MICROCHEMICAL REACTIONS OF THE COMMON ELEMENTS 325 



Precautions. 



An excess of the reagent must be avoided, otherwise the pre- 

 cipitate at first formed will be dissolved because of the formation 

 of a double iodide of the composition PbI 2 2 KI o;H 2 O. 1 Not 

 infrequently colorless crystals of this double iodide will be seen 

 in the immediate neighborhood of the reagent fragment. The 

 addition of a drop of water will usually cause the decomposition 

 of the double salt and a precipitation of the normal iodide. 



Double iodides of lead with many elements are known, most 

 of them crystallizing readily, 2 but it is not often that there will be 

 a sufficient separation of these interesting salts to interfere in 

 any way with the detection of lead. 



Too much nitric acid in the water employed for recrystallizing 

 the precipitate of lead iodide will cause partial decomposition 

 and consequently the separation of colorless octahedra of lead 

 nitrate. 



EXPERIMENTS. 



a. To a test drop containing Pb(NO 3 )2 add KI. Study the preparation, then 

 add a drop of water and heat to boiling. After the drop has cooled, study it 

 again. Repeat the experiment, but this time use an excess of KI. Try again in 

 acidified solutions. 



b. In like manner test a preparation of Pb(C2H 3 O2)2. 



c. Make a preparation of PbSO*. Decant the mother liquor, add to the sul- 

 phate residue a drop of water, acidify with HNO 3 , then add a fragment of KI. 

 After a few seconds examine the preparation. 



d. Make a mixture of Pb and Ag, test with KI. Then try in turn mixtures of 

 Pb and Sb; Pb and Bi; Pb, Sb and Bi; Pb and Cu; Pb and Sn. 



e. Test a preparation of HgCl 2 . Then one of Hg(NO 3 ) 2 . Make a mixture of 

 Pb(N0 3 ) 2 and Hg(N0 3 ) 2 and test. 



B. By Means of Hydrochloric Acid. 



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

 drop acidulated with a little nitric acid. 



This method of adding the reagent is not so good as allowing 

 two drops to flow together but is adopted so as to conform to that 

 for testing for silver and mercury. 



1 Brooke, Ch. N., 1898, 191. 



2 See Mosnier, Ann. chim. phys. (7) 12, 374; Comptes rend., 120, 444. 



