phuric acid cannot be employed, as by its action upon the iodide, iodine and sulphur dioxide are liberated, thus— 2KI+3H,SO1=2HKSO1+2H2O+SO2+I2. (3.) Hydriodic acid is produced by the action of sulphuretted hydrogen upon iodine (p. 410). At the ordinary temperature, and in the absence of water, these two substances do not react, hydriodic acid being an endothermic compound (p. 168); but if the iodine be suspended in water and sulphuretted hydrogen passed through, the heat of solution of the hydriodic acid supplies the necessary energy to enable the action to proceed. When, however, the solution reaches a sp. gr. of 1.56 the action ceases, because, as Naumann has shown, the heat produced by the solution of the product is insufficient to carry on the process beyond this degree of concentration. (4.) Hydriodic acid is most readily prepared by the action of phosphorus upon iodine in the presence of water P+51 +4H2O=H3PO4+5HI. The red phosphorus and iodine for this reaction may be placed in a dry flask, and water gradually dropped upon the mixture, when hydriodic acid is rapidly evolved. The gas is allowed to pass through a U-tube containing red phosphorus, in order to arrest any iodine vapour which may accompany it. Hydriodic acid may be collected over mercury or by displacement, as shown in Fig. 107. Properties. Hydriodic acid is a colourless, pungent-smelling gas, which fumes strongly on coming into the air. The gas is readily decomposed by heat into hydrogen and iodine. Thus, if a heated wire be thrust into the gas, or if a spiral of platinum wire be heated in the gas by means of an electric current, the violet vapour of iodine at once makes its appearance. When mixed with chlorine, hydriodic acid is at once decomposed, with the liberation of iodine, thus 2HI+Cl2=2HCl + I2. Hydriodic acid is one of the most readily liquefied gases; at o°, and under a pressure of four atmospheres, it condenses to a colourless liquid. The gas is extremely soluble in water. An aqueous solution of it is readily produced by allowing the gas, obtained by the method of preparation above described, to pass into water. In order to prevent the water from being drawn back into the generating flask, it is convenient to pass the gas through a retort arranged in the position seen in Fig. 108. Should there be any back rush of water, owing to the intermission of the evolution of gas in the apparatus, the liquid in the beaker will be drawn up into the retort and there lodge, leaving the end of the neck open to the air. A saturated aqueous solution of hydriodic acid at o° has a specific gravity of 2. At the ordinary pressure the strongest acid that can be obtained by distillation has a specific gravity of 1.67, and contains 57.7 per cent. of hydriodic acid. This solution boils at 127°. As in the case of the corresponding bromine and chlorine compounds, the particular strength of acid which has a constant boiling-point is a function of the pressure. FIG. 108. Aqueous hydriodic acid, when freshly prepared, is colourless, but it rapidly turns brown, owing to the oxidation of the compound and the solution of the liberated iodine in the acid 4HI+0,=2H2O+212. OXIDE AND OXYACIDS OF IODINE. One compound of iodine with oxygen is known, and three oxyacids, viz. : Mode of Formation. When iodic acid is heated to 170°, it loses water and is converted into the pentoxide 2HIO, H2O+IO Properties. Iodine pentoxide is a white crystalline solid body. It is soluble in water, combining with a molecule of the water to form iodic acid. Iodine pentoxide is more stable than any of the oxides of the other halogens, but, when heated to a temperature of 300°, it decomposes into its elements. IODIC ACID. Formula, HIO3. Modes of Formation.