أكتوبر 11th, 2018

 

Adsorption

A process in which atoms or molecules move from a bulk phase (that is, solid, liquid, or gas) onto a solid or liquid surface. An example is purification by adsorption where impurities are filtered from liquids or gases by their adsorption onto the surface of a high-surface-area solid such as activated charcoal. Other examples include the segregation of surfactant molecules to the surface of a liquid, the bonding…

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Adhesive

A material capable of fastening two other materials together by means of surface attachment. The terms glue, mucilage, mastic, and cement are synonymous with adhesive. In a generic sense, the word adhesive implies any material capable of fastening by surface attachment, and thus will include inorganic materials such as portland cement and solders. In a practical sense, however, adhesive implies the broad set of materials composed of organic…

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Adenosine triphosphate (ATP)

A coenzyme and one of the most impor­tant compounds in the metabolism of all organisms, since it serves as a coupling agent between different enzymatic reactions. Adenosine triphosphate is adenosine diphos­phate (ADP) with an additional phosphate group attached through a pyrophosphate linkage to the terminal phosphate group (see illustration). ATP is a powerful donor of phosphate groups to suitable acceptors because of the pyrophosphate nature of the bonds…

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acylation

A process in which a hydrogen atom in an organic compound is re­placed by an acyl group (R-CO, where R = an organic group). The reaction in­volves substitution by a nucleophile (electron donor) at the electrophilic carbonyl group (C=O) of a carboxylic acid derivative. The substitution usually proceeds by an addition-elimination sequence. Two common reagents, with the general formula RCOX, that bring about acylation are acid halides (X…

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Activation analysis

A technique in which a neutron, charged particle, or gamma photon is captured by a stable nuclide to produce a different, radioactive nu­clide which is then measured. The technique is specific, highly sensitive, and applicable to almost every element in the periodic table.

In neutron activation analysis (NAA), the most widely used form of activation anal­ysis, the sample to be analyzed is placed in a nuclear reactor where…

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activated carbon

A powdered, granular, or pelleted form of amorphous car­bon characterized by very large surface area per unit volume because of an enormous number of fine pores. Activated carbon is capable of collecting gases, liquids, or dis­solved substances on the surface of its pores.

Adsorption on activated carbon is selective, favoring nonpolar over polar substances.

Compared with other commercial adsorbents, activated carbon has a broad spectrum of adsorptive activity,…

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Actinium

A chemical element, Ac, atomic number 89, and atomic weight 227.0. Actinium was discovered by A. Debierne in 1899. Milligram quantities of the element

are available by irradiation of radium in a nuclear reactor. Actinium-227 is a beta­emitting element whose half-life is 22 years. Six other radioisotopes with half-lives ranging from 10 days to less than 1 minute have been identified.

The relationship of actinium to the element…

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Actinide elements

The series of elements beginning with actinium (atomic number 89) and including thorium, protactinium, uranium, and the transuranium el­ements through the element lawrencium (atomic number 103). These elements have a strong chemical resemblance to the lanthanide, or rare-earth, elements of atomic numbers 57 to 71. Their atomic numbers, names, and chemical symbols are: 89, ac­tinium (Ac), the prototype element, sometimes not included as an actual member of the…

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Acrylonitrile

An explosive, poisonous, flammable liquid, boiling at 171 °F (77.3°C), partly soluble in water. It may be regarded as vinyl cyanide, and its systematic name is 2-propenonitrile. Acrylonitrile is prepared by ammoxidation of propylene over various sorts of catalysts, chiefly metallic oxides.

Most of the acrylonitrile produced is consumed in the manufacture of acrylic and modacrylic fibers. Substantial quantities are used in acrylonitrile-butadiene-styrene (ABS) resins, in nitrile elastomers,…

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