Calcium in supersaturation

[Gustavo Duarte]

If calcium is in supersaturation, how it is possible to find 410 ppm of calcium in open oceanic waters? Why it don't precipitate? The poisoning of magnesium answer these question?

If i leave a glass of supersaturated calcium salt water, what is the expected value of calcium after a couple days? Why it don't precipitate?

Is is the same, why in aquarium without consumption (corals, Neogoliolithum algae and other calcareous organisms) the calcium levels tends to drop? Proteins "quelate" calcium?

 

[randy holmes-farley]

If calcium is in supersaturation, how it is possible to find 410 ppm of calcium in open oceanic waters? Why it don't precipitate? The poisoning of magnesium answer these question?


The answer is complicated. Whenever calcium carbonate begins to precipitate (which happens very slowly in the ocean; faster in a reef tank where things like heaters drive up the supersaturation in a reef tank), the growing calcium carbonate surface becomes clogged with phosphate, magnesium, and organics. This process changes the surface, and calcium and carbonate are no longer as attracted to the surface to precipitate.

So the ocean is in what one would call a metastable state: it is not the thermodynamically stable state (which would have much lower levels of calcium and carbonate), but it can be stable indefinately for the reasons given above. If you have a glass of seawater or tank water, chemical precipitation will not likely take place in a few days.

 

[CraigBingman]

If calcium is in supersaturation, how it is possible to find 410 ppm of calcium in open oceanic waters? Why it don't precipitate? The poisoning of magnesium answer these question?

If i leave a glass of supersaturated calcium salt water, what is the expected value of calcium after a couple days? Why it don't precipitate?

Is is the same, why in aquarium without consumption (corals, Neogoliolithum algae and other calcareous organisms) the calcium levels tends to drop? Proteins "quelate" calcium?

Supersaturation of calcium carbonate in near-surface waters of the ocean is driven largely by CO2 depletion by photosynthesis. Deep ocean waters (below the carbonate lysocline) are undersaturated and calcium carbonate dissolves there.

The reason why ocean surface waters can persist in this metastable state are largely kinetic. Nucleation of calcium carbonate particles is slow. There aren't many exposed calcium carbonate surfaces that aren't coated with organics.

The DOC level in most ocean water isn't what keeps calcium carbonate in solution. Most of the calcium present is free ions. There is some ion pairing with major anions, but organic complexation is an even smaller *miniscule* fraction. The influence of organics is mainly at calcium carbonate surfaces.

In some parts of the ocean, calcium carbonate actually will spontaneously precipitate from solution on a very large scale. It happens fairly frequently in the Bahama Banks, for example, where the water gets quite hot and is concentrated by evaporation.

Craig Bingman