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Thread: Diffusion of gas from pond

  1. #1 Diffusion of gas from pond 
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    Trying to determine diffusion rate of a gas emitted from a body of water. I believe Fick's first law may apply:

    J = -D * (dc/dx)

    where:
    J = diffusion rate [mg/scm2]
    D = diffusivity constant (can be looked up based on type of gas and local air temperature)
    dc = change in concentration from water surface to some distance above
    dx = height from water surface to some distance above

    So I can look up D, I know the concentration as measured at height "h" above the water, and dx = h. I don't know the concentration at the water surface. Can Fick's first law be used to compute diffusion rate in this situation, or is there a better approach?

    Thanks!


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  3. #2  
    Bullshit Intolerant PhDemon's Avatar
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    I think this is the right approach, to get the concentration at the waters surface you could use Henry's law if you know the concentration of gas dissolved in the pond.


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  4. #3  
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    Thank you for the reply. Let's assume I know nothing about the water or gas content in the water (because I don't). Perhaps a simplified version of this problem would be as follows. Let's say we have a large box with an open top filled with standard air (nitrogen, oxygen, etc). The bottom of the box has many pinholes through which a given gas "X" is seeping into the box. What is the diffusion rate of gas "X" over the distance from the bottom of the box to the top?
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    Bullshit Intolerant PhDemon's Avatar
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    You could assume that the water is saturated with the gas at the temperature and pressure you are doing the measurement. This will give you an upper limit for the diffusion rate.

    In your second model, to calculate a diffusion rate you would still need to know the concentration of X at the bottom of the box, so this doesn't help you...
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