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Re: [APD] Re: pH and CO2 questions

On Mon, 8 Dec 2003 14:47:08 -0600, Jim Seidman <js5 at seidman_net> wrote:

"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


I did in an earlier post suggest CO2 dissolved entirely as H2CO3, which I now accept as incorrect. Instead, a little bit of research at http://scifun.chem.wisc.edu/chemweek/CO2/CO2.html reveals the following equilibrium:

CO2 + H2O <-> H2CO3 (biased to the left for 'normal' conditions)

Which then dissociates as H+ and HCO3-. The HCO3- then further dissociates as H+ and CO3(2-). Obviously though, this only occurs in a tiny fraction of molecules of CO2. This (by adding CO3(2-), must add to KH, although since the level of dissolved CO2 in an average CO2-added tank is less than 30 ppm, and this is equivalent to <2 degrees hardness? so it will not have a huge impact on KH.
The H+ ions liberated (which will of course be in the form of H3O+ for the nitpickers) would then react with CO3(2-) in the water (which is present in an equilibrium with CaCO3 and HCO3-) to form HCO3-, thereby removing the H+ (which originally came from the water the CO2 reacted with). This would prevent a drop in pH, and so CO2 is being 'wasted'?
KH includes all carbonates CaCO3, MgCO3 etc. and HCO3- and CO3(2-)? So altering equilibrium from CO3(2-) to HCO3- would not affect KH, but then equilibrium would shift back at a constant pH?
So carbon is of course being added, not removed, slightly increasing KH (I would'nt like to guess whether this is significant), and I have confused myself. (And I'm supposed to have a A at A-level chemistry!)
People don't notice their KH dropping away to 0 as they add CO2, so it must be quite constant. I am sure lowering KH would prevent you from needing as much CO2 to reach a given level, but does pH correspond to CO2 added?
I need an Authority to speak on the subject, because it is really confusing.

--
Andrew McLeod
thefish at theabyssalplain_freeserve.co.uk
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