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Chapter II Solution Growth….
crystal nuclei form readily owing to spontaneous fluctuations in the
concentration of the substance. The boundary separating the metastable and
labile zones indicates the maximum super-saturation at which excess solute
does not crystallize spontaneously. Solutions are relatively stable in the
metastable state. This is because of considerable work done for the formation
of a critical size crystal nucleus; the energy barrier cannot be overcome by
means of natural fluctuations in the concentration of the substance. However,
the simple way to initiate growth from supersaturated solutions is to introduce
seeds, which provide nucleation centers. The boundary between the labile and
meta-stable zones, usually, can not be defined clearly because it depends on
the nature of the solution, the processes occurring in them and the degree of
the purity of the starting material.
Controlled crystal growth is possible only from metastable solutions.
The driving force of the process is the deviation of the system from
equilibrium, which can be characterized either by “super-saturation” ∆C or by
the value of “super- cooling” ∆T. The super cooling can be defined as the
difference between the temperature of saturation of the solution and that of
growth. If the possibility of the substance is unknown, the super cooling ∆T
serves as a rough estimate of the deviation from equilibrium.
The solubility of KDP in water at temperatures 0, 10, 20, 30, 40, 60, 80 and 90
0 C are 14.8, 18.3, 22.6, 28.0, 33.5, 50.2, 70.4 and 83.5 parts by weight per
100 parts by weight of water, respectively [41].
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