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Zone Refining is the method of choice for ultra-purification of organic chemicals.
Zone Refining is the method of choice for ultra-purification of organic chemicals.
MiniZone Refiner Theory
During zone refining or zone melting short molten zones move slowly through an elongated sample of crystalline material. A molten zone traversing a solid sample has two liquid-solid interfaces; a melting interface and a freezing interface. At the melting interface the sample is merely melted and mixed with the contents of the molten zone. At the freezing interface the crystals tend to be higher in purity than the liquid phase.
Extraordinarily high purification of chemicals can be obtained by repeatedly passing the molten zone or zones through the sample. The impurities are concentrated at the end of the sample, generally in the direction of the movement of the molten zone.
Mathematically this can be expressed as a constant, K, known as the distribution coefficient. If the concentration of impurity in the solid (crystalline) phase is Cs and the concentration of impurity in the liquid phase is Cl then K is Cs/Cl. If K is less than 1 (the usual case), the impurities will follow the movement of the liquid zone; if K is greater than 1, the impurities will travel in the opposing direction.