Purity of crystals

Phenomena, methods of operation, etc. have been studied extensively for the use of crystallization in separation processes. Although much remains to be learned about such processes, relatively little attention has been given to the other functions and the purpose of this work was to examine the role of various process variables in determining the purity of crystals recovered from a batch crystallizer. The system studied experimentally was a model system of amino acids, and the key variables were the composition of the liquor from which a key amino acid was crystallized, the rate at which supersaturation was generated by addition of an acid solution to reduce solubility, and the degree of mixing within the batch unit. 

The growth rates of crystals with partially broken surfaces or with fines adhered to it were larger in both directions than those of smooth surface. Further, the optical purities of crystals with broken surfaces and with fines adhered on the surfaces were lower than those with smooth surfaces. From these results, the kinetics of growth of D-SCMC crystals and the mechanism of inclusion of impurity during their growth process was considered. 

If the adsorption results in an organized deposit layer and a certain arrangement similar to the crystalline lattice we speak of epitaxy. A prerequisite of epitaxy is not only a two-dimensional lattice analogy of foreign lattice layers but also the mode of nucleus formation, fusing the nuclei, purity of crystal surface, and binding energy between lattice... 

Decrease in purity of crystal structure. Since larga single crystals are easily obtained, the amount impurity in a crystal can be reduced. However, in the case of consecutive phase transformations (6), the possibility of coexisting crystal... 

Besides the molar gel composition (what is an obvious factor of influence), a strong effect on phase purity of crystallization product and its crystallinity is attributed to the origin of the reagents used in synthesis of SAPO materials. We have found that DiPenA template is very selective in SAPO-31 formation for the case of both aluminum isopropoxide and Reheis F2000 alumina hydroxycarbonate as a source of alumina as well as for different kinds of silica sources. In Table 3 catalytic properties of SAPO-31 prepared with different reagents are presented. 

The detailed experimental investigations have, at least for CeCu2Si2 and UBej3, spoiled the original hope of finding triplet superconductivity in metals similar to the known case of superfluid He. Apparently the lower symmetry and the lower purity of crystals play an essential role for the superconducting states found in heavy-fermion compounds. We would like to recall a few key observations which underline this conclusion ... 

In commercial crystallization the yield and purity of crystals are not only important but also the sizes and shapes of the crystals. It is often desirable that crystals be uniform in size. Size uniformity is desirable to minimize caking in the package, for ease of pouring, for ease in washing and filtering, and for uniform behavior when used. Sometimes large crystals are requested by the purchaser, even though smaller crystals are just as useful. Also, crystals of a certain shape are sometimes required, such as needles rather than cubes. 

Another paper [143) deals with the optimization of conditions for Improving the purity of crystals. 

Stem layer adsorption was involved in the discussion of the effect of ions on f potentials (Section V-6), electrocapillary behavior (Section V-7), and electrode potentials (Section V-8) and enters into the effect of electrolytes on charged monolayers (Section XV-6). More speciflcally, this type of behavior occurs in the adsorption of electrolytes by ionic crystals. A large amount of wotk of this type has been done, partly because of the importance of such effects on the purity of precipitates of analytical interest and partly because of the role of such adsorption in coagulation and other colloid chemical processes. Early studies include those by Weiser [157], by Paneth, Hahn, and Fajans [158], and by Kolthoff and co-workers [159], A recent calorimetric study of proton adsorption by Lyklema and co-workers [160] supports a new thermodynamic analysis of double-layer formation. A recent example of this is found in a study... 

The tribromobenzene obtained in this way should be entirely free from unchanged tribromoaniline. To test its purity, dissolve a small quantity in hot dry benzene and pass in hydrogen chloride gas from a Kipp s apparatus no trace of crystals of tribromoaniline hydrochloride should appear. Note also that although the m.p.s of the two compounds are almost identical, that of the recrystallised product from the above preparation is considerably depressed by admixture with tribromoaniline. 

