502 PRINCIPLES OF CHEMISTRY 



tions as a hydrate, HIO 3 , which loses water at 170, and gives an 

 anhydride, I 2 O, 5 . Both these substances are crystalline (sp. gr. I 2 O 5 

 5*037, HIO 3 4*869 at 0), colourless, soluble in water, 83 decompose at a 

 red heat into iodine and oxygen, are in many cases powerfully oxidising 

 for instance, they oxidise sulphurous anhydride, hydrogen sulphide, car- 

 bonic oxide, &c. form chloride of iodine and water with hydrochloric 

 acid, and with bases form salts, not only normal MIO 3 , but also 

 acid, for example, KIO 3 HIO 3 , KIO 3 2HIO 3 . With hydriodic acid 

 iodic acid immediately reacts, disengaging iodine, HIO 3 + SHI = 

 3H 2 O + 31. 2 . 



As with chlorine, so with iodine, a periodic acid, HIO 4 , is formed. 

 This acid is produced, as its salts, by the action of chlorine on alkaline 

 solutions of iodates, and also by the action of iodine on chloric acid. 84 



oxidises iodine vapour, first forming the oxide L>O 5 , which is decomposed by water or on 

 heating into iodic anhydride and iodine. Iodic acid is formed at the positive poles 

 (Riche) when a solution of hydriodic acid is decomposed by a galvanic current. It is 

 also formed in the combustion of hydrogen mixed with a small quantity of hydriodic acid 

 (Salet). 



85 Kammerer showed that a solution of sp. gr. 2'127 at 14, containing 2HIO 3 ,9fL>O, 

 entirely solidified in the cold. On comparing solutions HI + ?HoO with HIO 5 + wH.>O, 

 we find that the specific gravity increases but the volume decreases (it is the same in 

 their passage to solutions HIO 5 + raH 2 O), whilst in the passage of solutions HCl + ??iH 2 O 

 to HClO 3 + ?nH 2 O the specific gravity augments, and so does the volume, which is re- 

 marked also in certain other cases (for example, H 3 PO 5 and H-PO 4 ). Observations on the 

 specific volume of solution of iodic and periodic acids were made by Thomson at 17/17. 

 He expresses-for HIO 3 + ?H 2 O by 18w + 39-l-l3-lw/(m + 18), and for H 5 IO 6 -i-wHoO 

 by 18m + 23'8, which shows the absence of contraction in mixing these acids with water 

 (see Mendeleeff, Investigation of Aqueous Solutions, p. 368). 



84 If sodium iodate be mixed with a solution of sodium hydroxide, heated, and chlorine 

 be passed through the solution, then a sparingly soluble salt separates out, which corre- 

 sponds with periodic acid, and has the composition Na 4 I 2 O 9 ,3H 2 O. 



ONaHO + 2NaI0 3 + 4C1 - 4NaCl + Na 4 I 2 O 9 + 3H,O. 



This compound is sparingly soluble in water, but easily dissolves in a very dilute 

 solution of nitric acid. If silver nitrate be added to this solution a precipitate? is formed 

 which contains the corresponding compound of silver, Ag 4 IoO 9 ,3HoO. If this sparin^ly 

 soluble silver compound be dissolved in hot nitric acid, then, on evaporating, orange 

 crystals of a salt having the composition AgIO 4 separate. This salt is formed from the 

 preceding by the nitric acid taking up silver oxide. Ag 4 I 2 O 9 + 2HNO 3 = 2AgNO 3 

 + 2AgIO 4 + H 2 O is decomposed by water, with the re-formation of the preceding salt, 

 while iodic acid remains in solution 



4 AgIO 4 + H 2 O = Ag 4 I 2 O 9 + 2HIO 4 . 



The structure of the first of these salts, Na 4 I 2 O 9 ,8H 2 O, presents itself in a simpler 

 form if the water of crystallisation is regarded as an integral portion of the salt; the 

 formula is then divided in two, and takes the form of IO(OH) 3 (ONa) 2 that is, it answers 

 to the type IOX 5 , or IX 7 (like AgIO 4 ) which is IO 3 (OAg). The;! composition of all the 

 salts of periodic acids are expressed by this type IX 7 . Kimmins (1889) classes all 

 the salts of periodic acid into four types the meta-salts of HIO 4 (salts of Ag, Cu, Pb), 

 the meso-salts of H 3 IO 5 (Pb, Ag 2 ,H, Cd, H),the para-salts of H 5 IO 6 (Na.JI-. Na.-.H ,'. and 

 the di-salts of H 4 I 2 O 9 (K 4 , Ag 4 , Ni 2 ). The three first are direct compounds of the type 

 IX 7 , namely, IO 3 (OH), IO 2 (OH) 3 , and IO(OH) 5 , and the latter are types of diperiodic 



