[Date Prev][Date Next][Thread Prev][Thread Next][Date Index][Thread Index]

Re: CO2 depleting buffers

> From: "S. Hieber" <shieber at yahoo_com>
> Subject: Re: CO2 Depleting Buffers
> > > True or false: with CO2 injection, a tank with a kH of 5 will be
> > depleted of
> > > calcium hardness in a matter of days by the carbonic acid if no
> > extra buffering is added, causing the pH and CO2 concentration
> > to drop?
> The CO2 put into an aquarium doesn't (all) stay there.  In fact, most
> CO2 leaves rather quickly into the atmosphere.  Part of the trick of
> doing CO2 injection well, besides using an efficient reactor like Tom
> Barr's 
> (  http://www.spectrumdesignplastics.com/petsup.htm  ), 
> is trying to minimize how quickly the CO2 is shed to the air.  The
> plants use some and the air takes much of the rest.  We keep adding CO2
> because much of it keeps leaving as we add it.

	We end up with a dynamic equilibrium.  It leaves at a rate which
will be roughly proportional to the concentration, so if this is above
the concentration that would be in equilibrium with atmospheric CO2,
then we must add CO2 to stop the concentration falling.

> If it never left and just kept building up (which it can't), then
> all/any alkaline buffers present in the tank would become "used up."  

	CO2 is what is called a weak acid, so it can only "use up"
alkaline buffers that are the anions of still weaker acids.

	The point is that a buffer system always consists of two parts.
If we have a weak acid HA, then the buffer would be a mixture of 
a salt containing A- and HA itself.  The pH would be set by the ratio
of the two, because [H+][A-]/[HA] is a constant.  If you have more than
one buffer system in there, then calculating where the pH will end up
is somewhat trickier!  If you drive the pH down, that is, increase [H+],
then you drive [A-] down, making more HA.  ("[]" means concentration of).
That could be said to be "using up" the "alkaline buffer", but I don't
think that's a very useful way of thinking of it.  There is always _some_
A- left.

> Even then, the plants can only use so much and the water can absorb
> only so much CO2, and after that the rest would still be passed on into
> the atmosphere and the CO2 level would drop, but would remain at that
> maximum saturation level. 

	The "saturation" concentration of a gas in a liquid is usually
regarded as the concentration at which bubbles of that gas start to form.
That is the concentration of the gas in equilibrium with the gas at
one atmosphere pressure.  For CO2, that would be something like 1500 ppm.
We don't get anywhere near that - we set up a dynamic equilibrium in 
which the gas is coming (CO2 system, fish, atmosphere) and going (plants,

Paul Sears        Ottawa, Canada