Impurities occlusion

Local variation in supersaturation is the most significant control issue that can cause non-reproducibility in PSD and other physical attributes, as well as solvent and impurity occlusion. These local variations occur both at the heating surface and at the boiling liquid/vapor interface. 

Intermediates in a chemical synthesis may also be produced by reactive crystallization and are also subject to impurity occlusion and poor downstream performance. 

Conductivity. Many workers have attempted to measure the dc conductivity of K2Pt(CN)4Bro.3(H20)x (40,166,246,253,258,270,273,309,310,364a, 435). Conductivity (303, 503) is a transport measurement and, therefore, is extremely sensitive to impurities and defects which may interrupt and/or interconnect chains. The defects may arise from cracks or fissures in the crystal or from solvent or impurity occlusions which affect the electron flow. A qualitative measure of crystalline quality is reflected by the anisotropic conductivity ratio, ct, jaj. Better morphology and lower impurity levels suggest fewer interchain bridges and intrachain breaks, allowing the anisotropic ratio to increase to its intrinsic value. Because the measured conductivity is a function of crystalline perfection, only the better characterized measurements are described, Table IV. 

This method of controlling supersaturation was also effective in rejecting impurities from the growing crystals, as shown in Figure 17-9, where impurity occlusion for a faster addition is shown to be increased substantially. 

Decolorisation by Animal Charcoal. It sometimes hap pens (particularly with aromatic and heterocyclic compounds) that a crude product may contain a coloured impurity, which on recrystallisation dissolves in the boiling solvent, but is then partly occluded by crystals as they form and grow in the cooling solution. Sometimes a very tenacious occlusion may thus occur, and repeated and very wasteful recrystallisation may be necessary to eliminate the impurity. Moreover, the amount of the impurity present may be so small that the melting-point and analytical values of the compound are not sensibly affected, yet the appearance of the sample is ruined. Such impurities can usually be readily removed by boiling the substance in solution with a small quantity of finely powdered animal charcoal for a short time, and then filtering the solution while hot. The animal charcoal adsorbs the coloured impurity, and the filtrate is usually almost free from extraneous colour and deposits therefore pure crystals. This decolorisation by animal charcoal occurs most readily in aqueous solution, but can be performed in almost any organic solvent. Care should be taken not to use an excessive quantity... 

Occlusions, which are a second type of coprecipitated impurity, occur when physically adsorbed interfering ions become trapped within the growing precipitate. Occlusions form in two ways. The most common mechanism occurs when physically adsorbed ions are surrounded by additional precipitate before they can be desorbed or displaced (see Figure 8.4a). In this case the precipitate s mass is always greater than expected. Occlusions also form when rapid precipitation traps a pocket of solution within the growing precipitate (Figure 8.4b). Since the trapped solution contains dissolved solids, the precipitate s mass normally increases. The mass of the precipitate may be less than expected, however, if the occluded material consists primarily of the analyte in a lower-molecular-weight form from that of the precipitate. 

Inclusions, occlusions, and surface adsorbates are called coprecipitates because they represent soluble species that are brought into solid form along with the desired precipitate. Another source of impurities occurs when other species in solution precipitate under the conditions of the analysis. Solution conditions necessary to minimize the solubility of a desired precipitate may lead to the formation of an additional precipitate that interferes in the analysis. For example, the precipitation of nickel dimethylgloxime requires a plT that is slightly basic. Under these conditions, however, any Fe + that might be present precipitates as Fe(01T)3. Finally, since most precipitants are not selective toward a single analyte, there is always a risk that the precipitant will react, sequentially, with more than one species. 

Example of copredpitation (a) schematic of a chemically adsorbed inclusion or a physically adsorbed occlusion in a crystal lattice, where C and A represent the cation-anion pair comprising the analyte and the precipitant, and 0 is the impurity (b) schematic of an occlusion by entrapment of supernatant solution (c) surface adsorption of excess C. 

In these cases, the product is reslurried with pure liquor or fresh solvent if the solubility is not too high, and refiltered. In order to meet the required product impurity level, several such washings may take place in series. See Coulson and Richardson (1991) and Mullin (2001) for design guides and examples calculations. It is noted that impurities retained within liquid occlusions are particularly difficult to remove without first crushing the crystals. 

When a precipitate separates from a solution, it is not always perfectly pure it may contain varying amounts of impurities dependent upon the nature of the precipitate and the conditions of precipitation. The contamination of the precipitate by substances which are normally soluble in the mother liquor is termed co-precipitation. We must distinguish between two important types of co-precipitation. The first is concerned with adsorption at the surface of the particles exposed to the solution, and the second relates to the occlusion of foreign substances during the process of crystal growth from the primary particles. 

The sample gets coloured due to the occlusion of bromine. It is recrystallized from 10 times its weight of water to obtain the pure form. It is always necessary to use the pure form by eliminating inorganic impurities which tend to increase the addition of bromine in reactions. 

Occlusion. Adsorbed impurities on the surface of the crystal are trapped by subsequent strata during crystal accretion. 

For solutions containing sulphuric acid or a sulphate the reagent commonly applied is barium chloride, both when the test is to be qualitative and when quantitative. Precipitation is effected by the gradual addition of barium chloride to the boiling solution containing a little hydrochloric acid, but for the production of pure barium sulphate, and therefore in order to ensure accuracy, certain precautions must be observed.4 Nitrates, perchlorates, phosphates, tervalent metals and large quantities of salts of the alkali metals (particularly potassium) and of the alkaline earth metals are to be avoided, as they cause the precipitated barium sulphate to be rendered impure by occlusion of otherwise soluble substances.5 Such impurities may be accounted for partly by... 

Adsorbed impurities are bound to the surface of a crystal. Absorbed impurities (within the crystal) are classified as inclusions or occlusions. Inclusions are impurity ions that randomly occupy sites in the crystal lattice normally occupied by ions that belong in the crystal. Inclusions are more likely when the impurity ion has a size and chaige similar to those of one of the ions that belongs to the product. Occlusions are pockets of impurity that are literally trapped inside the growing crystal. 

This kinetically dependent mechanism provides a means to develop a nanocomposite microstructure with the particles (or the majority of the particles) occluded within the matrix grains. On the other hand, occlusion can be prevented, for the most part, if glass-forming impurity elements are not introduced into the material during the processing stage. As we will see in the next section, the position of the particles (i.e. occluded or at grain boundaries) can influence the microstructurally dependent properties of nanocomposites. 

The purity of a crystalline product depends on the nature of the other species in the mother liquor from which the crystals are produced, the physical properties of the mother liquor, and the processing that occurs between crystallization and the final product (downstream processing). Impurities can find their way into the final product through a number of mechanisms the formation of occlusions, trapping of mother liquor in physical imperfections of the crystals or agglomerates, adsorption of species onto crystal surfaces, as part of chemical complexes (hydrates or solvates), or through lattice substitution. 

Crystallization from an overall viewpoint represents transfer of a material from solution (or even a gas) to a solid phase by cooling, evaporation, or a combination of both. But there is more to it. Of considerable importance are economics, crystal size distribution, purity, and the shape of the crystals. Impurities or mother solution are carried along only in the surface or occlusions in the crystals. The partical size distribution depends on the distribution of seed crystals, which are injected into the crystallizer prior to initiation of crystallization (batch) or continuously from recycled undersized particles, the mixing in the system, the crystal growth rate, and the degree of supersaturation of the mother liquor. As in shown in the figures, both batch and continuous crystallization are used in industry. 

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