Passive compounds

The temporal trend of the C/C ratio in soil humus (from prenuclear time to the present) makes it possible to separate the contribution of passive compounds that have very long soil residence times (measured in hundreds to thousands of years) and hence will be little changed by man s activities, and those active compounds which have relatively short chemical lifetimes (measured in decades). It also allows the characterization of the average turnover time of these active components. Further, by conducting such measurements on soils from different climate zones, it is possible to get a handle on how the turnover time of active compounds depends on temperature. Clearly, knowledge of this dependence is critical to the prediction of future global humus inventories. 

Passivation The production of a surface layer on a material that decreases its reaction with the ambient. Passivation can be accomplished by removing a reactive species from the surface (ASTM A380), increasing the thickness or density of a naturally forming oxide, chemically reacting the surface to form a passive compound, or by overlay techniques where the passivation layer consists of a different material. 

A piezo-composite consists of a piezoelectric active phase and a passive plastic phase [2]. In the 1-3-configuration adopted in our case, piezoelectric rods parallely aligned in thickness direction are imbedded in a three-dimensional plastic matrix (Fig. 1). The distance between the rods has to be chosen inferior to the half wave length of the shear wave in the matrix material ensuring that the whole compound is vibrating as a quasi-homogeneous material. 

Anotlier important modification metliod is tire passivation of tire external crystallite surface, which may improve perfonnance in shape selective catalysis (see C2.12.7). Treatment of zeolites witli alkoxysilanes, SiCl or silane, and subsequent hydrolysis or poisoning witli bulky bases, organophosphoms compounds and arylsilanes have been used for tliis purjDose [39]. In some cases, tire improved perfonnance was, however, not related to tire masking of unselective active sites on tire outer surface but ratlier to a narrowing of tire pore diameters due to silica deposits. 

Also, using dyes as laser media or passive mode-locked compounds requires numerous special parameters, the most important of which ate the band position and bandwidth of absorption and fluorescence, the luminiscence quantum efficiency, the Stokes shift, the possibiHty of photoisomerization, chemical stabiHty, and photostabiHty. AppHcations of PMDs in other technical or scientific areas have additional special requirements. 

MetaUic cobalt dissolves readily in dilute H2SO4, HCl, or HNO to form cobaltous salts (see also Cobalt compounds). Like iron, cobalt is passivated by strong oxidizing agents, such as dichromates and HNO, and cobalt is slowly attacked by NH OH and NaOH. 

Electrically assisted transdermal dmg deflvery, ie, electrotransport or iontophoresis, involves the three key transport processes of passive diffusion, electromigration, and electro osmosis. In passive diffusion, which plays a relatively small role in the transport of ionic compounds, the permeation rate of a compound is deterrnined by its diffusion coefficient and the concentration gradient. Electromigration is the transport of electrically charged ions in an electrical field, that is, the movement of anions and cations toward the anode and cathode, respectively. Electro osmosis is the volume flow of solvent through an electrically charged membrane or tissue in the presence of an appHed electrical field. As the solvent moves, it carries dissolved solutes. 

Euture electrotransport therapeutic systems will differ substantially from those just described. They will draw on advances in microelectronics and transdermal system technology to provide transdermal therapy for compounds with low passive permeation rates, patterned or pulsed dmg deHvery,... 

Aquatic organisms, such as fish and invertebrates, can excrete compounds via passive diffusion across membranes into the surrounding medium and so have a much reduced need for specialised pathways for steroid excretion. It may be that this lack of selective pressure, together with prey-predator co-evolution, has resulted in restricted biotransformation ability within these animals and their associated predators. The resultant limitations in metabolic and excretory competence makes it more likely that they will bioacciimiilate EDs, and hence they may be at greater risk of adverse effects following exposure to such chemicals. 

Certain chemical compounds in acids can influence f/ and U" considerably. Mainly Ni, Mo, and Cu as alloying elements improve passivation. With inereasing aeid eoneentration, U beeomes more positive. With increasing temperature U becomes more negative (see Chapter 21). 

Chemicals have to pass through either the skin or mucous membranes lining the respiratory airways and gastrointestinal tract to enter the circulation and reach their site of action. This process is called absorption. Different mechanisms of entry into the body also greatly affect the absorption of a compound. Passive diffusion is the most important transfer mechanism. According to Pick s law, diffusion velocity v depends on the diffusion constant (D), the surface area of the membrane (A), concentration difference across the membrane (Ac), and thickness of the membrane (L)... 

Chloro-oxazolo[4,5-/i]quinoline-2-carboxylic acid methyl ester was the most active compound in tests for inhibitors of antigen-induced release of histamine in vitro from rat peritoneal mast cells (IC50 of 0.3 p,M) and as inhibitors of IgE-mediated passive cutaneous anaphylaxis in the rat (ED50 (intraperitoneal) of 0.1 mg/kg in dose 0.5 mg/kg as an inhibitor of the test)—10 times and 60 times more potent, respectively, than the disodium salt of cromoglycic acid (85JMC1255). 

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