IODINE (symb. I. equiv. 127) is one of a group of four non-metallic elements to which the term halogens (q.v.) has been applied. It derives its name from Gr. violet-like, in consequence of its magnificent purple color when in a state of vapor. At ordinary temperatures, it usually occurs in solid dark-gray glistening scales; it is, how ever, crystallizable, 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 odor, which indicates its volatility. It fuses at 225°, and at about 350° it boils, and is converted into the purple vapor to which it owes its name; it has an acrid taste, and communicates a brownish-yellow color 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 vapor is the heaviest of all known vapors• its specific gravity being directly with phosphorus, sulphur, and the metals. Its behavior with hydrogen is analogous to that of chlorine and bromine (see HYDROCHLORIC Amu), but its affinities are weaker than those of the last-named eleinents. It likewise combines with numerous organic substances, and the compound which it forms with starch is of such an intense blue color, 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.
The following are some of the most important iodine compounds. With hydrogen, it forms only one compound, hydriodic acid (III), a colorless pumrent acid gas, which in most respects is analogous with hydrochloric acid. It is obtained bythe action of water on teriodide of phosphorus. The soluble iodides of the metals may be obtained by the direct combination of hydriodie acid with the metallic oxides, the resulting compounds being the metallic iodide and water. Some of these iodides are of extreme brilliancy, and others are of great value in medicine; amongst the latter must be especially men tiened iodide of potassium, iodide of iron, and the iodides of mercury.
Iodide of potassium is, next to quinine and morphia, the most important medicine in the pharmaeopeeia. It crystallizes in colorless 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.). There are various ways of obtaining this salt; the following is one of the best. If iodine be added to a warm solution of potash until a brown tint begins to appear, iodide of potassium (KI)and iodate of potash (KO, are formed. By gentle ignition of the residue obtained by evaporation, the iodate is decomposed into iodide of potassium and oxygen, so that all that remains is fused iodide of potassium, which is dissolved in water and allowed to crystallize. Iodide of iron is formed by digesting iron wire or filings in a closed vessel with four times the weight of iodine suspended in water. Direct combination takes place, and a pale-green solution is formed, which by evaporation in vacua yields crystals. It is the solution which is most commonly employed in medicine, but as, on exposure to the air, it becomes decomposed, and iodine is liberated, it is usually mixed with strong syrup, which retards this change.
There are two iodides of mercury, viz., the green sub-iodide and the red iodide (HgI). They may be formed either by the direct union of the two elements, or by the double decomposition of iodide of potassium and mercurial salts. There are two well defined compounds of iodine and oxygen, viz., iodic acid and periodic acid corresponding to chloric and perchloric acid, neither of which are of any special interest.
Iodine in small quantity, and usually in combination with sodium, magnesium, or calcium, 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, fdr example, in certain Mexican silver ores, in Silesian zinc ores, in phos phorite from the Upper Palatinate, and in coal.