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# Re: CO2 levels and KH

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On Wed, 29 Sep 1999, Michael Moncur wrote:

> Here's a question: Does it require more CO2 to change the pH, say, from 8.0
> to 7.0, than it does from 7.0 to 6.0? Would that explain my pH's seeming
> reluctance to change at pH 7.8 and 7 dKH?

It takes a smaller change in CO2 to change the pH from 8 to 7 than
it does to change the pH from 7 to 6.  Over most of the pH range we're
interested in, the change in CO2 needed to drop the pH by 1 is 9 times the
concentration of CO2.  For instance if you have 2 ppm CO2 at pH 8 and you
want to drop the pH to 7, then you need to add 18 ppm CO2, to a total of
20 ppm CO2.

This relationship falls apart at pH below 6 and can be changed by other
buffers in the system.

However, I'm not sure that's what you're asking.  I think this is what
happens...

In water with high pH the ratio between bicarbonate and dissolved CO2 is
high. When you bubble CO2 into water with high pH it reacts with the water
to form aqueous CO2 and bicarbonate in about the same proportions that are
there to start with.  For high pH water that means that a large part of
the added CO2 reacts to form bicarbonate and only a small part becomes
dissolved CO2, so you don't see much increase in the dissolved CO2.

In water with low pH the ratio between bicarb and dissolved CO2 is much
lower and a large part of the CO2 that you bubble in actually gets
converted to dissolved CO2; only a small part gets converted to bicarb.

So at low pH an increment in the amount of CO2 you're bubbling in creates
a relatively large increase in dissolved CO2 and at high pH the same
increment in the amount you're bubbling in creates only a small increase
in the dissolved CO2.

What this all means to the pH you measure in your tank depends on where
you are on the alkalinity-pH-CO2 chart and on how quickly the new CO2 is
lost or used.

Roger Miller

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