MICROCHEMICAL REACTIONS OF SODIUM 
323 
with parallel extinction which are the more characteristic and 
unless these are obtained the conclusion that sodium is present 
is unwarranted. 
Potassium sulphate yields plates having a hexagonal or coffin¬ 
like outline or six-pointed stars and rosettes. When first formed 
these plates appear as circular disks but they rapidly acquire six 
sides or grow into rosettes. Ammonium, rubidium and cesium 
form similar hexagons and rosettes. 
When both sodium and potassium are present, the rod-like 
crystals of the sodium double salt and the hexagons of the potas¬ 
sium salt each appear, permitting a simultaneous detection of 
sodium and potassium. 
The addition of a very minute quantity of nitric acid or of 
glycerine to the preparation before heating usually yields better 
crystals and more reliable results. 
Precautions. 
Tufts of fine radiating needles appearing greyish or brownish 
by transmitted light must not be regarded as indicating the 
presence of sodium; neither should stout prisms or elongated 
plates with forked or broken ends. 
It is always best to remove members of the calcium group by 
means of sulphuric acid before applying the bismuth sulphate 
test. Calcium is especially to be guarded against since calcium 
sulphate may assume forms which simulate tlje sodium double 
salt; for although the crystals CaS04 • 2 H2O are monoclinic and 
usually lie in positions yielding oblique extinction, the extinction 
angle is small and unless care is exercised the student may credit 
them with parallel extinction. 
Free mineral acids (especially nitric) greatly retard the sepa¬ 
ration of sodium bismuth sulphate. 
In the absence of bismuth sulphate the reagent may be pre¬ 
pared as follows: At the corner of a slide place a drop of dilute 
sulphuric acid; add to this drop a little basic bismuth nitrate 
and stir until the bismuth salt has completely dissolved. Heat 
carefully until the water has been mostly expelled, and crystal¬ 
lization of the bismuth sulphate takes place; then add a rather 
large drop of water and a very minute drop of dilute nitric acid. 
