IODINE (from Gk. ii:ks, iiidZR, like a vio let, so called from the color of its vapor). One of the 'halogen' elements, similar to chlorine and bromine. It was discovered by Courtois in 1811, in the ashes of seaweeds. It has been found free in nature, but occurs chiefly in combination, and is thus widely distributed both in the organic and inorganic kingdoms. It exists in sea-water, in sea plants and animals, and in mineral springs. Along with lead, mercury, silver, and zinc it occurs in ores from Chile, Mexico, and Spain. It is largely obtained from seaweeds, especially those thrown up on the north coast of France, the eeasts of Spain and Ireland, and the west coast of Scotland. 'When burned (formerly in the open air, now in closed retorts to prevent loss by volatilization) these weeds yield an ash called kelp, which contains from 0.1 to 0.3 per cent, of iodine. The kelp is dissolved in hot water, and the solution is evaporated until the bulk of the sodium car''onate contained in it has crystallized out. An excess of sulphuric acid is then added to the mother-liquor, and thus hydriodie acid is set free. Manganese dioxide (an oxidizing agent) is now added to the liquor in the distilling appa ratus, and thus the iodine is liberated. The product can be readily purified by sublimation, the first and last portions only being very im pure. The crude sodium nitrate from Peru and Chile, which contains about 0.2 per cent. of sodium iodate, is another important source of iodine. The iodine is extracted from the mother liquors from which the sodium nitrate has been separated by crystallization. These are run into wooden vats lined with lead, and the iodine is precipitated by a mixture of neutral and acid sodium sulphites. The resulting iodine is then washed with water and pressed into thick cakes, which are further purified by sublimation.
Iodine (symbol, 1; atomic weight, 126.S5) is a grayish-blaek soft solid, with a metal-like lustre. It has a specific gravity of 4.9 at 60° C.. melts at about 114° C.. and boils at 1S4° C., giving rise to a very heavy violet vapor with a peculiar and penetrating odor. Iodine finds extensive use in the arts, especially in the manufacture of coal-tar colors. Some iodine is used in photography. But the greater portion of the incline of commerce is employed in medicine. partly in the free state, partly in combination as iodoform, and as the iodides of potassium, sodium, ammonimn, stron tium, and zinc, all of which are official in the Pharmacopeia. Iodine itself is but little used internally, although repeated small doses of it, in form of the tincture well diluted with water. are often successfully used to stop vomiting. Ex ternally. however, iodine is in constant use as a disinfectant, irritant. counter-irritant, and parasitieide. The pharmaeopeial preparations are: (1) Compound solution of iodine, or Lugol's solution; an aqueous solution. 100 parts of which contain 5 parts of iodine and 10 parts of potas sium iodide. (2) Tincture of iodine. which is a 7 per cent. solution of iodine in alcohol. (3) Iodine ointment. which contains 4 per cent, of iodine, I per cent. of potassium iodide, 93 per cent. of benzoinated lard, and 2 per cent. of water. When applied to the skin, the prepara tions produce a stain, which disappears in time, largely owing to the volatility of iodine: the stain may, however, be readily washed off with alkalies or with sodium hyposulphite. As coun
ter-irritants the preparations mentioned are used fur pleurisy, chilblains, chronic inflammation of the joints, etc. As an antiparasitic, iodine is often used for ringworm, in the form of Coster's paste, which is made from 1 part of iodine and 4 parts of wood-tar. For the medicinal uses of iodides, see TomoEs.
Iodine is but sparingly soluble in water, 5000 parts of water dissolving only one part of the element. it is quite soluble in alcohol, and even more freely in ether. But it dissolves with great case in hydrocarbons, in carbon disulphide, and in chloroform. It is also freely soluble in iodine derivatives of the hydrocarbons, and in aqueous solutions of metallic iodides; but its solubility in all such substances is probably due to its supposed capacity for forming with them true chemical compounds. Among the com pounds of iodine may be mentioned hydriodie acid and its salts (see loomEs), iodic acid and its salts, termed iodates, and per iodic acid (MO) and its salts, termed per iodates. The anhydride of iodic acid is iodine pentoxide. The well-known compound of iodine with nitrogen is mentioned under the latter name. Very interesting are two compounds of iodine with chlorine—viz. the monoehloride of iodine. ICI, and the triehloride of iodine, 1C1,, both of which may be prepared by the direct action of chlorine on iodine. The triehloride has been rec ommended as an antiseptic.
It was mentioned above that iodine volatilizes very readily. The density of its vapor has been studied by Dumas, Victor Meyer, Crafts, of Boston, and Troost, of Paris. It will be remem bered that, according to Avogadro's rule, the molecular weight of a substance equals twice its vapor density referred to hydrogen. Up to about S00° C. iodine vapor is found to be in variably about 127 times as heavy as hydrogen under the same conditions of pressure and tem perature. Deuce the molecular weight of iodine up to S00° C. is about 254; and as the atomic weight of the element is about 127, the molecule of iodine vapor is considered as made up of two atoms and is represented by the symbol I,. Above S00° C.. however, iodine vapor grows lighter and lighter. At 1000° C. it is only 100 times as heavy as hydrogen: at 1400° C. only 75 times as heavy; and at 1500° C., and under reduced pressure, Dr. Crafts found it to be only 66 times as heavy as hydrogen under the same pressure and temperature. This indicates that the molecular weight of iodine vapor gradually diminishes above S00° C.. seemingly approach ing the limit 127 (i.e. 2 X 63.5), which is the relative weight of single atoms. In other words, the molecules of iodine seem to become gradually dissociated into single atoms. and at some limit ing temperature and pressure which have not yet been found, the particles of the vapor would all he single atoms. The phenomenon, ‘vhich has also been observed in the ease of certain other elements, presents considerable interest from many points of view. Van 't Iloff thinks that if the chemical elements can at all he decom posed, the method might be like that employed in bringing about the phenomenon just described.
Consult nUntersuchungen fiber das Jod von Gay-Lussac.," in Ostwald's Klassiker der exakten Wissensehaften (Leipzig, IsS9).