Solid solution occlusion

SrC03, as well as occlusions or solid solutions in the SrO. Contamination of the crucible during the heat cycle should also be included. Such small contamination errors are estimated to add up to not more than a contributing relative standard uncertainty of 2x10 4. 

Kolthoff proposed the concept that entrapment of foreign ions, involving growth of precipitate around adsorbed ions, is an important source of contamination, particularly of crystalline precipitates such as barium sulfate and calcium oxalate. Essential to this concept is that occlusion is not an equilibrium process and that recrystaUization during aging can effect purification. The foreign ions represent lattice imperfections unless they are actually held in solid solution. 

FIGURE 9-2 Occlusion of B by solid-solution formation. Solid lines, homogeneous solid solution with 17 = 0.1 or 10. Dotted lines, heterogeneous solid solution with X = 0.1 or 10. The straight line corresponds to 2 or /> = 1. (From Klein. )... 

The mechanism for adipic acid coprecipitation (surface adsorption versus occlusion or solid solution formation),... 

Sorption processes are very effective and include adsorption/desorption (reversible binding at the solid-water interface), absorption (diffusion of pollutants into the solid matrix), precipitation and coprecipitation (incorporation into a freshly formed solid), and occlusion (sequestration of adsorbed pollutants during mineral growth). The most important factors for retention processes are pollutant concentration, the composition of the solid matrix, solution composition (e.g., complexing agents) and E/pH conditions (Brady and Boms 1997). 

Because of their sizes, neither K+ nor Si + can enter into solid solution with the magnetite and so if some silica is present in the iron oxide used, small occlusions of alkali silicates are present as separate phases in the fused catalyst. Microscopic investigations of the milled catalyst showed that whereas the larger particles still contain alkali silicate occlusions, the finer particles consist of a mixture of separate alkali silicate and magnetite particles (20). Hence, the distribution of alkali in the milled, fused catalyst is heterogeneous. During reduction and FT synthesis, however, the alkali does to some extent spread over the catalyst surface ((7), chapter 3). 

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. 

Induced precipitation is a collective name for processes accompanying the formation of solid phase, such as occlusion, adsorption, compound formation, formation of isomorphous mixtures, mixed crystals, colloidal solutions, etc. In... 

Oil is to be leached from granulated halibut livers with pure ether as solvent. Content of oil in the feed is 0.321b/lb dry (oil-free) solids and 95% is to be recovered. The economic upper limit to extract concentration is 70% oil. Ravenscroft [Jnd. Eng. Chem. 28, 851 (1934)] measured the relation between the concentration of oil in the solution, y, and the entrainment or occlusion of solution by the solid phase, K lb solution/lb dry solid, which is represented by the equation... 

The test article is administered as solution or suspension to the back of six albino rabbits. The test material is administered to two sites onto the rabbits skin where the skin has to be clipped. On one site the skin remains intact, on the other the skin is mechanically abraded in order to remove the outer layer of the skin, the stratum comeum. The clipped areas should have a surface of about 1 square inch onto which the test material is administered. The dose level to be administered to the skin is 0,5 ml in case of liquids or 0,5 g in case of solid materials. The treated skin areas are covered with surgical gaze and fixed with a non-irritation tape. A second layer consisting of an occlusive material is wrapped over the entire trunk of the rabbits in order to avoid or retard the evaporation of the test material from the treated skin areas. After 24 hours, the wrapping is removed from the animals and the treated skin sites are evaluated for erythema and oedema, the classical signs of inflammation. 

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. 

Reality is often quite different. When a supercritical fluid mixture expands into pressures as high as ambient conditions, the resultant expansion plume can be a complex mixture it is a high velocity gas stream that entrains precipitated particles of extracted materials and often frozen carbon dioxide. Much adjustment needs to take place in the collection zone in order to achieve something close to 100 % recoveries of solutes with concentration ranges from parts per billion (PCBs) up to 50 % (total fat in a chocolate candy). Besides the flow dynamics of the expansion, several physicochemical parameters cause the deviation from the initial simple model. They include, but are not limited to, volatility of the solute, degree of co-precipitation of solid carbon dioxide (followed almost immediately with uncontrolled subhmation of the solid), aerosol formation, surface tension, occlusion in solid carbon dioxide, rebound from impinging surface, and many other interacting phenomena. 

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