Supersaturation, Metastable Zone, and Induction Time

A supersamrated solution is always located beyond its solubility line, just as a saUirated solution is always located at the equilibrium line. In order to provide more fundamental insight into the metastable phenomenon, a free energy-composition diagram, which provides further detail than is shown in Fig. 2-1, is presented in Fig. 2-8. The reader can find more detailed discussion in the following references (Balzhiser et al. 1972 Debenedetti 1995). 

As shown in Figs. 2-1 and 2-8, points A and B are equilibrium points at the lower part of two upward concave sections on curve II. In Fig. 2-8, two additional points C and D along curve II are identified. These are inflection points where the upward concave curve turns into 

Clearly, depending upon the nature of the system, a supersaturated solution could have a wide range of metastable zone width. Also, the supersaturated solution may remain metastable for a long time, i.e., a long induction time, before it forms the secondary solid phase. 

Similar to solubility, the metastable zone width and induction time of a supersaturated solution are affected by various factors, including temperature, solvent composition, chemical structure, salt form, impurities in the solution, etc. Therefore, although the spinodal point is a thermodynamic property, it is very difficult to measure the absolute value of the metastable zone width experimentally. Regardless, understanding the qualitative behavior of the metastable zone width and the induction time can be helpful for the design of crystallization processes. 

Other factors, such as the presence of seed of the desired compound, undissolved extraneous solid particles, and even agitation intensity can affect the metastable zone width and induction time. Clearly, these factors can perturb and alter the free energy-composition phase curve. In general, these factors will lower both the metastable zone width and the induction time. 

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