Iodine (sym. I, equiv. 127) is one of the four non-metallic elements. It was discovered in 1811, by Courtois, in the waste liquors produced in the manufacture of carbonate of soda from the ashes of seaweeds. A few years later Gay-Lussac discovered that it was a simple elementary body. While it is still obtained from the half-fused ash of dried seaweeds, which is known in Britain as Kelp (q.v.), it is much more largely prepared in South America from the iodate of sodium, which is found associated with nitrate of sodium in the native Chili saltpetre.
In small quantity, and usually in combination with sodium, magnesium, or calcium, iodine is very widely diffused over the earth's surface. It exists in sea-water, in marine animals and plants, and in certain mineral springs. It is also found in several minerals, as, for example, in certain Mexican silver ores, in Silesian zinc ores, in phosphorite from the Upper Palatinate, and in coal.
At ordinary temperatures it usually occurs in solid, dark-gray, glistening scales; it is, however, crystallisable, and sometimes appears as an octahedron with a rhombic base. It is soft, and admits readily of trituration, has the high specific gravity of 4.95, and evolves a peculiar and disagreeable odour, which indicates its great volatility. It fuses at ( C.), and at about ( C.) it boils, and is converted into the purple vapour to which it owes its name (Gr. iōdēs, 'violet-like'); it has an acrid taste, and communicates a brownish-yellow colour to the skin. It is very slightly soluble in water, but dissolves readily in watery solutions of iodide of potassium and of hydriodic acid, and in alcohol and ether. Iodine vapour is the heaviest of known vapours, its specific gravity compared with air as unity being 8.716. It combines directly with phosphorus, sulphur, and the metals. Its behaviour with hydrogen is analogous to that of chlorine and bromine (see HYDROCHLORIC ACID), but its affinities are weaker than those of the last-named elements. It likewise combines with numerous organic substances, and the compound which it forms with starch is of such an intense blue colour that a solution of starch forms the best test for the presence of free iodine. By means of this test one part of iodine may be detected when dissolved in one million parts of water.
With hydrogen iodine forms one compound, hydriodic acid (HI), a colourless, pungent acid gas, which in most respects is analogous with hydrochloric acid. It may be obtained by gently heating a mixture of amorphous phosphorus, iodine, and water. The soluble iodides of the metals may be obtained by the direct combination of hydriodic acid with the metallic oxides, the resulting compounds being the metallic iodide and water. Some of these iodides are of extreme brilliancy—e.g. the iodide of mercury, scarlet; the iodide of lead, yellow; and others are of great value in medicine. Amongst the latter must be especially mentioned iodide of potassium, iodide of iron, and the iodides of mercury.
Iodide of potassium is one of the most important medicines in the pharmacopœia. It crystallises in colourless cubes, which are sometimes clear, but usually have an opaque whitish appearance, and are soluble in water and spirit. It is decomposed and the iodine set free by chlorine, bromine, fuming nitric acid, and Ozone (q.v.). Iodide of iron is formed by shaking iron wire or filings in a closed vessel with four times the weight of iodine suspended in water. There are two iodides of mercury—viz. the green sub-iodide (HgI) and the red iodide (HgI2). There are several well-defined compounds of iodine and oxygen, but they are of no special interest.
The preparations of iodine are employed extensively in medicine and in Photography (q.v.). Iodine itself or its compounds may give rise to the symptoms known as iodism; most commonly running at the nose and eyes, with headache and sore throat; sometimes irritation of the intestinal canal, either alone or combined with the other symptoms. In the case of the iodine compounds these unpleasant results usually cease if the dose be increased.
Iodine and its compounds increase the activity of the absorbent system generally, and are useful in enlargement of the glands connected with that system (lymphatic glands, thyroid, spleen), and wherever absorption is deficient (hypertrophy of breasts, uterus, &c.; indolent inflammatory exudation in any organ). But they are perhaps of the greatest value in certain forms of chronic rheumatism, certain stages of syphilis, in scrofulous conditions generally, and in chronic poisoning by mercury and lead. In the last case they set free the metals from insoluble compounds in the tissues, and allow them to be eliminated from the body in solution in the urine.
Iodine is chiefly prescribed internally in combination, as iodide of potassium, iodide of iron, especially in strumous cases, and red iodide of mercury in syphilis. Free iodine is very apt to cause irritation of the intestinal canal, and can in general only be employed in small doses. But as an external application, in the form of ointment, tincture, or liniment, it is extensively used and is very valuable. It acts as a parasiticide in ringworm, removes muscular pains, and promotes the absorption of exudations and the subduing of chronic inflammations. In large doses iodine and most of the iodides act as irritant poisons; but very few fatal cases are on record. In the event of poisoning with the tincture of iodine the first point is to evacuate the stomach. See POISONS.