Reef Chemistry Question of the Day #226 Solubility of Calcium Carbonate

Randy Holmes-Farley

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Reef Chemistry Question of the Day #226

If I wanted to dissolve additional solid calcium carbonate into a water sample, which of the following solutions would dissolve the most, assuming I waited long enough for the solution to reach equilibrium with normal air?

A. Pure fresh water at 20 deg C
B. Pure fresh water at 80 deg C
C. 35 ppt normal seawater at 20 deg C
D. 35 ppt normal seawater at 80 deg C?

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Orm Embar

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B; seawater already has calcium and bicarbonate in solution, which pushes the equation towards precipitation (and the water has a slightly higher pH, which would also promote precipitation). Warmer solvents tend to solubilize more of a salt if I remember my chemistry correctly.
 

BrandonP

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My best educated guess is that due to ions already in saltwater CaCO3 shift to equilibrium will be much less than the shift that will occur in pure water since there are no ions present initially. Higher temperature should help with solubility. Also assuming saltwater has a pH around 8 and pure water will have a neutral pH of 7, CaCO3 will dissolve in better in a lower pH solution.

I believe the answer is B!
 

tgionet

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It appears that CaCO3 has lower solubility as temperature increases which would make the correct answer A. Though solubility also increases with an increase in salinity it requires a lower PH than would be found in standard seawater.
 

Beardo

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I chose A because the solubility of carbon dioxide increases with a decrease in water temp. With additional CO2 there should be more carbonic acid and resultant decrease in pH allowing more calcium carbonate to dissolve. I ruled out saltwater since the alkalinity in saltwater should buffer against a large pH drop.
 
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Randy Holmes-Farley

Randy Holmes-Farley

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And the answer is...

If I wanted to dissolve additional solid calcium carbonate into a water sample, which of the following solutions would dissolve the most, assuming I waited long enough for the solution to reach equilibrium with normal air?

A. Pure fresh water at 20 deg C

Normal seawater at any temperature is supersaturated with respect to calcium carbonate. That is, no more can dissolve into it. That is why coral skeletons are stable in the ocean. So answers C and D are incorrect.

In fresh water, the solubility of calcium carbonate decreases with increases in temperature. Thus, at a lower temp of 20 deg C the solubility is higher than at 80 deg C.

The same temperature effect translates into seawater. Even though it is supersaturated at all temperatures, it is even "more" supersaturated at higher temperatures, which is why calcium carbonate precipitates onto heaters and pump impellers.

The "why" of this is also interesting. In seawater, part of the reduced solubility with increasing temp is the same as in freshwater, and part fo the explanation is that at higher temperatures, more of the bicarbonate dissociates into carbonate and H+:

HCO3- ---> H+ + CO3--

and that increase in carbonate drives the apparent supersaturation of calcium carbonate even higher. I show that mathematically in the article linked below:

Chemistry and the Aquarium: Calcium ? Advanced Aquarist | Aquarist Magazine and Blog
http://www.advancedaquarist.com/2002/3/chemistry

from it:

Comparing Solubility and Acidity Changes on Supersaturation of Calcium Carbonate
Comparing the increase in supersaturation due to solubility and acidity changes between 25 and 40 °C for aragonite, we find a change from W = 3.0 to 3.3 due to solubility, and from 3.0 to 4.4 due to the bicarbonate pKa shift. Together these effects yield a supersaturation of 4.8 for aragonite.

Similarly for calcite, the solubility change between 25 and 40 °C causes an increases in W from 5.0 to 5.2 for the solubility change and from 5.0 to 7.3 for the bicarbonate pKa shift. Together these effects yield a supersaturation of 7.5 for calcite.

Comparing the increase in supersaturation due to solubility changes between 25 and 80 °C, we find a change from W = 3.0 to 5.4 due to solubility, and from 3.0 to 7.2 due to the bicarbonate pKa shift. Together these effects yield a supersaturation of 13 for aragonite.

Similarly for calcite, the solubility change between 25 and 80 °C causes an increases in W from 5.0 to 6.8 for the solubility change and from 5.0 to 12 for the bicarbonate pKa shift. Together these effects yield a supersaturation of 16.3 for calcite.

What do these values really mean? The following combinations of calcium and alkalinity have the same supersaturation in seawater:

  1. Normal seawater at 80 °C
  2. Seawater at 25 °C with calcium raised to 1300 ppm
  3. Seawater at 25 °C with the alkalinity raised to 8.2 meq/L
It makes sense that all three of the situations above could lead to precipitation of calcium carbonate, and that is exactly what happens on the surfaces of hot objects in our tanks.

Happy Reefing. :)
 

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