Precipitation of Lead as Lead Chloride in Presence of Mercuric Ions During teaching of a course in semimicro-qualitative inorganic analysis to freshmen students in the college of engineering we noticed an unexpected behavior from lead ions Ph2+. Unknown samples containing ions from groups 1and IIA were given to thestudents. Some of thestudents obtained unknowns that had Phz+,Cdz+,and HgZ+ions together. When thesestudents added dilute hydrochloric acid to their unknowns soas to precipitate any ion from group I, no precipitate was formed. Other students who had Pb2+ mixed with ions other than Hg2+ obtained a good precipitate immediately after addition of HCI. In the presence of Hg2+, it seems that PbClz does not precipitate easily unless the tube is cooled with continuous scratching of the walls. No reference to such a problem was found in Vogel's textbo0k.l The experiment was then repeated using twosamples,ane containinga mixtureof Pb2+and Cd2+,and the other Pb2+ plus Hg2+ions. When dilute HCI was added to both samples, immediate precipitate was formed in the tube containing P I P plus Cd2+.However, no precipitate was formed in the tube containing Hg2+.This last tube had to be cooled under the tap with scratching the walls to induce precipitation. We might mention here that failure to precipitate lead in group I does not harm the analysis, since lead is also detected in group IIA. 'Vogel, A. I., "A Textbook of Micro and Semimicro Inorganic Qualitative Analysis," Longman, London. T h a n u n Mohamad Pyriadi Aroosiak Polis University of Baghdad Baghdad, I r a q
Volume 54, Number 7,July 1977 1 439
