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D is the diffusion constant of the solute unit m 2 ⋅s −1, t is time unit s, c 2, c 1 concentration should use unit mol m −3, so flux unit becomes mol s −1. The flux is decay over the square root of time because a concentration gradient builds up near the membrane over time under ideal conditions.
(mol/s)/(m 2 ·mol/m 3) = m/s Note, the units will vary based upon which units the driving force is expressed in. The driving force shown here as ' Δ c A {\displaystyle {\Delta c_{A}}} ' is expressed in units of moles per unit of volume, but in some cases the driving force is represented by other measures of concentration with different units.
Typically, a compound's diffusion coefficient is ~10,000× as great in air as in water. Carbon dioxide in air has a diffusion coefficient of 16 mm 2 /s, and in water its diffusion coefficient is 0.0016 mm 2 /s. [1] [2] Diffusivity has dimensions of length 2 / time, or m 2 /s in SI units and cm 2 /s in CGS units.
In liquid chromatography, the mobile phase velocity is taken as the exit velocity, that is, the ratio of the flow rate in ml/second to the cross-sectional area of the ‘column-exit flow path.’ For a packed column, the cross-sectional area of the column exit flow path is usually taken as 0.6 times the cross-sectional area of the column.
The self-diffusion coefficient of neat water is: 2.299·10 −9 m 2 ·s −1 at 25 °C and 1.261·10 −9 m 2 ·s −1 at 4 °C. [2] Chemical diffusion occurs in a presence of concentration (or chemical potential) gradient and it results in net transport of mass. This is the process described by the diffusion equation.
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where D is the diffusion coefficient, Φ is porosity, n is the concentration, m > 0 (usually m > 1, the case m = 1 corresponds to Fick's law). Care must be taken to properly account for the porosity (Φ) of the porous medium in both the flux terms and the accumulation terms. [22]
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