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36. Iodine


This section is from the American Hand Book of the Daguerreotype, by Samuel D. Humphrey. Published S. D. Humphrey, 37 Lispenard Street 1858.

36. Iodine

History of Iodine.--This is one of the simple chemical bodies which was discovered in 1812 by M. Courtois, of Paris, a manufacturer of saltpetre, who found it in the mother-water of that salt. Its properties were first studied into by M. Gay Lussac. It partakes much of the nature of chlorine and bromine. Its affinity for other substances is so powerful as to prevent it from existing in an isolated state. It occurs combined with potassium and sodium in many mineral waters, such as the brine spring of Ashby-de-la-Zouche, and other strongly saline springs. This combination exists sparingly in sea-water, abundantly in many species of fucus or sea-weed, and in the kelp made from them. It is an ingredient in the Salt Licks, saline, and brine springs of this country, especially of those in the valley of the Mississippi. It is sparingly found in fresh-water plants, as well also in coal, and in combination with numerous other bodies.

Fermented liquors contain iodine; wine, cider, and perry are more iodureted than the average of fresh waters. Milk is richer in iodine than wine; independently of the soil, with which it varies, the proportion of iodine in milk is in the inverse ratio of the abundance of that secretion. Eggs (not the shell) contain much iodine. A fowl's egg weighing 50 gr. contains more iodine than a quart of cow's milk. Iodine exists in arable land. It is abundant in sulphur, iron, and manganese ores, and sulphuret of mercury: but rare in gypsum, chalk, calcareous and silicious earths. Any attempt to extract iodine economically should be made with the plants of the ferro-iodureted fresh waters. Most of the bodies regarded by the therapeutists as pectoral and anti-scrofulous are rich in iodine.

It is probably to the application of this body that we owe the discovery of the daguerreotype. There is no record of thep recise date when Daguerre commenced experimenting with iodine, but by the published correspondence between him and M. Neipce, his partner, it was previous to 1833. There is no doubt, however, that the first successful application was made in 1838, as the discovery was reported to the world early in January, 1839.

Preparation.--Iodine is mostly prepared from kelp, or the half vitrified ashes of seaweed, prepared by the inhabitants of the western islands, and the northern shores of Scotland and Ireland. It is treated with water, which washes out all the soluble salts, and the filtered solution is evaporated until nearly all the carbonate of soda and other saline matters have crystallized out. The remaining liquor, which contains the iodine, is mixed with successive portions of sulphuric acid in a leaden retort, and after standing some days to allow the sulphureted hydrogen, etc., to escape, peroxide of manganese is added, and the whole gently heated. Iodine distills over in a purple vapor, and is condensed in a receiver, or in a series of two-necked globes.

Properties.--Iodine is solid at the ordinary temperature, presenting the appearance of dark-grey or purple spangles, possessing a high degree of metallic lustre. It somewhat resembles plumbago, with which it is sometimes diluted, particularly when it is fine. Operators should endeavor to secure the larger crystals. It melts at 224.6 deg., forming a brown or nearly black liquid. It boils at about 356 deg., and emits a very deep violet colored vapor. It gives off a very appreciable vapor, sufficient for all purposes of forming the iodide of silver on the daguerreotype plate, at a temperature of 45 deg. or even lower. Iodine crystallizes readily. Every operator has found upon the side of the jar in his coating-box, perfectly regular crystals, deposited there by sublimation.

Water dissolves but a small proportion of iodine, requiring 7000 parts of water to dissolve one of iodine,

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or one grain to the gallon of water. Alcohol and ether dissolve it freely, as does a solution of nitrate or hydrochlorate of ammonia and of iodides.

The density of solid iodine is 4.95; that of its vapor 8.716. It greatly resembles chlorine and bromine in its combinations, but its affinities are weaker. It does not destroy the majority of organic substances, and vegetable colors generally resist its action. It combines with several organic substances, imparting to them peculiar colors. It colors the skin brown, but the stain soon disappears.

Chloride of Iodine--Is formed by passing chlorine into a bottle containing some iodine. This can be readily done by pouring one ounce and a half of muriatic acid upon a quarter of an ounce of powdered black oxide of manganese, and heat it gradually in a flask, to which is adapted a bent glass tube. This tube must connect with the bottle containing the iodine, and the yellowish-green gas disengaged will readily combine with the iodine, forming a deep red liquid, and the operation is complete. The use of chloride of iodine will be referred to in connection with the Accelerators.

