There has been some discussion about the rates of absorption of oxygen and CO2, and about the rate of outgassing of the latter. To help me understand this, I have several questions.

1. Which is more readily absorbed in water, O2 or CO2?

If a container of water that contains no gasses is exposed to air, one would expect that the levels of O2 and CO2 would eventually rise to the level at which they existed in the atmosphere. Which gas would reach that level first?

2. When water is agitated, dissolved gasses tend to be outgassed into the atmosphere. At the same time, the agitated water will absorb additional gasses from the atmosphere.

Will CO2 be outgassed at a more rapid rate than O2?

Is the rate of absorption of O2 and CO2 the same as the rate of outgassing of those two gasses?

3. In a non CO2 injected aquarium, is it ever possible for the CO2 level to be less than that in the local atmosphere?

Thank you!

Biill

Hey Bill, I just figured I would post these links over here also:

starting on page 7 has good info:
http://www.ieagreen.org.uk/oceanrep.pdf

also this one has good info
http://www.tos.org/oceanography/issu...eely_et_al.pdf

The CO2 uptake rate depends on the PH, temp and how much CO2 is in the water/atmosphere to begin with.

As for your question 3, I would deff say yes the CO2 levels will likely be less then atmospheric. If you have lots of plants and no surface agitation it will be much lower then atmospheric most of the time, at least during the day when plants are using up the CO2. I would say that also with great surface agitation and a high PH/temp if will not be as likely to bottom out and levels should stay closer to atmospheric. This is just my opinion as there are many variables/scenarios.

Now talking about El natural or some king of organic substrate then that is a big difference compared to regular substrate.

It would give Tom something to test with his nice CO2 meter, just testing same tank/plants/fish and changing the surface agitation.

CO2, which is pretty soluble, O2 is rather insoluble.


It'd be hard to get all the gas out. I'm not sure which might be first, nor that it matter that much. Most important is the rate of use/output and new inputs.


I think you are painting yourself into a corner here a bit.
When they discuss degassing, they mean it relative to the air ambient condition.
So say 100% with air is 7ppm for O2 say at 85F.
If you have 5ppm if the water, the air above will act as source for O2, if the plants have been growing well all day, and the O2 is not 10ppm, the water, not the air above, acts a source.

So it depends on what the differences are between the air and water which way this goes.

If they are both equal, agitation will not matter at all.
You have a larger amount of CO2 added in enriched systems, so the amount of CO2 degas will be more than the O2, which we do not enrich, just whatever extra the plants add. Generally not more than 150% say 2-5ppm of O2.

CO2 can be 20-30ppm difference.
Even within the aquariums, you can find regions of lower O2 and CO2.



I'm not sure.
I'd guess they are similar.
The diurnal time graphs suggest so from field site, but precise measurements and looking specifically at that question, I really do not know.


Certainly, it is most of the day/light cycle BTW. Why might that be?
It's likely a good thing.

Regards,
Tom Barr

A simple thought experiment should answer this. Assume that the rate of absorption of CO2, for example is greater than the rate of outgassing. That means that the amount of dissolved CO2 in the water would rise to a new equilibrium value. Then it would remain constant. But, to remain constant, the rates of absorption and outgassing would have to be equal, which is contrary to our assumption that the rate of absorption is greater. If the amount of dissolved CO2 in the water never did reach an equilibrium value, the concentration would increase without limit, which is physically impossible.

In reverse, assume the rate of outgassing is greater than the rate of absorption. That has to mean that the amount in the water would drop until an equilibrium is reached. And, at that equilibrium point, the rates would be equal, again contrary to the assumption that the rate of outgassing is greater than the rate of absorption. And, if an equilibrium is not reached, the water soon has zero CO2 in it, but that, too, isn't possible, since the rate of absorption is known not to be zero, so some must always be absorbed.

My conclusion is that there is always an equilibrium concentration of CO2 in water for any given condition of the water and the air.

The nice thing about this type of experiment is that it doesn't require buying any equipment or supplies, although a beer does seem to make the experiment go easier.

Yeah, I should have had my glass of wine before I read that. Thanks for reminding me.

Well, any time you talk about gas transfer and % saturation, take a sip
Solubility makes some difference, not a lot in most of the cases in aquariums really.

I have a nice O2 meter and have play with the CO2 meter, I suppose I could test and answer it if I still had the CO2 meter.

The O2 I can.
I'd have to bubble out all the other gases using N2 gas, then measure how long it takes for O2 to reach 100%. I'm too lazy
Your question, you answer it


Regards,
Tom Barr

Thanks for your answers! Below is what I've learned, along with a few more questions.



OK, CO2 is absorbed much more readily than O2.

Therefore, in a container of water that had all CO2 and O2 removed from it, and then was exposed to the atmosphere, CO2 would reach ambient before the O2 did.

Right?



Is the rate of absorption of O2 and CO2 the same as the rate of outgassing of those two gasses? Assume that the amount of each gas dissolved in the water is, say, twice that of ambient. After agitation, in the water will both return to ambient at about the same time?


Some say that it is, because the plants consume it faster than it dissolves in the water from the atmosphere. Is the rate of plant utilization of CO2 really greater than the rate of CO2 absorption from the atmosphere?

Then Vaughn said: My conclusion is that there is always an equilibrium concentration of CO2 in water for any given condition of the water and the air.[/quote]

Then the plants do not consume CO2 faster than it is absorbed?

Also, in Walstad-type tanks the concentration of CO2 is higher than ambient because of decomposition. That is why excessive surface agitation is to be avoided in such tanks.

Thanks again.

Bill

Plants will consume CO2 at a rate determined by how fast they are growing, which is determined by the availability of nutrients and light. So, CO2 can be consumed faster that it is absorbed, but only for awhile, and a short while. Then, unless there is very good water circulation in the tank, no more CO2 will be available for the plants to consume. Adding growing plants prevents an equilibrium from ever being reached, other than a dynamic one, with a gradient of CO2 concentration in the water.

In a Walstad-type tank the same reasoning would apply. Unless the light and other nutrient availabilities are low enough to restrict the plant growth rate, there may never be more CO2 in the water than ambient. Again, with very good water circulation, the amount available to the plants can be greater.

(One more sip of wine - Oh heck, one more glass!)

When it comes to which one would be absorbed faster, O2 is less soluble, however, there is a lot more concentration of O2 in the air above. CO2 is far more soluble, but there's a lot less of it.

Fick's 1st and second law of diffusion addresses this:

Fick's law of diffusion - Wikipedia, the free encyclopedia

The concentration gradient matters in other words, not just solubility.
The coefficient of diffusion will be the same for both gases however.
I'm not sure because I do not know how much the solubility and the concentrational gradient are influenced by each other with respect to time.

Regards,
Tom Barr

Please read Ms. Walstad's "Ecology of the Planted Aquarium", particularly pages 59 and 60 and 100 in edition 1. You are close to becoming a NPT enthusiast; this might the straw that does it.

Page 100 explains why surface agitation should be avoided in NPT's, since it drives off the over-ambient CO2.

Bill