Silicates European Pharmacopoeia

Since the majority of silicon salts are insoluble in water the anion first has to be liberated from the substance to be examined by incineration, often with an alkali carbonate. Silicone is purely covalent and has to be converted into a silicate by incineration. The ability of silicate to form a volatile complex with fluoride and the water insolubility of silicon dioxide is used in the identification. 

the substance (maybe pretreated) to be examined is mixed with about 10 mg sodium fluoride R and a few drops of sulfuric acid R to give a thin slurry. This leads to the formation of hydrofluorosilicic acid as shown below using silicon dioxide as an example. 

The analysis must be performed in a lead or platinum crucible, since glass and silica crucibles are silica based. In the second step, the crucible is covered with a thin transparent plate of plastic under which a drop of water is suspended and heat is applied. As a response to this, gaseous (boiling point 90°C) silicium fluoride is liberated. 

Fluoride should be present in a less then equivalent amoimt since the interfering F12F2 could be formed if present in excess. If heating is done in a lead crucible, care should be exercised since lead melts at 327°C. 

When the gas comes in contact with the water drop, the following reaction 

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