EXTRACTION OF IODINE FROM SEA-WEED.

171

substance which possessed properties different from those of any form of matter with which he was acquainted. He transferred it to a French chemist, Clement, who satisfied himself that it was really a new substance, and Gay-Lussac and Davy having examined it still more closely, it took its rank among the non-metallic elementary substances, under the name of iodine (ions, violet coloured), conferred upon it in allusion to the magnificent violet colour of its vapour.

This history of the discovery of iodine affords a very instructive example of the advantage of training persons engaged in manufactures to habits of accurate observation, and, if possible, of accurate chemical observation; for had Courtois passed over this new substance as accidental, or of no consequence, the community would have lost, at least for some time, the benefits derived from the discovery of iodine.

For some years the new element was only known as a chemical curiosity, but an unexpected demand for it at length arose on the part of the physician, for it had been found that the efficacy of the ashes of sponge, which had long been used in some particular maladies, was due to the small quantity of iodine which they contained, and it was, of course, thought desirable to place this remedy in the hands of the medical profession in a purer form than the ash of sponge, where it is associated with very large quantities of various saline substances. Much more recently the demand for this element has greatly increased, on account of its employment in photography, and large quantities of it are annually produced from kelp, the collection and burning of which affords occupation to the very poor inhabitants of some parts of the coasts of Ireland and Scotland, who would otherwise have been thrown out of work when soda began to be manufactured from common salt, and the demand for kelp as the source of that alkali had ceased. The sea-weed is spread out to dry, and burnt in shallow pits at as low a temperature as possible, for the iodide of sodium is converted into vapour and lost if the temperature be very high.* The ash, which is left in a half-fused state, is broken into fragments and treated with hot water, which dissolves about half of it, leaving a residue, consisting of carbonate and sulphate of lime, sand, &c. The whole of the iodide of sodium is contained in the portion dissolved by the water, but is mixed with much larger quantities of sulphate of soda, carbonate of soda, chloride of potassium, hyposulphite of soda, and sulphide of sodium. A portion of the water is expelled by evaporation, when the sulphate of soda, carbonate of soda, and chloride of potassium, being far less soluble than the iodide of sodium, crystallise out. In order to decompose the hyposulphite of soda and the sulphide of sodium, the liquid is mixed with an eighth of its bulk of oil of vitriol, which decomposes these salts, evolving sulphurous and hydrosulphuric acid, with deposition of sulphur, and forming sulphate of soda, which is deposited in crystals. The liquor thus prepared is next mixed with binoxide of manganese, and heated in a leaden retort (fig. 173), placed in a sand-bath, when the iodine is evolved as a magnificent purple vapour, which condenses in the globular glass receivers in the form of dark grey scales with metallic lustre, and having considerable resemblance to black lead. The liberation of the iodine is explained by the following equation

NaI + MnO, + 2(HO. SO,) = NaO. SO, + MnO. SO, + 2HO + I.

* The sea-weed is often only charred and not incinerated, so as to avoid loss of iodine.

172

PROPERTIES OF IODINE.

The distillation is conducted at a temperature below 212°, to avoid the liberation of chlorine from the chloride of sodium, and the consequent formation of chloride of iodine.

Several processes have been devised to render the extraction of the iodine from the concentrated solution of kelp easier and more economical. The most promising is very similar to that employed for separating bromine (p. 168). The iodine is liberated by chlorine, and extracted from the liquid by shaking it with benzole; by treating the benzole with solution of potash, the iodine is converted into a mixture of iodide of potassium and iodate of potash, from which the iodine may be precipitated by acidifying with hydrochloric acid.

6KO + I = 5KI + KO.IO

5KI + KO.IO + 6HC1 = 6KCI + 6H0+ Ig.

The features of this element are extremely well marked; its metallic lustre and peculiar odour sufficiently distinguish it from all others, and

AN

Fig. 173.-Extraction of iodine.

the effect of heat upon it is very striking, in first easily fusing it (at 225° F.), and afterwards converting it (boiling point, 347° F.) into the most exquisitely purple vapour, which is nearly nine times as heavy as air (sp. gr. 8.72), and condenses upon a cool surface in shining scales. It stains the skin intensely brown if handled. The specific gravity of solid iodine is 4.95.

When iodine is shaken with cold water a very small quantity is dissolved, forming a light brown solution. Hot water dissolves a larger quantity, but alcohol is one of the best solvents for iodine, producing a dark redbrown solution (tincture of iodine) from which part of the iodine may be precipitated by adding water. A solution of iodide of potassium also dissolves iodine freely. Benzole and bisulphide of carbon dissolve it abundantly, producing fine violet-red solutions, which deposit the iodine, if allowed to evaporate spontaneously, in minute rhombic octahedral crystals aggregated into very beautiful fern-like forms. If an extremely weak aqueous solution of iodine be shaken with a little bisulphide of carbon, the latter will remove the iodine from the solution, and on standing, will fall to the bottom of the liquid, having a beautiful violet colour. By dissolving a large quantity of iodine in bisulphide of carbon, a solution is obtained which is perfectly opaque to rays of light, though it allows heatrays to pass freely, and is, therefore, of great value in physical experiments. A solution of iodine in bichloride of carbon is also used for the same purpose.

