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Re: CO2 and Alkalinity
In response to the question about why CO2 affects pH, but not alkalinity-
Okay, it's like this. Alkalinity is caused by a number of compounds, but the
ones in greatest presence in the vast majorities of natural waters is
compounds containing carbonates. These alkaline carbonate ions are just
"waiting around" to dump their current "partners" of Calcium, Magnesium, or
whatever, and join up with a couple of H+ ions, leaving the Ca, etc. to then
associate with the NO3, PO4, Cl, or whatever the H+ ions were originally
associated with. They join with the H+ ions and form H2CO3, or carbonic acid.
This carbonic acid could also technically be called hydrogen carbonate.
CO2 dissolves in water. A portion of it combines with H2O to form H2CO3, or
carbonic acid. It can technically also be called hydrogen carbonate. The
compound is ionic in form, and the hydrogen separates off to become a pair of
H+ ions. Since pH is a measure of the concentration of H+ ions, this causes
the pH to go down (as acidity[H+ ion concentration} increases, pH goes down).
Now, the CO2 and H2CO3 are trying to establish a balance. As the
concentration of one goes up, some converts, and the concentration of the
other goes up. Like this;
H2CO3<=> CO2 + H2O
It goes back and forth, continually trying to establish a balance. However,
since the H2CO3 that is introduced because the concentration of CO2 is raised
introduces H+ ions, it affects pH. The H2CO3 that is caused by carbonates
combining with hydrogen on the other hand, uses up H+ ions by combining them
with the extra oxygen ions to form water. In highly simplified terms, this is
what alkalinity is, a willingness to react with and use up H+ ions to form H2O
and CO2. Since the CO2 that enters the water doesn't do this, it won't show
as alkalinity. It is already in the CO2-H2O-H2CO3 state.
Is this any better, or just even more confusing? It's late and I should be in
bed.
Bob Dixon