11-19-2012, 06:49 PM
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Join Date: May 2007
Location: Calgary
Posts: 2,461
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Have you taken in account the calcium bicarbonate precipitation that will occur with highly elevated levels of both calcium and HCO3-?
I have a feeling your pump would kick out on you in the span of a day with that much abiotic precipitation.
Quoted from Randy Holmes-Farley here:
Quote:
When Ω = 1, the solution is exactly saturated. When Ω exceeds one, it is supersaturated, and when Ω is less than 1, the solution is undersaturated. The higher the supersaturation, the more likely precipitation of CaCO3 will take place.
In normal seawater, Ω ~ 3 for aragonite and Ω ~ 5 for calcite, though these values have been steadily dropping as carbon dioxide has been added to the atmosphere, reducing the seawater's pH. Aragonite and calcite are just different crystalline forms of calcium carbonate. Calcite is slightly more stable, and hence slightly less soluble, than aragonite (i.e., has a lower Ksp*). Organisms can precipitate both aragonite (pteropods and corals) and calcite (foraminifera and coccoliths), but most of the precipitation in reef aquaria is aragonite (although certain organisms such as abalone form both).
Reef aquaria often have higher alkalinity and calcium levels than seawater, and hence are more supersaturated than seawater. Alkalinity is a measure of the bicarbonate and carbonate in solution. At a fixed pH, if the alkalinity is doubled, then the carbonate will also be doubled. Since many aquarists keep reef aquaria at alkalinity levels higher than natural seawater levels (2.5 meq/L; 7 dKH), the supersaturation is often higher than in the ocean.
The biggest driver of supersaturation in a reef aquarium, however, might be pH. In aquaria with a high pH (such as many aquaria using limewater) the supersaturation is much higher than in seawater. At the same alkalinity, if you raise the pH, you convert some of the bicarbonate into carbonate:
(7) HCO3 - + OH- → CO3- - + H2O
At pH 8.2 and 25°C, only 15% of the total carbonate and bicarbonate is present as carbonate. At pH 7.8, that value drops to 7%. But as the pH is raised, that portion increases to 50% at pH 8.93 and to 75% at pH 9.4. Consequently, as the pH is raised at a fixed alkalinity, the concentration of carbonate rises, thereby increasing the supersaturation of calcium carbonate. Within the pH range of most reef tanks (up to about pH 9 or so), the amount of carbonate present is approximately linear with the pH because of the relationship seen in equation (7). So if the pH rises from 7.5 to 8.5, there is approximately a ten-fold increase in the carbonate concentration. From pH 8.0 to 8.5, the increase in carbonate is about threefold. Above pH 9, the carbonate concentration continues to rise, but more slowly, and it levels off above about pH 10 as there is very little bicarbonate left at pH 10+ to convert into carbonate.
Calcium Carbonate Precipitation: Calcium, Alkalinity, and pH
Combining the various factors described above, here are some combinations of calcium, alkalinity and pH that have equal supersaturation (that is, equal propensity to cause calcium carbonate precipitation):
Ω = 1 (dissolution of aragonite takes place at all lower values of these parameters)
pH = 7.7
pH = 8.2
Calcium = 410 ppm
Calcium = 340 ppm
Alkalinity = 2.5 meq/L
Alkalinity = 1.0 meq/L
Ω = 3 (typical of normal seawater)
pH = 8.2
pH = 8.0
pH = 8.4
Calcium = 410 ppm
Calcium = 410 ppm
Calcium = 260 ppm
Alkalinity = 2.5 meq/L
Alkalinity = 4.0 meq/L
Alkalinity = 2.5 meq/L
Ω = 6 (non-biological precipitation is more likely)
pH = 8.2
pH = 8.2
Calcium = 410 ppm
Calcium = 820 ppm
Alkalinity = 5.0 meq/L
Alkalinity = 2.5 meq/L
pH = 8.0
pH = 8.7
Calcium = 410 ppm
Calcium = 410 ppm
Alkalinity = 8.0 meq/L
Alkalinity = 2.5 meq/L
pH = 8.45
Calcium = 410 ppm
Alkalinity = 4.2 meq/L
How should we think about supersaturation? The higher it is, the more likely it is that calcium carbonate will precipitate. The reason for this is straightforward: if the "pressure" to precipitate calcium carbonate becomes too high, certain inhibiting processes (described below) will be overwhelmed, and precipitation will take place.
If Ω is not too high, some precipitation will take place before the inhibiting mechanisms take control of the crystals' surface and prevent further precipitation. This is the process that happens in normal seawater. If Ω is too high, a bigger precipitation event can take place before being halted. In the worst cases, this can lead to a snowstorm of calcium carbonate particulates throughout the tank. Such snowstorms can occur, for example, when too much limewater is added to the tank. In that case, the pH rises and converts much of the bicarbonate to carbonate. Ω is then driven to unstable levels, and a massive precipitation event takes place.
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