Chemical natural selection

Selectivity Due to the chemical nature of the precipitation process, precipitants are usually not selective for a single analyte. For example, silver is not a selective precipitant for chloride because it also forms precipitates with bromide and iodide. Consequently, interferents are often a serious problem that must be considered if accurate results are to be obtained. [Pg.255]

The function of the essential oil in the plant is not fully understood. Microscopic examination of plant parts that contain the oil sacs readily shows their presence. The odors of flowers are said to act as attractants for insects involved in pollination and thus may aid in preservation and natural selection. Essential oils are almost always bacteriostats and often bacteriocides. Many components of essential oils are chemically active and thus could participate readily in metaboHc reactions. They are sources of plant metaboHc energy, although some chemists have referred to them as waste products of plant metaboHsm. Exudates, which contain essential oils, eg, balsams and resins, act as protective seals against disease or parasites, prevent loss of sap, and are formed readily when the tree tmnks are damaged. [Pg.296]

Reverse osmosis membrane separations are governed by the properties of the membrane used in the process. These properties depend on the chemical nature of the membrane material, which is almost always a polymer, as well as its physical stmcture. Properties for the ideal RO membrane include low cost, resistance to chemical and microbial attack, mechanical and stmctural stabiHty over long operating periods and wide temperature ranges, and the desired separation characteristics for each particular system. However, few membranes satisfy all these criteria and so compromises must be made to select the best RO membrane available for each appHcation. Excellent discussions of RO membrane materials, preparation methods, and stmctures are available (8,13,16-21). [Pg.144]

In the recent years intensive studies related to modification of silica with organic compounds of various chemical nature have being conducted in order to concentrate selectively metal ions from solutions and then to perform their analytical determination directly in the sorbent phase, or after that, to elute with appropriate reagents in solution. [Pg.277]

It is noteworthy that even a separate treatment of the initial data on branched reactions (1) and (2) (hydrogenation of crotonaldehyde to butyr-aldehyde and to crotyl alcohol) results in practically the same values of the adsorption coefficient of crotonaldehyde (17 and 19 atm-1)- This indicates that the adsorbed form of crotonaldehyde is the same in both reactions. From the kinetic viewpoint it means that the ratio of the initial rates of both branched reactions of crotonaldehyde is constant, as follows from Eq. (31) simplified for the initial rate, and that the selectivity of the formation of butyraldehyde and crotyl alcohol is therefore independent of the initial partial pressure of crotonaldehyde. This may be the consequence of a very similar chemical nature of both reaction branches. [Pg.46]

In addition, naturally growing plants resist plant pathogen and Insect attack because resistance develops over time via natural selection (35). Also, most natural and crop plants have, as a part of their basic physical and chemical makeup, a wide array of mechanisms that help them resist pest attack. These Include chemical toxicants, repellents, altered plant nutrients, hairiness, thorns, and diverse combinations of these (35). [Pg.315]

The occurrence of one of these situations rather than another depends on the chemical nature of the cyclophosphazene exploited and on the experimental conditions selected for the polymerization process. These facts are illustrated in Table 3. [Pg.181]

In electrochemical systems, many restrictions exist in the use of metal catalysts. Most metals other than the expensive noble metals are unstable at anodic potentials and cannot be nsed for anodic processes. The catalytic activity and selectivity of metal catalysts basically are determined by their chemical nature and are rarely open to adjustments. [Pg.542]

Additionally, it was deduced from experiments that the change in conductivity of a certain oxide (e.g., ZnO) caused by chemisorbtion of various alkyl radicals (the other experimental conditions being the same) is substantially dependent on the chemical nature of free radicals. The adsorbates can be put in the following activity row provided that the simplest alkyl radicals analyzed are ordered according to their effect on the conductivity of films made of the oxide selected ... [Pg.201]

Surfactant Transport in an Adsorbent Porous Medium. Chromatographic Aspects A first observation was made in all the tests in Table III. The breakthrough of both surfactants from the micellar slug always occurs simultaneously without any chromatographic effect (Figures 5 and 6). This stems both from the chemical nature of the two products selected and also from the fact that the injected concentration is much greater than the CMC of their mixtures. [Pg.285]

Buffer solutions are used to control retention and selectivity in the chromatography of ionisable solutes, and in addition the chemical nature of the buffering agent can affect secondary equilibria, eg interaction of the solute with silanol groups (see Section 4.2). [Pg.93]

IR spectroscopy is a powerful and readily available orientation characterization technique. It offers a high chemical selectivity since most functional groups absorb at distinct wavelengths (typically in the 2.5-25 pm range (4,000 00 cm-1 range)), which often depend on their local environment. IR spectroscopy thus provides qualitative and quantitative information about the chemical nature of a sample, its structure, interactions, etc. The potential of IR spectroscopy for orientation characterization stems from the fact that absorption only occurs if the electric field vector of the incident radiation, E, has a component parallel to the transition dipole moment, M, of the absorbing entity. The absorbance, A, is given... [Pg.305]

A core assumption of ultra-Darwinism is that if not all, then most observed characters must be adaptive, so as to provide the phenotypic material upon which natural selection can act. However, what constitutes a character - and what constitutes an adaptation - is as much in the eye of the beholder as in the organism that is beheld, as Gould and Lewontin pointed out in their famous paper on spandrels (Gould and Lewontin, 1979). Natural selection s continual scrutiny does not give it an a la carte freedom to accept or reject genotypic or phenotypic variation. Structural constraints insist that evolutionary, genetic mechanisms are not infinitely flexible but must work within the limits of what is physically or chemically possible (for instance, the limits to the size of a single cell occasioned by the physics of diffusion processes, the size of a crustacean like a lobster or crab by the constraints... [Pg.293]

Williams, R. J. P. and Frausto da Silva, J. R. R. (1996), The Natural Selection of the Chemical Elements, Oxford University Press, Oxford, UK. [Pg.367]

Mason, B. and Moore, C.B. (1982). Principles of Geochemistry (4th ed.). Wiley, New York Press, F.S. and Siever, R. (1986). Earth (4th ed.). W.H. Freeman and Company, New York Stanley, S.M. (2002). Earth System History. W.H. Freeman and Company, New York Wayne, R.P. (2002). Chemistry of Atmospheres (3rd ed.). Oxford University Press, Oxford Williams, R. J.P. and Frausto da Silva, J.J.R. (1996). The Natural Selection of the Chemical Elements - The Environment and Life s Chemistry. Clarendon Press, Oxford... [Pg.33]

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