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In chemical properties iodine resembles chlorine and bromine, although it is much less energetic. It bleaches, although slowly, and combines directly with hydrogen, chlorine, sulphur, phosphorus, and many of the metals. When a crystal of iodine is laid on a small piece of phosphorus the combination is attended with so much heat as to inflame the latter. As in the case of bromine, sodium and iodine can be heated together without change, while with potassium the action is explosively rapid.

With starch solution a trace of iodine forms a deep blue color. This color is destroyed on heating and reappears on cooling. The compounds of iodine do not produce this color with starch until a little chlorine is added to liberate the iodine; an excess of chlorine destroys the color.

Uses.-Iodine is used extensively in the manufacture of some of the coal-tar colors, in the preparation of iodides, in photography, and in medicine.

Impurities and Tests.-Fixed impurities, as coal, plumbago, etc., may be recognized by volatilizing a small quantity of the sample, which should leave no residue. An excess of moisture is indicated by the iodine adhering to the sides of the bottle, and by it failing to form a perfectly clear, limpid solution with chloroform. Iodine chloride is detected on agitating a sample with water, to which not more than a yellowish-brown color should be imparted. Compounds of iodine with chlorine, bromine, and cyanogen may be detected by triturating one part of iodine with twenty parts of water and filtering the solution. To one-half of the solution carefully add decinormal sodium hyposulphite solution until the solution is just decolorized. Then add a few drops of ferrous sulphate solution, and subsequently a little sodium hydrate, and heat the mixture gently. On now adding a slight excess of hydrochloric acid, the liquid should not assume a blue color, indicating the absence of iodine cyanide. To the other half of the aqueous filtrate add a slight excess of silver nitrate solution, shake actively and allow the precipitate to subside, and, having poured off the clear supernatant liquid completely, shake the precipitate with a mixture of 1 c.c. of ammonia water and 9 c.c. of water, and filter. Upon the addition of a slight excess of nitric acid to the filtrate not more than a slight opalescence should make its appearance, indicating the limit of chlorine or bromine.

PRACTICAL EXERCISES.

(1) Pass chlorine into a dilute (about one per cent.) solution of potassium iodide; the liquid will become dark in color, owing to separation of iodine. In stronger solutions the iodine would be precipitated.

(2) To a portion of the iodine solution, diluted with water, add solution of starch; a blue color will form, due to the combination of starch with iodine. When a precipitate is formed instead of a blue color, it is an indication that the iodine solution is too strong, and a more dilute one must be used. The blue color of the starch iodide is destroyed by heat and by excess of chlorine.

(3) To another portion of the iodine solution in a test-tube add a small quantity of chloroform and agitate; the chloroform will remove the iodine from the water, and settle to the bottom of the tube as a deep violetcolored liquid, and the water above will become nearly colorless.

IODINE AND HYDROGEN.

HYDROGEN IODIDE.

Formula, HI.

HYDRIODIC ACID.

Molecular Weight, 127.53.

Preparation. A mixture of equal volumes of hydrogen and iodine vapor, passed over platinized asbestos heated to redness, partly combines, forming hydrogen iodide. This reaction does not take place nearly so readily as in the case of hydrogen and chlorine, or even hydrogen and bromine; and the combination only imperfectly takes place when hydrogen is burned in iodine

vapor.

As with hydrogen bromide, the method with amorphous phosphorus is the best for obtaining the gas in quantity. One part of amorphous phosphorus is mixed with fifteen parts of water in a suitable flask, and twenty parts of iodine are gradually added, keeping the flask cool, with ice if necessary, and allowing the action after each addition to cease before adding more. When all the iodine has been added, and action has finally ceased, gentle heat may be applied, and the gas, which is very heavy, collected by downward displacement. The same precautions of passing the gas over a mixture of amorphous phosphorus and broken glass may be observed here as with hydrogen bromide. Mercury is acted on by hydriodic acid gas, and, therefore, cannot be used for the collection. The reaction is as follows:

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Another method, used when a solution of the gas in water is desired, consists in passing hydrogen sulphide into water containing iodine:

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The small quantity of iodine in solution is first converted into hydrogen iodide, which in turn dissolves more iodine to be acted Sulphur is deposited at the same time, and to prevent it enclosing the iodine, it is preferable to agitate, and add the latter in small portions at a time. The solution must be boiled to separate the excess of hydrogen sulphide, and filtered to remove sulphur.

Hydrogen iodide may also be prepared by heating together phosphoric acid and potassium iodide :

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The use of sulphuric instead of phosphoric acid in this process is precluded on account of the reduction of the sulphuric acid, according to the following reaction:

2KI + 3H2SO4 Sulphuric Acid.

Potassium
Iodide.

=

2KHSO4 + 2H2O + SO2 + Ig.
Water. Sulphur Iodine.
Dioxide.

Acid
Potassium

Sulphate.

Properties. Hydrogen iodide is a colorless, heavy gas, with a sharp, irritating odor, and an acid taste and reaction. At o° and a pressure of four atmospheres it forms a colorless liquid, which solidifies at 51°. Its specific gravity is 4.41. The gas is very soluble in water, and when saturated at o° the solution has a specific gravity of 1.99.

