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# re: CO2

```---------- Forwarded message ----------
From: psears at emr1_NRCan.gc.ca (Paul Sears)
Message-Id: <9701221659.AA19945@emr1.emr.ca>
Subject: Re: CO2
To: Aquatic-Plants at ActWin_com
Date: Wed, 22 Jan 1997 11:59:03 -0500 (EST)

* Subject: Re: CO2
* From: Neil Frank <nfrank at mindspring_com>

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

If they are the same ones I am thinking of, they will give you
the _ratios_ of CO2 to HCO3- and HCO3- to CO3-- over a range of pH.
They say nothing about actual amounts, though.

>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?

If you force the pH down, some of the HCO3- will be turned into
CO2.  The CO2 concentration in the solution will then be higher than it
was, but the situation will not last, because equilibrium with the
atmosphere will be re-established, ending up with the same CO2 concentration
you started with.  The net effect is the destruction of some HCO3-.
Some CO2 left the aquarium, but the concentration ended up unchanged.

> Then, this
>will cause HCO3 conc to decrease, right?

Yes.

> Assuming the other buffer keeps the
>pH the same, then what causes the % CO2 to go down.

If the other buffer you added does _not_ change the pH, then nothing
will happen to the HCO3- or CO2 concentrations.

> Is the HCO3
>concentration a factor, or at least until it goes below some threshold amount?

There are no threshold amounts; the equilibrium requirements must
be met.

>>  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.

I think my statement was ambiguous.  I'll try again:

If we have any significant bicarbonate hardness in the aquarium,
the pH is _determined_ the HCO3- and CO2 concentrations.  The CO2
concentration is decided by the CO2 system, and not by the reactions
going on in the water, whether there is bicarbonate hardness or not.

>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?

The CO2 concentration is fixed by the CO2 flowing into and
out of the water (from air, decaying organic material, into plants)
if the pH is decided by something _other_ than the CO2/HCO3- equilibrium,
then concentration of HCO3- will be decided by the pH.  It can be created
from CO2, or destroyed, as the pH changes.

--