Properties.—Iodine has a bluish-black color and metallic lustre; it stains the skin yellow, and has a pungent smell, like diluted chlorine. It is extremely volatile when moist, boils at 350°, and produces dense violet-colored fumes, which condense in brilliant plates. Specific gravity 4·946. Iodine is very sparingly soluble in water, 1 part requiring 7000 parts for perfect solution: even this minute quantity however tinges the liquid of a brown color. Alcohol and ether dissolve it more abundantly, forming dark-brown solutions. Iodine also dissolves freely in solutions of the alkaline iodides, such as the iodide of potassium, of sodium, and of ammonium.
Chemical Properties.—Iodine belongs to the chlorine group of elements, characterized by forming acids with hydrogen, and combining extensively with the metals (see chlorine). They are however comparatively indifferent to oxygen, and also to each other. The iodides of the alkalies and alkaline earths are soluble in water; also those of iron, zinc, cadmium, etc. The iodides of lead, silver, and mercury are nearly or quite insoluble.
Iodine possesses the property of forming a compound of a deep blue color with starch. In using this as a test, it is necessary first to liberate the iodine (if in combination), by means of chlorine, or nitric acid saturated with peroxide of nitrogen. The presence of alcohol or ether interferes to a certain extent with the result.
Iodide of Ammonium.
Symbol, NH{4}I. Atomic weight, 144.
This salt may be prepared by adding carbonate of ammonia to iodide of iron, but more easily by the following process:—A strong solution of hydrosulphate of ammonia is first made, by passing sulphuretted hydrogen gas into liquor ammoniæ To this liquid iodine is added until the whole of the sulphuret of ammonium has been converted into iodide. When this point is reached, the solution at once colors brown from solution of free iodine. On the first addition of the iodine, an escape of sulphuretted hydrogen gas and a dense deposit of sulphur take place. After the decomposition of the hydrosulphate of ammonia is complete, a portion of hydriodic acid—formed by the mutual reaction of sulphuretted hydrogen and iodine—attacks any carbonate of ammonia which may be present, and causes an effervescence. The effervescence being over, the liquid is still acid to test-paper, from excess of hydriodic acid; it is to be cautiously neutralized with ammonia, and evaporated by the heat of a water-bath to the crystallizing point.
The crystals should be thoroughly dried over a dish of sulphuric acid, and then sealed in small tubes containing each about half a drachm of the salt; by this means it will be preserved colorless.
Iodide of ammonium is very soluble in alcohol, but it is not advisable to keep it in solution, from the rapidity with which it decomposes and becomes brown.
The most common impurity of commercial iodide of ammonium is sulphate of ammonia; it is detected by its sparing insolubility in alcohol.
Iodide of Cadmium.