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 film 

 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, Na 2 SO 4 , HNa 2 PO 4 . 



b. NaKC 4 H 4 06; and in 3 (Na 2 C 2 O 4 )-Fe 2 (C 2 O 4 ) 3 . 



c. A mixture of NaCl and MgSO 4 and of NaCl and MgCl 2 . 



d. A mixture of Na 2 SO 4 and ZnSO 4 . 



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 3Na2SC>4 2Bi 2 (SO 4 )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 