Both the strong and weak solutions vary in boiling point until the strength of 57.75 per cent. is reached, when it distils at 127°, under ordinary atmospheric pressure. The gas commences to decompose at 180°, and if some of it be poured down on the flame of a Bunsen burner, clouds of iodine vapor are evolved. It also readily decomposes when a red-hot platinum wire is plunged into a vessel of it, or more slowly when simply exposed to the air. This accounts for the difficulty of preparing the gas by the direct union of the elements. The aqueous acid readily decomposes on exposure to light, and becomes dark brown in color; for this reason a syrup of it has been made official, instead

of the acid of 1860, in order to take advantage of the preservative

action of sugar.

Uses.-The acid has some use in medicine, and is also much employed in chemical operations where a reducing agent is desired, on account of the readiness with which it is decomposed, giving hydrogen in the nascent state.

Io dides. These salts, like the bromides, may be prepared from the acid, but it is cheaper to make them directly from the element. They are nearly all soluble in water, the important exceptions being the silver, mercurous, mercuric, and lead iodides. Heat fuses and decomposes the iodides, with formation of the metal or its oxide and free iodine.

Detection. The acid and its salts are detected by the addition of starch and chlorine water, when, if the starch solution be sufficiently dilute and the chlorine in not too great excess, there will be formed the blue starch iodide. This blue color disappears on heating and reappears on cooling, and is destroyed by an excess of chlorine water. When the blue mixture is agitated with ether, chloroform, or carbon disulphide, the iodine is withdrawn, imparting to the solvent a red or violet color, according to the amount present. Before deciding finally on the presence of hydriodic acid and iodides, one must be sure that such salts as the sulphites and thiosulphates are either absent or are completely oxidized by the chlorine, for the blue color will not appear until all readily oxidizable substances are converted. A few organic substances, notably tannin, interfere with the formation of the blue color. When bromides are present with iodides, the color resulting from the addition of starch and chlorine water will be green, from a mixture of the yellow starch bromide with the blue. starch iodide.

IODINE AND CHLORINE.

These two elements unite in two proportions, forming iodine monochloride, ICI, and iodine trichloride, ICl. The former is prepared by passing dry chlorine over dry iodine, until the latter is liquefied, or by distilling one part iodine with four parts potassium chlorate, or by boiling iodine with strong aqua regia, diluting with water, and agitating with ether. The ether dissolves the iodine chloride, and leaves it behind on evaporation. It is a thick, reddish-brown liquid, which solidifies, on standing, to well-defined crystals, which melt at 24.7°. It is decomposed by water, forming iodic acid, hydrochloric acid, and free iodine. might be expected, it does not color solution of starch, on account of the presence of chlorine.

As

lodine trichloride is prepared by continuing the passing of chlorine

over the iodine monochloride until a solid compound is obtained. This is a solid, crystalline compound, of a yellow color, which, when heated to 25°, gives off chlorine, forming the monochloride. It partly dissolves in water, and is in part decomposed into iodic and hydrochloric acids and free iodine. A solution is prepared for medicinal use by suspending 5.5 grammes of iodine in 22 grammes of water, and passing in chlorine as long as it is absorbed by the well-cooled mixture.

The solution contains 10 grammes of iodine trichloride.

For testing, the aqueous solution (1 to 20) is agitated with chloroform, which must not be colored violet at once, but the color should appear immediately on the addition of a few drops of stannous chloride.

FLUORINE.

Symbol, F.

Atomic Weight, 19.

Valence, I.

History. In the latter part of the eighteenth century, Marggraf noticed that his retort was powerfully attacked when he heated in it a mixture of fluor-spar and sulphuric acid. Ampère, in 1810, pointed out the analogy between hydrochloric acid and the compound at that time called fluoric acid; hence, when adapted to more modern nomenclature, the name of this peculiar compound became hydrofluoric acid.

Many attempts have been made in recent years to isolate the element. Its action on the vessels containing it and its intense affinity for moisture prevented its separation in the free state in such a quantity and degree of purity as to be of any value.

In 1886, Moissan, by passing a powerful electric current through anhydrous hydrofluoric acid, liquefied at o° and cooled to — 23°, and contained in a U-shaped tube of platinum, succeeded in obtaining the free element as a colorless gas. A small quantity of potassium fluoride was added to the acid to make it a better conductor, and the positive pole, at which the element appeared, was an alloy of platinum and iridium. The platinum tube in which the operation was carried out was not acted on perceptibly at such a low temperature.

Occurrence.-Fluorine is said to occur in the free state in the crevices of certain fluor-spar deposits in Bavaria. It is, however, only in minute quantity and recognized by its odor. It occurs combined in a number of minerals, chiefly as calcium fluoride or fluor-spar, CaF, and as sodium and aluminum fluoride or cryolite, 6NaF. Al,F. The former of these is pretty well distributed over the globe, the latter is found in Greenland. Fluorine has been found in small quantity in sea water, in many mineral waters, in the bones and teeth of man, and in milk.

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