Reef Chemistry Question of the Day #33 Calcium Carbonate Solubility

[Randy Holmes-Farley]

Reef Chemistry Question of the Day #33

Which of the following does not increase the solubility of calcium carbonate in seawater?


A. Reducing the pH with carbon dioxide
B. Reducing the pH with hydrochloric acid
C. Increasing the pressure
D. Decreasing the pressure




Good luck!



















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[Montireef]

I think C and D

 

[H2O]

I got $20 on A lol

 

[Montireef]

You lost your money ...

 

[DFW]

D!!

 

[Steven@]

A,b,d

 

[acromancer]

I'm going with D because an increase in pressure should increase the density therefore making it harder for solutes to stay in solution.

 

[Mat167]

Both A and B will increase hydronium ion concentrations, allowing for a better dissolution of calcium carbonate. I'm not sure what is meant by varying the pressure though, since we're talking about an aqueous medium, which is non-compressible.

 

[Randy Holmes-Farley]

Both A and B will increase hydronium ion concentrations, allowing for a better dissolution of calcium carbonate. I'm not sure what is meant by varying the pressure though, since we're talking about an aqueous medium, which is non-compressible.

To clarify, an incompressible fluid can be put under pressure. It just doesn't change volume.

For example, for answer C (increased pressure), imagine the conditions 1 mile down in the deep ocean. For answer D (decreased pressure), imagine the seawater under 0.5 atmospheres (such as at high altitude) instead of 1 atmosphere of atmospheric pressure.

 

[spartanman22]

D. Decreasing Pressure

 

[spartanman22]

Higher pH will push supersaturation of calcium carbonate to unusually high levels causing precipitation. Increased pressure will drive the dissolution of undissolved calcium carbonate (due to volume change caused by increased density of water molecules surrounding ions vs pure water). As a result solubility of calcium carbonate will be slightly greater at ocean depths than the surface.

 

[Mat167]

To clarify, an incompressible fluid can be put under pressure. It just doesn't change volume.

For example, for answer C (increased pressure), imagine the conditions 1 mile down in the deep ocean. For answer D (decreased pressure), imagine the seawater under 0.5 atmospheres (such as at high altitude) instead of 1 atmosphere of atmospheric pressure.

Okay then, that makes a lot more sense! I guess that's what I get for trying to think at 4 am

I'm thinking that putting it under pressure will precipitate it out easier then. So I'll go with C then.

 

[mainereefer]

I say D

 

[Randy Holmes-Farley]

And the answer is......D. Decreasing the pressure. Good job folks, this was a tough one!

It is also an important one.

A and B reduce the pH, convert the carbonate into bicarbonate or carbonic acid, and more calcium carbonate dissolves by Le Chatlier's Principle:

CaCO₃ <----> Ca⁺⁺ + CO₃^--

Remove CO₃^-- and the equilibrium shifts to the right, dissolving more CaCO₃. That is how CaCO₃/CO₂ reactors work, and how acids can clean pumps and such with calcium carbonate deposits.

The pressure is also very important in the ocean. I give details on that effect here:

http://www.advancedaquarist.com/2002/3/chemistry

One interesting aspect of calcium in seawater is that the calcium concentration can be higher in deep ocean water than in surface water. In the Pacific, for example, deep ocean water contains about 1% more calcium than surface water. The reason for this increase is that calcium carbonate becomes much more soluble at high pressure, preventing precipitation of calcium carbonate, and even permitting dissolution of calcium carbonate particulates formed higher in the ocean that settle into the depths.

The reason for this solubility change with pressure is fairly esoteric. When calcium and carbonate ions are dissolved in water, a number of water molecules become tightly attached to the ions. This hydration is discussed in detail in the following section. Overall, however, the volume occupied by undissolved calcium carbonate and water is larger than the volume occupied by calcium and carbonate ions dissolved in this same amount of water. This volume change is primarily due to the increased "density" that can be attained by water molecules around ions compared to pure water. When this dissolution is taking place under the tremendous pressures on the bottom of the ocean, the dissolution can actually be driven by the pressure, because of the volume change involved, resulting in greater solubility.