—(1.) Iodic acid can be prepared by adding to a solution of barium iodate the requisite amount of sulphuric acid demanded by the equation Ba(IO3)2+H2SO4 = BaSO4+2HIO. The aqueous solution of iodic acid is decanted from the precipitated barium sulphate, and may be concentrated at 100° without being decomposed. (2.) When chlorine is passed through water in which powdered iodine is suspended, a mixture of iodic acid and hydrochloric acid is produced 3H2O+1+5Cl=5HC1+HIO3. The hydrochloric acid may be removed by the addition of precipitated silver oxide to the solution, and separating the precipitated silver chloride by filtration. (3.) Iodic acid is most conveniently prepared by heating iodine with nitric acid, whereby the iodine is oxidised and a mixture of oxides of nitrogen is evolved as dense red vapours 3H NO,+1=HIO3+H2O+NO+2NO. Properties. Iodic acid is a white crystalline solid, soluble in water. The aqueous solution shows an acid reaction with litmus, but the colour is ultimately discharged by the bleaching action of the compound. Iodic acid does not form any blue colour with starch; being, however, an oxidising substance, it readily gives up oxygen to such reducing agents as sulphur dioxide, sulphuretted hydrogen, or hydriodic acid, with the liberation of iodine, thus 2HIO3+4H„O+5SO2=5H2SO4+I2. If, therefore, a small quantity of sulphurous acid be added to a dilute solution of iodic acid, previously mixed with starch, the blue iodide of starch will be formed. This reaction affords an excellent illustration of the time required for certain chemical changes to go forward. It is readily possible to obtain an interval of 30 to 60 seconds between the addition of the sulphurous acid and appearance of any visible result, when at the expiration of that time the whole mass of the liquid suddenly turns blue.* Iodates. When iodine is dissolved in potassium hydroxide, a mixture of potassium iodide and iodate is produced by an analogous reaction to that which takes place with either bromine or chlorine 6KHO+312=5KI+KIO2+3H2O. With the exception of the iodates of the alkali metals, the iodates are for the most part insoluble in water. On being heated they behave in a similar manner to the bromates, some being decomposed into an iodide and oxygen, while others leave a metallic oxide and evolve iodine as well as oxygen. The alkaline iodates are capable of uniting with iodic acid, forming salts which are termed acid and di-acid iodates, thus Normal potassium iodate Di-acid potassium iodate KIO3. PERIODIC ACID. Formula, HIO4,2H2O or H5106 Modes of Formation.-(1.) The compound represented by the formula HIO, has never been obtained; when aqueous solutions of periodic acid are evaporated, the compound which crystallises out has the composition HIO,,2H2O, or H2IO It may be obtained by boiling silver periodate with water, when an insoluble basic silver salt is produced 2AgIO,+4H,O=Ag2H2IO+HIO,,2H,O. The silver periodate is prepared by passing chlorine into an aqueous solu * See Experiment 246, "Chemical Lecture Experiments," new ed. tion of sodium iodate and sodium hydroxide, when the sparingly soluble disodium periodate separates out— NaIO3+3NaHO+Cl2=2NaCl+Na2H§IO This sodium salt is then dissolved in nitric acid and silver nitrate added, whereby AgIO, is formed, which crystallises out on concentration (2Na,H,IO+2HNO3 =2NaNO3+4H2O+2NaIO. 2NaIO1+2AgNO3=2NaNO3+2AgIO. (2.) Periodic acid is also formed by the addition of iodine to an aqueous solution of perchloric acid— 2HClO4+2H2O+I=Cl2+2HI04,2H2O. Properties. The acid having the composition HIO,,2H.O is a colourless, crystalline, deliquescent substance. It melts at 133°, and at 150° is decomposed into iodine pentoxide, water, and oxygen The acid cannot be converted into HIO by heat, for oxygen is evolved as soon as water begins to be given off. The Periodates constitute a numerous class of salts, many of them being of a highly complex composition. On the assumption that iodine is monovalent in these compounds, their classification is somewhat difficult, and they must be represented as associations of molecules of salts of the unknown monobasic periodic acid, HIO, with metallic oxide and water in various proportionsthus, the silver periodate in the foregoing equation, Ag2H310g, would be expressed by the formula, 2AgIO1, Ag2O,2H2O. The classification of these compounds is much simplified if we regard iodine as here functioning as a heptavalent element. On this assumption the periodates may be considered as the salts of various hypothetical acids, which are all derived from the compound I(HO), (itself hypothetical) by the withdrawal of varying quantities of water. Thus, by the successive removal of one molecule of water, the following three acids would be formed From these three acids the following salts may be regarded as being derived (1.) NaH3IO: AgeHзIO6; Ag5106; Bas(106)2 (2.) Ag.IO; Pb(IO3)2 (3.) KIO; AgIO. |