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> From: Neil Frank <aquarian.subjects at mindspring_com>
> Subject: Where does the Sodium go?
> Many of us use sodium bicarbonate (aka baking soda) to boost the KH in our
> tanks. The formula is NaHCO3. If the plants require more CO2 than what is
> available (especially when the CO2 tank is empty and we dont notice it for
> a while), then some/all of the plants will generally start using the
> AVAILABLE bicarbonates.
From what I have been able to find out, this means that they can
get CO2 by:
Ca++ + 2HCO3- -> CaCO3 (solid) + H2O + CO2
> If they are in the form of bicarbonates from
> additions of CaCO3, then we will observe biogenic decalcification (ie. a
> crust or calcium marl will form on the leaves).
> Now here is my question. Suppose that we have a combination of sodium
> bicarbonates and other calcium compounds. Suppose further that the Ca
> concentrations have gone down, so the HCO3 is still mostly in equilibrium
> with the Na+. Will the plants still be able to use the HCO3- as an
> alternative source of carbon, and if so, what happens to the Na+
I'm pretty sure that they can't. The pH has to be quite high
for the calcium deposition route (to get the CO3-- concentration high
enough). To do it without the CaCO3 deposition, the CO3-- concentration
would have to go _much_ higher, and the pH would go through the roof (10+).
> - --- does the Na+ go into the plant? or the same place that butterflys go
> when it rains?
In effect, you would have to make Na2CO3, but since that is quite
soluble in water, the ions would be separate.
> - --- does the Na+ find something else to dance with, or does it stay
> attracted to Na and then is the HCO3 unable to become CO2?
In solution, the ions don't "dance" with each other. They are
effectively separate, but there must be an overall charge balance,
i.e., there are the same number of positive and negative charges in
anything other than a really microscopic volume of solution. Bulk
charge separation takes lots of energy - it doesn't happen.
> Was this because of the sodium that had no where to go.
No. The question these cases isn't where the positive ion went,
but what happened to the charge on the HCO3-.
2HCO3- -> H2O + CO2 + CO3--
Paul Sears Ottawa, Canada