BROMINE, a deep red, liquid, non-metallic element of the halogen group, which takes its name from its pungent unpleasant smell l(ipc),uos, a stench). It was first recognized as an element and isolated by A. J. Balard in 1826 from the salts in the waters of the Mediterranean. It has the symbol Br, the atomic number 35, and the atomic weight 79.92. Bromine does not occur in nature in the uncombined condition, but in combination with various metals is very widely but sparingly distributed. Potas sium, sodium and magnesium bromides are found in mineral waters, in river and sea-water, and occasionally in marine plants and animals. Its chief commercial sources are the salt deposits at Stassfurt in Prussian Saxony, in which magnesium bromide is found associated with various chlorides, and the brines of Michi gan, Ohio, Pennsylvania and West Virginia, U.S.A. ; a scheme is being developed for the extraction of bromides from the Dead Sea (1927).
Manufacture.—The chief centres of the bromine industry are Stassfurt and the central district of Michigan. It is manufactured from the magnesium bromide contained in "bittern" (the mother liquor of the salt industry), by two processes, the continuous and the periodic. The continuous process depends upon the decomposition of the bromide by chlorine, which is generated in special stills. A regular current of chlorine mixed with steam is led in at the bottom of a tall tower filled with broken bricks, and there meets a descending stream of hot bittern : bromine is liberated and is swept out of the tower together with some chlorine, by the current of steam, and then condensed in a worm. Any uncondensed bromine vapour is absorbed by moist iron borings, and the resulting iron bromide is used for the manufac ture of potassium bromide. Commercial bromine is rarely pure, the chief impurities present in it being chlorine, hydrobromic acid, and bromoform. It is usually purified by repeated shaking with potassium, calcium or ferrous bromide, and subsequent redistillation, a process which removes the chlorine and a part of the other impurities.
Properties.—Bromine at ordinary temperatures is a mobile liquid of fine red colour, which appears almost black in thick layers. It boils at 59°C., forming a deep red vapour, which exerts an irritating and directly poisonous action on the res piratory organs. It solidifies to a dark brown solid which melts at —7.3°C. Its specific gravity is 3.18828 (11-°), latent heat of fusion i6.185 calories, latent heat of vaporization 45.6 calories, specific heat 0.1071. Its properties are altered to a remarkable degree by prolonged drying; a specimen which had been dried for eight years melted at and boiled at 118°C., a rise of 6o degrees (H. B. Baker) (see DRYNESS, CHEMICAL). Bromine is soluble in water, to the extent of 3.226 grammes of bromine per 1 oo grammes of solution at 15°C., the solubility being slightly increased by the presence of potassium bromide. The solution is of an orange-red colour, and is quite permanent in the dark, but on exposure to light, gradually becomes colourless, owing to decomposition into hydrobromic acid and oxygen. By cooling the aqueous solution, hyacinth-red octahedra of a crystalline hydrate of composition are obtained (Bakhuis Rooze boom). Bromine is soluble in chloroform, alcohol and ether.
Its chemical properties are in general intermediate between those of chlorine and iodine ; thus it requires the presence of a catalytic agent, a fairly high temperature, or actinic light to bring about its union with hydrogen. It does not combine directly with oxygen, nitrogen or carbon. With the other elements it unites to form bromides, often with explosive violence; phos phorus detonates in liquid bromine and inflames in the vapour; iron is occasionally used to absorb bromine vapour, potassium reacts energetically, but sodium requires to be heated to 200°C. The chief use of . bromine in analytical chemistry is based upon the oxidizing action of bromine water. Bromine and bromine water both bleach organic colouring matters. Bromine is used extensively in organic chemistry as a substituting and oxidizing agent, and also for the preparation of addition compounds. It is used in the liquid form, in vapour, in solution, and in the presence of the so-called "bromine carriers" (see below). The solvents in which bromine is employed are usually ether, chloro form, acetic acid, hydrochloric acid, carbon bisulphide or water. The choice of solvent and of other conditions is important, for the velocity of the reaction and the nature of the product may vary according to the solvent used, or according to whether the reaction is conducted in sunlight or not. The action of bromine is sometimes accelerated by the use of compounds which behave catalytically, the more important of these "carriers" being iodine, iron, ferric chloride, ferric bromide, aluminium bromide and phos phorus. For oxidizing purposes bromine is generally employed in aqueous and in alkaline solutions, one of its most important applications being by Emil Fischer, in his researches on the sugars. The most important determinations of the atomic weight of bromine are those of G. P. Baxter based on the ratios Ag: AgBr, and AgC1:AgBr; and of P. A. Guye and E. Moles, based on the density of hydrogen bromide.
