ديسمبر 6th, 2013

 

Brownian movement

Brownian movement The contin­uous random movement of micro­scopic solid particles (of about 1 micrometre in diameter) when sus­pended in a fluid medium. First observed by the British botanist Robert Brown (1773-1858) in 1827 when studying pollen particles, it was origi­nally thought to be the manifestation of some vital force. It was later recog­nized to be a consequence of bom­bardment of the particles by the continually moving…

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buffer

A solution that resists change in pH when small amounts of an acid or alkali are added over a cer­tain range or when the solution is di­luted. Acidic buffers consist of a weak acid with a salt of the acid. The salt provides the negative ion A-, which is the conjugate base of the acid HA. An example is carbonic acid and sodium hydrogencarbonate. Basic buffers have a…

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buna rubber

A type of synthetic rubber based in polymerization of butadiene (buta-1,3-diene). The name comes from Bu (for butadiene) and Na (for sodium, which was used as a catalyst in the original polymeriza­tion reaction). An improved form, known as Buna-S was developed by copolymerizing butadiene with styrene. In 1934, Buna-N was in­vented, in which the styrene was re­placed by acrylonitrile, giving a product with better oil resistance.

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Butler-Volmer equation

 An equation for the rate of an electro­chemical reaction; it describes the current density at an electrode in terms of the overpotential. The But­ler-Volmer equation is given by:

j = ja – jc = je

where ja and jc are the individual cathode and anode currents espect­tively, and je is the equilibrium cur­rent, called the exchange current density. By definition

je =…

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(B-Z reaction (Belousov-Zhabotin­skii reaction

 A chemical reaction that shows a periodic colour change between magenta and blue with a pe­riod of about one minute. It occurs with a mixture of sulphuric acid, potassium bromate(V), cerium sul­phate, and propanedioic acid. The colour change is caused by alternat­ing oxidation-reductions in which cerium changes its oxidation state (Ce3+ gives a magenta solution while Ce4+ gives a blue solution). The B-Z reaction is an example of…

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