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Thread: How fast do nitrates propagate throughout a water column?

  1. #1 How fast do nitrates propagate throughout a water column? 
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    Hi, I am an aquarist and have a real world problem that I cannot solve.
    If I add a nitrate solution to a tank of water containing no nitrates, will the nitrates mix in a similar fashion to say a dye in the water or as I suspect be very quickly if not instantly dispersed evenly throughout the water column via some sort of chemical reaction?


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    Bullshit Intolerant PhDemon's Avatar
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    They will not mix instantly but will diffuse (a physical process not a chemical one!) through the liquid until the concentration is constant throughout. The rate of diffusion will depend on the concentration and amount of the nitrate you add and the temperature of the water. If you stir it once the nitrate is added/dissolved it will be well mixed very quickly.

    The time taken, t, to diffuse a distance x, can be approximated as where D is the diffusion coefficient of the species at the temperature of the liquid.

    Most diffusion coefficients for ions at room temperature are around 10-5 cm2/s so if you use this value for D you will get a ball park figure for the time taken to diffuse a given distance.

    For example the time to diffuse 50 cm will be 502/2x10-5 = 125000000 seconds or about 1500 days!

    You might find this page useful http://www.physiologyweb.com/calcula...alculator.html


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    Many thanks for a most interesting and informative answer.

    I was at first gobsmacked at the figure of 1500 days, but then I realised that this was to reach complete equilibrium in a body of still water.

    I see now that if I want to reach equilibrium in a short time then stirring is the way to go.

    Thanks again
    Ed.
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    Bullshit Intolerant PhDemon's Avatar
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    No problem, glad to help.
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