re: CO2 (fwd)

---------- Forwarded message ----------
From: psears at emr1_NRCan.gc.ca (Paul Sears)
Message-Id: <9701211557.AA04734@emr1.emr.ca>
Subject: Re: CO2
To: Aquatic-Plants at ActWin_com
Date: Tue, 21 Jan 1997 10:57:26 -0500 (EST)

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

Paul Sears says:

>>       The CO2 concentration in the water will depend on the interaction
>>of the water with the atmosphere.  If you have no CO2 injection system,
>>it will be only a few ppm, and if you inject CO2, it may be as high as twenty
>>or thirty ppm.  The pH of the solution is irrelevant to this.
>>       If the pH is high, it just means that there is a lot of HCO3-
>>in the solution _as_ _well_.  There will be little CO3-- at any pH of
>>interest to us (below 8.5, say).
>Why does the concentration only depend on the interaction with the
>atmosphere? What if there is another source of CO2... like decomposition of
>organic matter (including fish food). Isn't this the source of C in natural
>systems where the CO2 concentration is higher than that predicted by the
>atmospheric equilibrium levels. I have tanks with organic substrates that
>run for years without CO2 injection. Secondly, once this CO2 is available in
>the water, doesn't it enter into equilibrium with HCO3... and the percent
>CO2/HCO3/CO3 is a funtion of pH. I had understood that the pH can be a
>funtion of many factors, only one of which is based on the CO2 entering the
>system. If the pH is lowered (from other factors), doesn't the HCO3 change
>to CO2?
	The point is that in the atmosphere or in our CO2 system, or even
in the fish food we put in, we have an essentially infinite source (or
sink) for CO2.  These sources/sinks do not change in concentration, and
will achieve equilibrium with our tank water, whatever side reactions
take place with finite quanitites of bicarbonate in the water. 
	One could temporarily depress the amount of CO2 in the water
by adding sodium hydroxide, but the CO2 would then flow into the 
system from the air or CO2 system until equilibrium had been re-established.
By that point, the OH- added would have been converted to HCO3-, and
the CO2 concentration would be back where it started.
	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.  Equilibrium is a two-way
street.  CO2 enters and leaves our tanks all the time, and if we
alter the CO2 concentration in the water, the rate at which it leaves
will change, but the rate at which it comes in will not.
	In our tanks, the major buffering system will be CO2/HCO3-.
If the water is _very_ soft, then organic acids might predominate,
but about the only other things that could do the job would be things
that we put in.  The HPO4--/H2PO4- system can control pH very
nicely in the range we want, but the side-effects are undesirable,
to say the least.  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.

Paul Sears        Ottawa, Canada

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