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Diffusion Coefficients

A substance will flow from a region of high concentration to a region of low concentration

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Fick's first law states that a substance will flow from region with high concentration to a region of low concentration. Fick's law can be expressed as

J = -D dφ / dx                                  (1)


J = diffusion flux - the amount of substance that flows through an unit area per unit time (mass or mol /m2s)

D = diffusion coefficient (m2/s)

dφ = concentration of substance (mass or mol / m3)

dx = length (m)

Diffusion Coefficients for some common Gases

SubstanceDiffusion Coefficient
- D -
10-5 (cm2/s)
Methane - Water 1.5
Ethane - Water 1.1
Air - Water 1.9


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