IODINE, a chemical element, belonging to the halogen group, was discovered in 1812 by B. Courtois when investigating the products obtained from the mother-liquors prepared by lixiviat ing kelp or burnt seaweed. It forms heavy, blackish scales with a violet lustre which gives it a metallic appearance. In 1815 L. J. Gay-Lussac showed it was an element. Iodine (symbol I, atomic number 53, atomic weight 126.92) is not in nature uncombined; it is found very widely but sparingly distributed, chiefly in the form of its sodium and potassium salts. It is also found in-small quantities in seawater, in some seaweeds, and in various mineral and medicinal springs. Deep-sea weeds, as a rule, contain more iodine than those which are found in the shallow waters.
Iodine is obtained either from kelp (the ashes of burnt sea weed) or from the mother-liquors obtained in the purification of Chile saltpetre. In the former case the seaweed is burnt in large heaps at as low a temperature as possible, and the product is known either as kelp or varec. Another method is to heat the seaweed in large retorts, whereby tarry and ammoniacal liquors pass over and a very porous residue of kelp remains. A later method consists in boiling the weed with sodium carbonate; the liquid is filtered and hydrochloric acid added to the filtrate, when alginic acid is precipitated ; this is also filtered off, the filtrate neutralized by caustic soda, and the whole evaporated to dryness and carbonized, the residue obtained being known as kelp sub stitute. The kelp obtained by any of these methods is then lixiviated with water, which extracts the soluble salts, and the liquid is concentrated, when the less soluble salts, which are chiefly alkaline chlorides, sulphates and carbonates, crystallize out and are removed. Sulphuric acid is now added to the liquid, whereby alkaline sulphides and sulphites present are decomposed and iodides and bromides are converted into sulphates, while hydriodic and hydrobromic acids are liberated and remain dis solved in the solution. The liquid is run into the iodine still and gently warmed, manganese dioxide in small quantities being added from time to time, when the iodine distils over and is collected. In the second method it is found that the mother liquors obtained from Chile saltpetre contain small quantities of sodium iodate, this liquor is mixed with the calculated quantity of sodium bisulphite in large vats, and iodine is pre cipitated :— The precipitate is washed and then distilled from iron retorts. Iodine may also be prepared by the decomposition of an iodide with chlorine, or by heating a mixture of an iodide and manganese dioxide with concentrated sulphuric acid. Commercial iodine may be purified by mixing it with a little potassium iodide and then subliming the mixture; in this way any traces of bromine or chlorine are removed.
Iodine's specific gravity is 4.948 melting at 114.2° C; it boils at C under atmospheric pressure (W. Ramsay and S. Young). The specific heat of solid iodine is 0.0541 (H. Kopp). Its latent heat of fusion is 11•7 calories, and its latent heat of vaporization is 2T95 calories (P. A. Favre and J. T. Silbermann). It volatilizes slowly at ordinary temperatures, but rapidly on heating. Iodine vapour on heating passes from a violet colour to a deep indigo blue.
Iodine possesses a characteristic penetrating smell, not so pungent, however, as that of chlorine or bromine. It is only very sparingly soluble in water, but dissolves readily in solutions of the alkaline iodides and in alcohol, ether, carbon disulphide, chloroform and many liquid hydrocarbons. Its solutions in the alkaline iodides and in alcohol and ether are brown in colour, whilst in chloroform and carbon disulphide the solution is violet, and in the latter cases there is some indication of combination with the solvent. Its chemical properties resemble those of chlorine and bromine ; its affinity for most other elements, how ever, is less than theirs. It will only combine with hydrogen in the presence of a catalyst, but combines with many other elements directly; for example, phosphorus melts and then inflames, anti mony burns in the vapour, and mercury when heated with iodine combines with it rapidly. It is completely oxidized to iodic acid when boiled with fuming nitric acid. It is soluble in a solution of caustic potash, a dilute solution most probably containing the hypoiodite, which changes slowly into a mixture of iodide and iodate ; change is rapid on warming. Iodine is readily detected by the characteristic blue coloration that it immediately gives with starch paste in the presence of soluble iodides ; the colour is destroyed on heating, but returns on cooling provided the heating has not been too prolonged. Iodine in the presence of water frequently acts as an oxidizing agent; thus arsenious acid and the arsenites, on the addition of iodine solution, are converted into arsenic acid and arsenates.
