Reef Chemistry Question of the Day #84 CaCO3/CO2 reactors

[Randy Holmes-Farley]

Reef Chemistry Question of the Day #84

Calcium carbonate reactors work by adding carbon dioxide to reef tank water in the presence of calcium carbonate chunks.

Describe the mechanism whereby the carbon dioxide allows the calcium carbonate to slowly dissolve and deliver calcium and alkalinity to the aquarium.

Good luck!





















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[tonizzy22]

Disolved carbon dioxide in water becomes carbon acid. The acid disolves the media away. The more co2 the more acidic it becomes the lower the ph will be then it starts to disolve the calcium carbonate that release the minerals back into the water.

 

[Randy Holmes-Farley]

OK, but why does low pH dissolve the media?

 

[Frozn]

What is calcium reactor.

Unless your looking for a more elaborate answer?

 

[Frozn]

Because cabonite saturation is lower at a lower ph which causes the coral skeletons (carbonate chunks) to dissolve.

 

[beaslbob]

Magic

 

[beaslbob]

OK, but why does low pH dissolve the media?

because it's more acid?

 

[Damon]

Mechanism? Hmm.. I think it's via acidification(pH manipulation) but I'm probably missing it..

The carbon dioxide reacts(infuses) with the water, lowering the pH, this in turn causes the chunks to break down into its simpler elements? I think it is the electrons from the carbon dioxide are easy to jump to and from. Nah, lol.. I can remember what i read on it.. Lol

Basic, but??

 

[Randy Holmes-Farley]

Trying to get at WHY low pH would dissolve calcium carbonate.

What actually happens on a molecular basis.

 

[beaslbob]

redox?
electron transfer?
missed that day in chemistry class?
oxidation-reduction?
woops that's redox

 

[H2O]

Not everything has to make sence I'm more confused about all of the twin towers concrete turning to baby powder on 911 200 metal and concrete floors turned to ground zero aka hole in the ground it took 5 months to clean up the site and 444 days to start investigation so my question is how did 160 bilion worth of gold evaporate from the basement volt if the fire burned only upstairs

 

[itsforte]

Le Chatelier's principle

H2CO3 <=> H(+) + HCO3(-)
HCO3(-) <=>H(+) + CO3(2-)

in acid solution, reaction will trend to left side, and reduce [CO3(2-)]

CaCO3 <=> Ca(2+) + CO3(2-)

that causes the reaction trend to right side, more CaCO3 resolves into Ca(2+) and CO3(2-)

 

[Randy Holmes-Farley]

And the answer is... Carbon dioxide lowers pH, which in turn lowers the concentration of carbonate, which is a controlling factor in the dissolution of calcium carbonate

itsforte's way of looking at it via Le Chatlier's Principle is a fine way to show the equilibrium effect. Good job!

In the case of dissolving calcium carbonate in a CaCO3/CO2 reactor, I think there are two effects, a kinetic one and and equilibrium one. The question was really aimed at the equilbrium effect, but I'll discuss the kinetic one as well.

First, kinetics. That is the speed that something happens. In this case, the speed that calcium carbonate dissolves.

Let's take a different example: pure calcium carbonate in pure fresh water. There is no carbonate present yet in the water, so the driving force is toward dissolution, and dissolution "should" be fairly fast, but it appears to be rather slow. Why?

Now take the same calcium carbonate in strong acid. It can dissolve almost instantly in a burst of CO2 bubbles. The initial carbonate concentration was equally low (same driving force), but it dissolved faster. Why?

There could be two reasons.

One is that once a little dissolution takes place in pure water, there is carbonate (and calcium) in the water near the solid surface, and the "off rate" of ions coming off the surface is quickly in competition with the "on rate' of calcium carbonate going back onto the surface. With a lot of acid around, there never is a substantial "on rate" since there never is carbonate around.

The second way has to do with a possible mechanism of ripping carbonate out of the solid structure of the calcium carbonate. At the surface of the solid, some carbonate is exposed. Adding acid protonates this exposed carbonate into bicarbonate or carbonic acid, making it no longer attracted to the surface. So the solid falls apart a lot faster.

The second one is, I think, the primary reason that a small chunk of calcium carbonate dissolves a lot faster in acid than in pure water, and it may be a substantial part of way lowering the pH in a CaCO3/CO2 reactor speeds the dissolution of calcium carbonate.

 

[Randy Holmes-Farley]

So, now to the equilibrium effect, which Le Chatlier's principle relates to.

itsforte's analysis showed how reducing the carbonate concentration causes the equilibrium to shift to the right, to replace the "lost" carbonate, causing more dissolution.

CaCO₃ <----> Ca⁺⁺ + CO₃^--


An alternative way of looking at the same process is to look at the guiding solubility product constant:

Ksp = [CO₃^--]x[Ca⁺⁺]

where Ksp is the solubility product constant of calcium carbonate in seawater, [CO₃^--] is the concentration of carbonate, and [Ca⁺⁺] is the concentration of calcium.

The reaction proceeds (either by dissolution or precipitation of calcium carbonate) until the conditions of the Ksp equation si exactly met (or the solid has fully dissolved and can't dissolve any more).

So lowering the carbonate concentration by adding CO₂ puts the product [CO₃^--]x[Ca⁺⁺] below the Ksp and dissolution proceeds until the Ksp is reached again. That is attained after more dissolution, raising calcium and alkalinity in the effluent.

The equilibrium argument onyl states what MAY happen if enough time is allowed, but not that it will happen. Wood (like a tree) in air "should" become CO2 and water, but it is very slow to do that at room temperature. Same for calcium carbonate: the equilibrium only tells you what should happen, not how fast it will actually happen.

Hope that all made sense!

Happy Reefing.