152

HYDRIODIC ACID-IODIDES.

changed at 262°. It is a powerful acid, and dissolves iodine freely, forming a brown solution: by exposure to the air, especially if placed in a strong light, it absorbs oxygen, water is formed, and the liquid becomes brown from the liberation of iodine. Chlorine effects its instant decomposition, whether it be in the gaseous form or in solution. Bromine and nitric acid act similarly and with almost equal rapidity; many other oxidizing agents also decompose it. Mercury decomposes it gradually, and combines with its iodine. A solution of hydriodic acid dissolves many of the metals, such as zinc and iron, with evolution of hydrogen and formation of a metallic iodide.

The composition of hydriodic acid may be ascertained by heating potassium in a measured volume of the gas. Iodide of potassium is formed, and hydrogen remains equal in bulk to half the acid gas employed; consequently its composition may be thus represented :

(396) Iodides.-The iodides of the metals are all solid at ordinary temperatures; they are less fusible and volatile than the corresponding chlorides and bromides. The iodides of gold, silver, platinum, and palladium are decomposed by heat alone, whilst the metals are left in a state of purity; but most of the iodides are converted into oxides when heated in the air,-the oxygen displacing the iodine. All the iodides, whether solid or in solution, are decomposed by chlorine and bromine, as well as by nitrous acid and concentrated nitric acid, with liberation of iodine. Iodine is also set free when an iodide is heated with oil of vitriol

and peroxide of manganese. Water dissolves the greater number of the metallic iodides freely; some of them are insoluble, and exhibit colours of great brilliancy. The iodides of some of the metals which form acids with oxygen, such as those of tin, arsenic, and antimony, are decomposed by water. The soluble iodides of the metals may be obtained by the direct action of hydriodic acid upon the metallic oxide, or by the action of iodine and water upon the metal itself. These iodides, when in solution, are characterized by the reaction with starch already mentioned. With a

upon causes similar to those traced in hydrochloric acid. Hydrofluoric acid evidently furnishes a true hydrate, but it was found to be partially decomposed by varying the temperature of evaporation.

OXIDES OF IODINE-IODIC ACID.

153

solution of corrosive sublimate (HgCl,) they give a beautiful salmon-coloured precipitate, which almost immediately changes to a brilliant scarlet: this is the biniodide of mercury: it is soluble in excess both of iodide of potassium and of corrosive sublimate. Mercurous nitrate gives a green precipitate in solutions of the soluble iodides. With nitrate of lead they yield a bright yellow precipitate of iodide of lead, which is slightly soluble in boiling water, especially if the lead salt be present in excess; on cooling, the iodide of lead is deposited in very beautiful silky scales. With nitrate of silver a buff-coloured iodide of silver, nearly insoluble in ammonia, is formed. If a mixture of green sulphate of iron and sulphate of copper be added to a solution of any iodide, a white subiodide of copper (EuI) is precipitated. With chloride of gold the iodides give a lemon-yellow precipitate; and with salts of palladium a brown iodide of palladium is produced, which is sometimes used for ascertaining the quantity of iodine present in a solution in which it occurs mixed with chlorine, since chloride of palladium is freely soluble in water (541).

(397) OXIDES OF IODINE.-Iodine has a more powerful attraction for oxygen than either chlorine or bromine, and forms with it two compounds which by their action upon water furnish welldefined acids, viz., the iodic (HIO) and the periodic acid (HIO) besides some other oxides but imperfectly known.

5

IODIC ANHYDRIDE (I2 = 334 or IO=167); Comp. in 100 parts; iodine, 76.04; oxygen, 23'96.-This body may be obtained by the cautious application of heat to iodic acid.

