74 § 58. BASES AND ACIDS WITH REAGENTS. 



ACIDS. 

 SILICIC ACID. HsSiOg. 



58. All silicates are insoluble in water and dilute acids, 

 except those of potassium and sodium. 



Silicates may be decomposed, and the metals contained 

 in them brought into a soluble form, by means of concen- 

 trated hydrochloric or sulphuric acid, by hydrofluoric acid 

 or ammonic fluoride, or by fusion with an alkaline carbon- 

 ate, and subsequent treatment with dilute hydrochloric^ 

 acid. 



Reactions. — If a solution of a soluble silicate is evapo- 

 rated to dryness, after addition of hydrochloric acid, the 

 residue gently ignited and treated with dilute acid, the 

 silica remains undissolved in the form of a white, gritty 

 powder. 



When a silicate in powder is fused in a bead of sodic 

 carbonate, on platinum wire, the carbonic acid is expelled 

 by the silicic, and its evolution causes the bead to froth. 



If a very small fragment of an insoluble silicate is 

 fused in a bead of phosphorus-salt, on platinum wire, 

 the bases are dissolved out, and the silica remains floating 

 about in the bead, retaining the form of the original 

 fragment. 



Quantitative estimation. — Silicic acid is always weighed 

 as such. 



a. 1. — If the acid is to be determined in a solution or 

 a soluble silicate, add an excess of hydrochloric acid to 

 the solution, or the very finely pow^dered solid, anct 

 evaporate the mixture to dryness on the water-bath with 

 frequent stirring to break up the lumps. 



If, as is sometimes the case, the solution analyzed con- 

 tains organic matter, or ferrous oxide, add a few drops of 

 nitric acid towards the close of the evaporation. If a solid 

 is being treated, the digestion should be continued, with 



