Towler, John. The Silver Sunbeam. Joseph H. Ladd, New York: 1864. Electronic edition prepared from facsimile edition of Morgan and Morgan, Inc., Hastings-on-Hudson, New York. Second printing, Feb. 1974. ISBN 871000-005-9

Chapter X.
PREPARATION OF THE IODIDES.

SEVERAL of the iodides are formed by the direct contact of the elements, as, for instance, the iodide of iron and the iodide of phosphorus. Others by double decomposition, as iodide of silver from a soluble iodide and nitrate of silver. And, finally, others are obtained by combining chemical equivalents of hydriodic acid with the carbonates of the bases required., as, for example, iodide of potassium from hydriodic acid and carbonate of potassa, iodide of barium from hydriodic acid and carbonate of baryta, etc. Iodine or hydriodic acid is the material from which the iodides may be and are prepared.

Iodine.

Symbol, I. Chemical Equivalent, 127 1/10; Specific Gravity, 4.948.

Iodine was discovered in 1812, by Courtois, a chemical manufacturer in Paris. This substance exists in nature combined with metals, such as calcium, magnesium, and sodium; and these are found in many saline springs and mineral waters, as also in sea-water. These salts are absorbed by several marine plants and animals; and it is from such plants that iodine is obtained in considerable abundance. The sea-plants are collected, dried, and burned in large pits, the ashes of which are called kelp. Formerly this kelp was collected on account of the carbonated alkali which it contains; its value no-~v is enhanced on account of the iodides and chlorides which are found in it. The powdered mass is dissolved in cold water, which is afterward evaporated until a scum forms on the surface. The solution is then set aside to cool, when a quantity of crystals will be deposited. By a further evaporation, more crystals may be obtained, until finally the mother-liquor ceases to yield any more. The dark-colored liquid contains the iodides, which may be precipitated by a mixture of five parts of sulphate of iron and two parts of sulphate of copper. The precipitate is subiodide of copper, which, by treatment with sulphuric acid, the deutoxide of manganese and heat, yields iodine in violet vapors, which by condensation form the metallic-looking crystals of iodine. There are other methods of separating the iodides.

Properties.

Iodine resembles plumago or black lead, in outward appearance; it is a crystalline substance, soft and brittle. It melts at 224°, and sublimes at 347°. Its taste is very acrid and astringent; its smell is somewhat like that of chlorine. Water dissolves about one part in seven thousand parts, and receives a brown color. Alcohol and ether dissolve it abundantly; and so do iodide of potassium and hydriodic acid, forming brownish red solutions. Iodine in solution, as tincture, or in iodide of potassium preferably, has very valuable medicinal properties. It is regarded as a specific in the reduction of glandular swellings, and in scrofulous diseases. It is said to cause the pustules of small-pox to abort. In photography, it is impossible to estimate its value; for without it, the art could not exist in its present state.

The impurities in iodine are plumbago, sulphide of antimony, and iodide of cyanogen. If by evaporation on apiece of porcelain there be any residue, one or both of the former impurities may be present; the latter impurity is of rare occurrence.

Tests: Free iodine is easily recognized by the formation of a deep blue color when mixed with a solution of starch; and this blue color is volatilized by heat. The iodine in an iodide has first to be set free before it can be thus tested. To effect this, either a current of chlorine is passed through the solution, or nitric acid is added to it; by boiling the solution afterward, the fumes may be obtained and thus tested.

Preparation of Hydriodic Acid.

Hydriodic Acid.--Symbol, I H. Combining Proportion, 128 1/10, Specific Gravity, 4.43.

This substance is a condensable gas; at a temperature of 59.8°, it solidifies into a transparent, colorless mass; and water absorbs a large quantity. The strongest liquid hydriodic acid has a specific gravity of 1 7/10, when it boils at a temperature between 257° and 262°. It is not a stable com pound; oxygen from the air is absorbed, and iodine is liberated and dissolved by it. Chlorine and bromine decompose it.

Hydriodic acid may be obtained by several methods. From the property which iodine possesses of abstracting hydrogen from several of its compounds, as from phosphide of hydrogen, hydrosulphuric acid, ammonia and organic compounds, methods have been devised to obtain hydriodic acid by their mixture. Thus, by diffusing iodine in powder through water, and then passing a current of hydrosulphuric acid through the solution as long as iodine is thus taken up and the fluid is rendered colorless. By this process, sulphur is deposited and iodine takes its place. By filtration, the sulphur is removed; by heat, the superfluous hydrosulphuric acid is driven away. The remaining transparent solution is hydriodic acid.

A solution of iodide of barium may be decomposed by an equivalent proportion of sulphuric acid, and by filtration from the insoluble sulphate of baryta, hydriodic acid is obtained in solution.

Phosphorus combines very vividly with iodine, and the iodide of phosphorus, when it comes in contact with water, is decomposed into hydriodic acid and phosphoric acid. Liebig has availed himself of this property in the preparation of the iodide of lithium, barium, calcium, potassium, sodium, etc.

Take one part of phosphorus, twenty-four parts of iodine, and forty of warm water; mix them intimately in a Wedgwood mortar by means of the pestle. The color of the fluid is at first dark brown, but becomes transparent as soon as the decompositions are effectuated. The heat of a water-bath and friction will soon complete the action. By this operation, iodine and phosphorus combine, so as to form iodide of phosphorus, which becomes resolved into hydriodic acid and phosphoric acid by the decomposition of the water. A little free iodine added to the transparent solution prevents the formation of phosphorous acid.

