A liquid composed of 10 per cent. of real or gaseous hydrobromic acid (HBr; 80.8) and 90 of water.

Characters. - A clear, colourless liquid, odourless, having a strongly acid taste and an acid reaction. Sp. gr. 1.077. By heat it is completely volatilised.

Reactions. - On adding chlorine or nitric acid to diluted hydrobromic acid, bromine is liberated, which is soluble in chloroform or in disulphide of carbon, imparting to these liquids a yellow colour. Test solution of nitrate of silver causes a white precipitate, insoluble in nitric acid and in water of ammonia, and sparingly soluble in stronger water of ammonia.

Tests. - On being kept for some time, the acid should not become coloured. Test-solution of chloride of barium should not produce a turbidity or precipitate (sulphuric acid).

Dose. - 15 to 50 min. B.P. Two fluid drachms contain 12 grains of bromine, which are equal to about 18 grains of bromide of potassium (United States Dispensatory). It may be given in syrup.

Action and Uses. - It appears to act as a sedative to the nervous system, diminishing reflex action and lessening tendency to spasm, in the same way as bromide of potassium, but differing from it in not producing the feeling of depression frequently caused by potassium bromide.

It has been employed in epilepsy, and to relieve nervousness.

It is useful in headache and singing in the ears, either idiopathic or due to the administration of quinine or of iron. It is used also to remove the bad effects of excess in tea or alcohol, and to quiet palpitation.