BROMIC ACID- -CHLORIDE OF BROMINE. 147 tageously substituted for ether, as it acquires a yellow tinge with an amount of bromine too small to act upon the ether; the presence of sulphuric or of sulphurous acid must however in this case be avoided. The bromides, when heated with black oxide of manganese and oil of vitriol, yield red vapours of bromine ; strong nitric acid has a similar effect. Nitrate of silver and nitrate of lead each give a white precipitate with solutions of the bromides, forming bromide of silver (AgBr), or bromide of lead (PbBr2); the bromide of silver is insoluble in cold nitric acid, but is dissolved by a large excess of ammonia; but the bromide of lead is dissolved by the addition of diluted nitric acid. Mercurous nitrate also gives a white precipitate of mercurous bromide (HgBr) when added to solutions of the bromides; it is soluble in chlorine water with liberation of bromine. Bromine often combines with the same metal in more than one proportion, and the compounds of bromine correspond almost always, both in number and composition, with those of chlorine with the same metal. Oxybromides may be formed resembling the oxychlorides; and the bromides of the alkaline metals form double bromides with the bromides of the metals which yield acids with oxygen. (392) BROMIC ACID (HBr→, or HO,BrO,=129).—The oxyacids of bromine are, with the exception of bromic acid, nearly unknown, and this compound even has never been obtained in the form of anhydride. The bromic corresponds to chloric acid in composition. Bromate of potassium is procured by acting on bromine with caustic potash, and from this salt the acid is obtained by a process similar to that employed in the preparation of chloric acid (382). By the action of heat, bromate of potassium is decomposed, bromide of potassium being formed, whilst oxygen is liberated. Any solid bromate, when mixed with concentrated sulphuric acid and heated, gives off red fumes of free bromine, while oxygen is evolved. Bromate of silver and mercurous bromate are anhydrous and sparingly soluble: bromate of lead retains I atom of water (PbBr,,H,→); it likewise is but slightly soluble. When heated with hydrochloric acid these precipitates evolve free bromine. All the salts of bromic acid which have as yet been prepared are monobasic. 2 (393) Chloride of Bromine is easily obtained by transmitting chlorine gas through liquid bromine: it is a volatile, reddishyellow liquid, with a very pungent, irritating odour. Water dissolves it, forming a deep yellow solution possessed of considerable bleaching power. Bromide of Nitrogen may be obtained by digesting bromide of potassium with the so-called chloride of nitrogen; it forms a detonating oily-looking liquid, resembling chloride of nitrogen in appearance and properties. § III. IODINE: I=127.* Atomic or Comb. Vol. of Vapour, Theoretic Sp. Gr. of Vapour, 8.756; Observed, 8.716: Sp. Gr. of Solid, 4'947; Melting-pt. 225°; Boiling-pt. 347°. (394) IODINE, the third element in the group which we are now examining, is still denser than bromine, as it assumes the solid form at the ordinary temperature of the air. Its discovery dates back to the year 1811, when it was found by Courtois accidentally, in the waste liquors produced in the manufacture of carbonate of sodium from the ashes of sea-weed. Iodine exists in the ocean in quantities still smaller than bromine. It is, notwithstanding, obtained with less difficulty, since the fuci, algæ, sponges, and other marine plants, extract it from sea-water, and store it up in their tissues. These, when burnt, give an ash which is technically known as kelp; it contains iodine in the form of iodide of sodium. In the mineral kingdom, iodine has been found in one or two rare ores; thus it occurs combined with silver in Mexico, and with zinc in Silesia. Extraction.-Iodine is at present largely manufactured at Glasgow, from kelp made on the coasts of Scotland and Ireland; the following is an outline of the process adopted in procuring it :-The sea-weed having been dried in the sun, is burned in shallow excavations, at a low heat; owing to the volatility of the iodide of sodium at a red heat, the loss of this salt would be considerable if the temperature were allowed to rise too high. The half-fused ash, or kelp, which remains, is broken into fragments and treated with boiling water, which dissolves about one-half of the ash. The liquid thus obtained is then evaporated in open pans, and all that can be separated by crystallization is removed; a double sulphate of potassium and sodium, carbonate of sodium, and chloride of potassium are thus extracted. The iodine remains in the mother-liquor, which still retains sulphide of sodium, besides hyposulphite, and some carbonate of sodium. This liquor, or iodine ley, is now mixed with one-eighth of its bulk of oil of vitriol, * Taking the molecule of iodine when free to be (I I) or I2, its molecular volume in the state of vapour will be EXTRACTION AND PROPERTIES OF IODINE. 