MICROCHEMTCAL REACTIONS OF SODIUM 
321 
indication of the probable presence of K, Rb, Cs or NH4, can be 
obtained at the same time Na is being searched for. 
In the presence of magnesium there will be obtained in addi¬ 
tion to the tetrahedra of the sodium double salt large monoclinic 
crystals of a triple salt 
NaC2H302 • Mg(C2H302)2 • s (U02(C2H302)2) • 9 H2O, 
taking the form of rhombs or appearing to be octahedra, dodeca- 
hedra or having a more or less triangular outline with incurving 
sides. When, however, the amount of sodium is very small with 
reference to that of magnesium, only the triple salt will appear. 
As might be expected any of the other elements in the magne¬ 
sium group in the Periodic System, Gl, Zn, Cd, can replace Mg 
in the triple salt. 
Precautions. 
Carbonates or hydroxides must first be converted into acetates 
or chlorides. 
Too much free acid interferes with the test — a further reason 
for evaporation to dryness before applying the reagent. 
Much magnesium gives rise to a film of salts so hygroscopic 
that a dry film cannot be obtained unless the salts are first 
converted into sulphates by evaporation with a little dilute 
sulphuric acid. 
Members of the calcium group often cause trouble. If, there¬ 
fore, an unsatisfactory test for sodium is obtained and subse¬ 
quent testing reveals the presence of Ca, Sr or Ba, these ele¬ 
ments should be removed by precipitation with sulphuric acid, 
the solution filtered or decanted from the precipitate and the 
filtrate evaporated to dryness on platinum (why ?) and again 
tested for sodium. 
Any compounds present in the material to be tested which 
will yield an insoluble precipitate with uranyl acetate, as, for 
example, phosphates, will naturally seriously interfere with the 
test or may absolutely prevent the detection of Na. In such an 
event the amount of uranyl acetate employed must be slightly 
more than sufficient to satisfy all the PO4 present and to unite 
with the sodium to form the double salt. Under these condi- 
