Barium Hydroxide, Ba(OH)₂

Barium oxide, like the other alkaline earth oxides, combines with water with considerable evolution of heat, forming powdered Barium Hydroxide, Ba(OH)₂. It may be obtained commercially from the sulphide by the action of metallic oxides, for example zinc, copper, and lead, in the presence of water, by treatment with superheated steam, or by oxidation of the warm solution by air in the presence of Weldon mud. It may also be prepared from the carbonate by the action of superheated steam at red heat. The electrolysis of barium chloride solution with a mercury cathode, by methods similar to those employed for the production of alkali hydroxides, gives barium hydroxide. An electrolytic process has also been suggested for the preparation of barium hydroxide from barium sulphide. Barium hydroxide is formed at the anode through the interaction of hydroxyl ions with barium sulphide. A method of preparation for commercial use by the hydrolysis of tribarium silicate, or, more probably, an intimate mixture of one molecule of metasilicate with three molecules of barium carbonate, previously obtained by the fusion of barium carbonate with silica, has been described.

The heat of formation of barium hydroxide from the oxide and water is 24.24 Cal. It is an amorphous white powder of density 4.495, and it melts at 325° C. A high temperature is required to dehydrate the hydroxide. The following values have been found for the vapour pressure at different temperatures: -

Temperature, °C.	630	670	710	749	789	829	870	910	951	998
Pressure, mm. Hg	9.2	17.4	31.5	55	92	149	234	355	526	760

Under the influence of the cathode rays, barium hydroxide fluoresces a bright orange-yellow.

The heat of solution of barium hydroxide is 11.40 Cal. It is more soluble than either strontium or calcium hydroxide, and its solubility increases considerably with temperature. The solution is strongly basic. The following values for the solubility have been found: -

Temperature, °C.	0	6	12	16	21	30	41	54	64	70	77	80
Grams hydroxide as BaO in 100 grams water	1.5	1.8	2.4	3.0	3.6	5.0	7.6	14.0	23.8	31.9	70.0	90.77

The densities of barium hydroxide solutions, and the electrical conductivities of dilute solutions, and of concentrated, have been determined.

The solubility is considerably diminished by alkali hydroxides, and to a less extent by ammonia. It is also decreased by barium chloride, but increased by barium nitrate, probably through the formation of a complex ion. Acetone and alcohol decrease the solubility.

On electrolysing a barium hydroxide solution, less oxygen is evolved than is equivalent to the hydrogen formed, probably owing to the production of barium peroxide or hydrogen peroxide.

Carbon disulphide reacts with barium hydroxide solution at 100° C., forming barium carbonate and hydrosulphide.

Several hydrates of barium hydroxide have been described, but the existence of these has not in all cases been confirmed. At low temperatures Artus obtained the hydrate Ba(OH)₂.16H₂O. From solutions at ordinary temperatures the octahydrate crystallises out. The formulae, Ba(OH)₂.7H₂O and Ba(OH)₂.9H₂O, have also been assigned to the compound thus obtained.

The octahydrate may form monoclinic or tetragonal crystals, the latter being isomorphous with the corresponding strontium compound. The density is 1.656, and the heat of solution -14.5 Cal. At 78° C. it melts in its water of crystallisation, and if the temperature be raised it boils at 103° C. As the water evaporates, the boiling-point rises to 109° C., at which temperature it remains constant, whilst large brilliant rhombic crystals of the trihydrate separate out. From vapour pressure measurements, Lescoeur concluded that a compound, 3Ba(OH)₂.10H₂O, probably existed. This is no doubt the trihydrate. The lustre rapidly disappears on exposure to air, and in dry air the crystals effloresce, forming the monohydrate. By heating the solution in equilibrium with the trihydrate to a still higher temperature, a white crystalline powder of the monohydrate is formed. The latter is also obtained from the octahydrate by drying in vacuo or by heating for some time in a current of hydrogen at 45° C., or at 80° C. The heat of solution is 7.06 Cal. The monohydrate is changed to the hydroxide by heating in a current of hydrogen above 90°-95° C.

The monohydrate will combine with carbon dioxide at fairly low temperatures, but a slight excess of water is apparently necessary.

From vapour pressure measurements Muller-Erzbach concluded that a dihydrate can exist at 15° C.

Barium oxide forms compounds with methyl and ethyl alcohol, and also with other alcohols, such as glycerol and mannitol. According to de Forcrand these are not true alcoholates, but addition compounds, for example, 3BaO.4CH₃OH and 3BaO.4C₂H₅OH. In aqueous methyl alcohol a hydrated compound is formed.

If ethyl alcohol and a little water be added to a methyl alcoholic solution of barium oxide, and the solution be slowly evaporated in vacuo, a compound, Ba(OCH₃)₂, separates out in transparent needles. It is soluble in water, slightly soluble in ethyl alcohol, and insoluble in ether or acetone.

Barium hydroxide was formerly used in the beet and cane sugar industries for separating otherwise uncrystallisable sugars by precipitating the sucrate and then decomposing it by carbon dioxide, but this process has now been abandoned.

It is also used as a depilatory in tanning, in the white pigment and dry colour trades for the manufacture of permanent white for example, and for the softening of water for industrial purposes.