326 ELEMENTARY CHEMICAL MICROSCOPY 



Lead chloride PbC^ separates at once in the form of char- 

 acteristic, white, long acicular crystallites belonging to the ortho- 

 rhombic system. There are also seen feathery dendritic X's and 

 long irregular ragged prisms. 



The appearance of the lead chloride separating varies with the 

 concentration of the solution being tested and with the nature 

 of the substances present. If the test drop is not sufficiently 

 concentrated the lead chloride will not separate at once in the 

 form of the characteristic crystallites, but will appear more 

 slowly, prismatic forms being the rule. This question of con- 

 centration becomes a most important one if the substance con- 

 tains salts with which lead chloride can unite to form double 

 salts, as for example chlorides of the alkali metals and ammonium, 

 for in such an event dilute or even moderately concentrated 

 drops fail to yield recognizable forms. Indeed it may be said 

 that testing for lead with hydrochloric acid is not advisable in 

 the presence of members of Groups I and II. 



In neutral solutions of lead acetate there may be precipi- 

 tated in the presence of members of Group I and no excess of 

 the reagent, colorless, highly refractive prisms of the formula 

 Pb(OH)Cl (n = 2.08 to 2.16) belonging to the orthorhombic sys- 

 tem but some tunes also appearing as monoclinic prisms. 



Lead chloride is slightly more soluble in water containing a 

 little nitric acid than in pure water, hence the separation of lead 

 as chloride is never complete. 



Lead chloride differs from the chlorides of silver and mercurous 

 mercury in being easily soluble in hot water, thus affording a 

 simple method of separation. On cooling, the lead chloride no 

 longer appears in the forms stated above but assumes that of 

 thin pseudohexagonal prisms, rhombs and hexagons. 



Recrystallized in the presence of Group I, double chlorides 

 result, which generally separate more slowly. The crystal form is 

 quite different from that of the normal salt. It is quite impor- 

 tant that the student should be familiar with at least the double 

 chloride of cesium and lead (cesium chloroplumbate) , since this 

 compound not infrequently makes its appearance when testing for 

 tin with cesium chloride and is quite apt to puzzle the beginner. 



