Solute impurities

The earlier discussion on the effects of additives or impurities on crystal growth (Section 11.3.2) suggests that impurity incorporation is often surface specific. Black and Davey (1988) have reviewed much of the available information on the crystallization of amino acids. Amino acids are interesting model systems because of their common zwitterion group coupled with a variety of side chains, which may be present on a particular crystal face. L-asparagine can accommodate some 15% of L-aspartie acid as a mixed crystal (solid solution). From the effects of aspartic acid on the habit of asparagine crystals it is believed that aspartic acid primarily is incorporated at the 010 face whose growth rate is considerably reduced. 

The fact that impurities can be incorporated by growth on a particular face is exploited in doping of organic materials for electronic use. For example, triglycine sulfate crystals are doped throughout the lattice with alanine by growth only on the 010 face held in a crystal holder (White et al. 1976). A number of amino acids (L-aspartic, L-valine, L-leucine and L-phenylalanine) have been observed to be incorporated into the lattice of glutamic acid 

Mole % additive with respect to paracetamol in crystallizing solution 

These few examples indicate the importance of understanding the molecular and crystal structure of a given system to understand impurity incorporation and habit modification in the crystallization of these complex molecules. 

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The lead buUion, ready to be shipped to the refinery, contains in solution impurities such as silver, gold, copper, antimony, arsenic, bismuth, nickel, 2inc, cadmium, tin, tellurium, and platinum metals. 

Haidness decreases with increasing porosity and increased grain size. SoHd solution impurities influence hardness, but it is often hard to separate the effect of the impurity on the hardness, from the effect of the impurity on other microstmctural effects that influence hardness such as grain size. Further information on hardness of ceramics is available (45). 

One further point needs to be mentioned when probing the feasibility of a particular experiment. Apart from its dependence on temperature and concentration (for instance of ions, solutes, impurities, isotopes), relaxation times - in particular the longitudinal relaxation time Tj - depend on the field strength. This can be understood from the concept that energy exchange is most efficient if the timescale of molecular motion is equal to the Larmor frequency. Often, molecular motion takes place over a wide range of frequencies, so that the func-... 

The occurrence of a persistent absorbance change (AA ) for the [M, Fe] hybrids is a slight complication that requires an extended kinetic model (Scheme II) in which (MP) is reduced not only by Fe P (regenerating the [MP, Fe P] state), but also by an as yet unidentified amino acid residue X and/or solution impurities (Eq. (3)) leading to [M, Fe " ] ... 

In preparing cyclohexene from cyclohexanol, the impure cyclohexene obtained during the distillation of the reaction mixture can be further purified by solvent extraction. This can be done by adding the crude cyclohexene to a separating funnel along with an equal volume of sodium chloride solution. Impurities in the cyclohexene are extracted into the lower aqueous sodium chloride layer. The cyclohexene layer is then run off and treated with anhydrous magnesium sulfate to remove any remaining water. 

NaCi + NaCI grown grown in presens of a solution impurity of 10 % urea... 

Benzylidenearginine is quite insoluble in ether but appreciably soluble in methyl alcohol and in water. Attempts to recrystallize it from the latter solvents lead to a product of inferior quality, owing to decomposition in solution. Impure or contaminated samples may be purified by hydrolysis with hot hydrochloric acid and reprecipitation with benzaldehyde after neutralization. 

Extrinsic Crystal Self-Diffusion. Charged point defects can be induced to form in an ionic solid by the addition of substitutional cations or anions with charges that differ from those in the host crystal. Electrical neutrality demands that each addition results in the formation of defects of opposite charge that can contribute to the diffusivity or electronic conductivity. The addition of aliovalent solute (impurity) atoms to an initially pure ionic solid therefore creates extrinsic defects.10... 

SPERRY PROCESS. An electrolytic process for the manufacture of lead carbonate, basic (white lead) from desilivenzed lead containing, some bismuth, The impure lead forms the anode. A diaphragm separates anode and cathode compartments, and carbon dioxide is passed into the solution. Impurities, including bismuth, remain on the anode as a slime blanket,... 

