[APD] Re: pH and CO2 questions

["Jim Seidman" <js5 at seidman_net>]

"Tarah Nyberg" <Tarah_Nyberg at UTSouthwestern.edu> writes:

>> basically every molecule of CO2 that is used to reduce the pH is
wasted/converted to bicarbonate.
If your water has a low KH and is already at, for example, pH 6.35 most
of the CO2 you inject will remain as free CO2  (because of the
equillibrium point btw CO2 and HCO3- is pH 6.35).

If you start with a pH of 7.5 and aim to reduce it to pH6.8, all the
Hydrogens used to drop the pH from 7.5 to 6.8 came from "wasted" CO2
molecules. 

So If you are concerned about every molecule of CO you input into the
system (which is not the case if you use a CO2 tank) a lower KH will
help. <<

I believe you are correct about all of this. However, the effect isn't really
meaningful enough to make a difference. There is so little bicarbonate in the
water compared to CO2 (just as there is so little carbonic acid compared to
CO2) that the amount of CO2 lost is not worth worrying about.

Even if it were a noticeable, if you inject CO2 continuously this loss will
only occur once. That is, you'd lose some CO2 as the pH dropped. Once the pH
stabilized, it would be because the levels were in equilibrium, at which point
the KH wouldn't matter.

- Jim


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