Question

Explain post precipitation with examples what is the remedy of this problem ?

Answer 1

In this case, as the name suggests the primary precipitate separates out in a pure form and the second phase of the foreign substance which is slightly soluble forms afterwards. The second phase is therefore, not coprecipitated but postprecipitated. Postprecipitation can occur only when the supernatant is supresatureated, with respect to some component which crystallizes fairly slowly.

Ex:

Post precipitation of magnesium oxalate occurs if a precipitate of calcium oxalate is allowed to stand too long before being filtered.

1) If the precipitate is left in connect with the mother liquor the contamination increases with time in postprecipitation the contamination increases the faster the solution is agitated by either the mechanical or thermal means.

2)  In the case of postprecipitation the contamination increases the faster the solution is agitated by either the mechanical or thermal means.