MICROCHEMICAL REACTIONS OF LEAD 
371 
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 Pbl2 • 2 KI • xH20d 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,^ 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(N03)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(C2H302)2. 
c. Make a preparation of PbS04. Decant the mother liquor, add to the sul¬ 
phate residue a drop of water, acidify with HNO3, 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 Ind Sb; Pb and Bi; Pb, Sb and Bi; Pb and Cu; Pb and Sn. 
e. Test a preparation of HgCb- Then one of Hg(N03)2. Make a mixture of 
Pb(N03)2 and Hg(N03)2 and test. 
B. By Means of Hydrochloric Acid. 
Apply the reagent by Method III, page 300, 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. 
* Brooke, Ch. N., 1898 , 191. 
2 See Mosnier, Ann. chim. phys. (7) 12 , 374; Comptes rend., 120 , 444. 
