322 
ELEMENTARY CHEMICAL MICROSCOPY 
tions this test becomes unsatisfactory as applied above since it 
requires too much time. It is then better to flood the dry fihn 
with reagent, allow a few seconds to elapse for the establish¬ 
ment of equilibrium and decant the clear solution from the pre¬ 
cipitate of uranyl phosphate. The decanted solution must then 
be allowed to evaporate spontaneously until crystallization sets 
in, or the evaporation may be hastened by gentle heating. 
This test for sodium is also apt to prove unsatisfactory in the 
presence of much potassium. To remove the latter add per¬ 
chloric acid in slight excess. Evaporate to dryness, moisten the 
residue with perchloric acid and again evaporate. Extract the 
residue with alcohol; potassium perchlorate is insoluble; so¬ 
dium perchlorate passes into solution (Schoorl). Evaporate the 
clear alcoholic extract to dryness and test for sodium. 
A further caution is necessary relative to the possible inter¬ 
ference of elements such as Fe, Mn, Ni and Co, which can form 
double acetates with uranyl acetate and thus reduce the amount 
of the reagent available to form the double sodium compound. 
EXPERIMENTS. 
Test for Na in 
a. NaCl, Na2S04, HNa2P04. 
b. NaKC4H406; and in 3(Na2C204) •Fe2(C204)3. 
c. A mixture of NaCl and MgS04 and of NaCl and MgCL. 
d. A mixture of Na2S04 and ZnS04. 
B. By means of Bismuth Sulphate. 
First convert the compound to sulphate by evaporations 
to dryness with sulphuric acid. Dissolve the residue in water 
and add a trace of nitric acid. 
Apply the bismuth sulphate by method II, page 300. 
Immediately after the addition of the unknown to the reagent, 
gently warm the preparation over the micro-burner, cool, and 
examine at once. 
Sodium bismuth sulphate 3Na2S04 • 2Bi2(S04)3 separates in, 
the form of colorless slender rods or prisms with almost rounded 
ends, uniting in crosses, X’s, or more or less star-like radiating 
clumps. The crystals separating near the circumference of the 
drop are usually shorter, stouter and more prismatic, while those 
nearer the center are more rod-like. It is these rod-like crystals 