If the filtrate is of value, it should be transferred to another vessel immediately the crystals have been drained. Frequently, the mother liquor may be con centra ted (suitable precautions being, of course, taken if it is inflammable), and a further crop of crystals obtained. Occasionally, yet another crop may be produced. The crops thus isolated are generally less pure than the first crystals which separate, and should be recrystaUised from fresh solvent the purity is checked by a melting point determination. 

The normal boiling point of 2-methylthiazole is 17 0= 128.488 0.005°C. The purity of various thiazoles was determined cryometrically by Handley et al. (292), who measured the precise melting point of thiazole and its monomethyl derivatives. Meyer et al. (293, 294) extended this study and, from the experimental diagrams of crystallization (temperature/degree of crystallization), obtained the true temperatures of crystallization and molar enthalpies of fusion of ideally pure thiazoles (Table 1-43). 

Acetyl chloride frequently contains 1—2% by weight of acetic acid or hydrochloric acid. Phosphoms or sulfur-containing acids may also be present in the commercial material. A simple test for purity involves addition of a few drops of Crystal Violet solution in CHCl. Pure acetyl chloride will retain the color for as long as 10 min, but hydrochloric, sulfuric, or acetic acid will cause the solution to become first green, then yellow (34). 

Lake Texcoco. Lake Texcoco, a few miles northeast of Mexico City, is in the lowest part of the Valley of Mexico. The lake is mostly dry and alkaH is recovered from brine weUs that have been drilled into the underlying stmcture. The brine is concentrated first in a spiral flow solar evaporation pond and further in conventional evaporators. This strong brine is carbonated and then cooled to crystallize sodium bicarbonate which is subsequently filtered and calcined to soda ash. Purity of this product is similar to Magadi material (9,29). 

The test methods used by industry to determine if a sample of maleic anhydride is within specifications (165) ate ASTM methods D2930, D1493, and D3366. These methods describe procedures for the determination of maleic acid content, the crystallization point, and the color properties of the maleic anhydride sample, respectively. By quantitative deterrnination of these properties, a calculation of the overall purity of the maleic anhydride sample can be made. 

The other two methods used by industry to examine the purity of maleic anhydride are the crystallization point (168) and color deterrnination of the sample (169). These tests determine the temperature at the point of solidification of the molten sample and the initial color properties of the melt. Furthermore, the color test also determines the color of the sample after a two-hour heat treatment at 140°C. The purpose of these tests is to determine the deviation in properties of the sample from those of pure maleic anhydride. This deviation is taken as an indication of the amount of contaminants in the maleic anhydride sample. 

Unfortunately, both EEEM and EIM microscopes require a conducting sample, usually metaUic, capable of being fashioned into a very tine point. The microscopes are used for study of crystal defects, purity, and, with EIM, the identification of single impurity atoms. 

Several commercial grades are available fine crystals of 99 to 100% purity, large crystals, pressed lumps, rods, and granular material. Double-Decomposition Methods. Double-decomposition processes all iavolve the reaction of sodium chloride, the cheapest chlorine source, with an ammonium salt. The latter may be suppHed directiy, or generated in situ by the reaction of ammonia and a supplementary iagredient. Ammonium chloride and a sodium salt are formed. The sodium salt is typically less soluble and is separated at higher temperatures ammonium chloride is recovered from the filtrate by cooling. 

The manufacture of silver nitrate for the preparation of photographic emulsions requires silver of very high purity. At the Eastman Kodak Company, the principal U.S. producer of silver nitrate, 99.95% pure silver bars are dissolved in 67% nitric acid in three tanks coimected in parallel. Excess nitric acid is removed from the resulting solution, which contains 60—65% silver nitrate, and the solution is filtered. This solution is evaporated until its silver nitrate concentration is 84%. It is then cooled to prepare the first crop of crystals. The mother Hquor is purified by the addition of silver oxide and returned to the initial stages of the process. The cmde silver nitrate is centrifuged and recrystallized from hot, demineralized water. Equipment used in this process is made of ANSI 310 stainless steel (16). 

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