Crystals purity

Value depends on crystal purity. For example, the presence of 0.1 atom % boron causes a lattice constriction of about 0.03%. 

In practice, industrial crystallization processes are subject to a number of constraints, which tend to limit equipment selection. For example, since particle size and purity tend to be such important variables, equipment and operating conditions that induce minimum particle breakdown or achieve maximum crystal purity are normally desirable. 

Redman, T. and Roliani, S., 1992. Control of crystal purity in a continuous KCl crystallizer. In First separations division topical conference on separations technologies New developments and opportunities, 2-6 November 1992, Miami Beach FI. (New York American Institute of Chemical Engineers), p. 685. 

Tables Assignment and wavenumbers (cm ) of the external and torsional vibrations of a-Ss based on polarization dependent studies [106, 107]. In the first two columns the type and symmetry classes of the molecular and crystal vibrations, respectively, are given. The wavenumbers of the vibrations are listed in the columns infrared and Raman corresponding to the order of symmetry species given in the second column (crystal). " S means orthorhombic Sg with natural isotopic composition, while stands for isotopically pure Sg crystals (purity >99.95%)... 

Re-crystallisation from a solution or a melt is, therefore, widely employed to increase crystal purity. 

The purity of amino acids recovered by batch crystallization has been examined using L-isoleucine as a model system. The concentration of impurities in the feed solution were shown to affect crystal purity, as were variables that affect crystallization kinetics (e.g., agitation, precipitant addition rate, and cooling rate). 

The second mechanism by which supersaturation was thought to infiuence crystal purity is through crystal growth an increase in supersaturation leads to higher growth rates which means greater likelihood of impurity entrapment. This is considered to be the most probable cause of the relationship between supersaturation and purity for crystallizations of the type examined here. 

In the optical resolution of DL-threonine mixtures by batch preferential crystallization, changes of solution concentration and crystal purity were measured. The mechanism of nucleation of the un-seeded enantiomer was discussed to explain the purity decrease of the resolved crystals. From the observation of crystallization behavior of the seed crystals of L-threonine, it was concluded that the existence of the D-enantiomer on the surface of the seed caused the sudden nucleation when they grew to attain sufficient amounts. 

Changes of Solution Concentration and Crystal Purity during Crystallization... 

Figure 2. Changes of crystal purity with time for crystallization with washed and unwashed seed crystals.

In Figure 4 the corresponding changes in the crystal purity are shown. For the solutions with the initial supersaturation At=5 C, the purity was always 100% for three hours, while purity decrease started very soon for the solution having higher initial supersaturation such as At=10 C. Since with different initial supersaturations, the crystallization rate are very different, the purity decrease should be compared on the basis of either the supersaturation change or the crystal mass grown in the solution rather than the crystallization time. 

When the crystal purity is plotted against the total crystal mass in the slurry calculated from the mass balances, the purity decrease seems to start at some constant value of the crystal mass as shown in Figure 5. As mentioned earlier in the text, there are possibilities of existence of the D-enantiomer as small particles on the surface of the seed crystals. If we assume that the breeding of the D-enantiomer starts only when that enantiomer has grown to a certain size, the amount of the L-enantiomer crystals must have also increased to a certain value, the latter being proportional to the former. The crystallization kinetics of the both enantiomers are believed to be the same, the relative amounts of crystals of the both enantiomers must therefore be constant before nucleation of the D-enantiomer starts. 

Figure 4. Decrease of crystal purity during resolutions. Figure 3 for the notation of the keys.

From the consecutive measurements of solution concentrations and crystal purities during the optical resolution by preferential crystallization, the crystallization of D-threonlne other than the seeded component (L-threonine) was observed In the later stage of the resolution. Washing of the seed crystals was found to be effective to delay the purity decrease. D-threonine was believed to be Introduced... 

Batch crystallizers, applications, 102 Batch preferential crystallization, purity decrease of L-threonine crystals in optical resolution, 251-259 Bayer proce description, 329 Benzene, hi -pressure crystallization from benzene-cydohexane mixture, 281-289... 

DSC, melting point DSC methods may be developed to determine crystal purities, and transformation of one polymorph to another during heating. 

Available forms Small ingots or bars, shot, pencils, wire, sheets, powder, single crystals. Purity Technical, high purity (below 10 ppm impurities). 

The dielectric constant of alumina is given in Table 19 as a function of temperature and crystal orientation. The dielectric constant increases slightly up to 500°C, and is quite dependent on orientation. Very low dielectric loss values for sapphire have been reported [2], but are questionable. With reasonable purity, loss tangents below 0.001 are likely. Actual values probably depend strongly on crystal purity. 

Impurities in liquid crystal materials can induce the appearance of a mesophore where none exists in a purer sample, or can inhibit the formation of a mesophore that would be present (68). Small amounts of impurity may broaden and shift the mesophore transition to lower temperatures by several degrees. Hence, the DSC method is useful for the determination of liquid crystal purity. 

As stated above, control of the crystallization or precipitation process is essential to obtain crystals of biochemical compounds having appropriate properties. The phenomena, techniques, and analysis discussed in many of the previous chapters solubility and supersaturation, nucleation and growth kinetics, population balance methods, batch and continuous crystallizers, and factors governing crystal purity, habit and morphology are all relevant to the discussion of the crystallization of pharmaceuticals. We shall analyze the crystallization/precipitation of biomolecules in terms of these concepts. 

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