Existing, as iodine does, in very minute quantity in the water from various natural sources, it would often be overlooked if the chemical analyst did not happen to possess a test of the most delicate description for it.

Iodine, in the uncombined state, dyes starch of a beautiful blue colour,

as may be proved by heating a grain or two of the element with water, and adding to the solution a little thin starch (see p. 15), or by placing a minute fragment of iodine in a stoppered bottle, and suspending in it a piece of paper dipped in thin starch. This test, however, though sensitive to the smallest quantity of free iodine, gives no indication whatever with iodine in combination, as it always exists in nature; in order, therefore, to test for iodine, a little starch-paste is added to the suspected liquid, and then a drop of a weak solution of chlorine, which will set free the iodine, and cause the production of the blue colour. Characters written on paper with a brush dipped in a mixture of iodide of potassium and starch, are brought out in blue by pouring a little chlorine-gas upon them. It is necessary, however, carefully to avoid adding too much chlorine, since it would immediately destroy the colour of the iodised starch. Alkalies also bleach it, and the colour of a mixture of the iodised starch with water is removed by heating, but returns in great measure when the solution cools.

Though very closely connected with chlorine and bromine in its general chemical relations, there are several points in the history of iodine which cause it to stand out in marked contrast by the side of these elements. The attraction which binds it to hydrogen and the metals is certainly weaker than that exerted by chlorine and bromine, so that either of these is capable of displacing it from its compounds, and its bleaching properties are very feeble. On the other hand, it exhibits a more powerful tendency to unite with oxygen, for boiling nitric acid converts it into iodic acid (101), though this oxidising agent would not affect chlorine or bromine.

Some of the compounds of iodine with the metals are remarkable for their beautiful colours. The iodide of mercury, produced by mixing solutions of iodide of potassium and chloride of mercury, forms a fine scarlet precipitate, which dissolves in an excess of iodide of potassium to a colourless solution.

If this iodide of mercury be collected on a filter, washed and dried, it will be found, on heating a portion of it in a test-tube, that it acquires a fine yellow colour and sublimes in golden yellow crystals, which resume the original red colour when rubbed with a glass rod. If it be spread upon paper and gently heated, the scarlet iodide becomes yellow, but the red colour returns on rubbing it with the thumbnail. These changes of colour are attended by an alteration in crystalline form, but not in the chemical composition of the iodide of mercury.

Iodide of lead has a bright yellow colour, as may be seen by precipitating iodide of potassium with a solution of acetate of lead. The precipitate is dissolved by boiling with water (especially on adding a little hydrochloric acid), forming a colourless solution, from which the iodide of lead crystallises in very brilliant golden scales on cooling. Iodide of silver is produced as a yellow precipitate when nitrate of silver is added to iodide of potassium. The bromide and chloride of silver would form white precipitates.

126. Oxides of iodine.-Although the compound IO, corresponding to hypochlorous acid, is believed to exist, it has never yet been obtained in a separate state, the only known oxides of iodine being iodic acid (IO) and periodic acid (IO, ?) which has only been obtained in the hydrated state. Iodic acid, like the corresponding chloric and bromic acids, is formed when iodine is dissolved in solution of potash or soda

It is most easily prepared by boiling iodine with the strongest nitric acid in a long-necked flask, when it is dissolved in the form of iodic acid which is left on evaporating the nitric acid, as a white mass. This may be purified by dissolving in water and crystallising, when the iodic acid forms.

white hexagonal tables, which have the composition HO. 10, + 2Aq. Heated to 266° F., they become HO. IO,, and at 360° F. the whole of the water is expelled, leaving anhydrous iodic acid, which is decomposed at about 700° F. into iodine and oxygen. The anhydrous iodic acid oxidises combustible bodies, but not with any great violence. The hydrate is far more stable than the hydrated chloric and bromic acids. Its solution first reddens litmus paper, and afterwards bleaches it by oxidation. Its salts, the iodates, are less easily soluble in water than the chlorates and bromates, which they resemble in their oxidising action upon combustible bodies. They are all decomposed by heat, evolving oxygen, and sometimes even iodine, showing how much inferior this element is to chlorine and bromine in its attraction for metals.

