200 METALS AND THEIR COMBINATIONS. 



Mercurous iodide, Hydrargyri iodidum flavum, Hg 2 I 2 = 652.6 

 ( Yellow iodide, green iodide, or protiodide of mercury). Both iodides 

 of mercury may be obtained either by rubbing together mercury and 

 iodine in the proportions represented by the respective atomic weights, 

 or by precipitation of soluble mercurous or mercuric salts by potas- 

 sium iodide. 



According to the U. S. P., mercurous iodide is made by the pre- 

 cipitation of a 4 per cent, solution of mercurous nitrate, to which 1 

 per cent, of nitric acid has been added, by a 2.4 per cent, solution of 

 potassium iodide: 



Hg 2 (N0 3 ) 2 + 2KI = 2KN0 3 + Hg 2 I 2 . 



The precipitate is collected on a filter, well washed with water and 

 alcohol, and dried between paper at a temperature not exceeding 

 40 C. (104 F.). During the whole operation light should be ex- 

 cluded as much as possible, as it decomposes the compound. 



Mercurous iodide is a yellow, tasteless powder, almost insoluble in 

 water. It is easily decomposed into mercuric iodide and mercury, 

 becoming darker and assuming a greenish-yellow tint (Plate 

 IV., 5.) 



Mercuric iodide, Hydrargyri iodidum rubrum, HgI 2 452.8 

 (Hed iodide or biniodide of mercury). Made by mixing solutions of 

 potassium iodide and mercuric chloride, when a pale-yellow precipi- 

 tate is formed, turning red immediately (Plate IV., 6) : 



HgCl 2 + 2KI = 2KC1 + HgI 2 . 



Mercuric iodide is soluble both in solution of potassium iodide and 

 mercuric chloride, for which reason an excess of either substance will 

 cause a loss of the salt when prepared. It is a scarlet-red, tasteless 

 powder, almost insoluble in water and but slightly soluble in alcohol ; 

 on heating or subliming it turns yellow in consequence of a molecular 

 change which takes place ; on cooling, and, more quickly, on pressing 

 or rubbing the yellow powder, it reassumes the original condition 

 and the red color. 



Mercuric sulphate, Hg-SO 4 . When mercury is heated with strong 

 sulphuric acid (the presence of nitric acid facilitates the formation) 

 chemical action takes place between the two substances, sulphur 

 dioxide being liberated and mercuric sulphate formed, which is ob- 

 tained as a heavy, white, crystalline powder : 



Hg + 2H 2 SO 4 = HgS0 4 -j- 2H 2 O + SO 2 . 