Iodine finds application in organic chemistry, forming additional products with unsaturated compounds, the combination, how ever, being more slow than in the case of chlorine or bromine. It rarely substitutes directly, because the hydriodic acid produced reverses the reaction ; this can be avoided by the presence of pre cipitated mercuric oxide or iodic acid, which react with the hydriodic acid as fast as it is formed, and consequently remove it from the reacting system. As a rule it is preferable to use iodine in the presence of a carrier, such as amorphous phosphorus or ferrous iodide or to use it with a solvent. It is found that most organic compounds containing the grouping or in the presence of iodine and alkali give iodoform, (q.v.).
Hydriodic Acid, HI, is formed by passing hydrogen and iodine vapour through a hot tube, but the reaction is reversible, On shaking iodine with a solution of sulphuretted hydrogen in water, a solution of hydriodic acid is obtained, sul phur being at the same time precipitated. The acid cannot be prepared by the action of concentrated sulphuric acid on an iodide on account of secondary reactions taking place, which result in the formation of free iodine and sulphur dioxide. The usual method is to make a mixture of amorphous phosphorus and a large excess of iodine and then to allow water to drop slowly upon it ; the reaction starts readily, and the gas obtained can be freed from any admixed iodine vapour by passing it through a tube containing some amorphous phosphorus. It is a colourless sharp-smelling gas which fumes strongly on exposure to air. It readily liquefies at o° C under a pressure of four atmospheres, the liquefied acid boiling at C (730-4 mm.) ; it can also be obtained as a solid melting at —50.8° C. It is readily soluble in water, one volume of water at o° C dissolving 425 volumes of the gas. The saturated aqueous solution is colourless and fumes strongly on exposure to air; after a time it darkens in colour owing to liberation of iodine. When boiled the solutions lose either acid or water until a solution containing 58% of HI is obtained, which distils unchanged at 127° C. This has a specific gravity of 1.7 and is used in the Zeisel method of determining methoxyl groups. The gas is readily decomposed by heat into its constituent elements. It is a powerful reducing agent, and is frequently employed for this purpose in organic chemistry ; thus hydroxy-acids are readily reduced on heating with the concen trated acid, and nitro-compounds are reduced to amino-corn pounds, etc. It is preferable to use the acid in the presence of amorphous phosphorus, for the iodine liberated during the reduc tion is then utilized in forming more hydriodic acid, and conse quently the original amount of acid goes much further. It forms addition compounds with unsaturated compounds.
It has all the characteristics of an acid, dissolving many metals with evolution of hydrogen and formation of salts, called iodides. The iodides can be prepared either by direct union of iodine with a metal, from hydriodic acid and a metal, oxide, hydroxide or carbonate, or by action of iodine on some metallic hydroxides or carbonates (such as those of potassium, sodium, barium, etc.;
other products, however, are formed at the same time). The iodides as a class resemble the chlorides and bromides, but are less fusible and volatile. Silver, mercurous, mercuric and cuprous iodides are insoluble in water; lead iodide is sparingly soluble, whilst most of the other metallic iodides are soluble. Cupric iodide is unstable and breaks down to cuprous iodide and iodine. Strong heating decomposes the majority of the iodides. Nitrous acid and chlorine readily decompose them with liberation of iodine ; the same effect is produced when they are heated with concentrated sulphuric acid and manganese dioxide. The soluble iodides, on the addition of silver nitrate to their nitric acid solu tion, give a yellow precipitate of silver iodide, which is insoluble in ammonia solution. Hydriodic acid and the iodides may be estimated by conversion into silver iodide.
Iodine combines with chlorine to form iodine monochloride, IC1, which may be obtained by passing dry chlorine over dry iodine until the iodine is completely liquefied. It is readily de composed by water. The trichloride, results from the action of excess of chlorine on iodine, or from iodic acid and hydro chloric acid, or by heating iodine pentoxide with phosphorus pentachloride. It crystallizes in long yellow needles and decom poses readily when heated into the monochloride and chlorine. It is readily soluble in water, but excess of water decomposes it. Iodine monochloride in glacial acetic acid solution is used for obtaining the "iodine value" of fats and oils in the Wijs method.
Iodine pentoxide, 120,, the best known oxide, is obtained as a white crystalline solid when iodic acid is heated to i7o° C; it is easily soluble in water, combining to regenerate iodic acid; and when heated to 300° C it breaks up into its constituent elements. It is used in the detection of carbon monoxide (see CARBON), which it oxidizes with liberation of iodine: 1205-1-5C0=12+5CO2. Iodine dioxide, or 1204, is a lemon-yellow solid obtained by acting on iodic acid with sulphuric acid, or by grinding iodine with cold fuming nitric acid.
