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 hr own 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 KNO2. Cu(N 02)2 • Pb(N 02 ) 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 
