Occlusion and adsorption

The precipitated metallic hydroxides or hydrated oxides are gelatinous in character, and they tend to be contaminated with anions by adsorption and occlusion, and sometimes with basic salts. The values presented in Table 11.2 suggest that many separations should be possible by fractional precipitation of the hydroxides, but such separations are not always practical owing to high local concentrations of base when the solution is treated with alkali. Such unequal concentrations of base result in regions of high local pH and lead to the precipitation of more soluble hydroxides, which may be occluded in the desired precipitate. Slow, or preferably homogeneous, precipitation overcomes this difficulty, and much sharper separations may be achieved. 

During the adsorption or occlusion of various molecules, the micropores fill and empty reversibly. Adsorption in zeoHtes is a matter of pore filling, and the usual surface area concepts are not appHcable. The pore volume of a dehydrated zeoHte and other microporous soHds which have type 1 isotherms may be related by the Gurvitch rule, ie, the quantity of material adsorbed is assumed to fill the micropores as a Hquid having its normal density. The total pore volume D is given by... 

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]). 

The influence of temperature on the occlusion by palladium has also been investigated by Firth.6 He employs the term adsorption to denote the surface phenomenon of rapid occlusion of hydrogen, diffusion not being a determinable factor limits absorption to slow occlusion in which the rate of diffusion or solution is a determinable factor and uses the collective term sorption to include both adsorption and absorption.7 Firth observed adsorption only below 0° C., between 0° and 150° C. he also observed absorption but above 150° C. found absorption only. 

There are four types of coprecipitation surface adsorption, mixed-crystal formation, occlusion, and mechanical entrapment." Surface adsorption and mixed-crystal formation are equilibrium processes, whereas occlusion and mechanical entrapment arise from the kinetics of crystal growth. 

Arsenic is ubiquitous in nature and is found in detectable concentrations in all environmental matrices. The occurrence of As in the continental crust of Earth is usually given as 1.5 to 2.0 mg/1. The distribution of arsenic in nature is extremely variable, showing little correlation with geological formation, climate, or soil. Numerous minerals, rocks, sediments and soils contain arsenic partly as constituent of sulfide minerals or complex sulfides of metal cations and partly as a constituent retained by soils and/or sediments in occluded or adsorbed forms. The latter is manifested primarily by the adsorption or occlusion of As on hydrous A1 and Fe oxides, but these are not necessarily the only source. Arsenic is also adsorbed on clay colloid, is bound to organic matter and may form slightly water soluble compounds with Al, Fe, Ca and Mg in the soil matrix. Some of the more common minerals in soils are arsenopyrite (FeAsS), Orpiment (AsgSg) etc. 

Gas adsorption is a phenomenon in which a gas molecule is removed from the gaseous phase by a solid surface. (Fig. 3.4.3) The gas itself is called the adsorptive and the concentrated gas on the surface in high density is called the adsorbate. The substance adsorbing the gas is called the adsorbent and the phenomenon in which the gas adheres to the surface (pores) is called adsorption. The phenomena in which the adsorbents are taken into the solid are called absorption or occlusion. Occlusion is a special case where gas is reversibly adsorbed into a crystal lattice with intrinsic saturated composition. These phenomena are further divided into physisorption and chemisorption based on the strength of the interaction energy between the gas and the solid. When the heat of adsorption is greater than 20 kj mol-1 it is classified as chemical adsorption, but the classification is not strict. 

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). 

Gas storage on a particular substance occurs in accordance with four different mechanisms namely, physical adsorption, chemisorption, absorption, and occlusion. The first two mechanisms take place on the surface while the other two proceed within the cracks and empty spaces in... 

The results of this study demonstrate the varying physical differences between the selected wound dressings assessed arxl average results are shown in Table 1. Two of the dressing types showed very low WVTR (Water Vapour Transference Rate) and air permeability. The Carbopad VC specimens showed very low/minimal w permeability, exhibiting an occlusive characteristic this presented problems during the test to evaluate the efficiency of odour adsorption and was therefore with-drawn from this test. 

This review shows the important contribution of MN4 macrocycUc compounds in the electroanalytical chemistry of hydrazine. Electrodes with MN4 macrocyclic compounds confined on their surfaces can be obtained by simple physical adsorption, chemical adsorption of self-assembled monolayers, electrochemical polymerization, and occlusion or entrapment. The electroanalytical performance of the modified electrode for hydrazine oxidation depends on the substrate, pH, type of electrolyte, and measurement technique. 

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. 

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. 

Freeze Conceit tret ion Hater sample is partially frozen, concentrating the dissolved substances in the unfrozen portion. All sample types Minimizes sample losses due to volatilization or chemical modification. Principal losses occur due to occlusion, adsorption, evaporation and channelling in the ice layer. Limited sample size. 

N2 adsorption-desorption isotherms show instead occlusion of mesopores between 2 and 9 hours of immersion, possibly due to reaction of silica with SBF. This seems to vanish between 24 and 27 hours, only in the case of ibuprofen-loaded spheres. We suggest that in this latter case the entrapped molecules play a role in the temporary reopening of the pores. 

High-pressure liquid chromatographic (HPLC) analysis performed with a chiral mobile phase (57,58) confirmed in all the conglomerate systems that the S inhibitors are selectively occluded only in the bulk of the S substrate crystals, typically in amounts of 0.5-1% (and, by symmetry, occlusion of R occurs only in R crystals). The selective adsorption causes, furthermore, a drastic decrease in the growth (and possibly nucleation) rate of the affected enantiomer, leading to efficient kinetic resolution various conglomerate systems have been resolved by this method (54). 

Liquid-liquid partitioning constitutes tlie most common form of solvent extraction. It is a relatively simple, rapid, and flexible procedure that is readily applicable to all types of matrices and a wide range of analytes ranging from fairly polar to nonpolar compounds (54). Despite the fact that they are simple and rapid, liquid-liquid extractions may result in highly selective isolation (55, 56). However, they also necessitate use of toxic and inflammable solvents, favor formation of emulsions, may cause sample losses by occlusions or adsorption onto glass surfaces, and are often laborious and costly. 

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