Chemical Alteration and Separation of the Analyte

The physical separation of an analyte from a sample so that its weight can be measured, which the above examples typify, can be difficult, if not impossible, to accomplish. It is important to recognize that in many cases there are no physical means by which such a separation can take place. For example, if you wished to determine the sodium sulfate present in a mixture with sodium chloride, it would not be possible to separate it from the sodium chloride by physical means to determine its weight. 

However, a gravimetric procedure might still be used if a chemical reaction is employed to convert the analyte to another chemical form that is both able to be separated cleanly and able to be weighed accurately. In the example of determining sodium sulfate in the presence of sodium chloride, one can dissolve the mixture in water and precipitate the sulfate with barium chloride to form barium sulfate. Sodium chloride would not react. 

Na2S04 + BaCl2 — BaS04 ( l ) + NaCl NaCl + BaCl2 — no reaction 

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