Considerable difficulties are presented when solid impurities (dirt) are separated from the diethylaluminumchloride solution. Impurities as a rule consist of finely dispersed particles of sodium chloride and highly flam-... 

Another thermal separation unit often used for the laboratory scale purification of ionic liquids is recrystallization [125]. It is an attractive option for those ionic liquids that can form solids with a high degree of crystallinity. Crystals of ionic liquids are expected to be pure because each molecule or ion must fit perfectly into the lattice as it leaves the solution. Impurities preferentially remain in solution as they do not fit as well in the lattice. The level of purity of the crystal product finally depends on the extent to which the impurities are incorporated into the lattice or how much solvent is entrapped within the crystal formed. 

Kinds of matter material, substance (pure substance), mixture, solution, impure substance, impurity. 

We have also pursued electrochemically back-plating of the copper sample to reduce the copper ion concentration and leave in solution impurities such as thorium and uranium, which should not plate out at the half-cell potential of copper. Theoretically, the amount of sample that can be processed in this manner is not limited. All materials including any non-sample electrodes must not add contamination and must be of extreme purity. Also, the amount of copper remaining in solution must be back-plated to <10 (xg/ml, and if a sulfate system is used, which is useful in support of further developing the predictive rejection rate information, then the sulfate ion should be <10 mmol as well. This approach hinges on the rejection rate remaining sufficiently high as to not introduce an undue amount of error. We have measured rejection rates as low as 10 but even at 10 this would only represent a 1% error in the assay result. 

In 2003, the world zinc production was 9 880000 tons [55]. The most important zinc production process is the electrolytic or roast-leach-electrowinning (RLE) process. This was first used in 1916 by Anaconda and Cominco. The industrial processes of zinc production use zinc oxides as raw materials. The most important natural raw material is zinc sulfide, and, therefore, it needs to be roasted and converted to oxide. The main problem in leaching and liquor purification is separation of zinc and iron. As dissolution of iron cannot be avoided, it must be precipitated from the zinc sulfate solution. Impurities still present after the iron precipitation stage can lead to lower current efficiency and impurities in the zinc cathode. Therefore, the solution is further purified by cementation with zinc powder. 

Design (velocity of freezing front)(liquid boundary layer thickness that depends on mixing)/(dif-fusivity of solute impurity in the liquid) = 1. 

The initiator may be preformed or generated in situ simply by combining the organolithium compound with the amine. Despite its high reactivity, the resulting complex can be easily handled as a hydrocarbon solution. Impurities such as water, air, and carbon dioxide must be rigidly excluded because of their rapid reaction with organolithium compounds. 

Figure 13.6.1 Cyclic voltammogram for a smooth platinum electrode in 0.5 M H2SO4. Peaks formation of adsorbed hydrogen. Peaks H oxidation of adsorbed hydrogen. Peaks Oq formation of adsorbed oxygen or a platinum oxide layer. Peak Oc reduction of the oxide layer. Point 1 start of bulk hydrogen evolution. Point 2 start of bulk oxygen evolution. The shape, number, and size of the peaks for adsorbed hydrogen depend on the crystal faces of platinum exposed (62), pretreatment of electrode, solution impurities, and supporting electrolyte. See also Figure 13.4.4.

For a simple system in which a solid solution is not formed and an ideal solution in the melt is formed, the mole fraction of solute impurity, component 2) in the solvent [component 1K is given by... 

Lastly, the mass transport processes at the crystal-liquid interface play a central role in crystallization. The influence of solvent and impurities on the structure and growth rates of faces is considered in this chapter along with its effect on the incorporation of impurities. The solvent solute-impurities interactions in solution will also be shown to interact in subtle, but important, ways with the interface during the crystallization process. With appropriate thermodynamic analysis it is shown how these interactions ultimately affect crystallization as a purification process. 

As has been discussed throughout this book, crystallization can generally result in very large purification factors as well as being useful as a concentration step that removes most of the solvent from the desired solute. Impurities within crystals that limit purification efficiency arise from ... 

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