149 and allowed to stand for twenty-four hours; carbonic anhydride, sulphurous anhydride, and sulphuretted hydrogen gases escape, and sulphate of sodium crystallizes out, mixed with a considerable quantity of deposited sulphur. The supernatant liquid is next transferred to a stoneware or leaden retort, which, if of lead, is of cylindrical form, a, fig. 300, supported in a sand-bath, and gently heated from beneath by a small fire; the head of the retort, b, c, is luted on with clay, and the contents of the retort, having been heated to about 140°, a FIG. 300. quantity of powdered black ox object of the second tubulure, shown at c, is to facilitate the clearing of the neck of the retort in case it should become. obstructed by the formation of crystals. If the temperature be allowed to rise as high as 212°, the chloride of sodium retained in the ley is decomposed, chlorine is disengaged, and combines with part of the iodine, forming chloride of iodine, which is wasted. In the foregoing process, the addition of the sulphuric acid occasions the decomposition of the carbonate and hyposulphite of sodium, which still remain in solution, as well as of any sulphide of sodium that may be present, forming sulphate of sodium which is removed by crystallization. The liquid retains an excess of sulphuric acid, and all the iodide of sodium. When this mixture is heated with peroxide of manganese, the iodine is liberated, whilst sulphates of sodium and manganese remain in the retort. The process resembles that for bromine : 2 NaI + MnᎾ + 2 HᎦᎾ = Na,ᎦᎾ, + MnᎦᎾ, + 2 H,Ꮎ + Ig. 150 PROPERTIES OF IODINE-STARCH TEST. Properties. The crystalline form of iodine is an octohedron with a rhombic base; but it is generally obtained in bluish-black scales, resembling plumbago in lustre. It is a non-conductor of electricity. At ordinary temperatures, and especially when in a moist condition, it is sensibly volatile, emitting an odour like that of chlorine, but much weaker; when heated it undergoes fusion at 225°; and at about 347° it boils and is converted into a magnificent purple vapour, whence it derives its name (from wdns, violetcoloured). Iodine, when taken internally, acts in large doses as an irritant poison; but in small quantities it is a very valuable medicine, particularly in glandular swellings, and in certain forms of goître. It stains the skin and most organized substances of a brown colour, and gradually corrodes them. Water forms with it a yellow solution, but dissolves it only in very small quantity. Its bleaching properties are very feeble. Alcohol, ether, hydriodic acid, and solutions of the iodides, dissolve it freely, forming brown solutions. Lugol's Solution, which was formerly much used in medicine, consists of 20 grains of iodine and 30 of iodide of potassium, dissolved in an ounce of water; it is of a deep brown colour. Iodine is also soluble in bisulphide of carbon, to which a minute quantity of iodine imparts a characteristic rich violet colour. Chloroform and benzol likewise dissolve it, forming red solutions. Iodine attacks many of the metals rapidly, forming compounds termed iodides; iron or zinc is readily dissolved by it if placed with it in water, an iodide of the metal being formed. The compounds of iodine with the metals and with hydrogen are decomposed by chlorine, and even by bromine, while the iodine is set free. Advantage is taken of this fact in ascertaining the presence of iodine. The most delicate test for it, when uncombined, is the intense blue colour which it strikes with starch; by its means, with due precaution, I part of iodine, when dissolved in one million parts of water, may be discovered. There are various modes of applying this test: the simplest consists in mixing a little cold starch paste with the liquid which is suspected to contain iodine; if it be present in an uncombined form, a beautiful blue colour shows itself. If the iodine be in combination, this colour does not appear until a drop of chlorine water or of solution of bleaching powder be added to set the / iodine free. An excess of chlorine must be avoided, as it forms chloride of iodine, and prevents the action of the test: David Price recommends the use of a solution of nitrite of potassium as a substitute for the chlorine, the liquid to be tested being slightly acidulated no inconvenience arises from the presence of the nitrite in slight excess. As The colour fades away if the solution be heated, but it is partially restored as the temperature falls. Solutions of the alkalies, as well as of sulphurous acid, sulphuretted hydrogen, and reducing agents generally, destroy the colour. starch paste cannot be long kept without undergoing decomposition, it is often convenient to substitute for the freshly made paste, paper which has been smeared with the starch, and allowed to become dry. If kept in a dry place, such paper may be preserved for an indefinite length of time, and is ready for use at any moment. (395) HYDRIODIC ACID (HI =128); Theoretic Sp. Gr. 44125; Observed, 4'443; Mol. Vol.-By heating iodine in hydrogen, the volume of the gas becomes doubled, and a colourless acid gas is produced; but it is never prepared for use in this manner. A better mode is the following:-Place in a small retort 10 parts of iodide of potassium with 5 of water, and 20 of iodine; then drop in cautiously I part of phosphorus cut into small fragments, and apply a gentle heat. Hydriodic acid gas will be extricated abundantly, and may be collected, by displacement, in dry bottles. The result of the reaction is explained by the following equation :— 8 KI + 10 I, + P1 + 16 H2→ yield 4 K2HP✪ + 28 HI. 2 Hydriodic acid gas is not combustible, nor does it support combustion. It fumes in the air, and possess a powerfully acid irritating odour. It is reduced under strong pressure to a yellowish liquid, which freezes at 60°. Water dissolves the gas with great avidity. A solution of hydriodic acid may be easily prepared by suspending iodine in water, and transmitting a current of sulphuretted hydrogen gas through the mixture until the brown colour of the iodine disappears; sulphur is deposited in abundance, and hydriodic acid is formed. The liquid gradually becomes clear if left at rest; it may then be decanted from the precipitated sulphur: the decomposition consists simply in the displacement of the sulphur by the iodine; 2 H2S + 2 I, becoming 4 HI + S. This liquid may be concentrated till it acquires a density of 17, when it consists of (2 HI,11 H2O; Bineau).* It then distils un * Roscoe has shown that, both in this case and in the analogous one of the solution of hydrobromic acid, the constancy of the boiling-point, as well as the apparent definite character of the hydrate, is accidental, and dependent |