# Calculating CO2 invassion/diffusion into water

Hi,

I trying to help someone with an aquarium Experiment. Anyways I'm trying to find sources on how to calculate how fast CO2 should diffuse through their still water surface in response to CO2 demand due to their plants. I have the aquarium dimensions, plant biomass produced over the time period in question but I'm having trouble tracking down appropriate physics info to work it out.

I'm expecting the plants to have been drawing down the CO2 to very low levels such that I think it was being used about as fast as it could enter the system. Any recomendations on formula or good sources to find the appropriate physics solution to calculate how fast/slow the CO2 would entering the system accross a stagnant air/water boundary and then diffuse through the water to the plants about 15 cm away?

Cheers,

Johan

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Diffusion is an extremely slow process at distances greater than a few microns. In an aquarium you can forget about it. It is convection a currents induced by aerators which mix the water.

Hi,

My friend had no aerators or anything mixing the water. He was trying to look at stable Isotope lableing of water by aquatic cellulose with basic Idea that CO2/cellulose should take on the isotopic signature of the water due to 18O exchange between water and dissolved CO2 (which is supposed to be fast). However his results indicate more of an atmosphereic CO2 labeling was being incorparating than expected so I'm trying to work out if the plant CO2 uptake from the water was faster than 18O exchange between dissolved CO2 and water.

Cheers,

Johan

Even if there are not mixing devices, convection is almost impossible to stop. It would ask to maintain the temperature of the aquarium and the room at the same temperature, really the same or maybe at no more than 0.001°. Even when you enter a room, you change the temperature more than this.

I have not understood the " at stable Isotope lableing of water by aquatic cellulose".
In fact a read your explanations several times and still I do not see exactly the situation. You tried to be short, but it is too short for me. Please give a longer explanation. Anyhow, I insist, forget about diffusion. It is really inexistent at room temperature for distances bigger than a few microns (for solids and liquids, for gases in gases it works better).

You should have a look to this paper. It is about oxygen, but it could give you a starting point:
http://www.css.cornell.edu/compost/oxygen/oxygen.diff.water.html [Broken]

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