Page:Encyclopædia Britannica, Ninth Edition, v. 5.djvu/508

Rh 496 CHEMISTKY [IODINE Reaction. Units of heat dereloped or absorbed. Remarks. T3 r ci 2, o -18,040 Gaseous oxide. 1 C1 3 0, Aq + 9,440 ( Absorption of gas by 01 water. 3 2. CL, 0, Aq ... - 8,600 ( Formation of aqueous o ( acid. 3 o &( C1,0,H, Aq + 29,880 1 Ditto from Cl, 0,H, and &amp;gt;&amp;gt; . ClOHAq, KOHAq + 9,980 Heat of neutralization. f ! Formation of aqueous C1 2 ,0 5, Aq, -20,480 acid from chlorine, oxygen, and water. ( Ditto from chlorine, oxy Cl,0 3 ,H,Aq + 23,940 gen, hydrogen, and 9 ( water. H3 c ClOgHAq, KOHAq + 13,760 Heat of neutralization. c C10 3 K, Aq - 10,040 Heat of dissolution. c KC1, 0.,.. 9,770 Oxidation of dry KC1. f-&amp;gt; c ( Formation of crystalline ^ K,C1, 3 + 95,840 &amp;lt; KClOj from its ele- (^ mcnts. IIClAq,0 3 -15,380 ( Oxidation of HC1 in ( solution. KClAq,O 3 -15,370 ( Oxidation of KC1 in k. I solution. In this table, and in all similar tables, the reactions are represented empirically, and the figures after the symbols merely indicate the number of atoms which enter into the composition of the compound. Thus, the reaction ex pressed as Cl s, O, merely refers to the formation of a mole cule (in grammes) of hypochlorous anhydride, C1 2 O, from chlorine and oxygen, and the arrangement of the symbols in this manner is not intended to indicate that the com pound is formed from a molecule of chlorine and an atom of oxygen. The symbol Aq is intended to indicate that an excess of water is employed, i.e., such an amount that no appreciable development of heat would be caused by the addition of a further quantity. When bromine is added to an aqueous solution of sodium hydroxide, sodium hypobromite, NaOBr, is formed, but this is much less stable than the corresponding hypochlorite, and is readily converted into bromate and bromide. The bromates and bromic acid resemble the chlorates and chloric acid, but are much less stable ; thus, a solution of bromic acid is decomposed at 100 C., giving off bromine and oxygen, and all reducing agents decompose it with facility. The inferior stability of the oxidized com pounds of bromine as compared with those of chlorine appears to indicate that the affinity of bromine to oxygen is less than that of chlorine ; this is entirely confirmed by thermochemical investigation, Thomsen having obtained the following values for bromic acid : Br 3, 4 , Aq Br,0 3 ,H, Aq BrHAq , 3 = - 43 5*&amp;gt;0 Formation of the aqueous acid from ( bromine, oxygen, and water. Formation of the aqueous acid from its elements. Oxidation of hydrobromic acid. Heat of neutralization. = +12,420 = -15,960 KOHAq, Br0 3 HAq= +13,750 When iodine is added to solutions of alkaline hydroxides or carbonates, alkaline hypoiodites corresponding to the hypochlorites are perhaps formed, but as yet have not been isolated. By dissolving iodine in a warm solution of potassium hydroxide a mixture of potassium iodide and iodate is produced 3I 2 Iodine. + 6KHO = KI0 3 + 5KI + 30H 2 . Potassium hydroxide. Potassium iodate. Potassium iodide. Tater. Iodine is converted into iodic acid by heating with the strongest nitric acid, and by the action of many other oxidizing agents ; but iodic acid is usually prepared from barium iodate, which is decomposed with the necessary quantity of sulphuric acid. Potassium iodate is most readily prepared by passing chlorine into water in which iodine is suspended until the latter is dissolved, then adding a corresponding quantity of potassium chlorate and warming; the iodine is converted into monochloride, which enters into reaction with the chlorate, and chlorine is evolved By adding a solution of barium hydroxide, insoluble barium iodate is precipitated, from which, as pointed out, iodic acid may be prepared. Iodic acid separates from its solution on spontaneous evaporation in crystals of the composition HI0 3 ; when heated to 120 C. these crystals lose water, and appar ently form the compound HI 3 O 8 or HI0 3 + 1 2 5 ; at about 170 C. this compound is resolved into water and iodic anhydride, I 2 O 5. Iodic anhydride is a crystalline sub stance, which readily dissolves in water, forming iodic acid ; when heated to about 370 C. it is resolved into oxygen and iodine. Iodic acid is a powerful oxidizing agent, being easily decomposed by deoxidizing agents ; hydriodic acid reduces it to iodine and water The chlorates and bromates are mostly easily soluble, and the corresponding salts are isomorphous ; but the iodates are mostly difficultly soluble, and with the excep tion of barium iodate, are not isomorphous with the cor responding chlorates and bromates. Moreover, chloric and bromic acids furnish but one class of salts, of which potassium chlorate, KC10 3, is typical ; but iodic acid, in addition to the normal iodates, such as potassium iodate, KI0 3, forms acid salts. These differences appear to in dicate that iodic acid differs in constitution from the other acids, and Thomsen has pointed out that it is probably a dibasic acid of the formula If 2 I 2 O G, since it is readily converted into water and the anhydride I 2 O 5, a property which is characteristic of dibasic but not of monobasic acids ; and since it furnishes acid salts, which is also characteristic of polybasic but not of monobasic acids, and is isomorphous with succinic acid, which is dibasic. Normal potassium iodate would therefore have the formula K I 2 O 6, and the so-called diiodate is probably the acid salt KHI 2 O 6. This view of the constitution of iodic acid is rendered the more probable by the behaviour of periodic acid, the disodium salt of which separates as a crystalline powder on passing chlorine into a solution of sodium iodate and sodium hydroxide ; the reaction is empirically represented by the equation NaIO 3 + 3XaOH + C1 2 = Na 2 IH 3 O 6 + 2NaCl. Sodium iodute. Disodium perioolate. To prepare the acid from this salt, it is dissolved in nitric acid, and silver nitrate added ; the precipitate of disilver periodate is dissolved in hot dilute nitric acid, and the solution concentrated at a moderate heat until a salt of the composition AgI0 4 crystallizes out. By treating this salt with cold water it is decomposed, disilver periodate antf periodic acid being produced : 2 AgI0 4 + 40H 2 = Ag 2 IH 3 6 + H 5 I0 6 ; to obtain the latter the solution is evaporated. Periodic acid separates from its aqueous solution in colourless crystals of the composition H 5 IO 6, which are not altered at 100 C., but melt at 130 C., and when heated to a higher temperature give off water and oxygen, leaving iodic anhydride. Periodic acid furnishes a complex but extremely in teresting series of salts. Thus, it forms two potassium salts, one of which has the composition KIO 4, and is ob tained on passing chlorine into a solution of potassium iodate and potassium hydroxide ; the second is represented
 * water.