and Laboratory Methods. 



1851 



Fig. 79. 



7! Hydrochloric Acid added to solutions containing Lead causes the separation of 

 Lead Chloride. 



Pb(N03)2+2HCl^PbCl2 + 2HN03 



Method. — Into a drop of a moderately concentrated solution of the substance to 

 be tested, cause a drop of dilute hydrochloric acid to flow. This flowing in method 

 is far preferable to the direct iaddition of the reagent to the test drop. In a few 

 seconds, if the concentration is right, there will ap- 

 pear characteristic white, long, acicular crystals, 

 crystallites in the form of Xs and more or less 

 feathery masses (Fig. 79). By transmitted light the 

 crystallites often appear to be black. 



Remarks. — The appearance of the lead chloride 

 separating varies with the concentration of the solu- 

 tion being tested and with the nature of the sub- 

 stances 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 concentration becomes 

 a most important one if the substance contains 

 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 solutions or even moderately 

 concentrated ones fail to yield recognizable forms. Indeed it may be said that 

 testing for lead with hydrochloric acid is not advisable in the presence of mem- 

 bers of Groups I and II. ' 



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 shown in Fig. 79, 

 but assumes that of thin orthorhombic prisms, rhombs and hexagons (Fig. 80). 

 Recrystallized in the presence of chlorides 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 important that the worker should be familiar 

 with at least the double chloride of cesium and lead (cesium chlorplumbate), 



since this compound not infrequently makes its 

 appearance when testing for tin with cesium 

 chloride and is quite apt to puzzle the beginner. 

 Alkalies convert lead chloride into a basic 

 chloride to which the formula PbCl,* 3?bO 

 4H2O is generally assigned. 



Thallous salts yield with hydrochloric acid 

 star and cross-like crystallites differing con- 

 siderably from those given by lead. There is 

 _ . ^1^^ little danger of confusing these two elements, 



p-jQ gQ since recrystallizing thallous chloride from hot 