It is a remarkable feature of the iodates, that some of them contain two or even three equivalents of iodic acid to one of base. Thus there are three iodates of potash, KO. IO,, KO. 210,, and KO. 310,. No such compounds are known in the cases of chloric and bromic acids.

Periodic acid has not been satisfactorily obtained in the anhydrous state. The hydrated periodic acid is obtained from the basic periodate of soda formed by passing chlorine through a mixture of iodate of soda and free soda, when the latter is decomposed, its sodium being abstracted by the chlorine, whilst its oxygen converts the iodic acid into periodic acidNaO. IO, 3NaO + + Cl2 2NaO. IO, = 2NaCl. + Basic periodate of soda.

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This periodate of soda is deposited, being sparingly soluble in water, a most unusual circumstance with salts of soda. By dissolving it in nitric acid, and adding nitrate of silver, a basic periodate of silver is obtained, which is yellow when precipitated from cold, and red from hot solutions-2NaO. IO, + 2(AgO. NO1) 2AgO. 10, + 2(NaO.NO) .

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When the silver salt is dissolved in nitric acid, it is decomposed into nitrate of silver, which remains in solution, and neutral periodate of silver, which is deposited in crystals

2Ag0.10, HO. NO, = AgO. IO, + AgO. NO, + HO.

When neutral periodate of silver is boiled with water, it again yields the insoluble basic periodate of silver, and hydrated periodic acid is found in the solution

2(AgO. 10) + HO

=

2Ag0.10, + HO.IO,.

On evaporating the solution, the hydrated periodic acid is deposited in prismatic crystals having the composition HO. 10, + 4Aq, which lose their water at about 320° F., and are decomposed into iodic acid and oxygen at 400° F. The solution of periodic acid, of course, exhibits

oxidising properties.

The periodates are remarkable for their sparing solubility in water; they are easily decomposed by heat, like the iodates. It will have been remarked, in the above account of the preparation of periodic acid, that this acid exhibits a great tendency to the formation of basic salts, whilst iodic acid is remarkable for its acid salts.

127. Hydriodic acid.-Iodine vapour combines with hydrogen, under

the influence of heated platinum, to form hydriodic acid gas. The gas is best prepared by decomposing water with iodine in the presence of phosphorus, so as to produce hydriodic acid and phosphoric acid, which is allowed to act upon iodide of potassium in order to produce more hydriodic acid

100 grains of iodide of potassium are dissolved in 50 grains of water in a retort (fig. 174), and 200 grains of iodine are added; when this has dissolved, 10 grains of phosphorus are introduced, and the mixture

heated very gradually, the gas being collected by downward displacement in stoppered bottles, which must be placed in readiness, as the gas comes off very rapidly. A loose roll of dry filter paper in the neck of the retort will be useful to retain drops of liquid. These quantities will fill four pint bottles with the gas.

Hydriodic acid gas is very similar in its properties to hydrochloric and hydrobromic acids, fuming strongly in moist

acid.

air, very readily absorbed by water, lique- Fig. 174.-Preparation of hydriodic fied only under strong pressure, and soli

dified by extreme cold. It is much heavier, its specific gravity being 4-44. If a bottle of hydriodic acid gas be placed in contact with a bottle containing chlorine or bromine vapour diluted with air (fig. 133), it will be instantly decomposed, with separation of the beautiful violet vapour of iodine.

The aqueous solution of hydriodic acid is most conveniently prepared by passing hydrosulphuric acid gas through water in which iodine is suspended, HS + I = HI + S, the separated sulphur being filtered off, and the solution boiled to expel the excess of hydrosulphuric acid. Solution of hydriodic acid differs greatly from hydrochloric and hydrobromic acids, in being decomposed by exposure to air, its hydrogen being oxidised and iodine separated, which dissolves in the liquid and renders

it brown.

This tendency of the hydrogen of hydriodic acid to combine with oxygen renders that acid a powerful reducing agent. It is even capable of converting hydrated sulphuric acid into hydrosulphuric acid

so that when iodide of potassium is heated with concentrated sulphuric acid, hydrosulphuric acid is evolved in considerable quantity.

The action of hydriodic acid upon the metals and their oxides is generally similar to that of the other hydrogen acids.

When potassium is heated in a measured volume of hydriodic acid, the iodine is removed, and the hydrogen occupies half the original volume. Hence 1 volume of hydrogen is combined with 1 volume of iodine vapour in 2 volumes of hydriodic acid. One equivalent (47 grains) of potash is neutralised by 128 grains of hydriodic acid. This quantity occupies 4 volumes (8 grains O 1 volume), so that 1 eq. or 4 volumes or 128 parts by weight of hydriodic acid, will contain 1 eq. or 2 volumes or 1 part by weight of hydrogen, and 1 eq. or 2 volumes or 127 parts by weight of iodine vapour.

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