THE HALOGEN FAMILY 203 



IODINE I 2 



Sources. Iodine was formerly all obtained from seaweed 

 (kelp), certain species of which use the traces of organic com- 

 pounds of iodine in sea water as part of their food. The dried 

 seaweed is carbonized in retorts, and sodium iodide remains in the 

 residue, along with much sodium carbonate and carbon. In an 

 improved process the iodine compounds are dissolved out of the 

 kelp, and from the latter a sort of gelatin, named algin, is extracted. 



The greater part of our supply of iodine is at present obtained 

 from sodium iodate NaIO 3 , which forms about 0.2 per cent of 

 crude Chile saltpeter. 



Preparation. The processes for obtaining iodine from an 

 iodide are precisely the same as those for bromine. 

 In France chlorine is used to displace the iodine: 



The precipitate of iodine is pressed free from the solution. 



In Great Britain the iodide is mixed with manganese dioxide 

 and sulphuric acid and heated: 



3H 2 S0 4 + Mn0 2 + 2NaI -* MnSO 4 + 2NaHSO 4 + 2H 2 O + I 2 f . 



Iodine vapor condenses upon a cold surface, not to the liquid, 

 but directly to the solid, crystalline form. Distillation which 

 gives a solid product is called sublimation. The crude iodine is 

 purified by repetition of this process. 



Physical Properties. Iodine forms black, shining crystals. 

 The vapor, visible even at the ordinary temperature, is violet in 

 color (hence the name of the substance, from Greek, meaning like 

 a violet} and has a density corresponding to the formula I 2 . The 

 substance is very slightly soluble in water. It dissolves, however, 

 easily in carbon disulphide (violet solution), in alcohol or potas- 

 sium iodide solution (brown solution), and even in starch, upon 



