A test to see what the gas is inside the CO2 reactor chamber

Tom Barr

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I've long pondered this question and tried a few things, all of which were inconclusive.

1. Observations:
I have observed the levels inside the reactors build up rapidly if I turn off the return pump and allow the sump water to recirculate through the CO2 reactor.
The pH drops to about 5.3-5.4, but goes no lower. Typically tank pH's run about 5.9 with CO2 addition. 7.2 pH without.
The reactor fills up in less than hour.

Once the water change is done and the new water is circulated through the sump, the chamber gas dissapears in about 2-3 minutes.
It cannot be N2 gas and dissolve that fast with the new water after a water change.

Many suggest that it's N2. I say it's CO2.
It has to be a very water soluble gas.


So I thought of another way to use the rate of dissolving to predict AND TEST what gas it might be.
I use to make CO2 reactors for sale on line out of acrylic.
I can still do this.

I have a couple of reactors I kept also.
I have plenty of power heads also.

2.
So I could make a few of the hang on type reactors and add say 8 inches of gas inside the tubes and then try to see how fast the gas dissolves in an aquarium before the lights come on, and then repeat this same dissolution test after the CO2 ppm s have built up over the day in the SAME planted aquarium.
I just need to add the CO2 and measure the time.

3.
Next, I can add some N2 gas. I have a N2 gas regulator I used to purge some water samples free of O2.
I can do this and repeat this SAME TEST.

4. Add O2. Without getting a $$$ O2 tank and reg, I'll see if I can borrow a friend's welding set up and O2 tank.
I did this back around 2005. O2 takes a logn time to dissolve, N2 is likely even tougher.

5. Try the same amount of gas using air.

Measure the rates dissolution.

Lastly, take a CO2 reactor that has built up a gas level to the same depth over the course of the day and measure how fast it dissolves in the AM right when the lights come on. I and many folks have done this and noted that it only takes a quick minute for the gas to dissolve away.
Which is the fundamental basis for the hypothesis I suggest: the gas is really CO2 and that it does not dissolve so readily and easily as the concentration increases over the day. I also hypothesize that the rate of dissolution is not linear, rather, a decaying exponential.

-J is the Flux(rate of degassing) = Change Concentration / change distance




Aquarist seem to suggest that the rate is linear, eg, you can keep adding more and more CO2 and it'll immediately dissolve even at high ppm's. This cannot be true.
The above shows a distance the concentrational gradient to be 10 cm. But in reality, the distance is pretty much a very very small distance between the water and the air layer above.
But..........this can be affected by a scum or a film layer on the water. The distance is not really different, but the rates change dramatically because the material the gas exchange takes place is radically different than a clean water's surface.
A protein scum layer has a difference diffusion coefficient than clean water.

So let's say it's clean water only.............

So each treatment and test will have the same clean water surface etc.

We can now reduce the flux down to this equation:

-J = Change in concentration

We can also add a oil like layer to a tank and then see how that influences CO2 and dissolution.
 

ltb420

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I often wondered this also. When I first started injecting CO2 into a reactor it would not completely degas over night and would take longer to get to its usual point the next day but it would also go past the regular max limit by the end of the CO2 period. I think this is where people have issues with PH controllers as it takes longer for the CO2 to be absorbed once in the upper limits.
 

Tom Barr

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I think the sump observations when the water change is done late in the day pretty much says it all.
N2 gas would stay there. But that's not the case once you restart the return pump and filter.

The flux changes at an incredible amount with concentration. So even a small change in concentration, say 3 ppm vs say 60 ppm is huge.
But similoar to the scum layer, the diffusion between a liquid like water and the air above is 10,000X slower even if clean and without a scum layer. So the constants change for the air and the water since the media also changes. This is often left out in discussions of Fick's 1st law AND THE EXAMPLE ABOVE.
We know this as hobbyists, but do not discuss it as it applies to the gas build up inside a reactor.

The difference between the media effectively increases the "distance" of X in the above example.
 

Tom Barr

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Jan 23, 2005
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I think the sump observations when the water change is done late in the day pretty much says it all.
N2 gas would stay there. But that's not the case once you restart the return pump and filter.

The flux changes at an incredible amount with concentration. So even a small change in concentration, say 3 ppm vs say 60 ppm is huge.
But similoar to the scum layer, the diffusion between a liquid like water and the air above is 10,000X slower even if clean and without a scum layer. So the constants change for the air and the water since the media also changes. This is often left out in discussions of Fick's 1st law AND THE EXAMPLE ABOVE.
We know this as hobbyists, but do not discuss it as it applies to the gas build up inside a reactor.

The difference between the media effectively increases the "distance" of X in the above example.
 

BenFishin

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Hey Tom,

Long time reader first time poster (my membership age does not reflect how much time I have spent gathering free information from the best group of fellow hobbiests) It's lhere in Indiana as I am stricken with insomnia, lying here next to my passed out, cold ridden wife, tapping away on the dim glow of the screen on my iPhone. But anyways, if you had a bleeder valve on your reactor hooked to an air hose you could possible gather enough gas to do a test. I remember some flaming splint tests in HS Chem labs.