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Re: Where does the Sodium go?
Neil Frank wrote:
.....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+
--- does the Na+ go into the plant? or the same place that butterflys go
when it rains?
--- 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?
Once, my tank got really out of whack after my CO2 tank was depleted. My
bolbitus never recovered and many plants were not the same for months. I
even had some algae. :-) I never tested for Ca, but it could have been
low. Was this because of the sodium that had no where to go. Did my plants
have a heart attack?
This issue of aquatic equilibrium is clearly a question for one of our
Plants capable of using bicarbonate (HCO3-) take it in, keep the CO2, and
release the remainder, which is hydroxide (OH-) ion. If there is a certain
amount of calcium bicarbonate in the vicinity of the plant, the released
hydroxide reacts with some of the bicarbonate forming water plus carbonate
ion (CO3--). The carbonate reacts with Calcium ion (Ca++) to form
insoluble CaCO3 which precipitates out on the plant. The reaction is
something like this:
Ca (HCO3)2 + OH- --------> CaCO3 + HCO3- + H2O.
If the Ca++ and Mg++ concentrations are quite low, then I suppose that some
free carbonate (CO3--) ions would accumulate in the water. Any sodium ions
also remain in the water. The pH must be quite high before the
concentration of free carbonate can become significant. Whether Ca and Mg
are present or not, as the plant consumes bicarbonate and releases
hydroxide, the pH will go up and up. Plants that utilize bicarbonate can
bring the ph up to around 11 in good light. This would be lethal for most
fish and also pretty hard on non bicarbonate utilizing plants that are not
tolerant of high pH's.
If you started off with just a solution of sodium bicarbonate in the water
and let the bicarbonate-utilizing plants do the best they can in good
light, you would wind up with a solution that still had all the sodium
ions, but in addition a lot more carbonate ions and more free hydroxide
ions. There would still be some bicarbonate ions. As the hydroxide
concentration increases, the concentration of H+ ions decreases, and, thus,
the pH decreases.
I once had a tank, lit with 4 watts per gallon, that had a lot of Eigeria
densa and a school of zebras. I had not been adding any CO2, just food for
the fish. One day I noticed that the fish were swimming weakly and
erratically and didn't seem to be breathing at all (no visible movements of
the gill covers). They also had very little interest in food. I measured
the pH, getting a value of 9.6. I immediately added CO2, bringing the pH
down to around 7.0. One of the fish died, but the other 5 or so fish
recovered and their behavior returned to normal.
Paul Krombholz, in cool, dry, central Mississippi.