M1CROCHEM1CAL REACTIONS OF LEAD 369 



With simple mixtures, this test is a very beautiful one, but 

 with complex material it is sometimes difficult to adjust the con- 

 ditions, especially as regards the quantity of potassium thio- 

 cyanate required. 



EXPERIMENTS. 



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



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



c. Try this test with CuSO* hut 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. AN ISOTROPIC. 



Hexagonal. Iodide. 



Tetragonal. 



Orthorhombic. Bromide; chloride; 2 sulphate; tartrate. 



Monodinic. Acetate; chromate; thiocyanate. 



Triclinic. 



DETECTION. 



A . By Means of Potassium Iodide. 



Apply the reagent, by Method ///, page 300, 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. 



* Recrystallized from hot water PbCl 2 is pseudohexagonal. 



