Precipitate formation, occlusion

Coprecipitation — The -> precipitation of a normally soluble substance that is carried down during the precipitate formation of the desired substance. The coprecipitation of a substance arises from processes such as - adsorption, -r mixed-crystal formation, - occlusion and/or mechanical entrapment [i]. 

Both occlusion and mechanical entrapment are at a minimum when the rate of precipitate formation is low—that is, under conditions of low supersaturation. In addition, digestion is often remarkably helpful in reducing these types of coprecipitation. Undoubtedly, the rapid solution and reprecipitation that go on at the elevated temperature of digestion open up the pockets and allow the impurities to escape into the solution. 

M Mixed-crystal formation may occur in both colloidal and crystalline precipitates, whereas occlusion and mechanical entrapment are confined to crj s-talline precipitates. 

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... 

Trace analysis has its special hazards for the unwary. The most important of these are loss of material in the analytical process and contamination by outside sources. Everyone realizes that trace constituents can be lost from samples, but few are aware of the many ways in which this can occur. For example, phosphate has been observed to disappear mysteriously from water samples in polyethylene bottles (10). Nitric acid, used to clean plastic vials, has been observed to convert these surfaces to ion exchangers, which readily take up as much as 10 12 moles per sq. cm. of trace metals (16). Lead nitrate solutions unless made distinctly acidic, plate out much of the lead on the walls of glass bottles. While everyone realizes that formation of a precipitate is liable to carry out trace constituents either by adsorption or occlusion, it is not as well-known that vanishingly small amounts of precipitates—amounts likely to be overlooked on casual observation—may also do this. The fly-ash and soot, which seem to be inescapable components of city air,... 

If catalysts are prepared by coprecipitation, the relative solubilities of the precipitates and the possibility for the formation of defined mixed phases are essential. If one of the components is much more soluble than the other, there is a possibility that sequential precipitation occurs. This leads to concentration gradients in the product and less intimate mixing of the components. If this effect is not compensated by adsorption or occlusion of the more soluble component, the precipitation should be carried out at high supersaturation in order to exceed the solubility product for both components simultaneously. Precipitation of the less soluble product will proceed slightly faster, and the initially formed particles can act as nucleation sites for the more soluble precipitate which forms by heterogeneous precipitation. The problem is less crucial if both components form a defined, insoluble species. This is for instance the case for the coprecipitation of nickel and aluminum which can form defined compounds of the hydrotalcite type (see the extensive review by Cavani et al. [9] and the summary by Andrew [10]). 

Adverse effects. When bleeding is induced by heparin, the heparin action can be instantly reversed by protamine. Against fractionated heparins and fondaparinux, protamine is less or not effective. Heparin-induced thrombocytopenia type II (HIT II) is a dangerous complication. It results from formation of antibodies that precipitate with bound heparin on platelets. The platelets aggregate and give rise to vascular occlusions. Because of the thrombocytopenia, hemorrhages may occur. Fondaparinux is also contraindicated in HIT II. 

Gel Formation, Entrapment of enzymes in solid matrices composed of synthetic or natural polymers or inorganic gels is a relatively simple process. The basic technique involves occlusion of the enzyme within the lattice of a solid matrix as the matrix is formed by polymerization, precipitation, or coacervation. Gels... 

Aberrant thrombus formation and deposition on blood vessel walls imderlies the pathogenesis of acute cardiovascular disease states which remain the principal cause of morbidity and mortality in the industrialized world [1,2,3]. Plasma proteins, proteases and specific cellular receptors that participate in hemostasis have emerged as important risk considerations in thrombosis and thromboembolic disorders. The clinical manifestations of the above disease states include acute coronary artery and cerebrovascular syndromes, peripheral arterial occlusion, deep vein thrombosis and pulmonary/renal embolism [3]. The most dilabilitating acute events precipitated by these disorders are myocardial infarction and stroke. In addition, the interplay between hemostatic factors and hypertension (4) or atherosclerosis (5) dramatically enhances the manifestation of these pathologic states. 

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. 

Prior to the consolidation of fouling, which represents the penetration of the solute molecules of the feed fluid in the pores of the membrane, blocking them, there is an increased concentration of solutes on the membrane surface due to the concentration of solutes in solution, resulting from transport by convection, which is known as concentration polarization. This polarization leads to the precipitation of solute molecules on the surface of the membrane, a phenomenon known as the formation of gel layer. Later, the adsorption of small molecules on the inner wall of pores, and a complete occlusion by the molecules of solute leads to consolidated fouling. These phenomena determine a rapid reduction in the permeate flux. 

Among the impurities by coprecipitation, one can also distinguish those by occlusion. This is the case when a small volume of solution containing some solutes is trapped within the precipitate. As a result, the mass of the precipitate is higher than expected. The formation of impurities by occlusion is minimized by maintaining the precipitate in equilibrium with the solution from which it was formed for as long as possible. This is the process of digestion. It can be carried out at any temperature. 

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