Occluded compounds, formation

Closed cavities. In these, occluded (impurity) atoms, molecules or ions, trapped during compound formation, may be present. Their removal will necessitate the breaking up of the crystal lattice. An example is provided by Na3P04-nHjO jcNaOH in whieh the NaOH is imprisoned [4,5] (Figure 5.38) (this particular compound has detergent applications). Closed cavities are also present in the heteropoly anions (see below). 

Harbauer (1984) first described a venous model of thrombosis induced by mechanical injury and stenosis of the jugular vein. In a modification, both arterial and venous thrombosis is produced in rabbits by stenosis of the carotid artery and the jugular vein with simultaneous mechanical damage of the endothelium. This activates platelets and the coagulation system and leads to changes in the bloodstream pattern. As a consequence, occluding thrombi are formed as detected by blood flow measurement. The dominant role of platelets in this model is shown by the inhibitory effect of an antiplatelet serum in both types of vessels (Just 1986). The test is used to evaluate the antithrombotic capacity of compounds in an in vivo model of arterial and venous thrombosis where thrombus formation is highly dependent on platelet activation. 

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. 

Hydrogen peroxide as supplied is normally stabilised with phosphates and sometimes tin(IV) compounds, the latter being effective at the product s natural (weakly acid) pH by hydrocolloid formation, which occludes adventitious transition metal ions and reduces their catalytic activity. In many cases, extra stabilisation is not required when H2O2 or its derivatives are used in synthesis. However, elevated temperature, high alkalinity and increased metal impurities all tend to destabilise peroxy-gens, and where these conditions are unavoidable, additional stabilisers may be employed added either to the peroxide (subject to reactivity) or separately to the reaction mixture. Such stabilisers fall into two categories. 

Several authors have reviewed established and promising applications in this field [2-A]. The present paper is not comprehensive, but is intended to offer some perspective, specifically, concerning methods of functionalizing the intracrystalline surfaces of zeolites, the chemical and physical nature of the interactions between the occluded species and the zeolite framework, and some principles describing the formation and behavior of organometallic compounds within the internal void volumes of zeolites. 

Three classes of interactions can be envisaged to lead to formation of organometallic compounds within zeolite frameworks all three are illustrated schematically in Fig. 1. The structure include exchangeable cationic species, occluded complexes, and species incorporated in framework sites. 

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