Long ago in one of my marine science classes, the lab instructor
demonstrated the carbonate-bicarbonate buffering system of seawater by
placing a pH probe in a container of sea water (pH 8.2) then bubbling
nitrogen gas into the water. This, he explained, would drive off the
CO2 from the water and the carbonate-bicarbonate equilibrium would
shift, causing a pH change. (Nitrogen gas is inert and will not
affect the pH by itself).
After a few minutes of nitrogen bubbling, the pH changed
dramatically--it shot up to around 10 (as I recall--it's fuzzy), and
then once the nitrogen was turned off and regular air was bubbled
through, the pH dropped back down to 8.2. (8.2 is the normal pH of
sea water). The nitrogen drove off all the O2, as well.
Doesn't this empirically demonstrate that an excess of one gas can
drive off the others? Or is it just that the sea water was exposed to
nothing but nitrogen (I'm sure there was a layer of nitrogen gas over
the surface of the sea water, as well) and it reached a new
equilibrium--100% N2, 0% CO2 and 0% 02? If this is the case, then
excess 02 (or CO2) in our aquariums would probably NOT displace other
gases, as they are not in equilibrium with the atmosphere. BTW, I'm a
biologist, not a chemist.