CO2 and KH

Michael Irlbeck <u7211aa at sunmail_lrz-muenchen.de> writes:

    is the ammount of CO2 needed (as in bubble rate or 'how fast does my
    CO2 bottle empty') to hold a dissolved CO2 concentration of 18 mg/l the
    same in 

        - -water with a KH of 10 
        - -water with a KH of 4 ???

    assuming one doesn't care about the difference in resulting pH and all
    other factors (temperature, surface motion, plant consumption etc.)
    being the same.

My answer would be Yes, when you've adjusted pH to keep the free CO2

    I wrote 'hold' that CO2 concentration, because additonal CO2 would be
    produced, of course, by titration with acid to bring down KH. As you
    have explained in your post.

This reminds me that I didn't think through part of the matter, which is
embarrassing, since the part I omitted was probably the main point to
begin with.  When you add acid to the system, the KH goes down directly,
since it's really a measure of alkalinity, not carbonate. In the short
term, though, the concentration of carbonate (in all forms, from H2CO3
to CO3--, combined) is unaffected.  (Unless you add so much acid that
the tank effervesces :-)

After that, the H2CO3 starts breaking down, and the system injects more
CO2 to hold the pH constant, and I see that I need to think more about
what's going to happen.  Just 2 things are now clear to me:

 . If you really hold the free CO2 constant, the loss rate will be
 . Use of a buffering system with a pK anywhere between the pH of your
   aquarium and the pH used in either the KH test or the free-CO2 test
   (a system such as phosphate) will tend to invalidate the two tests and
   make them give results that aren't consistent with the values in the
   tables in places like The Optimum Aquarium.  This wasn't clear to me
   till now.  It won't affect you when using hydrochloric acid, though,
   or even (I think) acetic acid.

Dan Drake
dandrake at nbn_com