MICROCHEMICAL REACTIONS OF SODIUM 325 



hexagonal or pseudo-hexagonal modification of potassium sili- 

 con 1 uoride 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 barium silicofluoride BaSiF 6 , 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 CaSiF 6 2 H 2 O (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 MgSiF 6 6 H 2 O 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. 



