510 PRINCIPLES OF CHEMISTRY 



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

 This acid is produced in the form of its salts, by the action of chlorine 

 on alkaline solutions of iodates, and also by the action of iodine on 

 chloric acid. 84 It crystallises from solutions as a hydrate containing 

 2H 2 O (corresponding with HC1O 4 ,2H 2 O), but as it forms salts con- 

 taining up to 5 atoms of metals, this water must be counted as water 

 of constitution. Therefore IO(OH) 5 = HIO 4 ,2H 2 O corresponds with 

 the highest form of halogen compounds, IX 7 . 85 In decomposing (at 



acid be added to the solutions. Analogous salts of cadmium, silver, and copper give 

 compounds of the type 2Me1O 6 4NH 3 and Me"(IO 3 ) 2 4NH 3) with gaseous ammonia (Me' 

 and Me" being elements of the first (Ag) and second (Cd, Zn, Cu) groups). With an 

 aqueous solution of ammonia the above salts give substances of a different composition, 

 such as Zn(10 3 ) 2 (NH4) 8 0, CdflO^NH^O. Copper gives Cu(IO 3 ) 2 4(NH 4 ) 2 O and 

 Cu(I0 5 ) 2 (NH 4 ) 2 O. These salts may be regarded as compounds of I 2 O 5 , and MeO and 

 (NH 4 ) 2 ; for example, Zn(IO 5 ) 2 (NH 4 ) 2 O may be regarded as ZnO(NH 4 ) 2 OI 2 O 5 , or, as 

 derived from the hydrate, I 2 O 5 2H 2 O = 2(HI0 3 )H 2 O. 



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

 passed through the solution, a sparingly soluble salt separates out, which corresponds, 

 with periodic acid, and has the composition Na 4 I 2 O0,8H 2 O. 



GNaHO + 2NaIO 5 -j- 4C1 = 4NaCl + Na 4 I 2 O 9 + 3H 2 0. 



This compound is sparingly soluble in water, but dissolves easily 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 I 2 09,8H 2 O. If this sparingly 

 soluble silver compound be dissolved in hot nitric acid, orange crystals of a salt having 

 the composition .AgIO 4 separate on evaporation. This salt is formed from the preceding 

 by the nitric acid taking up silver oxide Ag 4 I 2 O 9 + 2HNO 3 = 2AgN0 3 + 2AgIO 4 + H 2 O. 

 The silver salt is decomposed by water, with the re-formation of the preceding salt, 

 whilst iodic acid remains in solution 



4AgIO 4 + H 2 O = Ag 4 I 2 O9 + 2HIO 4 . 



The structure of the first of these salts, Na 4 I 3 O 9 ,3H 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 aU the 

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

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

 the raeso-saltsof H 3 IO 5 (PbH, Ag 2 H, CdH),the para-salts of H 5 IO 6 (Na 2 H 3 , Na 3 H 2 ), 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 last are types of diperiodio 

 salts, which correspond with the type of the meso-salts, as pyrophosphoric salts corre- 

 spond with orthophosphoric salts i.e. 2H3iO 5 -H 2 O = H 4 I 2 Og. 



85 Periodic acid, discovered by Magnus and Ammermiiller, and whose salts were 

 afterwards studied by Langlois, Rammelsberg, and many others, presents an example of 

 hydrates in which it is evident that there is not that distinction between the water of 

 hydration and of crystallisation which was at first considered to be so clear. In HClO^H-jO 

 the water, 2H 2 O, is not displaced by bases, and must be regarded as water of crystallisa- 

 tion, whilst in HIO 4 ,2H 2 O it must be regarded as water of hydration. We shall after- 

 wards see that the system oi the elements obliges us to consider the halogens as 

 substances giving a highest saline type, OX lt where G signifies a halogen, and X oxygen 

 (O = X 2 ), OH, and other like elements. The hydrate IO(OH) 5 corresponding with many 

 of the salts of periodic acid (for example, the salts of barium, strontium, mercury) does 

 not exhaust all the possible forms. It is evident that various other pyro-, meta-, &c., forma 

 are possible by the loss of water, as will be more fully explained in speaking of phosphoric 

 acid, and as was pointed out in the preceding note. 



