repeated precipitation with alcohol.

The heat of formation of barium chloride is 197.08 Cal. The density of the salt dried at 220° C. is 3.856 at 24° C. The melting point is 958.9° C., and there is a transition point to another form at 924.5° C. The specific heat is 0.0902.

The specific electrical conductivity at the melting-point is 1.91 reciprocal ohms, and increases to 2.31 at 1100° C.

The highest value found for the decomposition potential of barium chloride is 3.3 volts, but this is probably too low owing to the difficulty of avoiding polarisation. At 650° C. the value for barium chloride, mixed with lithium chloride to reduce the temperature of fusion, is 3.05 volts, and at 750° C., 1.45 volts.

By heating with bromine at 200°-300° C., part of the chlorine is removed. At the temperature of fusion it is partly decomposed by oxygen or by sulphur with loss of chlorine. It is decomposed by water vapour with the formation of hydrochloric acid, but less readily than the other alkaline earth chlorides.

Barium chloride is readily soluble in water, and the heat of solution is 1.92 Cal. Its solubility has been studied by a number of investigators, and the following values were given by Mulder: -

Temperature, °C.	10	20	30	40	50	60	70	80	90	100
Grams BaCl2 in 100 grams water.	33.3	35.7	38.2	40.8	43.6	46.4	49.4	52.4	55.6	58.8

The solubility is considerably diminished in presence of hydrochloric acid, sodium chloride, or barium nitrate, due to the action of a common ion. Engel found that, whereas 10 c.c. of pure water dissolve 28.9 milli-equivalents of barium chloride, 10 c.c. of hydrochloric acid solution, containing 50-5 milli-equivalents of acid, only dissolve 0.29 milli-equivalents of the salt.

Values for the solubility in different water-alcohol mixtures have also been determined, but it is almost insoluble in absolute alcohol. It is soluble in methyl alcohol to the extent of 2.18 parts in 100 parts of alcohol at 15.5° C.

Determinations of the transport number of the barium ion in chloride solutions of different concentrations have been made, and seem to indicate the presence of the complex ions BaCl₃' and BaCl₄''. Powdered quartz absorbs barium chloride from its solutions.

Two hydrates of barium chloride, the mono- and the di-hydrate, have been isolated. On the ground of vapour pressure measurements, the existence of a hexahydrate, only stable in contact with the aqueous solution below about 10° C., has been suggested, but the evidence scarcely seems sufficient to justify this supposition.

Barium Chloride Dihydrate, BaCl₂.2H₂O, separates from solution at ordinary temperatures in colourless monoclinic crystals of density 3.097 at 24° C. and specific heat 0.171. The vapour pressure at 25° C. is 5.8 mm. The heat of solution is -4.93 Cal., and the heat of hydration is 7.00 Cal. The dihydrate melts at 100° C., giving the anhydrous salt.

The existence of Barium Chloride Monohydrate, BaCl₂.H₂O was first suspected by Thomsen, because he found that the heat of reaction of anhydrous barium chloride with one grm.-mol. of water, 3 17 Cal., is less than half the heat of reaction with two grm.-mol. Vapour pressure measurements also indicate its existence.

It may be formed by heating the dihydrate at 60°-65° C., or by shaking the dihydrate with 99 per cent, of methyl alcohol, filtering at once through a Buchner funnel to remove undissolved salt, and leaving to crystallise. After a few minutes the monohydrate separates from solution in colourless rhombic plates and can be dried over calcium chloride in vacuo. The vapour pressure of the monohydrate at 25° C. is 2.5 mm.

Barium chloride may be used for softening boiler water, especially for removing permanent hardness.

Barium chloride absorbs ammonia under pressure, but gives it up again when exposed to the atmosphere. The compound, BaCl₂.4NH₃, is formed. It has a dissociation pressure of 541 mm. of mercury at 0° C., and 1850 at 28.4° C. Ammonia also forms a compound with the double salt of mercury cyanide and barium chloride, 2Hg(CN)₂.BaCl₂.4NH₃. Several addition compounds with ammonia derivatives have been isolated, with hydroxyiamine, BaCl₂.NH₂OH.2H₂O and BaCl₂.NH₂OH, with glycine, BaCl₂.2NH₂CH₂COOH, and with monobetaine, BaCl₂.C₅H₁₁O₂N.4H₂O.

Barium chloride forms mixed crystals in all proportions with lead chloride.