IODINE, ra-din or -din, a non-metallic ele ment, analogous in its general properties to chlorine and bromine. It was discovered by Courtois in 1811, in the mother-liquor of kelp that had been used for the production of sodium carbonate; occurring, there in combination with sodium and magnesium. It is still obtained from the ashes of certain seaweeds, but the principal supply is now obtained from °caliche,* a crude nitrate of sodium that occurs in im mense quantities in northern Chile. In the preparation of the commercially pure nitrate of soda from caliche, the mother liquors, after the removal of the nitrate by crystallization, are found to contain large quantities of iodine, chiefly in the form of iodate of sodium, NaI08; and it is from this substance that the iodine of commerce is now chiefly prepared. The richest caliche contains about 3.5 pounds of iodine per ton.
In its ordinary form, iodine is a solid sub stance, melting at 237° F., and boiling at about 380° F. In a vacuum, iodine sublimes without melting. Solid iodine is soft and dark gray in color, with a metallic lustre. The vapor is vio let in color, from which circumstance the ele ment takes its name (Greek, °like a violets). Chemically, iodine has the symbol I, and an atomic weight of 126.92 for 0=-16. Solid iodine has a specific gravity of about 4.95 at ordinary temperature and a specific heat of about 0.05412. Its volume increases, on account of thermal expansion, by about 0.00013 of its own value for a rise of temperature of 1° F. At temperatures not far above its boiling point, the vapor of iodine has a specific heat (at constant pressure) of 0.03369; and in the same region of temperature the ratio of its specific heat at constant pressure to the specific heat at constant volume is about 1.294. Iodine shows an important change in its vapor density at high temperatures. Thus, below about 1,200° F. the vapor has a density about 126 times as great as that of hydrogen under the same con ditions of temperature and pressure; but as the temperature rises the density of the vapor, relatively to hydrogen, falls off, until it is only about 68 at 2,700° F. It is believed that this change in density indicates that the molecules of iodine vapor split in two as the temperature rises; a molecule, just above the boiling point, containing two atoms, while at the higher tem perature the molecules are monatomic. Iodine is freely soluble in alcohol, ether, carbon di sulphide, chloroform and glycerin. It is only slightly soluble in pure water, but dissolves readily in aqueous solutions of the iodides. It is also soluble in benzine, acetic acid and nu merous other organic fluids. Iodine is a non
conductor of electricity.
With hydrogen, iodine forms the important compound HI, known as hydriodic acid. (See HYDRIODIC ACID). With the metals it forms binary compounds called "iodides," which may also be regarded as salts of hydriodic acid. Of these the most important is potassium iodide, KI, which is largely used in medicine. It is prepared by dissolving iodine in a solution of caustic potash, evaporating to dryness and iit niting. This salt is very soluble and crystal lizes in cubes. The iodides of ammonium, sodium, strontium and zinc are also used to a more limited extent. Iodoform, a yellow crys talline powder with a peculiar characteristic odor when warmed, is also much used as a dressing in surgery. It has the formula CHI,, and is analogous in its chemical structure and deportment to chloroform. Iodoform may be prepared by dissolving iodine in an alcoholic solution of caustic potash, the iodoform that is produced separating out as a precipitate. It is also prepared in Germany, to a certain extent, by the electrolysis of a similar solution. (Con sult Lab,- (Electrolysis and Electrosynthesis of Organic Compounds)). Iodine and its com pounds are used to some extent in photog raphy, and to a larger extent in synthetic chemistry, for the preparation of the coal-tar colors (q.v.), and other organic substances..
Iodine forms two important oxy-acids, known respectively as iodic acid, HI0s, and periodic acid, HI0.4.2.1.1b0. These are anal ogous, in their chemical deportment, to chloric and perchloric acids. It also forms the inter esting chlorides IO and IC16.
Free iodine combines with starch to form a remarkable deep blue compound, whose pro-. duction is a well-known test for the presence, in a given substance, of either starch or free iodine. To detect the presence of iodine in a solution, a few drops of thin, clear starch paste. are added to the solution to be tested (which should be cold), and hydrochloric acid is added until the reaction is acid. A couple of drops of , a concentrated solution of potassium nitrite are then added, when the dark blue color of . iodide of starch will instantly be produced, if iodine is present. This test may readily be modified so as to serve for the detection of starch. The reaction is not given by dextrin, nor by other isomers of starch.
Iodine is used medically in a compound solution with KI, in alcoholic solution or as a component in an ointment. These preparations are valuable counterirritants and antiseptics. A preparation of iodine and wood tar is used in ringworm. See Hvinuorac Acia; IODINE IN. MEDICINE.