Exercise 59. Fluosilicic Acid

Select an old flask A (Fig. 21) of about 1 1. capacity (it will be etched in the experiment) and place in it a mixture of 50 g. of powdered fluorspar and 50 g. of fine sand. Provide the flask with a stopper carrying a funnel 

The excess of silicic acid is deposited as a gelatinous precipitate which would easily stop up a narrow delivery tube. It is to prevent this that the mercury trap is used. Continue the heating as long as the silicic acid 

Heat a portion of the solution to boiling and neutralize it with potassium carbonate. On cooling, the potassium fluosilicate separates as a very transparent crystalline product, which should be filtered off and preserved. It will serve just as well to add potassium chloride, since potassium fluosilicate is not decomposed by hydrochloric acid. 

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