# Re: CO2/KH/pH

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> From: George Booth <booth at hpmtlgb1_lvld.hp.com>
> Subject: Re: CO2/KH/pH Questions
>
> > The next sentence talks about calcium precipitating out of the water
> > if free CO2 falls.  Will calcium precipitate at 15 dKH if free CO2
> > falls below 50 mg./l?  How does carbonic acid fit into the equation?
>
> I think there are some generalizations here.  If a lot of the KH is
> due to calcium carbonate (instead of sodium bicarbonate, for example),
> the water may be saturated with calcium carbonate and that saturation
> point is partly determined by the amount of dissolved CO2 (more CO2,
> higher saturation point).  If CO2 drops, the saturation point drops
> and calcium carbonate comes out of solution.
>
The solubility product of CaCO3 is pretty low, meaning that
the limit of the concentrations of the two ions Ca++ and CO3-- multiplied
together is low.  If the product of the two concentrations goes above
that, then CaCO3 precipitates out.

I've done some rough calculations to provide an example:

For a solution with GH and KH 4, the concentration of Ca++ ions
is about 0.71 x 10^-3 molar.  The solubility product of CaCO3 is about
10^-8, so the maximum allowable CO3-- concentration is about 1.4 x 10^-5 molar.
The HCO3- concentration is close to 1.43 mM (double the Ca++ concentration),
so the second ionization constant of carbonic acid gives us the max. pH (about
8.3) from the ratio of CO3-- to HCO3-.  From the pH, the first ionization
constant of carbonic acid gives us the CO2 concentration: about 0.7 ppm.

The pH controls whether the CaCO3 comes down, but we are controlling the
pH by varying the CO2 concentration.

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