Volatilisation or evolution methods

Another example is the determination of pure silica in an impure ignited silica residue. The latter is treated in a platinum crucible with a mixture of sulphuric and hydrofluoric acids the silica is converted into the volatile silicon tetrafluoride  

The residue consists of the impurities, and the loss in weight of the crucible gives the amount of pure silica present, provided that the contaminants are in the same form before and after the hydrofluoric acid treatment and are not volatilised in the operation. Although silicon is not the only element that forms a volatile fluoride, it is by far the most abundant and most often encountered element consequently the volatilisation method of separation is generally satisfactory. 

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The separation of the element or of the compound containing it may be effected in a number of ways, the most important of which are (a) precipitation methods (b) volatilisation or evolution methods (c) electroanalytical methods and (d) extraction and chromatographic methods. Only (a) and (b) will be discussed in this chapter (c) is considered in Part E, and (d) in Part C. 

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