أرشيف يوم: 06/12/2013

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 …

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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 …

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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 …

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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[exp(1 – a)Fη/RT- exp(- aF/RT)] where ja and jc are the individual cathode and anode currents …

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