Rapid formation of precipitates
When crystals form slowly, each ion or molecule finds
its correct place in the crystal lattice. Very careful crystallization
is the best method of preparing materials that are extremely pure. In environmental
engineering, there is no need for pure crystals or precipitates. The rate
of precipitation depends, in part, on the degree of supersaturation. You
can select the time for the crystal to form in the following Java applet.
If the material that precipitates is fairly insoluble and forms crystals
at a moderate rate, a long time to form the crystal can deliver high purity.
Note that a few red circles stand for a low level of impurities.
When the precipitate is highly insoluble and precipitates
very rapidly (e.g., ferric hydroxide), there is not enough time for ions
to find their place in the lattice, and foreign ions can take their places.
The crystal can be quite impure.
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