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I think the confusion swirling around about KH/CO2 interactions results
from flawed explanations of buffer systems. People love to say they
stabilize the pH, but this is only partly true. What they really do is
control the strongest possible acid or base you can add to a solution.
In water, the strongest acid or base you can add is normally H3O+ or
OH-. Anything stronger will immediately react with the water; e.g. if
I had some superacid HA and added it to water, it would rapidly
protonate the water:
HA + H2O ---> A- + H30+
So water sort of sets maxima on the strength of the acids and bases
you can have in solution. But we can lower these maxima by including
a weak acid (e.g. H2CO3) and its conjugate weak base (e.g. HCO3-).
Now, these are the strongest acids and bases that can exist, in
exactly the same way; if I tried to add some strong acid like HCl,
it would rapidly react with the HCO3-
HCl + HCO3- ---> Cl- + H2CO3
And similarly, the H2CO3 will scavenge out any strong bases.
What this means is that the playing field has been levelled -- when
we try to add HCl to make large pH swings, our buffer changes all of
the HCl we add into H2CO3, which is a weaker acid and makes a smaller
pH swing. The amount of HCO3- and H2CO3 we have in solution determines
how much strong acid/base the buffer can cope with in such a
situation -- you need to have at least as much HCO3- present initially
as the amount of HCl you are adding, or you will run out of buffering
capacity and the HCl will win.
So why does adding H2CO3 (via CO2 injection) cause pH swings that are
independent of the KH levels? The buffer is mostly irrelevant to H2CO3
addition! The buffer has made it so that H2CO3 is the strongest acid
that can exist in our solution, but how much buffer we have doesn't
matter, since the buffer won't react with H2CO3 (or any weaker acid).
The effect of KH levels is not rigorously zero, because having some
HCO3- already present in the water will inhibit the dissociation of
H2CO3 to form more HCO3- by Le Chatelier's principle. This effect is
not large, and can be neglected for the ranges we talk about, although
it is barely measurable.