MICROCHEMICAL REACTIONS OF THE COMMON ELEMENTS 323 



EXPERIMENTS. 



a. Test as above HgCl 2 , using ZnSO 4 . 



b. Try again, this time introducing a trace of CuSO 4 . 



c. Try this test with CuSO 4 but with no ZnSO 4 present (which method is most 

 satisfactory?). 



LEAD. 1 



Crystal Forms and Optical Properties of Common Salts 

 of Lead. 



A. ISOTROPIC. Nitrate (I). 



B. ANISOTROPIC. 



Hexagonal. Iodide. 

 Tetragonal. 



Orthorhombic. Bromide; chloride; 2 sulphate; tar- 

 1 f crate. 



Monoclinic. Acetate; chromate; sulphocyanate, 

 Triclinic. 



DETECTION. 



A. By Means of Potassium Iodide. 



Apply the reagent, by Method ///, page 252, to the test 

 drop slightly acidified with nitric acid. 



Lead iodide PbI 2 is at once formed as a bright yellow precipi- 

 tate in a circular band about the reagent fragment. The circle 

 gradually becomes larger and larger and at its outside circum- 

 ference beautiful hexagonal plates appear. These plates and 

 flakes of lead iodide appear greenish or brownish yellow by 

 transmitted light, sometimes even gray, according to their thick- 

 ness. By reflected light lead iodide plates glow and glisten and 

 display the iridescent colors of thin films, an extremely charac- 

 teristic feature of this salt. 



These hexagons of lead iodide do not belong, according to 



1 Lead, silver and copper are introduced at this point rather than in their 

 proper position in the Periodic System because of their close relations in qualita- 

 tive analysis. 



2 Recrystallized from hot water PbCU is pseudohexagonal. 



