Just found this. Ccapt, according to this you are wrong.
Marine Reef Aquarium Handbook - Google Books
I read that article. Unless I missed something, where does it say what I said was wrong?
I know your not into reading these articles, but here ya go.
Chemistry and The Aquarium
Dissolution of CaCO3
in Aquaria
If calcium carbonate is supersaturated in marine aquaria, how can it also dissolve? The answer lies in the fact that while the water column is supersaturated, other parts of the tank may not be. Specifically, the interstitial water of sand and rock if often lower in pH than the water column. For example, if I put a pH probe into my oolitic aragonite sand bed, I get a pH in the 7’s, when the water column itself has pH = 8.4.
The reasons for the pH being lower in the sand are beyond the scope of this article, but it relates to the breakdown of organics (and some nitrogen compounds). Both aerobic and anaerobic oxidation of organics in seawater can lead to the production of acid, especially carbonic acid derived from CO2. This
link is to a thread that shows some of the reactions that can (and cannot) produce acid in sand beds.
As the pH is lowered, the equilibrium between carbonate and bicarbonate shifts towards bicarbonate (i.e., shifts to the left in equation 15):
(15) HCO3-
ßà H+ + CO3--
This shift considerably lowers the carbonate concentration. Using equation 8, one can calculate that the concentration of carbonate drops by about a factor of 3 for a pH drop of 0.5 and by a factor of 10 for a full pH unit drop. Consequently, aragonite first becomes soluble in seawater when the pH drops below about 7.7 (this value might be more like 7.5-7.7 in reef tanks where the alkalinity is often higher than in seawater). That level is attained in some sand beds, and permits the dissolution of some of the sand.
The rate of dissolution is fairly low, however, because the rate of delivery and degradation of organics (or certain nitrogen compounds) deep enough in the sand to permit a pH drop is fairly low. The rate will, however, vary from tank to tank as the different ways of delivering organics to deeper parts of the sand will vary (diffusion; movement by organisms; death of organisms, etc). Note that the need to oxidize the organics in deeper parts of the sand to permit dissolution of the sand has nothing to do with the oxygenation of the sand. It has more to do with the fact that at near surface regions of the sand, the pH will be closer to that of the tank water by acid and base transfer from the water column, and you need to be deep enough to permit a lower pH to become established.
i think many people think it dissolves but in reality it is being taken out little by little every water change. sand will not dissolve unless the ph is very low. in a very deep sand bed as far as i know the ph has a chance to get that low but that i dont know for shure.
Simple yet accurate statement. :bowdown: What may dissolve deep within a sand bed precipitates back out of solution because of the high concentration of carbonates in a small area. Sort of like a mini snow storm (precipitation event) deep in the sandbed.