3

Iodic Acid (HIO, or HO,IO=176).—This acid corresponds in composition to chloric and bromic acids. It may be prepared in several ways. 1.-It may be procured by long boiling of iodine in concentrated nitric acid. 2.-Equal parts of chlorate of potassium and iodine may be mixed with 5 parts of water and a little nitric acid; chlorine is thus evolved in abundance, whilst iodate of potassium is formed and dissolved in the liquid: the chloric acid which is set free in the first instance by the nitric acid imparts its oxygen to the iodine, chlorine gas escaping, while the iodic acid liberates a fresh portion of chloric acid from the chlorate, and this undergoes a similar decomposition (Millon). 3.—Liebig's plan of preparing iodic acid consists in suspending iodine in water, and transmitting through it a current of chlorine gas till the iodine is dissolved; the liquid is then neutralized by carbonate of sodium, when a copious effervescence ensues, attended by a precipitate of iodine, which may be again treated similarly by chlorine. In this case the chlorine combines directly with

154

IODIC ACID-IODATES.

the iodine and forms terchloride of iodine, which is dissolved by water unaltered. It is decomposed on the addition of the alkaline carbonate in the following manner :

5 IC+9 Na€Ð ̧ = 15 NaCl + 3 NaI0, + I + 9¤¤2. The neutralized liquid contains iodate and chloride of sodium. Chloride of barium is next added; an abundant precipitate of iodate of barium, which is but sparingly soluble, is formed; this is washed from adhering salts, and is decomposed by a quantity of sulphuric acid just sufficient to combine with the barium: iodic acid is dissolved by the water, whilst the insoluble sulphate of barium is separated.

2

Iodic acid may be obtained by spontaneous evaporation of its aqueous solution in crystals composed of 3 HIO, at a temperature of 266° it loses two-thirds of its water, and becomes (HIO,,I,) or (HO,310); by raising the heat to 360° it is converted into the anhydride, and at about 700° it is decomposed into iodine and oxygen. Its solution is destitute of odour, and has a sour, astringent taste: many organic bodies decompose it, and owing to this circumstance litmus-paper is first reddened and afterwards bleached by it.

5

Iodates.-Iodic acid offers some anomalies in its combination with bases. Some iodates contain atom, some 2, and some 3 atoms of anhydride to 1 atom of base: for example, there are three iodates of potassium; they may represented by the following formula:-the normal iodate, KIO,; the acid iodate, often called the biniodate, KH 2 IÐ ̧: and the teriodate, [2 (KH 2 IÐ3), I?]. All the iodates are decomposed by heat, and give off oxygen. If the metal have a stronger attraction for iodine than for oxygen, an iodide of the metal is formed; thus iodate of potassium 2 KIO becomes 2 KI+30,: but if the attraction of the metal be greater for oxygen than for iodine, the oxide is left behind: iodate of barium, for example, is converted into baryta, oxygen gas, and free iodine, the latter escaping with oxygen in violet vapours; 2 (Ba 2 I03) yielding 2 Ba✪ +5 + 2 I. The iodates of the metals of the alkaline earths, if not heated too strongly, leave a basic periodate, the barium salt consisting of (Ba 2 I, 4 BaÐ) or 5 BaO,IO,. The aqueous solutions of the iodates are decomposed by sulphurous acid; for example, KIÐ,+3 H2SO3=KI+ 3H2SO; an iodide of the metal is formed, and then the iodine may be discovered by the starch test in the usual way. With the exception of the iodates of the alkali-metals the iodates are

2

PERIODIC ACID-CHLORIDES OF IODINE.

155 but sparingly soluble. The calcium salt retains 6 H2O; those of strontium, and barium (Ba✪ 2 103, H2O) if precipitated from hot solutions, retain H2O; whilst the iodates of lead and silver are anhydrous iodate of silver is insoluble in diluted nitric acid. Iodic acid forms crystallizable compounds with the sulphuric and many other acids.

