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I few days back Dave Gomberg asked for opinions about the value of the
Ca++ + 2HCO3- <-> CaCO3 + CO2 + H2O
He also was surprised at the conclusion by Falkowski and Raven that this
reaction was to the net detriment of aquatic plants.
In a wider sense (aquatic plants inclusive of algae, and growing under
natural conditions) Falkowski and Raven are probably right. In an earlier
book Raven seemed to believe that HCO3- and CO2 were functionally
equivalent as carbon sources, and it sounds like that idea has carried on
to Falkowski and Raven. Given that, the reaction would be to the net
detriment of aquatic plants when it runs to the right because the reaction
causes a net loss of one dissolved carbon. That's the carbon that ends up
in the CaCO3.
That may not be a disadvantage in an aquarium. Few of us actually want
plants to use HCO3- as their carbon source because of the pH rise it
causes. Also, some of the plants we grow may not be sufficiently adapted
to submerged growth to use HCO3-. So in an aquarium there may be an
advantage to the reaction because it produces CO2 at the cost of HCO3-.
I wonder just what other effects the reaction might have. To answer my
wonderings, I considered the case of an aquarium containing 50 gallons of
water, with CO2 generated by this reaction to a total of 10 mg/l. In this
hypothetical tank if the reaction is stopped (say the CO2 generator is
turned off) the CO2 content of the water drops to 5 mg/l in 2 hours. That
strikes me as being a fairly normal behavior, and I think it means that
the reaction is adding at least 5 mg/l of CO2 every two hours. That
doesn't strike me as being very unusual. But what does it mean to other
components over the course of a week?
In a week of operation, the reation will produce 79.8 grams of CO2.
That's 1.8 moles of CO2. The reaction will also use 1.8 moles of Ca++,
3.6 moles of HCO3- and produce in addition to the CO2 1.8 moles of CaCO3
and 1.8 moles of H2O. That means the reaction would consume dissolved
calcium at the rate of 379 mg/l (53 degrees GH) per week. It would also
consume bicarbonate at the rate of 1,158 mg/l (53 degrees KH) per week.
It would produce 947 mg/l per week of CaCO3 (that's 180 grams, or 66 cubic
centimeters per week) and 170 mg/l per week of water.
Jumpin' Jimminy! Can that be right? Calcium would disapper, alkalinity
would be a dim memory and the generator would be an encrusted rock.
There must be something else going on in the Carbo-plus.