Partitioning of impurity

This experiment demonstrates that a distinction between the r and z faces based on the size difference becomes impossible. Also, the growth sector of the z face takes on a violet color. In an isotropic environment, the violet coloring appears selectively in the r growth sectors, and the z growth sectors are colorless. This experiment also demonstrates that the partitioning of impurity Fe is affected by the growth rate. 

Solvent extraction—purification of wet-process phosphoric acid is based on preferential extraction of H PO by an organic solvent vs the cationic impurities present in the acid. Because selectivity of acid over anionic impurities is usually not sufficient, precipitation or evaporation steps are included in the purification process for removal. Cmde wet-process acid is typically concentrated and clarified prior to extraction to remove post-precipitated sludge and improve partition of the acid into the solvent. Concentration also partially eliminates fluoride by evaporation of HF and/or SiF. Chemical precipitation of sulfate (as Ba or Ca salts), fluorosiUcates (as Na salt), and arsenic (as sulfides) may also be used as a prepurification step preceding solvent extraction. 

In some metal components it is possible to form oxides and carbides, and in others, especially those with a relatively wide solid solubility range, to partition the impurity between the solid and the liquid metal to provide an equilibrium distribution of impurities around the circuit. Typical examples of how thermodynamic affinities affect corrosion processes are seen in the way oxygen affects the corrosion behaviour of stainless steels in sodium and lithium environments. In sodium systems oxygen has a pronounced effect on corrosion behaviour whereas in liquid lithium it appears to have less of an effect compared with other impurities such as C and Nj. According to Casteels Li can also penetrate the surface of steels, react with interstitials to form low density compounds which then deform the surface by bulging. For further details see non-metal transfer. 

The effect of temperature on distribution ratios has already been mentioned on page 91. Although the separation proceeds more quickly at elevated temperatures, resolution suffers because of increased rates of diffusion. However, in adsorption TLC only small increases in Rt values are observed even with a 20°C rise. Strict temperature control is not necessary if samples and standards are run at the same time, although large fluctuations should be avoided. The quality of the thin-layer materials, and in particular the presence of impurities in them, determine the extent to which partition, adsorption, ion-exchange and exclusion participate in the sorption process. These factors affect Rr values in an unpredictable manner. Thin layers should be of uniform thickness, between 0.2 and 0.3 mm with thinner layers, local variations in thickness can result in appreciable variations in Rf values. 

It is clear from the applications described above that the strength of CE in the field of impurity profiling lies in the various selectivities that can be applied. Not only can one take advantage of electrophoretic phenomena but also of the principle of chromatographic separation and partitioning that is present in techniques such as MEKC and MEEKC. The resolution of substances based on size differences as in CGE is also very useful in the field of biopharmaceuticals. CE can be seen as a complementary technique to other commonly used separation techniques as LC, and in view of its capacity to separate large numbers of compounds, it should certainly always be kept in mind when an analytical problem has to be solved. 

At the third level, the most detailed partition of luminescence minerals is carried out on the basis of metals in the mineral formulae, hi rare cases we have minerals with host luminescence, such as uranyl minerals, Mn minerals, scheelite, powellite, cassiterite and chlorargyrite. Much more often luminescent elements are present as impurities substituting intrinsic cations if their radii and charges are close enough. Thus, for example, Mn + substitutes for Ca and Mg in many calcium and magnesium minerals, REE + and REE substitutes for Ca, Cr substitutes for AP+ in oxygen octahedra, Ee substitutes for Si in tetrahedra and so on. Luminescence centers presently known in solid-state spectroscopy are summarized in Table 4.2 and their potential substitutions in positions of intrinsic cations in minerals in Table 4.3. 

In some systems an impurity may partition itself in such a way that it is swept to the top of the ingot, and of course both types of impurity may occur in the same material. In such cases only the middle part of the ingot has the required purity. (The author thanks Prof. J. N. Sherwood for some personal advice on this point.)... 

I.4. Adsorption. The process of adsorption involves the partitioning of the adsorbed substance between the bulk phase in which it is dissolved and the surface of the adsorbant, and it is a reversible, equilibrium process, unless the adsorbed species is transformed or bound chemically the use of solid reagents to trap impurities by chemical reaction will be discussed in the following section. 

If we consider NaOH, KOH, and NaF, which act as mineralizers, to be the solvent components, and other minor amounts of elements such as Fe +and Al " to be impurity elements, then the partitioning of these impurity elements is controlled principally by kinetics. The impurity partitioning is related to color, or radiation-induced color, and crystal morphology. 

Angus J.G., Rayner B.J. and Robson M. (1979) Reliability of experimental partition coefficients in carbonate systems evidence for inhomogeneous distribution of impurity cations. Chem. Geol. 27, 181-205. 

The bulk of the hexane extract is a clear, yellow oil that partitions into the hexane layer. The small amount of impurity which accompanies the pesticide into the acetonitrile layer is removed easily later by column adsorption chromatography. The acetonitrile is removed by evaporation on a hot water bath with a gentle stream of filtered air. In this way the extract may be evaporated just to dryness without loss, the small amount of oil remaining being sufficient to prevent losses by volatilization. 

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