(398) PERIODIC ACID (HIO, or HO,10,=192).—This acid corresponds in composition to the perchloric: the anhydride is at present unknown. The acid is obtained by transmitting a current of chlorine gas through a solution of iodate of sodium, to which caustic soda has been added in the proportion of 3 atoms of free alkali to 1 atom of iodate of sodium: a sparingly soluble basic periodate of sodium is formed. The reaction which occurs may be represented by the following symbols :

2 Nale,+6 NaHe+2 Cl, Na,0, 2 NaIe, + 4 NaCl + 3 H20.

=

The basic periodate of sodium, which when cry tallized contains (Na,, 2 NaIO,, 3 H2O) is dissolved in diluted nitric acid, and precipitated by the addition of nitrate of silver; the periodate of silver is then dissolved in boiling nitric acid; a normal periodate of silver (AgI) crystallizes as the liquid cools, and this salt when treated with water is decomposed into basic periodate of silver, which is insoluble, and periodic acid, which is dissolved. By evaporation of the solution the periodic acid may be obtained in deliquescent oblique rhombic prisms (HIO,,2 H2O), which are somewhat soluble in alcohol and in ether. The periodates are most of them sparingly soluble in water, but are dissolved freely by diluted nitric acid. The normal sodium salt (NaIO) causes, with solutions of normal salts of barium, a precipitate of a basic periodate 2 (Ba 2 IO), 3 BaÐ,5 H,O. Analogous precipitates are formed with salts of calcium, of lead, or of silver, whilst the liquid becomes acid. The basic silver salt is pale yellow (2 AgIÐ, Ag2, 3 H2O) if precipitated from cold, but red (2 AgIЄAg‚¤‚Í‚Ð) if from hot solutions.

(399) CHLORIDES OF IODINE.-Two compounds of iodine with chlorine, a protochloride (ICI) and a terchloride (IC), may be obtained.

The protochloride is a very irritating, volatile, yellowishbrown liquid, which is obtained by distilling 1 part of iodine with 4 parts of chlorate of potassium; the distilled chloride of iodine is soluble, apparently without change, in alcohol, and in ether.

156

IODIDE OF NITROGEN.

This chloride when hot dissolves iodine readily, and deposits it in beautiful crystals.

Terchloride of iodine is procured by acting upon iodine with excess of dry chlorine gas. It forms magnificent ruby red crystals, which undergo spontaneous sublimation in closed bottles: the vapour is extremely irritating to the eyes. If exposed to the air it attracts moisture, and is dissolved by water without experiencing decomposition. Alkaline solutions decompose it, and iodine is precipitated as in Liebig's method of preparing iodic acid (397).

Bromides of Iodine.-Iodine also combines with bromine, and forms compounds with it which are possessed of properties similar to those of the chlorides of iodine.

(400) IODIDE OF NITROGEN? (NHI). This substance may be obtained as a black powder by digesting iodine for half an hour in a cold solution of ammonia. The brown supernatant liquid, which contains an excess of iodine held in solution by iodide of ammonium, is decanted, and the insoluble powder is placed upon filtering-paper in quantities of a grain or less, and allowed to dry spontaneously. It may also be prepared by mixing alcoholic solutions of iodine and ammonia, and diluting with water, when it falls as a black powder which may be washed with an aqueous solution of ammonia. When dry, it explodes upon the slightest touch, and indeed it often detonates without any assignable cause: the explosion is remarkably sharp and sudden, fumes of iodine are produced, and a faint light is emitted.

2

The experiments of Bineau, the results of which have been subsequently confirmed by Gladstone, have shown that this detonating compound is not a mere iodide of nitrogen, but that it contains hydrogen also, having the formula NHI,. The mode of its preparation admits of easy explanation by the following equation: 2 I,+3 H2N=2 H2NI+NHI,, the reaction of 4 atoms of iodine upon 3 of ammonia producing 2 of iodide of ammonium, and I atom of the detonating substance. Bunsen assigns to the compound obtained by precipitating an alcoholic solution of iodine with ammonia the formula (NH,,NI), but it is more probable that the composition attributed to it by Bineau (Ann. de Chimie, III. xv. 71) is correct.

Iodide of nitrogen becomes slowly decomposed in water; ammonia retards, but potash and the acids accelerate the decomposition; chlorine, bromine, and strong nitric acid destroy it rapidly; sulphuretted hydrogen also effects its decomposition quietly but completely. The results of the reaction last mentioned afford a