Although hypoiodous acid is not known, it is extremely probable that on adding iodine or iodine monochloride to a dilute solution of a caustic alkali, hypoiodites are formed, the solution obtained having a characteristic smell of iodoform, and being of a pale yellow colour. It oxidizes arsenites, sulphites and thiosulphates immediately. The solution is readily decomposed on the addition of sodium or potassium bicarbonates with liberation of iodine. The hypoiodite disappears gradually on standing, and rapidly on warming, being converted into a mixture of iodide and iodate.
Iodic Acid, can be prepared by dissolving iodine pent oxide in water; by boiling iodine with fuming nitric acid, by decomposing barium iodate with the calculated quantity of sulphuric acid, previously diluted with water; or by suspending iodine in water and passing in chlorine, 12+502+6H20= It is a white crystal line solid, dissolving readily in water to give an acidic solution. It is a most powerful oxidizing agent, readily changing phosphorus to phosphoric acid, arsenic to arsenic acid, silicon at 250° C to silica, and hydrochloric acid to chlorine and water. It is readily reduced, with separation of iodine, by sulphur dioxide, hydriodic acid or sulphuretted hydrogen, thus :— The salts, known as the iodates, can be prepared by the action of the acid on a base, or sometimes by the oxidation of iodine in the presence of a base. They are mostly insoluble or only very slightly soluble in water. The iodates of the alkali metals are, however, readily soluble in water (except potassium iodate). They are decomposed on heating, with liberation of oxygen, in some cases leaving a residue of iodide and in others a residue of oxide of the metal, with liberation of iodine as well as of oxygen. It is note worthy that, although chlorine or bromine will displace iodine from an iodide, yet in these highly oxygenated acids the reverse obtains and iodine displaces chlorine (see the preparation of periodic acid).
Periodic acid, is only known in the hydrated form. It can be prepared by the action of iodine on perchloric acid, or by boiling normal silver periodate with water : It is a colourless, crystalline, deliquescent solid which melts at 135° C, and at 14o° C is completely decom posed into iodine pentoxide, water and oxygen. A remarkable feature of iodic and periodic acids is the ease with which they give complex salts, e.g., KH(I03)2, KH2(103)3, K5106 (compare etc.
Iodine has extensive applications in volumetric analysis, being used more especially for the determination of copper in virtue of the reaction: 2K2SO4+Cu212+12.
In medicine iodine is frequently applied externally as a counter irritant, having powerful antiseptic properties. In the form of certain salts iodine is very widely used, for internal administra tion in medicine and in the treatment of many conditions usually classed as surgical, such as the bone manifestations of tertiary syphilis. The most commonly used salt is the iodide of potassium; the iodides of sodium and ammonium are almost as frequently employed, and those of calcium and strontium are in occasional use. The usual doses of these salts are from five to thirty grains or more. Their pharmacological action is as obscure as their effects in certain diseased conditions are consistently brilliant and unexampled. Our ignorance of their mode of action is cloaked by the term deobstruent, which implies that they possess the power of driving out impurities from the blood and tissues. Most notably is this the case with the poisonous products of syphilis. In its tertiary stages—and also earlier—this disease yields in the most rapid and unmistakable fashiori to iodides; so much so that the administration of these salts is at present the best means of determining whether, for instance, a cranial tumour be syphilitic or not. No surgeon would think of operating on such a case until iodides had been freely administered and, by failing to cure, had proved the disease to be non-syphilitic. Another instance of this deobstruent power—"alterative," it was formerly termed—is seen in the case of chronic lead poisoning. The essential part of the medicinal treatment of this condition is the administration of iodides, which are able to decompose the insoluble albuminates of lead, which have become locked up in the tissues, rapidly causing their degeneration, and to cause the excretion of the poisonous metal by means of the intestine and the kidneys. The following is a list of the principal conditions in which iodides are recognized to be of definite value : metallic poisonings, as by lead and mercury, asthma, aneurism, arteriosclerosis, angina pectoris, gout, goitre, syphilis, haemophilia, Bright's disease (nephritis) and bronchitis.
A minute proportion of iodine in the diet appears to be essen tial for health. This was formerly supplied as an accidental im purity in table salt, but the highly purified article now used as a condiment is free from iodides, and in certain States (e.g., some cantons of Switzerland) iodides are added to the drinking water. The deficiency is of greater significance in the case of small children, for the iodine is required for the synthesis of thyroxine, the active principle of the thyroid gland, which, however, has been artificially prepared by C. R. Harington and G. Barger (Biocliem. J., 1927, 21, p. 169).