Calculating CO2 invassion/diffusion into water

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Discussion Overview

The discussion revolves around calculating the diffusion of CO2 into water in an aquarium setting, particularly in relation to plant CO2 demand and the dynamics of gas exchange at the air/water interface. Participants explore the implications of diffusion versus convection in this context, as well as the effects of temperature and mixing on CO2 uptake.

Discussion Character

  • Exploratory
  • Technical explanation
  • Debate/contested

Main Points Raised

  • Johan seeks formulas or sources to calculate the rate of CO2 diffusion into still water, considering aquarium dimensions and plant biomass.
  • One participant asserts that diffusion is very slow over distances greater than a few microns and suggests that convection currents from aerators are more significant in mixing the water.
  • Johan clarifies that the aquarium lacks aerators and discusses a stable isotope labeling experiment involving CO2 and water, questioning the rate of CO2 uptake by plants compared to the isotopic exchange.
  • Another participant emphasizes the challenges of stopping convection in the aquarium, noting that even minor temperature changes can induce mixing, and requests further clarification on Johan's experimental setup.
  • A participant recommends a paper related to oxygen diffusion in water as a potential resource for understanding similar processes for CO2.

Areas of Agreement / Disagreement

Participants express differing views on the significance of diffusion versus convection in the aquarium context. There is no consensus on the mechanisms at play or the effectiveness of diffusion over the distances involved.

Contextual Notes

Participants note the potential limitations of diffusion at room temperature and the need for precise temperature control to minimize convection effects, but do not resolve these issues.

jarvik
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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 across a stagnant air/water boundary and then diffuse through the water to the plants about 15 cm away?

Thanks for any advice,

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
 
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