and Laboratory Methods. 1933 



these interesting salts to interfere in any way with the detection of lead. 



From neutral solutions containing lead in the form of lead acetate, potassium 

 iodide will generally precipitate, in addition to the normal iodide, basic iodides 

 of variable composition, such as, Pblj • PbO ; Pbl2 • 2PbO. 



In most cases it will be found advantageous, before adding the reagent, to 

 acidify the test drop with acetic or with nitric acid. 



Lead iodide can be recrystallized from hot water, best if acidified with -nitric 

 or acetic acid. On cooling, large, beautifully formed hexagons separate. A 

 large drop of water is necessary in order that good results may be obtained. 



Heated with hydrochloric acid lead iodide dissolves, and on cooling crystals of 

 the normal iodide Pbl2, a chloriodide, PbCl2 • Fhl^ or 'iPbClj • Pbia (or both), 

 and the normal chloride PbClj separate. The chloriodides appear in the form 

 of needles of a faint yellow color. 



Too much nitric acid in the water employed for recrystallizing the precipi- 

 tate of lead iodide will cause partial decomposition and consequently the separa- 

 tion of the colorless octahedra of lead nitrate. 



Silver iodide separates as a yellowish amorphous mass insoluble in hot water 

 and in hot nitric acid. 



Mercuric iodide takes the form of ruby red rhombs. Mercurous salts acidi- 

 fied with nitric acid usually give in addition to the heavy precipitate of mercur- 

 ous iodide the ruby colored rhombs of the mercuric salt. 



If cuprous salts are present a white granular precipitate of cuprous iodide is 

 formed and iodine is set free. 



Thallium is precipitated as an exceedingly fine granular precipitate. 



Antimony and bismuth salts interfere with the reaction for lead. These ele- 

 ments yield with potassium iodide, double iodides which separate in neat, well 

 formed crystals (see Antimony and Bismuth). Solutions containing lead, anti- 

 mony and bismuth, when treated with potassium iodide, yield a dark reddish 

 brown, sandy precipitate wholly unlike in appearance anything obtained with the 

 different elements alone. Boiling the mixed product with water will generally 

 cause a partial decomposition, and on cooling hexagons and irregular plates of 

 lead iodide will appear. Solutions of lead and bismuth yield orange red disks 

 and plates. 



Selenium and tellurium under certain conditions are precipitated as dark red- 

 dish brown iodides. 



Exercises for practice. 



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. 



In like mannertesta preparation of Pb(C2H302)2- 



Make a preparation of PbSO^. Draw of? the mother liquor, add to the sul- 

 phate residue a drop of water, acidify with nitric acid, then add a fragment of 

 KI. After a few seconds examine the preparation. 



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. 

 Cornell University. E. M. ChaMOT. 



