This section is from the book "Materia Medica And Therapeutics Inorganic Substances", by Charles D. F. Phillips. Also available from Amazon: Materia medica and therapeutics.
Iodine occurs in the form of iodide, with magnesium and sodium, in sea-water, and in many mineral waters, such as those of Kreuznach, Cau-terets, etc.; also in sponges and sea-weeds, in water-cress, beans, potatoes, etc. Molluscs, and the liver of the cod and other fish, contain iodine, and in the human organism minute quantities are commonly found.
Iodine is prepared from kelp, the residue of burnt sea-weed, soluble iodides being extracted by water, treated with sulphuric acid, and distilled with manganese oxide. Free iodine volatilizes and is condensed in receivers - 2HI (hydriodic acid) + MnO2+H1SO4=MnS04+ 2H1O+I2.
Iodine forms heavy, dark, glistening scales, which stain yellow or brown, and have a peculiar, irritating odor. It is volatile, rising in violet-purple vapor at 400°. The sp. gr. of this vapor is 8.7, that of the crystals 4.9. It is soluble in alcohol, ether, and chloroform, and in water containing salt or iodide of potassium, but very slightly soluble in pure water (1 part in 7,000). The best test for free iodine is starch solution, which forms with it a dark blue iodide. In testing an alkaline iodide, nitric acid or solution of chlorine must be added before the starch, which should be cold, for the iodide loses its color on heating. The addition of caustic alkali also decolorizes the solution, iodide and iodate of the alkali being formed - 6I + 6KHO=5KI + KIO3 + 3H1O. Iodine is closely related, chemically, to bromine and chlorine (Halogens).
It has a stronger affinity for oxygen than these latter; but, for all other elements besides oxygen, a weaker affinity.