Morphological structure and purity of precipitates

29 MORPHOLOGICAL STRUCTURE AND PURITY OF PRECIPITATES Precipitation is probably the most frequently used method in the practice of qualitative analysis. The occurrence of a precipitate as the result of the addition of a certain reagent may be used as a test for a given ion. In such 

The size of crystals formed during the course of precipitation depends mainly on two important factors these are the rate of nucleation and the rate of crystal growth. 

The morphological structure of precipitates can often be improved by after-treatment. It is known that the solubility of very small particles is considerably greater than that of a larger crystal of the same substance. If a mixture made of the mother liquor and the precipitate is simply allowed to stand for a longer time, the small particles will slowly dissolve in the mother liquor, while the larger particles will in turn grow thus a recrystallization occurs. This process 

Some precipitates are deposited slowly and the solution is in the state of supersaturation for a considerable time. Thus, when calcium oxalate is precipitated in the presence of larger amounts of magnesium ions, the precipitate is practically pure at first, but if it is allowed to remain in contact with the solution, magnesium oxalate forms slowly (and the presence of calcium oxalate precipitate tends to accelerate this process). Thus, the calcium oxalate precipitate becomes contaminated owing to post-precipitation of magnesium oxalate. Post-precipitation often occurs with sparingly soluble substances which tend to form supersaturated solutions, they usually have an ion in common with the primary precipitate. Another typical example is the precipitation of copper or mercury(II) sulphide in dilute acid solution, which become contaminated, if zinc ions are present, by post-precipitation of zinc sulphide. Zinc ions alone may not be precipitated with sulphide ions under identical circumstances. 

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