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Re: CO2 and KH

On Thu, 30 Sep 1999, Wayne Jones wrote:

> >I have a digital model that I can use
> to solve
> >chemical problems under fairly complex
> conditions like in aquariums. I ran
> >some tests with the model and it
> appears that the increment in dissolved
> >CO2 that you get from increasing the
> bubble rate should be the same
> >regardless of what the starting pH is
> Does this mean that you agree that the
> addition of baking soda to raise the kH
> should do nothing to prevent pH swings
> caused by CO2 injection?

I do agree that increasing the KH doesn't change the pH swing that's
caused by a change in CO2.  I find this *really* unintuitive and difficult
to see on the pH-KH-CO2 charts, but that's what the formulas say.  But I
don't see how you would draw this conclusion from the paragraph you

> >The model did confirm the pattern
> (observable on the CO2-KH-pH charts)
> >that the change in pH that comes from
> an increase in CO2 is larger at high
> >pH than it is at low pH; the lower the
> pH is to start with, the more
> >difficult it is to lower the pH.
> I am afraid I do not understand this. I
> thought you just said that the change
> would be the same regardless of the
> initial pH. Of course once the pH has
> been lowered using CO2 an additional
> change of the same amount will require a
> much greater amount of CO2.

In the first paragraph I said that the change in CO2 caused by an increase
in the bubble count was the same regardless of pH.  In the second
paragraph (which apparently you agree with) we are talking about the
change in pH caused by a change in CO2 -- that's a different thing.  It
change in CO2 needed to lower the pH from 8 to 7 is smaller than the
change in CO2 needed to lower pH from 7 to 6. 

> In the end though this does not explain 
> why the pH in Mr. Moncurs tank does not
> drop significantly when he adds CO2.
> Surely, one bubble per second in that
> size tank with low suface turbulence
> should be enough to make a pH change
> greater than .2. I have also heard many
> other people state that it is more
> difficult to lower the pH of hard water
> than soft water using CO2. Is there
> typically some other ingredient in hard
> water that prevents CO2 injection from
> lowering the pH?

No, there's not anything generally present in hard water that would change
the pH-KH-CO2 relationship.  Any buffer present at levels comparable to
the bicarbonate concentration will mess with the relationship, but that is
mostly a problem at low pH with low bicarbonate and relatively high
organics - the peat water or black water case.

I can't really explain the effects in Mr. Moncur's tanks, partly because I
don't know how big his bubbles are.  The bubbles going into my tanks are
mostly small, and my bubble counts are 3-4/second when I switch out
the yeast batch.  One bubble/second doesn't create much of an effect

Losses of CO2 across the water surface (even when it isn't turbulent)
increase with CO2 concentrations and tend to offset increases in the
bubble count.  Also, if the CO2 is initially low then an increase in the
bubble count will also be partly offset by an increase in CO2 uptake by

I suppose there may be other offsets as well.  Any more ideas?

Roger Miller