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Re: carbonate pH buffering for CO2



> From: Matt Rhoten <mrhoten at oz_net>
> >> add sodium bicarbonate to keep my carbonate hardness up and my pH
> >> stable
> 
> > I don't think this is a good idea.  Sodium bicarbonate will also raise
> > the pH, not stabilize it.  Again, DIY CO2 is your best bet IMHO.
> 
> Then what is the recommended way to raise carbonate hardness? I'm not
> likely to inject CO2 with a carbonate hardness of roughly 30ppm
> (that's around 1.7dKH). I realize that without CO2 injection, adding
> sodium bicarbonate will raise the pH (since it will change the
> equilibrium value), but since my plants appear to be sucking all or
> nearly all the CO2 out of the water anyway, it can't go too much
> higher, right?
> 
> Anyway, it seems to me I should increase the hardness before risking
> large pH swings with injected CO2.

You should not see very large pH swings with CO2 injection for KH above
2 or so. Dave Gomberg gave the following formula:

for KH=1, CO2(ppm)=3*10**(7-pH)

for other KH values, just multiply by KH, so
CO2(ppm)=KH*3*10**(7-pH)
which looks remarkably like the equilibrium
equation for:

     H2O + CO2 = 2H+ + CO3--

at KH = 1, 1 degree of pH corresponds to 30 ppm of CO2 which is about
.5 delta pH for an optimum CO2 of around 15 ppm. At KH of 2 you have
60 ppm of CO2 for 1 degree of pH change which gives .25 delta pH for
the optimum of 15 ppm. Almost not enough to worry about. Still, you
have to calculate the amount of sodium bicarbonate to add to your
water and you need to know your KH pretty accurately too. Anybody
know how many tsps or grams of sodium bicarbonate give 1 degree KH
for 10 liters or 10 gallons of water? Remember too that with soft
water, a small addition of sodium bicarbonate also causes a change
in pH! Maybe a better way to do it is with less soluble calcium 
carbonate? If I recall, at one time I was adding about 1/4 tsp
of baking soda to balance 10 gal of 6.5 pH tap water to about 7.
It wasn't too precise; anybody have a better figure?

Steve