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The solubility of calcium compounds is extremely poor. The exceptions are calcium halides (i.e. CaCl2 for aquarists). As the mineral composition of aragonite is calcium carbonate, it's insoluble.[...] I think it is safe to say that while [aragonite] doesn't dissolve at the pH of our systems, it has a solubility so it contributes to the ionic balance of the tank.
Just to play devils advocate, this thread was about best substrate to maintain stable pH ie. less change not pH8.0-8.2. Aragonite is by far the best choice to maintain stable pH as it is the best buffer out of all the substrates mentioned. it will just help maintain a stable pH at 7.4. Starboard and styrofoam have no affect on buffering capacity and therefore they do not draw the pH down, however they do not maintain it at high levels either. This lack of affect could be attributed to the scientific results mentioned. This means that whichever substrate is the best buffer maintains a stable pH the best. BTW the only time I ever measure pH is when I am acclimating inverts...Like I said above - I too would have said aragonite. I think it is safe to say that while it doesn't dissolve at the pH of our systems, it has a solubility so it contributes to the ionic balance of the tank. I also think its safe t to say that the pH of our systems varies considerably depending on where we take the measurement. Is the pH the same an inch down in your substrate? In my system, the return water flows up through the sand-bed, so the pH is close to the main body of water, but a foot away next to the sump, the dephosphate/denitrate chamber is 7.5 (or less).
The action of a buffer is to resist a change in pH away from a natural balance point. It does this by either pulling in a H+ ion or liberating a H+ ion depending on which side of the balance point the pH of the solution is. So if you add an acid (net gain of h+), the buffer sucks up H+, if you add a base, the buffer disassociated and frees a h+ ion...always around the point where pH=pK for the buffer. We have many such buffers in our tank. Its the same principle by which ammonium NH4 turns into ammonia NH3 in base environments (releases a H+ ion) and turns back into ammonium in acid (grabs hold of H+ ions). Its a buffer too.
The carbonate system in a tank with an aragonite substrate has a natural buffer balance point around pH7.5. So as your trying to push the pH up, the ions liberated from the aragonite are trying to pull it down,
Frank - your right to question of all the various studies out there - you have to read carefully to see what they are really trying to tell you. Its easy to draw wrong conclusions, but your statement that you have a thick aragonite bed coupled with a picture of your pH controller says volumes. If aragonite kept your pH stable why the expensive monitor? I bet your dosing like crazy to keep your pH up - I am.
I'm old. I kept saltwater tanks in the 70's when quartz substrate was more the rule than the exception. We mostly only did fish, but I don't remember ever really worrying about pH. I'd test once a month or so and it was almost always good. Very rarely did I have to add any buffer to the system. So when I read the article - it resonated with my own experiences.
As one more thing to consider - bicarbonate is one of the primary buffers of the human blood stream - its natural pH balance point - 7.4.
Just to play devils advocate, this thread was about best substrate to maintain stable pH ie. less change not pH8.0-8.2. Aragonite is by far the best choice to maintain stable pH as it is the best buffer out of all the substrates mentioned. it will just help maintain a stable pH at 7.4. Starboard and styrofoam have no affect on buffering capacity and therefore they do not draw the pH down, however they do not maintain it at high levels either. This lack of affect could be attributed to the scientific results mentioned. This means that whichever substrate is the best buffer maintains a stable pH the best. BTW the only time I ever measure pH is when I am acclimating inverts...
Calcium reactors work because calcium carbonate solubility increases in acidic environments. AZDesertRat shared this with us in his post a few before mine. Essentially, CO2 (gas) + H2O (liquid) = H2CO3 (acid).I won't pretend to understand the Carbonate mineral solubility in the Na-K-Mg-Ca-H-Cl-SO4-OH-HCO3-CO3-CO2-H2O system. But I would have to ask how calcium reactors work if crushed coral is so stable - or how sulphide oxidizing bacteria free calcium from aragonite rock while consuming alkalinity - sulphide reducing bacteria do the same or how organics change the precip points of aragonite over reefs. Isn't it a mistake to overlook biological activity?
How would you otherwise explain the study cited?
Again, straight CaCO3 solubility is a fraction of a percent. RHF wrote an article (link) in Advanced Aquarist that offers an alternate explanation. To paraphrase: aquarium water is supersaturated with calcium and carbonate. When you add a bit of solid calcium carbonate (i.e. aragonite), it causes calcium carbonate to precipitate on the surface (ever grow crystals as a kid?). This crystal growth consumes alkalinity from the water column, reducing buffering capacity. As dolomite is a "mixture" of calcium/magnesium carbonate, it - likewise - is a surface where precipitation can occur. Since the inert substrates are not carbonate based, they do not provide the same surface for carbonate precipitation. As they do not consume the buffer, the authors of the article experienced less need for maintenance. Hence, starboard and foam look like "better" substrates.It is evident from our experience and the understanding of buffers described above that the addition of calcium ions in the form of carbonates (lime, crushed coral) will ultimately lead to a pK drop, and ultimately, to a buffer at pH 7.6.
It looks natural, helps in nitrogen fixation, and reflects a bit of light, I suppose. The value of an aragonite sandbed is in the sandbed, not the aragonite. These are mostly just my opinions, though. Sorry for the long post.Finally - If aragonite is inert in the reef tank - what is its value?
Calcium reactors work because calcium carbonate solubility increases in acidic environments. AZDesertRat shared this with us in his post a few before mine. Essentially, CO2 (gas) + H2O (liquid) = H2CO3 (acid).
For CaCO3 in neutral water, you can get a miniscule amount of calcium to dissolve. Even though some of the carbonate will form bicarbonate, driving the solvation of CaCO3 further, it's still a super-small amount that will be dissolved. Randy Holmes-Farley (hereafter, "RHF") suggests that a little aragonite sand in your freshwater reservoir could have the tiniest of effect this way, but he is quite clear that aragonite will not dissolve in the reef aquarium.
To attempt to explain the article, let me first say this disclaimer: I am an armchair chemist -- I could be wrong on some of these points and will happily take correction. Now, it seems to me that the investigators misattributed their finding to the release of calcium ions released from crushed coral substrate. Again, straight CaCO3 solubility is a fraction of a percent. RHF wrote an article (link) in Advanced Aquarist that offers an alternate explanation. To paraphrase: aquarium water is supersaturated with calcium and carbonate. When you add a bit of solid calcium carbonate (i.e. aragonite), it causes calcium carbonate to precipitate on the surface (ever grow crystals as a kid?). This crystal growth consumes alkalinity from the water column, reducing buffering capacity. As dolomite is a "mixture" of calcium/magnesium carbonate, it - likewise - is a surface where precipitation can occur. Since the inert substrates are not carbonate based, they do not provide the same surface for carbonate precipitation. As they do not consume the buffer, the authors of the article experienced less need for maintenance. Hence, starboard and foam look like "better" substrates.
It looks natural, helps in nitrogen fixation, and reflects a bit of light, I suppose. The value of an aragonite sandbed is in the sandbed, not the aragonite. These are mostly just my opinions, though. Sorry for the long post.