242 NON-METALS AND THEIR COMBINATIONS. 



substance known as tbyro-iodine, which is of great value in certain diseases, 

 especially cretinism, resulting from deficient development of the thyroid gland. 

 Bauman discovered (1895) iodine in this gland. The thyroid of sheep, which 

 in the dried form is official, contains 0.17 per cent, of iodine. 



Hydriodic acid, Acidum hydriodicum, HI = 126.9. This is a 

 colorless gas readily soluble in water ; the solution is unstable, being 

 easily decomposed with liberation of iodine. It may be obtained by 

 processes analogous to those mentioned for the preparation of hydro- 

 bromic acid. The action of hydrogen sulphide upon iodine in the 

 presence of water is as follows : 



H 2 S + 21 2HI + S. 



The official method for making diluted hydriodic acid depends on the decom- 

 position of an aqueous solution of potassium iodide by an alcoholic solution of 

 tartaric acid in the presence of a small quantity of potassium hypophosphite, 

 which acts as a preservative. Upon cooling the mixture to the freezing-point, 

 acid potassium tartrate separates, while hydriodic acid remains in solution, 

 which is further diluted until a 10 per cent, acid is obtained. The decomposi- 

 tion taking place is this : 



KI + H 2 C 4 H 4 O 6 : KHC 4 II 4 O 6 -j- HI. 



While hydriodic acid itself is not of much importance, many of 

 its salts, the iodides, are of great interest. 



At C. an aqueous solution can be obtained containing as much as 90 per 

 cent. HI. Nearly all iodides are soluble in water. The insoluble ones are of 

 silver, mercury, copper (ous). Lead iodide is sparingly soluble. 



The ionic reactions for iodine compounds are analogous to those for chlorine 

 compounds. 



Tests for iodine and iodides. 



(Any soluble iodide may be used.) 



1. Add to solution of an iodide, solution of silver nitrate: a pale- 

 yellow precipitate of silver iodide, Agl, falls, which is insoluble in 

 nitric acid, very sparingly soluble in ammonia water, but soluble in 

 solution of sodium thiosulphate or potassium cyanide. (See Photog- 

 raphy in Chapter 31, under Silver.) 



2. Add lead acetate to a solution of an iodide : a yellow precipi- 

 tate of lead iodide, PbI 2 , is produced. When the precipitate is dis- 

 solved in a large volume (200 c.c.) of boiling water, and the solution 

 is cooled slowly, beautiful golden spangles are formed. 



3. Add mercuric chloride solution to a solution of an iodide : a red 

 precipitate of mercuric iodide, HgI 2 , is produced, which is soluble in 

 solutions of mercuric chloride and potassium iodide. Note that the 

 corresponding chloride and bromide of mercury are soluble and white. 