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Iodides.--The iodide treated with the oil of vitriol, instantly produces a considerable deposit of iodine; and if the mixture be heated, intense violent vapors are disengaged. The reaction is due to the decomposition of oil of vitriol by iodohydric acid, water and sulphurous acid being formed, and iodine set free. The iodides in solution are decomposed by chlorine, iodine being precipitated, the smallest quantity of which in solution is instantly detected by its imparting to starch an intensely blue color.

Iodide of Potassium.*--This compound is easily made in the following manner: Subject to a moderate heat a mixture of 100 parts of iodine, 75 of carbonate of potash, 30 of iron filings, and 120 parts of water. This mass must be thoroughly dried and then heated to redness; the resulting reddish powder is to be washed with water, and the solution obtained filtered, and evaporated to dryness. It is found that 100 parts of iodine yield 135 parts of very white, but slightly alkaline, iodide of potassium.

* I shall present the preparation of only a few iodides, and such as are more intimately connected with the Daguerreotype.

Experiment.--On projecting dry pulverized iodide of potassium into fused anhydrous phosphoric acid, a violent disengagement of iodine takes place, attended by a transient ignition; fused hydrate of phosphoric acid liberates iodine abundantly from iodide of potassium; this reaction is accompanied by the phenomenon of flame and formation of a considerable quantity of hydriodic acid.

Iodide of Mercury.--For the preparation of iodide of mercury, Dublanc recommends to cover 100 grms. of mercury with 1 kilogrm. of alcohol, to add 124 grms. of iodine gradually in portions of ten grms., and agitating between each fresh addition, until the alcohol becomes colorless again. After the addition of the last 4 grms. the alcohol remains colored, the whole of the mercury having become converted into iodide. The resulting preparation is washed with alcohol; it is crystalline and of a hyacinth color.

Iodide of Silver.--This compound is formed upon every plate upon which a Daguerreotype is produced. The vapor of iodine coming in contact with the silver surface, forms an iodide which is peculiarly sensitive to light.

The various colors produced are owing to the thickness of the coating, and the maximum sensibility of the coating, as generally adopted, is when it assumes a deep yellow, or slightly tinged with rose color.

This compound is largely employed in most photographic processes on paper, and may be easily prepared by the following formula: By adding iodide of potassium to a solution of nitrate of silver, a yellowish-white precipitate of iodide of silver is obtained, which is insoluble in water, slightly soluble in nitric acid, and soluble in a small degree in ammonia, which properties seem easily to distinguish it from the chloride and bromide of silver. Chlorine decomposes it and sets the iodine free, and chlorohydric acid converts it into a chloride. It fuses below a red heat. Although the effect of light on the iodide is less rapid than on the chloride, the former sooner turning black, assuming a brown tinge; but when in connection with gallic acid and the ferrocyanate of potash, it forms two of the most sensitive processes on paper.

Iodide of silver dissolves easily in a solution of iodide of potassium, and the liquid deposits in evaporation crystals of a double iodide.

Iodide of Gold.--If a solution of potassium be added to a solution of chloride of gold, a precipitate of iodide of gold takes place, soluble in an excess of the precipitate. A little free potash should be added to combine with any iodide that may chance to be set free by the chloride of gold.

Iodide of Lime is prepared by adding iodine to hydrate of lime (which will be referred to farther on) until the mixture assumes a light yellow shade, when wanted for combinations with accelerators, or to a dark brown when employed for the first coating. This latter mixture has been sold in our market under the name of "Iodide of Brome."

Iodide of Bromine.--(See page 76.)

Experiments with Iodine.--Place a plate which bas been exposed in the camera over the vapor of iodine for a very brief period, and it will present the appearance of the impression having been solarized.

b. Upon a Daguerreotype plate, from which an impression has been effaced by rubbing or otherwise, the picture may be made to reappear by merely coating it over with iodine.

c. Place in a vessel a little water, into which put the smallest possible quantity of free iodine and add a little starch, and the liquid will instantly assume a blue color. Advantage is taken of this fact in the laboratory to detect the presence of iodine in liquids. The starch should be dissolved in boiling water and allowed to cool. There are numerous other interesting experiments that can be performed by the aid of iodine, but it is unnecessary here to consume more space.


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