Reef Chemistry Question of the Day #60 Gas Exchange 1

[Randy Holmes-Farley]

Reef Chemistry Question of the Day #60

This is the first of several questions relating to gas exchange in coral reef aquariums. This first one addresses how much total oxygen and carbon dioxide are present in our coral reefs.

Imagine that I have a device that can strip out all of a single gas from an aquarium. I can accomplish this, for example, by completely aerating the water with normal air that has none of the gas in question in it.

I start with two totally identical aquariums. I use that device over a period of a couple of hours to strip out all of the oxygen (as O₂) from the water of one of them, and I use it similarly on the second one to strip out all of the carbon dioxide (as CO₂).

When I measure what was removed by analyzing the used air that I collected after the aeration, what do I find?

A. The amount of O₂ removed is far higher than the amount of CO₂.
B. The amount of CO₂ is far higher than the amount of O₂.
C. The amount of O₂ and CO₂ is roughly the same (within a factor of 2).
D. Since the amount of these varies tank to tank and with the time of day, even in two identical tanks the answer could be A or B or C depending on the nature of the two identical tanks.

Good luck!












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[Paintguru]

My mostly uneducated guess....B.

 

[Cory]

At 20celcius seawater holds 9.1mg/l of oxygen, but at 20c seawater holds grams of carbon dioxide. So is guess b, co2 would be more.

But since you diddnt say what the salinity was, or the temperature, or the pressure, since these things effect the concentration, the answer could also be e, it depends on other factors, but I'm sticking with b, final answer.

 

[Cory]

Actually disregard the co2 being grams in seawater that's not true haha.

 

[saltyhog]

Are the tanks in the same room? CO2 is in equilibrium (as is O2) with the air surrounding it but CO2 levels in air vary a whole lot more than 02. So I say D

 

[Randy Holmes-Farley]

Are the tanks in the same room? CO2 is in equilibrium (as is O2) with the air surrounding it but CO2 levels in air vary a whole lot more than 02. So I say D

Yes, they are in the same room.

 

[johnanddawn]

A
Atmospheric O2 is like 16% or something like that whereas co2 is like 380 ppm - far far less so assuming equilibrium between the atmosphere and the water (I'm not sure you can do this) O2 would be many times greater

 

[Cory]

I think most of the co2 in seawater is hco2 at ph 8.2?

 

[Randy Holmes-Farley]

And the answer is...B. The amount of CO₂ is far higher than the amount of O₂. Paintguru was first to get the right answer.

In normal seawater the amount of dissolved O2 at saturation with normal air at 25 deg C is about 206 umole O2 per kg of seawater, or about 6.6 ppm.

In normal seawater the total amount of dissolved CO2 at saturation with normal air at 25 deg C varies a bit with latitude and other factors, but in the equatorial Pacific it is about is about 2050 umole CO2 per kg of seawater, or about 90 ppm.

Much of the carbon dioxide in seawater is held in the bicarbonate and the carbonate. As you strip away the CO2, the bicarbonate and carbonate fall apart into hydroxide and CO2, leaving behind higher pH seawater:

HCO3- --> CO2 + OH-
CO3-- + H2O --> CO2 + 2OH-