Tutoring chemistry remains interesting because it’s full of neat concepts and applications.

Zone refining is a technique that can produce highly pure samples; the semiconductors germanium and silicon can be zone-refined.

For zone refining to work, the substance must reject impurities as it cools. Mathematically, the ratio between the % impurity it will tolerate in solid form versus liquid form is k, the distribution coefficient. When k<1, the molten substance, as it solidifies, will exclude impurities; the impurities will be pushed to the boundary between solid and liquid. Zone refining is carried out as follows:

1. A long rod of the impure substance, desired to be refined, is obtained.
2. A cylindrical heater, much shorter than the rod, is fitted around one end.
3. The heater melts the portion of the rod it surrounds, then is moved along to the adjacent section, which it also heats and melts.
4. The portion of the rod that has been left behind now cools and solidifies.
5. Because k<1, the solidifying part of the rod excludes the impurities, which are drawn to the adjacent, molten section of the rod.
6. As the process continues, the impurities are drawn along the rod, eventually to its far end, which can be discarded.
7. With each melting and re-hardening, perhaps not all impurities are removed. However, the process can be repeated to gain increased purity.

Source:

Mortimer, Charles E. Chemistry, sixth ed. Belmont: Wadsworth, 1986.

www.princeton.edu

www.princeton.edu

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