Reef Chemistry Question of the Day #63 Gas Exchange 4 CO2 and the Superbowl

[Randy Holmes-Farley]

Reef Chemistry Question of the Day #63 Gas Exchange 4

It is Superbowl Sunday, and you have a big crowd over to watch the game. It is really cold out and the the house is closed up tight.

It is a very exciting finish and everyone is standing and shouting and milling about.

Being the reef geek that you are, you have two reef aquariums in your living room, in addition to the big screen TV with the game on.

The aquariums are identical in every way, except that one sits on the floor (waiting for a new stand) and the other is up near the ceiling (I can't imagine why you did that, but you did).

Just after the last seconds of the game clock tick down, you remember hearing that big crowds can produce a lot of CO2 and that CO2 can lower tank pH. You rush to the tanks and check the pH that has been logged for each tank since just before game time when no one was around.

What do you see?

A. The pH started the same and then declined by exactly the same amount in the two tanks.
B. The pH started the same and then declined in both, but the one on the floor declined more.
C. The pH started the same and then declined in both, but the one on the floor declined less.
D. The pH started higher in the tank on the floor and then the pH declined by exactly the same amount in the two tanks.
E. The pH started lower in the tank on the floor and then the pH declined in both, but the one on the floor declined more.
F. The pH started higher in the tank on the floor and then the pH declined in both, but the one on the floor declined less.
G. The pH rose in at least one of the tanks.

Assume no supplements have been added during the pH measurement period, and that contributions to pH changes from photosynthesis and respiration in the reef tanks are insignificant during this time period relative to the effects of the CO2 from the people.

Good luck!


















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[beaslbob]

C- hot air rises

 

[beaslbob]

nothing happens because both tanks have massive 3,000 water changes per hour circulation to a common sump in the basement. :tongue:

 

[Randy Holmes-Farley]

You only get one choice.

 

[dccordell]

F, assuming that before the game started and all of the guests came over, there were still SOME people at home, such as the people that live there.

 

[sluicebox]

A. The density of air with such small differences in CO2 concentrations will be the same on the floor as the ceiling simply due to air mixing in the room caused by the movements of people in the room and general air circulation.

 

[Cory]

I'd say e.

Because co2 is a heavier gas I believe.

 

[beaslbob]

depends on how much methane was released.

 

[saltyhog]

I have no idea but I will say E....my reasoning...the air near the floor will be slightly more dense and therefore have a slightly higher partial pressure of CO2 causing a lower pH. This will also result in a greater additional increase in CO2 near the floor. If none of that is true then I think it's A! :wink:

Thanks for doing these Randy, it's a great way to have fun while learning a lot of useful chemical principles that relate to our hobby!

 

[H2O]

A . air and co2 weight is about the same the crowd is moving the air around so both should be the same

 

[Randy Holmes-Farley]

Anyone else before I give the answer later today?

 

[Randy Holmes-Farley]

And the answer is...A. The pH started the same and then declined by exactly the same amount in the two tanks.

Unless you have a localized source of CO2, it never forms layers of different concentration in something like a room. So the entire room will essentially be at the same CO2 concentration, hence the same pH for the aquariums.

While it is true that CO2 is much heavier than the main constituents in air (N2 and O2), if you start with well mixed air, CO2 will not settle out at room temperature because the random motion of the air molecules is enough to keep it well mixed, even with no air currents at all.

In the situation where people will be exhaling localized CO2, the air currents in the room from people moving will be enough to rapidly mix the new air (exhaled) in with the old air.

CO2 will travel along the ground if you put a localized source down low (like a chunk of dry ice) and don't stir it up much with air currents, but that's a different situation.

In terms of air pressure alone, the effect between the floor and the ceiling is too low to detect in a room. Localized pressure differences from air flow will be higher than the pressure difference based on altitude in the room.