Reef Chemistry Question of the Day # 173 Dissolved Gases in Seawater

[Randy Holmes-Farley]

Reef Chemistry Question of the Day # 173

There are two factors that determine how much of a gas dissolves into seawater. These are

1. How much the gas "wants" to be in the water relative to the air
2. How much is present in the air

This question focuses only on the first factor by making the second one the same for all gases.

Starting with normal seawater from the ocean, the seawater is equilibrated with a large amount of gas that has the following composition:

Nitrogen 20% (by volume)
Carbon Dioxide 20%
Oxygen 20%
Helium 20%
Argon 20%

Some of the equilibrated seawater is then taken into a lab and put under vacuum high enough for long enough to remove all gases.

When the gases that came off are analyzed, which will be found in highest concentration?

In other words, which gas wants to be in seawater the most?

A. Nitrogen
B. Carbon Dioxide
C. Oxygen
D. Helium
E. Argon

Ignore water vapor for this question. Yes, there will be a lot coming off (nearly all of it), but our device cannot detect it.

Good luck!





















.

 

[nervousmonkey]

Co2. It has the lowest percentage in air, minus helium, of all the gases listed (0.033), yet the highest percentage in NSW (1.4) relative to the other gases listed. I.e., CO2, by percentage in air compared to percentage in NSW, has the highest affinity for saltwater of all gases listed. All others certainly dissolve in saltwater but not to the extent that CO2 does.
Edit: In fact, once the gases are analyzed, we will find that we have recovered all of the CO2 that was used to pressurize the water.

 

[beaslbob]

d

helium (it's the smallest)

last answer was d also.

 

[Taservices]

I don't have a good memory for this but I did some lab tech work in my undergrad on a program to try and sequester flue gas produced in the oil and gas sector with algae.

It worked with the exception of some of the VOCs but was not deemed cost effective to afford funding for full scale.

Lon story short I'm going to guess CO2 because it was some of the most readily absorbed gasses by the fluid (I know having a ton of algae reducing the co2 content in the water skews this a bit but it's my best guess)

 

[Randy Holmes-Farley]

d

helium (it's the smallest)

last answer was d also.

Why didn't helium rise to the top to be "A"?

Inquiring minds want to know!

 

[Ryengoth]

F. hydrogen

 

[Cory]

Id say A because its the most abundant gas in the atmosphere.

 

[beaslbob]

Why didn't helium rise to the top to be "A"?

Inquiring minds want to know!

I give up.

Why?

 

[beaslbob]

ok ok

B carbon dioxide.

there has to be something important learned by these questions applicable to our tanks.

 

[Ryengoth]

It's CO2 due to it being a molecule with a polarized link structure between the 3 atoms. The others are monatomic or diatomic elements so they aren't as "magnetic" to the water molecule.

 

[Randy Holmes-Farley]

I give up.

Why?

Not sure. Maybe give it more time.

meanwhile...

 

[Ryengoth]

You make no sense..

 

[beaslbob]

You make no sense..

thanks you
I try real hard to make no cents

 

[Ryengoth]

I don't see how N2O or NO plays into any of this, much less Helium. Just go ahead and give the solution Randy! If it's not CO2 I'm walking in the wrong area.

 

[JimWelsh]

B. Carbon dioxide has the greatest solubility in water: http://sites.chem.colostate.edu/div...ference data/solubility of gases in water.pdf

 

[Myka]

I'm going with B, but I have no scientific reasoning behind this. All I know is that crap gets in my tank, and it's freaking annoying!

 

[Randy Holmes-Farley]

And the answer is...B. Carbon Dioxide and by a very big margin

Carbon dioxide is a lot more soluble in seawater for several reasons.

1. First is that it really likes to be in water far more than any of the other gases because it interacts strongly with water molecules (none of the others do). It first hydrates to carbonic acid (H2CO3)

CO2 + H2O --> H2CO3

and then it strongly hydrogen bonds to nearby water molecules.

2. Second is that much of the carbon dioxide in seawater breaks up into H+ and bicarbonate (HCO3-) and some of the bicarbonate even breaks up into H+ and carbonate (CO3--)

H2CO3 --> H+ + HCO3- --> 2H+ + CO3--

So the bicarbonate and carbonate act as a big reservoir for the dissolved carbon dioxide.

Happy Reefing!

 

[beaslbob]

And the answer is...B. Carbon Dioxide and by a very big margin

Carbon dioxide is a lot more soluble in seawater for several reasons.

1. First is that it really likes to be in water far more than any of the other gases because it interacts strongly with water molecules (none of the others do). It first hydrates to carbonic acid (H2CO3)

CO2 + H2O --> H2CO3

and then it strongly hydrogen bonds to nearby water molecules.

2. Second is that much of the carbon dioxide in seawater breaks up into H+ and bicarbonate (HCO3-) and some of the bicarbonate even breaks up into H+ and carbonate (CO3--)

H2CO3 --> H+ + HCO3- --> 2H+ + CO3--

So the bicarbonate and carbonate act as a big reservoir for the dissolved carbon dioxide.

Happy Reefing!

justa a fwiw, I feel that co2 is extremely important most especially for fish. This is based on my experience with planted freshwater and marine systems. I think (well sometimes just speculate) that the plants immediately (or at least very quickly) consuming the co2 from the fish (and I guess ammonia as well) creates a very healthy environment.

It seems to be much more effective than circulation, filters, and so on. But than my FW planted have no mechanical filtration at all.

But that's just my .02