Page:The American Cyclopædia (1879) Volume IX.djvu/339

 IODINE 327 ence of iodine in solutions containing only T.!nri.?nri7 ' '* ^ is supposed that the iodine is merely mixed in a finely divided state with the starch. It must be free for the test to suc- ceed ; and to insure this, where the iodine may be in the state of an iodide, it is recommended to add to the solution a drop of sulphuric acid, and then a little vapor of chlorine, or instead of the chlorine a drop or two of nitric acid may he used. Though iodine is detected in a multitude of organic bodies, principally those connected with the sea or in plants growing near the salt water, it is found in largest proportion in the fuci or common seaweeds, and other marine plants which grow at great depths, fhefucus palmatus and saccharinus are especially rich in it. The preparation of the iodine of com- merce is principally carried on at Glasgow, Scotland, at Donegal, Ireland, and at Cher- bourg, France, to which places are brought the half vitrified ashes produced by burning the seaweeds collected on the coast. These ashes, called kelp, or on the continent varec, being coarsely powdered, are digested some hours in water, and the solution is then drawn off upon a fresh portion, and from this upon a third, fourth, fifth, and it may be a sixth, until the liquor has attained a density of 1'257. The various soluble salts, including the iodides and bromides of all the alkalies, are thus taken up and separated from the earthy salts. The solution is then drawn off into broad evaporating pans, and concentrated to 60 Twaddell, or sp. gr. 1-30. At this point the sulphate of soda and chloride of sodium begin to crystallize ; they are ladled out as they sepa- rate from the solution, and placed so that the drainings from them run back into the pans. When they cease to appear, the liquor is left to repose and to deposit more chloride in the pans. It is then drawn off into coolers, and left for five days for the sulphates to crystallize, as also chloride of potassium. The liquor is then again evaporated in the pans, and at 68 T., or sp. gr. 1'34, deposits carbonate of soda, and more chlo- ride of sodium and sulphate of soda. It is again run into a cooler to cause a further separation of chloride of potassium. The process is some- times again repeated, and the liquor brought by evaporation to 74 T. After removing all the crystals that appear, there frequently remain in the solution some chlorides, hyposulphite of soda, and sulphide of sodium. Strong sulphuric acid in the proportion of one seventh of the whole is then added, and after agitation the mix- ture is left to stand for two days. The sulphur- ous compounds are in this time decomposed, and sulphates are produced with liberation of sul- phur in a free state, in sulphuretted hydrogen, and in sulphurous acid. The iodine is fixed by its combination with sodium, and, unless too much acid has been added, cannot be liberated except by oxygen. The lye is now poured into an iron still lined with lead, and when heated in a sand bath to 140, a quantity of manga- nese dioxide is introduced, and the still is luted and connected with its condensers. Iodine vapors, come off at a temperature below the boiling point, and condense in the receivers. Particular care is required that the temperature does not exceed 212, in which case the iodine is apt to combine with chlorine with loss. Cy- anide of iodine often collects in white, prismatic crystals in the receiver furthest from the retort. A portion of iodine remains in the retort in combination with lead and sodium, which is recovered by first converting it into an iodide of copper by the addition of sulphate of cop- per, and, when this is separated by filtration, decomposing it by sulphuric acid and oxide of manganese, and collecting the vapor. The pro- cess above given is somewhat modified at dif- ferent localities. A method has been proposed by Dr. Kemp to dispense with the burning of the plants, by which much iodine is volatilized and lost, and to crush the roots in which the largest proportion of iodine is concentrated, and set them to ferment; after which the iodine with other salts may be dissolved out by water acidulated with hydrochloric acid, and finally separated by proper reagents. It is also proposed to distil the seaweeds instead of incinerating them. Large quantities of iodine are now recovered from the nitrate of soda of Chili, according to a method invented by Thiercelin. The mother liquors resulting from the manufacture of saltpetre are treated with a mixture of sulphurous acid and sulphite of soda, and the iodine is precipitated as a black powder. This is placed in earthen jars, on the bottom of which are layers of quartz sand, fine at the top and coarse at the bottom ; from these jars the iodine is removed by earth- en spoons lined with gypsum, and the greater part of the water is thus separated. It is fur- ther purified by sublimation, but is often sold before undergoing the last named process. The amount of iodine thus reclaimed from Chili saltpetre in 1870 amounted to 30,000 Ibs. Iodine is useful as a test for starch, and also as an ingredient of various chemical reagents. Some of these are of great importance in the photographic art. From the iodide of potassi- um is prepared the iodide of silver, which con- stitutes the sensitive film upon the plates. Iodine forms two important acids, iodic acid, HIO>, and periodic, IIIO t. Hydriodic acid closely resembles hydrochloric acid, as the oxy- gen acids correspond respectively to chloric and perchloric acids. Iodine has been employed in medicine since 1819, although burnt sponge, which depends upon iodine for its efficacy, had been previously used with advantage in treat- ment of goitre. A large number of preparations are employed both for external and internal ap<- plication ; the most important are solutions of iodine in alcohol, iodine dissolved in water by the aid of iodide of potassium, iodoform, the iodide of potassium, and iodides formed with mercury, iron, sulphur, sodium, and arsenic. Iodoform is a yellow salt in hexagonal flat crystals, which contains more than 96 per