IODINE

From Agepedia

IODINE (from to5%, vioalacens, in allusion to the beautiful violet color of its vapor) is the name of an undecornpounded.principle or element in chemistry. It had escaped the observation of chemists until 1812, when a manufacturer of saltpetre, at Paris, detected it in the ashes of seaweeds, in the following manner. In evaporating the ley from these ashes, to procure the carbonate of soda which they contain, he noticed that the metallic vessels, with which he operated, were powerfully corroded, and that the corrosion was increased as the liquor became more concentrated. Having at hand, one day, a bottle of sulphuric acid, he added some of it to a portion of the motherwater, and was surprised to see a rich violet vapor disengaged; this vapor was the IODINE. He at once communicated the observation to M. Clement Desormes, who set about collecting some of the vapor, and, after examining its leading properties, announced it to the royal institute of France as a new body. Its real nature was soon after unfolded through the accurate researches of GayLussac and sir H. Davy. Its history proved singularly interesting in modifying the then prevailing theory of chemistry. Sir H. Davy had, a few years previously, promulgated the new theory of chlorine, which was still received with suspicion among chemists. The strong analogies, however, between this substance and chlorine, in their relations to combustibles, both bodies forming compounds by uniting with them, similar to acids containing oxygen, or oxides,were conceived to give great weight to the views of sir H. Davy, and operated completely to overthrow the erroneous hypothesis of oxygenation, invented by Lavoisier. Its investigation, therefore, may be said to have formed a new era in chemistry. The physical properties of iodine are as follow : It is a soft, friable, opaque solid, of a bluishblack color, with a metallic lustre, usually in scales, but sometimes in distinct crystals of the form of rhomboids or rhomboidal tables, referable to an octahedron, with a rhombic base as their primary form ; its specific gravity is 4.946. It possesses an odor somewhat analogous to that of chlorine. It is a nonconductor of electricity, and possesses in an eminent degree the electrical properties of oxygen and chlorine. present, it sublimes rapidly at a temperature considerably below 212°, and gives rise to a dense vapor of the usual violet hue. It is scarcely at all soluble in water, but is readily taken up by alcohol and ether, to which it imparts a reddishbrown color. It extinguishes vegetable colors, but with less energy than chlorine. It is not inflammable. Its range of affinity for other bodies is very extensive ; the most important compounds it forms with these we shall describe after alluding to its natural state and preparation. It exists most abundantly in the various species of fucus, which form the greatest part of the seaweeds of our coast; it also occurs in the sponge, and in the coverings of many molluscous animals, and has been found in a great number of mineral waters, as those of Salz in Piedmont, Saratoga in New York, &c, and more recently has been detected in some silver ores from Mexico, and in an ore of zinc from Upper Silesia. But it is from the incinerated seaweed or kelp, that the iodine, in large quantities, is obtained. As the soapmanufacturers are in the habit of obtaining their soda from kelp, iodine may be procured, very economically, from the residu urns of their operation, according to the process invented by doctor lire, which is as follows: The brown iodic liquor of the soapboiler, or the solution of kelp from which all the crystallizable ingredients have been separated by concentration, is heated to about 230° Fahr., poured into a large stoneware basin, and saturated with diluted sulphuric acid. When cold, the liquor is filtered through woollen cloth ; and to every 12 oz. (apothecaries'measure) of it, is added 1000 grains of black oxide of manganese in powder. The mixture is put into a glass globe, or large matrass with a wide neck, over which a glass globe is inverted, and heat is applied, which causes the iodine to sublime copiously, and to condense in the upper vessel. As soon as the balloon becomes warm, another is substituted for it; and when the second becomes heated, the first is again applied. The iodine is withdrawn from the globes by a little warm water, which dissolves it very sparingly; and it is purified by undergoing a second sublimation. The test made use of for the detection of iodine in any solution, when it is suspected to be present, is starch, with which iodine has the property of uniting, and of forming with it a compound, insoluble in cold water, which is recognised with certaintyby its deep blue color. The solution should be cold at the time of adding the starch; and, if the color does not become apparent simply on the addition of the starch, a few drops of sulphuric acid should be cautiously added, when, if any iodine is present, the blue color will make its appearance. This test is so exceedingly delicate, that a liquid, containing 4 J7T1,~O~O~O of its weight of iodine, receives a blue tinge from a solution of starch. Iodine has a powerful affinity for hydrogen, which it takes from animal and vegetable substances, in the same manner as ehlorine, and, uniting with it, forms hydriodic acid. The following are the methods for obtaining this acid in the gaseous and in the liquid state: Into a flask, to which a recurved tube is fitted, dipping under a jar of mercury, are introduced eight parts of iodine and one of phosphorus, and to the mixture a few drops of water are added ; the water is immediately decomposed; the phosphorus, seizing its oxygen, forms phosphoric acid, while the hydrogen combines with the IODINE. As there is not water present in sufficient quantity to