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**To**:**Aquatic-Plants at actwin_com****Subject**:**Re: CO2/KH/pH****From**:**psears at nrn1_NRCan.gc.ca (Paul Sears)**- Date: Tue, 1 Apr 1997 14:52:10 -0500 (EST)
- In-Reply-To: <199704010839.DAA04917 at acme_actwin.com> from "Aquatic Plants Digest" at Apr 1, 97 03:39:03 am

> 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. -- Paul Sears Ottawa, Canada Finger ap626 at freenet_carleton.ca for PGP public key.

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