Re: CO2

To: Aquatic-Plants at actwin_com
Subject: Re: CO2
From: Neil Frank <nfrank at mindspring_com>
Date: Wed, 22 Jan 1997 06:56:59 -0500
>	The same thing applies to the shifting of pH by addition of 
>another buffer system.  The CO2 concentration will move when the change
>is made, but will return to where it was. 

I am still trying to understand those equilibrium diagrams. You know, the
ones with overlapping bell type curves - one for CO2, HCO3 and CO3.

Suppose you have a constant source of CO2 (from atmosphere, or substrate or
whatever). And that you have a finite supply of HCO3. If you lower the pH
(say, by adding a different buffer), will percent CO2 increase? Then, this
will cause HCO3 conc to decrease, right? Assuming the other buffer keeps the
pH the same, then what causes the % CO2 to go down. Is the HCO3
concentration a factor, or at least until it goes below some threshold amount?

In nature, I assume that the supplies of CO2 and HCO3 are large enough to
allow the process to be stable. In the aquarium it is obvious that without a
flow thru system, the supplies are much more finite. In the example above,
maybe the HCO3 is limiting??

>  For us, the pH is _determined_ by the HCO3- and
>CO2 concentrations, if we have any significant bicarbonate hardness,
>however, that condition is not necessary for the CO2 concentration
>to be decided by the CO2 system and not the other water chemistry.

I can understand this statement if there is nothing else controlling the pH.
And in aquarium, it does seem reasonable that the HCO3/CO2 would be it. Do
you think there is something else (other buffers) in natural systems that
affect the pH and corresponding CO2 concentration according to the
equilibrium curves?

Neil Frank      TAG editor         Raleigh, NC