IODIDES. 731 lODimE. iodide of sodium or of strontium, or preferably by free hjdriodic acid. I'ODINE (from Gk. luitis, iodes, like a vio- let, so called from the color of its vapor ) . One of the 'halofiifn' 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, l)ut occur.s 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 coasts 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 kflp. 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 carbonate contained in it has crystallized out. An excess of sulphuric acid is then added to the mother-liquor, and thus hydrioilic 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 ■nooden 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. 126.85) is a grayish-black 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 184° 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 p-jrtion of the iodine of commerce is employed in medicine, partly in the free state. jiartly in combination as iodoform, and as the iodides of potassium. s(Klium, anmioniuin, stron- tium, and zinc, all of which are official in the Phamiacopiria. Iodine itself is but little used internally, although rc|iented 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, coimterirritant. and parasiticide. The pharmacopoeial preparations are: (1) Comjtninid solution of iodine, or Lugol's solution; an aqueous solution. 100 parts of which contain 5 parts of iodine and 10 parts of pota.s- sium iodide. (2) Tinriure of iodine, which is a 7 per cent, solution of iodine in alcohol. (3) Iodine oinlmfnt. which contains 4 per cent, of iodine. 1 per cent, of potassium iodide. 9.3 per cent, of bonzoinated lard, and 2 per cent, of water. When applied to the skin, the prepara- tions produce a stain, which disapjx^ars in time, largely owing to the volatility of iodine: the Vol. X.— 47. stain may, however, be readily washed off with alkalies or with sodium hyposulphite. As coun- ter-irritants the preparations mentioned arc used for pleurisy, chilblains, chronic inflammatioD 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 Iodides. 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 ease 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 hydriodic acid and its salts (see Iodides), iodic acid (HlOj) and its salts, termed iodates, and per- iodic acid (HIO,) and its salts, termed per- iodates. The anhjdride of iodic acid is iodine pentoxide. The well-known compound of iodine w ith nitrogen is mentioned under the latter name. Very interesting are two compounds of iodine with chlorine — viz. the monochloride of iodine. ICI, and the trichloride of iodine, ICI„ both of which may be prepared b}- the direct action of chlorine on iodine. The trichloride 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 800° 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. Hence the molecular weight of iodine up to 800° 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 Ij . .Vbovc 800° C, however, iodine vapor grows lighter and lighter. At 1000° C. it is only 100 times as hea-y as hjdrogen : at 1400° C. only 75 times as heavy; and at 1.500° C, and under reduced pressure. Dr. Crafts found it to ye only 60 times as hea^-j- as hydrogen under the same pressure and temperature. This indicates that the molecular weight of iodine vapor gradually diminishes above 800° C. seemingly approach- ing the limit 127 (i.e. 2X63.5). 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 be single atoms. The phenomenon, which has also been observed in the case of certain other elements, presents considerable interest from many points of view. Van 't HofT thinks that if the chemical elements can at all be decom- jiosed. the method might be like that employed in brmging about the phenomenon just describeil. Consult "Untersuchungen iiber das Jod von Gay-Lussac." in Ostwald's Klassiker der exakten ^yisscnscha|ten (Leipzig, 1889).
