Field of Supersaturation

The solubility chart divides the field of the solution into two regions: the subsaturated region where the solution will dissolve more of the solute at the existing conditions, and the supersaturated region.

Before Miers identified the metastable field, it was thought that a solution with a concentration of solute greater than the equilibrium amount would immediately form nuclei. Miers' research and the findings of subsequent researchers determined that the field of supersaturation actually consists of at least three loosely identified regions (Fig. 1):

Metastable region—where solute in excess of the equilibrium concentration will deposit on existing crystals, but no new nuclei are formed.

Intermediate region—where solute in excess of the equilibrium concentration will deposit on existing crystals and new nuclei are formed.

Labile region—where nuclei are formed spontaneously from a clear solution.

The equipment designer wishes to control the degree of supersaturation of the solution in the metastable region when designing a batch crystallizer. In this region, where growth takes place only on existing crystals, all crystals have the same growth time and a very uniform crystal size is obtained.

When designing a continuous crystallizer, the designer wishes to control the degree of supersaturation in the lower limits of the intermediate region. In continuous crystallization, it is necessary to replace each crystal removed from the process with a new nuclei. It is also necessary to provide some degree of crystal size classification if a uniform crystal size is to be obtained.

Solutions of most organic chemicals can, as a general rule, attain a considerably higher degree of supersaturation than inorganic chemicals. The formation of crystalline nuclei requires a definite orientation of the molecules

Temperature

Figure 1. Solubility curve.

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