Iodide of Barium.

To the transparent solution above obtained, by decantation from any remaining phosphorus, add, in the first place, carbonate of baryta as long as effervescence ensues, and afterward a little water of baryta, so that the mixture becomes slightly alkaline. By this decomposition phosphate of baryta is formed from the phosphoric acid and the carbonate of baryta; and from the hydriodic acid, and the carbonate of baryta, iodide of barium is the resulting formation; and carbonic acid is liberated as gas. The iodide of barium, being soluble, is separated from the insoluble phosphate by filtration. A current of carbonic acid is now passed through the filtrate, in order to combine with any remaining solution of baryta, and the mixture is again filtered.

Iodide of Calcium.

This salt is obtained precisely in the same way as the preceding substituting only milk of lime for the barytic salt. Both these salts crystallize, when slowly evaporated; they are, too, both deliquescent. Froth either iodide of barium or iodide of calcium the alkaline iodides are easily formed.

Iodide of Lithium.

Add two ounces of carbonate of lithia to the iodide of either barium or calcium solutions produced from seven ounces of iodine by the preceding manipulation. The carbonate is previously levigated in water to an impalpable consistency. The mixture is frequently stirred during the twenty-four hours it is allowed to stand, in order to effect the complete precipitation of baryta or lime. The solution of iodide of lithium is now separated by filtration from the insoluble carbonate of baryta or lime. If the iodide of barium or of lime has not been thoroughly decomposed, add a cold solution of carbonate of lithia as long as any precipitate is formed.

Iodide of Potassium.

Digest a hot solution of sulphate of potassa in a solution of iodide of calcium in the proportion of their equivalents for six or eight hours. Double decomposition ensues, the sulphuric acid and oxygen of the potassa combine with the lime to form sulphate of lime, whilst the iodine and potassium enter into combination to form iodide of potassium. By filtration through cloth these two salts are separated. The liquid, containing probably still some iodide of calcium and solution of sulphate of lime, is evaporated and then treated with pure carbonate of potassa as long as any precipitate is produced. The insoluble lime is again separated, and the filtrate is evaporated to crystallization. The mother-liquor is afterward evaporated to dryness.

Iodide of Sodium and Iodide of Ammonium.

These two salts may be prepared in like manner, either from the iodide of barium or of calcium, by the substitution in one case of sulphate and carbonate of soda, and in the other of sulphate and carbonate of ammonia. The results are better with the iodide of barium, owing to the more perfect insolubility of the sulphate of baryta after decomposition. Both of these iodides, as well as that of potassium, may be obtained by the direct action of iodine on the caustic alkalies. In this way iodine is added to a solution of potassa, for instance, until the latter becomes slightly colored; the solution so obtained contains iodide of potassium and iodate of potash; it is evaporated to dryness, and then heated to redness, in order to convert the iodate of potash into iodide of potassium by driving off its oxygen. The fused mass is afterward dissolved and crystallized. Sulphuretted hydrogen is sometimes used to decompose the iodate.

Another method, similar to the first, consists in first obtaining either the iodide of iron or of zinc, by mixing iodine, water, and iron-filings, or iodine, water, and zinc-filings, together, and then heating the mixture until the combination is complete, which is indicated by its becoming colorless. The filtered solution is next decomposed completely by adding solution of carbonate of potassa as long as any precipitate takes place. The precipitate, which is either carbonate of iron or of zinc, is removed by filtration; and the filtrate is evaporated to crystallization.

Iodide of Cadmium.

This very important iodide is formed precisely in the same way as iodide of iron or of zinc, by gently heating a mixture of the filings of cadmium, water, and iodine, until the solution becomes colorless.

Impurities of the Iodides.

The iodides which are formed by the direct contact of the two elements are quite pure if the materials are pure; whereas, if the iodides arise from double decomposition, the combination may sometimes fail in accuracy, in which case carbonates and sulphates of foreign ingredients and iodates of the same base nay be found in such iodides; chlorides may be present, too, in the decomposing carbonates and sulphates, so that we may sometimes expect to find them with the other impurities.

Tests of the Purity of the Iodides.

No precipitate is produced in a pure iodide by solution of chloride of barium. If a precipitate results from the introduction of this test, one or all of the following acids are probably indicated: carbonic, iodic, and sulphuric. Other acids might be indicated, but not probably, because materials are not used in the preparation of the iodides containing the acids hinted at, as, for instance, oxalic, sulphurous, silicic, chromic, hydrofluoric, phosphoric, and boracic. Supposing, however, a precipitate is formed when the test is added, then a carbonate, iodate, or sulphate may be one or all present. The next test is to find out which or how many of the three are present. Add, therefore, nitric acid to the precipitate; if it becomes dissolved, there is no sulphate in the iodide. Carbonic acid or an alkaline carbonate added to lime-water produces a milkiness caused by the formation of the insoluble carbonate of lime; and an iodate in solution is recognized by the addition of chlorine-water, or citric, or tartaric acid, which liberates free iodine, afterward made manifest by solution of starch. The chlorides are tested for as follows: in a given quantity of the iodide precipitate with solution of nitrate of silver, until nothing more falls as sediment; dissolve this sediment in ammonia, and then add nitric acid; if a chloride is present, a white flocculent precipitate will be produced, which is chloride of silver.