Chemical perspective classification

Rivers transport material in several phases dissolved, suspended particulate and bed load. Physical and chemical processes within an estuary infiuence the transportation and transformation of this material, thereby affecting the net supply of material to the oceans. Several definitions and geomorphologic classifications of estuaries have been reviewed by Pe-rillo (1995). From a chemical perspective, an estuary is most simply described as the mixing zone between river water and seawater characterised by sharp gradients in the ionic strength and chemical composition. Geographic distinctions can be made between drowned river valleys, fjords and bar-built estuaries. They can alternatively be classified in terms of the hydrodynamic regime as ... 

This entry presents a discussion of the principles of respiratory toxicology including (1) an historical perspective, (2) approaches used to evaluate respiratory responses to inhaled chemicals, (3) classification of airborne chemicals, (4) concepts of dose-time relationships, (5) factors influencing toxicity of airborne substances, (6) the basic biology of the respiratory system with emphasis on those structures and functions that are involved in toxicological responses, (7) biomarkers of pulmonary effects, (8) toxicological response associated with inhaled chemicals, and (9) assessing the human risk of airborne chemicals. 

Classification of the chemical constituents of seawater into conservative, bioactive and adsorbed (Chapter 1) revealed much about the processes that control concentration distributions in seawater of the latter two categories, but little about the conservative elements. Concentrations of the elements that make up most of the salinity of the oceans provide clues to the mechanisms that control their sources and sinks. Thus, the chemical perspective of oceanography revealed by conservative element concentrations is about processes that occur at the ocean boundaries weathering reactions on land, authigenic mineral formation in marine sediments and reactions with the crust at hydrothermal areas. The amount of time some of the dissolved constituents remain in solution before they are removed chemically is very long, suggesting the possibility for chemical equilibrium between seawater and the minerals in the ocean... 

In this work the state-of-the-art and perspectives of column characterization and compai ison have been presented and discussed. All information about physico-chemical properties of RP HPLC Cl8 and C8 columns as porosity, average surface area, free silanol concentration, binding ligand density and others, were summarized. The points of views about column classifications, its advantages and disadvantages were discussed. It was shown that Cl8 and C8 HPLC column classification processes do not allow selecting the column with the same or preai range selectivity. 

The present article reviews the photochemical deactivation modes and properties of electronically excited metallotetrapyrroles. Of the wide variety of complexes possessing a tetrapyrrole ligand and their highly structured systems, the subject of this survey is mainly synthetic complexes of porphyrins, chlorins, corrins, phthalocyanines, and naphthalocyanines. All known types of photochemical reactions of excited metallotetrapyrroles are classified. As criteria for the classification, both the nature of the primary photochemical step and the net overall chemical change, are taken. Each of the classes is exemplified by several recent results, and discussed. The data on exciplex and excimer formation processes involving excited metallotetrapyrroles are included. Various branches of practical utilization of the photochemical and photophysical properties of tetrapyrrole complexes are shown. Motives for further development and perspectives in photochemistry of metallotetrapyrroles are evaluated. 

Previous sections have presented technical and historical information on radiation and chemical risk assessment and on classification of radioactive and hazardous chemical wastes. This information provides important perspectives for establishing the foundations of a new hazardous waste classification system. Before establishing these foundations, it is useful to specify the attributes that an ideal waste classification system should possess. The following sections identify the desirable attributes of a waste classification system including that the system should be risk-based, it should allow for exemption of waste, and it should be comprehensive, consistent, intrinsic, comprehensible, quantitative, compatible with existing systems, and flexible. These attributes should be recognized as goals that are not all likely to be fully realized in a practical waste classification system. 

Flocculation processes are complicated phenomena because of the varieties of both particle morphology and chemical reactions they encompass.34 A few concepts of a general nature have emerged, however, and they will be the focus of this chapter. From the perspective of kinetics, perhaps the most important of these broad generalizations is the distinction that can be made between transport-controlled and reaction-controlled flocculation, parallel to the classification of adsorption processes described in Section 4.5. Flocculation kinetics are said to exhibit transport control if the rate-limiting step is the movement of two (or more) particles toward one another prior to their close encounter and subsequent combination into a larger particle. Reaction control occurs if it is particle combination instead of particle movement (toward collision) that limits the rate of flocculation. 

The above-discussed classification of isomers is depicted schematically in the upper half of Fig. 2. Such a classification, which is considered classical and widely accepted, nevertheless fails to be fully satisfactory, as aptly demonstrated by Mislow [18]. Thus, this classification considers diastereoisomers to be more closely related to enantiomers than to constitutional isomers. In fact, diastereoisomers resemble constitutional isomers in that their energy content is different, and therefore they differ in their chemical and physical properties. In this perspective, diastereoisomers differ from enantiomers which have identical energy contents and thus display identical physical and chemical properties. 

While we have used a classification based on chemical structure, alternative classifications based on experimental techniques or chemical stability are frequently encountered. These afford a somewhat different and certainly useful perspective of the field, and are outlined below. 

The chemist may prefer to classify toxicants according to their chemical structure, the doctor according to the organ they harm, the environmentalist according to their stability in the environment, and so forth. The biochemist may use a different classification, and we will approach the toxicology of pesticides from the biochemist s perspective. Because of point 1 above, and because the cells in all organisms are very similar, it is possible to classify... 

GRAS. See generally regarded as safe, green chemical. A popularized classification for a chemical or material that exhibits the attributes associated with human safety and environmental friendliness as seen from the perspective of its designated application. 

Hegnauer, R., Chemical evidence for the classification of some plant taxa, in Perspectives in Phytochemistry (J. B. Harborne and T. Swain, eds.), 121-138, Academic Press, London, 1969. 

Rather than considering the relative virtues of these placements in chemical terms, the argument for the removal of hydrogen and hehum from the main body of the table can be examined from the perspective of the elements as basic substances. The widely held behef among chemists is that the periodic system is a classification of the elements as simple substances that can be isolated and whose properties can be examined experimentally. However, as emphasized in the present book, there is a long-standing metaphysical tradition of ako regarding the elements as unobservable basic substances. 

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