Hydrobromic Acid.—This acid, HBr, the only compound of hydrogen and bromine, is in many respects similar to hydrochloric acid, but is rather less stable. It may be prepared by passing hydrogen gas and bromine vapour through a tube containing a heated platinum spiral. It cannot be prepared with any degree of purity by the action of concentrated sulphuric acid on bromides, since secondary reactions take place, leading to the liberation of free bromine and formation of sulphur dioxide. The usual method employed for the preparation of the gas consists in dropping bromine on to a mixture of amorphous phosphorus and water, when a violent reaction takes place and the gas is rapidly liberated ; an aqueous solution may be obtained by passing sulphuretted hydrogen through bromine water. At ordinary temperatures hydrobromic acid is a colourless gas which fumes strongly in moist air, and has an acid taste and reaction. It can be condensed to a liquid, which boils at —68.7°C. (under a pres sure of 76o mm.), and, by still further cooling, gives colourless crystals which melt at —88.5°C. It is readily soluble in water, forming the aqueous acid, which when saturated at o° C. has a specific gravity of 1.78 and contains 82.02% HBr. When boiled, the aqueous acid loses either acid or water until a solution of constant boiling point is obtained, containing 48% of the acid and boiling at 126°C. under atmospheric pressure; should the pressure, however, vary, the strength of the solution boiling at a constant temperature varies also. Hydrobromic acid is one of the "strong" acids, being ionized to a very large extent even in concentrated solution, as shown by the molecular conductivity increasing by only a small amount over a wide range of dilution.
Bromides.—Hydrobromic acid reacts with metallic oxides, hydroxides and carbonates to form bromides, which can in many cases be obtained also by the direct union of the metals with bromine. As a class, the metallic bromides are solids at ordinary temperatures, which fuse readily and volatilize on heating. The majority are soluble in water, the chief exceptions being silver, mercurous, palladous and lead bromides ; the last is, however, soluble in hot water. They are decomposed by chlorine, with liberation of bromine and formation of metallic chlorides; con centrated sulphuric acid also decomposes them, with formation of a metallic sulphate and liberation of bromine and sulphur dioxide. The non-metallic bromides are usually liquids, which are readily decomposed by water. Hydrobromic acid and its salts can be readily detected by the addition of chlorine water to their aqueous solutions, when bromine is liberated; or by warming with concentrated sulphuric acid and manganese dioxide, the same result being obtained. Silver nitrate in the presence of nitric acid gives with bromides a pale yellow precipitate of silver bromide, AgBr, which is sparingly soluble in ammonia. For their quantitative determination they are precipitated in nitric acid solution by means of silver nitrate, and the silver bromide is well washed, dried and weighed.
Acids.—No oxides of bromine have been isolated, but three oxy-acids are known, viz., hypobromous acid, HBrO, bromous acid, and bromic acid Hypobromous acid is obtained by shaking together bromine water and precipitated mercuric oxide, followed by distillation of the dilute solution in vacuo at low temperature (about 40°C.). It is a very unstable compound, breaking up, on heating, into bromine and oxygen. The aqueous solution is light yellow in colour, and possesses strong bleaching properties. Bromic acid is obtained by the addition of the calculated amount of sulphuric acid (previously diluted with water) to the barium salt; by the action of bromine on the silver salt, in the presence of water, = or by passing chlorine through a solution of bromine in water. The acid is only known in the form of its aqueous solution ; this is, however, very unstable, decomposing on being heated to ioo°C. in water, oxygen and bromine. By reducing agents such, for example, as sulphuretted hydrogen and sulphur dioxide, it is rapidly converted into hydrobromic acid. Hydrobromic acid decomposes it according to the equation Its salts are known as bromates, and are as a general rule with difficulty soluble in water, and decom posed by heat, with evolution of oxygen. Bromine forms com pounds with fluorine and iodine (IBr), but only a series of mixed crystals with chlorine.
Applications.—The salts of bromine are widely used in pho tography, especially bromide of silver. In medicine it is largely employed in the form of bromides of potassium, sodium and ammonium, as well as in combination with alkaloids and other substances.
Medicinal Use.—Bromide of potassium is the safest and most generally applicable sedative of the nervous system. Whilst very weak, its action is perfectly balanced throughout all nervous tissue, so much so that Sir Thomas Lauder Brunton has suggested its action to be due to its replacement of sodium chloride (com mon salt) in the fluids of the nervous system. Hence bromide of potassium (or bromide of sodium, which is possibly somewhat safer still though not quite so certain in its action) is used as a hypnotic, as the standard anaphrodisiac, as a sedative in mania and all forms of morbid mental excitement, and in hyperaesthesia of all kinds. Its most striking success is in epilepsy, for which it is the specific remedy. It may be given in doses of from Do to 5o grains or more, and may be continued without ill effect for long periods in grave cases of epilepsy (grand mal). Of the three bromides in common use the potassium salt is the most rapid and certain in its action, but may depress the heart in morbid states of that organ; in such cases the sodium salt (of which the base is inert) may be employed. In whooping-cough, when a sedative is required but a stimulant is also indicated, ammonium bromide is often invaluable. The conditions in which bromides are most frequently used are insomnia, epilepsy, whooping-cough, delirium tremens, asthma, migraine, laryngis mus stridulus, the symptoms often attendant upon the climacteric in women, hysteria, neuralgia, certain nervous disorders of the heart, strychnine poisoning, nymphomania and spermatorrhoea. Hydrobromic acid is often used to relieve or prevent the headache and singing in the ears that may follow the administration of quinine and of salicylic acid or salicylates.