374 ELEMENTARY CHEMICAL MICROSCOPY 



isomorphous. The size of the crystals obtained decreases as 

 their solubility decreases. 



This test is a most convenient one if alloys or substances sus- 

 pected of containing 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. 



The nickel salt also forms squares, rectangles and cubes but 

 these are light brown by transmitted light not 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 very tiny cubes and 

 octahedra. 



The triple nitrite may be written thus: 



2 KN0 2 • Cu(N0 2 ) 2 • Pb(N0 2 ) 2 . 

 Precautions. 



In very dilute solutions the test fails unless rubidium or cesium 

 chlorides are added because of the too great solubility of the 

 potassium salt. Concentration may sometimes yield the typical 

 black 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 be very slightly in excess of 

 that called for by theory. It is therefore necessary to proceed 

 somewhat cautiously. Add a tiny fragment of nitrite, then 

 after waiting a few moments, if no crystals appear add a little 

 more. 



Too concentrated solutions of lead yield sandy black precipi- 

 tates 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. 



Free mineral acids must be absent. 



When the amount of lead is relatively great and cesium chloride 



