MICROCHEMICAL REACTIONS OF SODIUM 
325 
hexagonal or pseudo-hexagonal modification of potassium sili- 
cofluoride is also known but is formed only at low temperatures. 
There is no possible danger, therefore, of confusing sodium and 
potassium. It is well to remember, however, that undue de¬ 
velopment of the diagonally opposite faces of an octahedron 
yields a crystal giving an image hexagonal in outline. The 
color of the crystal and its action on polarized light should leave 
no room for doubt as to its identity. 
From very concentrated solutions, in addition to potassium, 
Li, Ca, Sr, Mg, Mn, Fe, etc., may possibly separate. 
Barium, if present, is always precipitated with sodium, form¬ 
ing barimn silicofluoride BaSiFe, which cannot be confused with 
the sodium salt since the barium compound crystallizes in 
rods or fusiform crystals singly, in crosses or in irregular masses. 
Neither calcium nor strontium are precipitated by ammonium 
silicofluoride, but each salt is liable to separate from too concen¬ 
trated solutions. The calcium salt CaSiFe • 2 H2O (monoclinic) 
forms spindle-shaped crystals, and though these are grouped in 
rosette-like masses, they are not to be mistaken for sodium. 
The magnesium salt MgSiFe • 6 H2O is so much more soluble 
than those above mentioned as to never separate save upon 
evaporation or from very concentrated solution. Its crystals 
are rhombohedra, polarize strongly and do not have a six-sided 
outline. The silicofluoride of iron is isomorphous with the 
magnesium salt. 
It is evident that if silicon is present in the material under 
examination, we can test for sodium and silicon in one operation 
by adding ammonium fluoride and then acidifying. A pre¬ 
cipitation of crystals resembling sodium silicofluoride would 
point to the presence of sodium and silicon, or an element be¬ 
having, under like conditions, similarly to silicon. Thus we 
have titanofluorides, zirconofluorides and stanofluorides from 
elements of the fourth group; and from the transitional ele¬ 
ments, glucinum in the second group and boron in the third, 
we may have glucinofluorides and borofluorides of sodium. Of 
these compounds the titanofluoride is known to be isomorphous 
with the silicofluoride of sodium. 
