Barium Iodide, BaI₂

Barium Iodide, BaI₂ may be obtained by the neutralisation of hydriodic acid by barium carbonate, hydroxide, or sulphide. Instead of hydriodic acid, iodine and a reducing agent, for example iron or barium sulphite, may advantageously be employed. By the action of iodine on barium hydroxide, barium iodide in solution and practically insoluble barium iodate are formed, and can be separated by filtration. Instead of removing barium iodate it may be reduced to barium iodide by sulphuretted hydrogen. Barium iodide is also formed by the action of iodine on barium peroxide in the presence of water. The anhydrous salt is obtained by evaporating to dryness and heating in a current of gaseous hydriodic acid. It is a white, deliquescent compound of density 5.150 at 25° C. and melting-point 740° C. The heat of formation is 149.9 Cal., and the heat of solution 10.3 Cal.

Barium iodide is very readily soluble in water, and the following values have been obtained: -

Temperature, °C	-22	-9	-5	15	32	40	67	87	96	113	140	165
Grams BaI2 per 100 grams solution.	58.5	60.0	60.9	66.0	68.3	69.8	71.9	72.1	72.7	73.6	73.7	74.1

The solubility is reduced by the presence of salts with a common ion. Barium iodide is readily soluble in alcohol.

A solution of pure barium iodide has a neutral reaction. When exposed to air and sunlight it soon becomes yellow and deposits barium carbonate.

Barium iodide is stable in dry air at ordinary temperatures, but when heated in air it ultimately gives up all its iodine and forms the oxide.

It is sometimes used for therapeutic purposes.

There are apparently several hydrates, but they are difficult to identify on account of their deliquescent nature. The salt crystallising out at ordinary temperatures has been described as a heptahydrate, BaI₂.7H₂O, but, according to Mugge, commercial barium iodide, in large hexagonal prisms melting in the water of crystallisation at 25.7° C., is a hexahydrate, isomorphous with strontium chloride hexahydrate. By evaporation of concentrated solutions Lescoeur obtained a hexahydrate.

There is also a very hygroscopic dihydrate which is rapidly coloured reddish brown in the air owing to the liberation of iodine. It is isomorphous with barium bromide dihydrate, and is apparently the stable form in contact with saturated solutions at 85° C. At 125° C. the higher hydrates are converted into the monohydrate, which begins to lose water and forms the anhydrous salt at 150° C.

The heat of solution of the heptahydrate is –6.850 Cal.

Barium iodide forms addition compounds with ammonia containing 1, 2, 4, and 6 molecules of the latter respectively. The compound, BaI₂.6NH₃, has a dissociation pressure of 50 mm. of mercury at 20° C., and the heat of formation from the iodide and ammonia is 10.80 Cal.

Barium iodide forms a number of double salts: BaI₂.ZnI₂.4H₂O, BaI₂.2ZnI₂, BaI₂.HgI₂, BaI₂.2HgI₂, BaI₂.3HgI₂, BaI₂.5HgI₂.8H₂O, 2BaI₂.3HgI₂, 2BaI₂.3HgI₂.16H₂O, 3BaI₂.5HgI₂.2IH₂O, BaI₂.HgI₂.5H₂O, BaI₂.2Hg(CN)₂.4H₂O, BaI₂.2PbI₂.7H₂O, 2BaI₂.SbI₃.18H₂O.