dissolve the hydriodic acid, it passes over in the gaseous state, and is collected over the mercury. In contact with air, it smokes, or fumes, like the muriatic acid, and, like it, reddens vegetable blues. It is distinguished, however, from that acid, by the superior affinity possessed by chlorine for hydrogen, in consequence of which, if chlorine and hydriodic acid gases are mingled together, the yellow color of the former disappears, and the violet vapor of iodine makes its appearance, which proves the decomposition of the hydriodic acid by the chlorine. If the decomposition is complete, the vessel will be wholly occupied by muriatic acid gas. To obtain the hydriodic acid in a liquid state, we have only to conduct the gas through water, until it is fully charged with it; or it may be obtained by transmitting a current of sulphureted hydrogen gas through water in which iodine, in fine powder, is suspended. The iodine, from a greater affinity for hydrogen than the sulphur possesses, decomposes the sulphureted hydrogen ; and hence sulphur is set free, and hydriodic acid produced. The constitution of hydriodic acid is,By volume. By weight. Iodine .... 50......124 Hydrogen . . 50...... 1 TOO "125 The solution of hydriodic acid is easily decomposed. Thus, on exposure for a few hours to the air, the oxygen of the atmosphere forms water with the hydrogen of the acid, and liberates the iodine Nitric and sulphuric acids likewise decompose it by yielding oxygen, the former being converted into nitrous and the latter into sulphurous acid. The free iodine becomes obvious on the application of the abovementioned test. The compounds of hydriodic acid with the salifiable bases may be easily formed, either by direct combination, or by acting on the basis in water with iodine. Sulphurous and muriatic acids, as well as sulphureted hydrogen, produce no change on the hydriodates, at the usual temperature of the air; but chlorine, nitric and concentrated sulphuric acid, instantly decompose them, and separate the iodine. The hydriodates of potash and soda are the most interesting of their number, because they are the chief sources of iodine in nature. The latter salt is probably the one which affords the iodine . obtained from kelp ; while it is believed, that it is the hydriodate of potash, which is most generally found in mineral springs. (Hence the necessity of adding sulphuric acid to the residual liquor of the soapboiler, in order to procure the iodine, which requires to be separated from its combination with the alkali to which it is united, in the condition of hydriodic acid ; and peroxide of manganese is also added, in order to facilitate the decomposition of the hydriodic acid.)Iodine forms acids also by uniting with oxygen and with chlorine. When it is brought into contact with protoxide of chlorine, immediate action ensues ; the chlorine of the protoxide unites with one portion of iodine, and its oxygen with another, forming two compounds,a volatile orangecolored matter, the chloriodic acid, and a white solid substance, which is iodic acid. Iodic acid acts powerfully on inflammable substances. With charcoal, sulphur, sugar, and similar combustibles, it forms mixtures which detonate when heated. It enters into combination with metallic oxides, giving rise to salts called iodates. These compounds, like the chlorates, yield pure oxygen by heat, and deflagrate when thrown on burning charcoal. Iodic acid is decomposed by sulphurous, phosphorous and hydriodic acids, and by sulphureted hydrogen. Iodine, in each case, is set at liberty, and may be detected, as usual, by starch. Chloriodic acid, which is also formed by simply immersing dry iodine in chlorine gas, deliquesces in the open air, and dissolves verv freely in water. Its solution is very sour to the taste ; and it reddens vegetable blues, but afterwards destroys them. It does not unite with alkaline bases ; in which respect it wants one of the characteristics of an acid, and has hence been called by GayLussac a chloride of iodine. Iodine unites with nitrogen, forming a dark powder, which is characterized, like chloride of nitrogen, by its explosive property. In order to form it, iodine is put into a solution of ammonia; the alkali is decomposed ; its elements unite with different portions of iodine, and thus cause the formation of hydrioclic acid and iodide of nitrogen. Iodine forms, with sulphur, a feeble compound, of a grayishblack color. With phosphorus, also, it combines with great rapidity at common temperatures, attended with the emergence of heat. It manifests little disposition to combine with metallic oxides; but it has a strong attraction for the pure metals, producing compounds which are called iodurets, or iodides. The iodides of lead, copper, bismuth, silver and mercury, are insoluble in water, while the iodides of the very oxidizable metals are soluble in that liquid. If we mix a hydriodate with the metallic solutions, all the metals which do not decompose water will give precipitates, while those which decompose that liquid will give none. Iodine, besides being employed for philosophical illustration, is used in the arts, for pigments, dyes and medicine. The protoioduret of mercury is used in England as a substitute for vermilion, in the preparation of paperhangings ; and a compound of hydriodate of potassa 65, iodate of potassa 2, and ioduret of mercury 33, is employed in printing calico. The tincture of iodine, 48 grs. to 1 oz. of alcohol, is a powerful remedy in the goitre and other glandular diseases; but it is so violent in its action on the system as to require great caution in its administration. The hydriodate of potash, or of soda, is also applied to medical uses ; and it is inferred, that the efficacy of many mineral springs, in certain diseases, is owing to the presence of one or the other of these salts.