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# Re: CO2 and KH

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Hello George,
thank you for the discussion on the issue.

>
> How about Limnobium laevigatum ("Amazon Frogbit")?

Yes, could be. I think it's Limnobium.

> > CO2 and KH: I once thought that I could save CO2 by lowering KH.
>
> True
>
> > If you want a certain CO2 level in the tank, say 15 mg/l, you would
> > have to add 15 mg/l plus whatever is needed to get past the buffering
> > capacity (KH).
>
> No, it's a continuous function, not a threshold.

Yes, of course, you're right. It's not a threshold. I didn't explain my
question very well.
The question is:
given that

CO2+H2O<-->[H2CO3]<-->HCO3-+H+

How much CO2 do you have to add to get a certain level of free CO2. How
is this a function of HCO3-? I know the tables that show the
relationship, I even found an equation that let's you calculate your own
table for any given value of pH and KH. From the table it would appear
that 15 mg/l of CO2 will result in a certain pH (7.3 at a KH of
10). At a KH of 5, the pH will be lower at 15 mg/l of CO2. But how much
CO2 do I have to add to get 15 mg/l in both cases. Is it the same? This
would mean that no CO2 can be saved by lowering KH. Or does the available
pool of HCO3- influence how much CO2 you have to add to get a certain
ammount of free CO2?
I think it does, because the HCO3-+H+ part of the equation does not come
from CO2 (or not mainly) in my tap water. it comes from Ca- carbonate.
That's why the water is alkaline. So the HCO3- will take away a lot of H+
ions which would be generated by adding CO2 to the water. That's the
buffering and the flow in the equation would go more to the right side.
Hence, I need more CO2 at a higher HCO3- concentration to achieve a
certain 'free' level of CO2. Am I right? But how much more would it take?

> > So _I think_ it would take more CO2 to get to a certain level of free
> > CO2 when the KH is high.
>
> Still true.

We agree here.

> > Maybe it's just the same ammount of CO2
> > needed to reach 15 mg/l, no matter how high the KH is. Maybe the
> > difference would just be in a different resulting pH.
>
> True.  But you have a poor mental model of what's going on.  KH will
> raise pH and CO2 will lower pH.  You can select any two of the three
> variables to determine the third.  The KH/pH/CO2 relationship is fixed.

This and the previous statement seem to be contradictory.

> Since I have soft water, I can add baking soda to raise the KH to 4.5
> and inject 17 ppm of CO2 to achieve a pH of 6.9.

Lucky you.

I will be away for a few days at a congress, so I won't be able to check
my email until sunday.
But geuess what, folks: the algae problem seems to have slowed down a
little. At least the green algae on the glass and on the gravel. Still
some hairy algae on the plants, however.

Michael, Munich
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