2054 Journal of Applied Microscopy 



taining both lead and copper are being examined. It is then only necessary to 

 add to the solution, sodium acetate, potassium nitrite and acetic acid. If, after 

 waiting a reasonable time, no triple nitrite separates, cesium chloride or thallous 

 nitrate can be added. 



Nickel forms with lead an analogous salt of the composition ['iKNOo^Ni 

 (N02)2 •Pb(N02)2] also crystallizing in cubes, but much smaller than those of 

 the copper salt, and yellowish brown in color instead of black. 



Cobalt is immediately precipitated by potassium nitrite as a very insoluble 

 double nitrite of potassium and cobalt in the form of a reddish brown powder, 

 or in well defined cubes. 



With potassium nitrite alone this test for lead fails in dilute solution. Care- 

 ful evaporation will sometimes yield recognizable crystals. 



The addition of an excessive amount of potassium nitrite is objectionable 

 because of the fact that the triple nitrite is quite soluble in solutions of this 

 reagent. On the other hand, it is essential that the amount added shall be very 

 slightly in excess of that called for by theory. It is therefore necessary to pro- 

 ceed somewhat cautiously. 



Too concentrated solutions of lead yield dense sandy black precipitates 

 requiring recrystallization. Recrystallization can be effected by adding to the 

 preparation a little water, a trace of acetic acid and a slight excess of potassium 

 nitrite, then heating the preparation to boiling. Good crystals should appear 

 on cooling. 



Exercises for Practice. 



Test a preparation containing Pb. 

 Try another preparation, this time introducing CsCl. 

 Try again with TINO3. 



By a series of careful dilutions determine the limit of the precipitation of 

 lead as the K salt, the Cs salt and Tl salt. 



Test a mixture of Pb and Ni ; Pb and Co ; Pb and Ag, etc. 



In addition to the five tests given above, the worker must be familiar with 

 the appearance and properties of the precipitates given by lead with such reagents 

 as oxalic acid, primary sodium carbonate, ammonium carbonate, primary sodium 

 tartrate, secondary sodium phosphate, potassium antimonyl tartrate, etc. 



And lastly, the reduction of lead by means of powdered metallic magnesium 

 (or zinc) must not be overlooked. By means of a little magnesium, lead can be 

 easily separated from many complex and annoying liquids, as for example exces- 

 sive amounts of the elements of Groups I and II. Moreover, the appearance of 

 the lead precipitated in crystalline form by the magnesium is intensely interest- 

 ing and very characteristic. The precipitated lead is easily separated and 

 washed. This method is not of universal application, however, as many other 

 metals are also reduced by metallic magnesium. The appearance of the precipi- 

 tate will serve as a guide as to what metals are present. 

 Cornell University. E. M. ChamOT, 



