Purification Using Zone Refining

When a crystal freezes from the melt, it tends to reject impurities. If Zone- Refining is applied to such a cr3rstal, the resulting single crystal can be purer than the original. If we make more than one pass, we can 

It has been determined that there is a distribution coefficient for the impurities between crystal and melt which favors the melt. We can see how this arises when we reflect that impurities tend to cause formation of intrinsic defects within the crystal and lattice strain as a result of their presence. In the melt, no such restriction applies. Actually, each impurity has its own distribution coefficient. However, one can apply an average value to better approximate the behavior of the majority of impurities. 

One calculates the concentration of impurities left in the single aystal, as a function of a single pass, from  

We plot Ci / Co VS L / Z, the niunber of zone-lengths solidified. This is illustrated in the following diagram, given as 4.2.34. on the next page. In this case, we have shown how the impurities change as a function of the distribution coefficient for one pass. Note that at valu of ko 1, we do not obtain any significant improvement in impurity content. For example, we start at k = 5.0, but end up at 1.0. Even at kp = 0.5, the further we 

Measuring particle size and growing single crystals 

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K2C03 CaH2, CaO or sodium, then fractionally distd. Near-dry alcohol can be further dried by refluxing with magnesium activated with iodine, as described for ethanol. Further purification is possible using fractional crystn, zone refining or preparative gas chromatography. 

H. Oka, M. Suzuki, K.-I. Harada, M. Iwaya, K. Fuii, T. Goto, Y. Ito, H. Matsumoto andY. Ito, Purification of Food Color Red No. 106 (acid red) using pH-zone-refining counter-current chromatography. J. Chromatogr.A, 946 (2002) 157-162. 

Pyrene and naphthalene were purified by zone refinement for more than 130 passes (Bridgmen method). After such purification the central part of the glass ampoule was extracted and used in the sample preparation. Drop coating of the pyrene (naphthalene) with SWNT suspension in toluene on a quartz substrate was used to form a thin film for Raman measurements. The film was deposited onto the quartz substrate from nanotube suspension in toluene (0.1 mg/mL) and from suspension of nanotube with pyrene (or naphthalene) after short sonication (20 minutes 44 kHz). The weight ratio was 1 1 and 1 4 for mixture with pyrene (samples PI and P2, respectively) and 1 1 with naphthalene (sample N). 

NMA was obtained from Eastman Kodak Co., fractionally crystallized once, and then continuously zone-refined 10-20 times. The zone refiner had three purification zones the product from the first zone was used,... 

In the case of the solids,initial experiments showed that the conventional purification techniques such as recrystallization from solutions do not offer sufficiently pure materials. For example,PMDA samples, after several recrystallizations and vacuum sublimations appear colorless,but show high levels of ionic contaminations when analyzed for inorganic ions. To eliminate this problem,the technique of zone refining as used in semiconductor materials purifications has been used to purify the starting materials. This technique has been applied for a number of organic materials in our laboratory. PMDA for example,when subjected to a simple zone refining process ( as few as 25 zones), shows removal of impurities as high as 3% even if preceded by recrystallization and sublimation. In syntheses of polyamic acids,a variation as small as 3% in stoichiometry can cause considerable variation in the final batch-to-batch synthesis. 

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