Reef Chemistry Question of the Day #136 Diffusion of oxygen in water

Randy Holmes-Farley

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Reef Chemistry Question of the Day [HASHTAG]#136[/HASHTAG]

From the past two questions of the day we know that oxygen molecules in air are moving about 1000 miles an hour and get only about 180 of their diameters before hitting another molecule and bouncing off in another direction.

In fact, that bouncing is why diffusion in air is so slow.

To put this in perspective, let's consider an aquarium that is 24 inches deep. When it is filled with air only, it takes almost 3 hours for an average oxygen molecule to diffuse from the tank top to the tank bottom! (we'll have more on this surprising result in later questions)

But unless you keep lizards, we are interested in water, not air. Liquid water obviously is more dense than air, and a diffusing oxygen molecule will take longer to diffuse through it because of the increased number of collisions taking place.

About how long do you think it would take an average oxygen molecule to diffuse from the tank top to the tank bottom of a 24" deep tank filled with still water?

A. 3.5 hours
B. 7 hours
C. 25 hours
D. 250 hours
E. 2,500 hours
F. 25,000 hours

Good luck!






































.
 

beaslbob

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c-- 25 hours.

(also means I don't know).

If your algae is consuming co2 and returning oxygen does it not take the same long period of time for co2 to travel from the surface down to the substrate?

So is it not possible that with no circulation the tank can become a low co2 environment with the possibility of a higher o2 as well. Kinda like an oxygen tent that also remove co2?

(this may explain why my FW planted tanks with no circulation have heavy bioloads for years and years.)
 

leptang

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I'll guess A
Of the top of my head because water is more dense and molecules have to travel less distance before they bounce back. But it still takes longer then air because maybe there are more molecules in water in the space of the tank. I'm just guessing.
 
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Randy Holmes-Farley

Randy Holmes-Farley

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c-- 25 hours.
If your algae is consuming co2 and returning oxygen does it not take the same long period of time for co2 to travel from the surface down to the substrate?

So is it not possible that with no circulation the tank can become a low co2 environment with the possibility of a higher o2 as well. Kinda like an oxygen tent that also remove co2?

(this may explain why my FW planted tanks with no circulation have heavy bioloads for years and years.)

Yes, that would be true in a true no circulation situation. That's why a power failure is so bad.
 

JimWelsh

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Assuming your value of "almost 3 hours" is correct for an oxygen molecule in air, then since the diffusion coefficient of oxygen in air is 0.176, and that of oxygen in water is 0.000021, then it would take 3 hours * 0.176 / 0.0000021 = about 25,000 hours. Answer is F.
 

reefbeard

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The O2 diffusion coefficient in air is thousands of times greater than in H2o. My answer is F.
 

beaslbob

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Yes, that would be true in a true no circulation situation. That's why a power failure is so bad.
Thanks and interesting.

Might also explain why I had pH over 8 (purple on api high range test kit) even with a peat moss in the substrate.

FWIW I have had 10g FW planted tank with 30 guppies (6 reproducing adults) run for 9 years from the original cycle trio. With no circulation, no mechanical filters, no water changes. And have done that in multiple cities as I traveled around in the air force.

Of course the fish provide some circulation but the tanks never became stagnant, never had foul odors, no surface scum.

Again thanks for helping me to understand how such "unproven, experimental, and dangerous" techniques worked so well.

But that's just me and my .02
 

brandon429

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lm linking this thread to plantedtank.net we are discussing how much bubbling water actually adds o2 to an aquarium, vs degassing of co2 as the primary benefit. i was under the impression that o2 doesnt get input very well at all using airstones, that it might stay rather constant unless there is a huge percentage of bacteria command from mulm and detritus if applicable, but that the main benefit to airstones is the driving off of co2 not because it made that much of an 02 benefit. im wondering if this slow transition or dissolution rate means that bubbling water doesnt change the 02 much if there wasnt a large gradient to begin with.
 
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klp

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Are we to assume a constant temperature 24/7? Is there a light source and if so how much and how long would it be on for?
 

brandon429

why did you put a reef in that
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Just to get an idea of how much o2 goes into soln in a bubbling container vs a still one I think I'd like to know how it worked in a bare glass container in the dark...as few skews towards o2 production or consumption as possible where just the mechanical motion is the difference between two equal test vats
 

Cory

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Ill say f just a guess. If true thats astonishing. Setting up the tank soon! Woot!
 

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