Reef Chemistry Question of the Day #109 CaCO3/CO2 Reactors

Randy Holmes-Farley · Mar 11, 2015

Reef Chemistry Question of the Day #109

In a calcium carbonate/carbon dioxide reactor that is used to supply calcium and alkalinity to a reef aquarium, what is the molecular level function of the carbon dioxide?

A. It increases the concentration of magnesium, thereby increasing the solubility of the solid media
B. It increases the concentration of carbonate, thereby increasing the solubility of the solid media
C. It reduces the concentration of bicarbonate, thereby increasing the solubility of the solid media
D. It reduces the concentration of carbonate, thereby increasing the solubility of the solid media

Good luck!

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redfishbluefish · Mar 11, 2015

Gee whiz Randy, you always get me scratching my head and thinking about stuff I'd otherwise never think about. I'm not a calc reactor user and therefore never gave much thought to the molecular level stuff. My simplistic thought was CO2 dropped the pH which dissolved the calcium carbonate "rocks." So with your query, I looked for CO2 dropping pH....and that choice wasn't there, verbatum.

So after scratching my head, and now thinking with a bit more chemistry in mind, the CO2 drops the pH by forming carbonic acid (carbonate).....so the molecular choice would be B, It increases the concentration of carbonate, thereby increasing the solubility of the solid media.

OK, I'm done using my brain today.....let me go back to thinking simple thoughts.

beaslbob · Mar 11, 2015

It decreased pH increasing the solubility of the media.As a wild guess
c.

JimWelsh · Mar 11, 2015

But, redfishbluefish, I think you may have made a mistake. There are basically three species of carbonate in aqueous solutions: Carbonate (CO3--), bicarbonate (HCO3-), and carbonic acid (H2CO3). The distribution of these three species is dependent on the pH of the solution. As the pH drops, carbonate is converted into bicarbonate, and then as the pH drops even further, bicarbonate is converted into carbonic acid. At the pH at which most calcium reactors are run, there is still plenty of bicarbonate in solution, but no more carbonate. I say the answer is D -- the removal of carbonate is what makes the calcium carbonate solid media more soluble.

redfishbluefish · Mar 11, 2015

Curse you JimWalsh.....I'm retired and have already exceeded by brain usage for the day. :thumb:

Let me review.....molecular level function of carbon dioxide.....what does carbon dioxide do in water?

The simple formula is: CO2 + H2O --> H2CO3

and as you stated, dependent on pH, it might be carbonic acid, bicarbonate or carbonate. Now maybe it's a matter of semantics, but in any case the carbon dioxide INCREASES one, two or all three of these "carbonate" entities. My guess is that bicarbonate and hydronium are most likely the two guys present, with the bicarbonate a weak base, and hydronium "acid" beating up and solubilizing the calcium carbonate "rock."

Now I could be wrong, because I took chemistry years ago, but either way, I'm done.....I need a nap!

beaslbob · Mar 11, 2015

whatever you two guys come up with looks like my wild guess of bicarbonate is wrong.
So another wrong guess on my part. :cry:

JimWelsh · Mar 12, 2015

Since Randy specifically asked "what is the molecular level function" of CO2, I'll express my thoughts this way: CO2 + H2O = H2CO3. H2CO3 is a diprotic acid. To the degree that there is carbonate (CO3--) in solution, then the carbonic acid molecule can give up a proton to neutralize a carbonate ion, with each becoming a bicarbonate ion in the process: H2CO3 + CO3-- = 2 HCO3-. In terms of the question as asked, the reduction in the carbonate concentration is what is happening, I think.

EDIT: That is, I believe, a different way of saying that the addition of the CO2 lowered the pH, and thus, changed the distribution of the different species to one where carbonate is either greatly reduced or nonexistent.

Randy Holmes-Farley · Mar 12, 2015

Think of the problem this way...

The solubility of calcium carbonate is a balancing act between calcium and carbonate landing on the surface and sticking, and calcium and carbonate ions desorbing from the surface into solution.

How does CO2 impact that specific process?

redfishbluefish · Mar 12, 2015

I'm not sure where you want us to go. Unless I'm totally out in left field, we've shown the simplified reaction of CO2 to carbonic acid. This most likely, at the pH we are talking about, is in the form of bicarbonate and hyrdonium (and yes, some carbonate and some carbonic acid, pH dependent). This proton then "reacts" with the calcium carbonate producing yet another bicarbonate. So the addition of CO2 to a calcium reactor causes for an increase in bicarbonate. But I don't see this as one of your choices!......unless in B you're simply talking about carbonate in general.

Your Honor, my final agruement, if it would so please the court, is that if the addition of CO2 did not INCREASE some carbonate species, than how could it contribute to the increase in the alkalinity of your tank. CO2 has to cause for an increase of something for there to be an increase in alkalinity....and the only choice that shows INCREASE (other than the Mag choice, which is out), is choice B. I rest my case!

And finally, to quote Edith Ann, "And that's the truth.....pbbbbbbbt."

JimWelsh · Mar 12, 2015

I maintain that the addition of CO2 to the water has a net effect of dropping pH, which reduces the carbonate concentration, which makes the carbonate (and calcium along with it) in the solid media more likely to, as Randy put it, desorb from the surface into solution. The increase in alkalinity comes not directly from the addition of CO2, but rather, from an increase in the availability of protons, which ultimately causes the carbonate in the media to go into solution. Adding CO2 to water drops the pH, but does not affect the alkalinity. Please see: https://www.reef2reef.com/forums/re...eef-chemistry-question-day-59-soda-water.html

redfishbluefish · Mar 12, 2015

Jim, I'm at a loss. After reading the link provided, it would appear that D would be the correct answer. My little pea-brain never saw the dissociation of the calcium carbonate to be caused by the reduction in the carbonate present. I always looked at it as the presence of the acid....the protons....that reacted with the calcium carbonate to knock it apart. I can't wrap my little brain around that.

Looking at fractional alpha graphs doesn't help me either.

I think I'll go back to scratching my head and talking to my fish!

Randy Holmes-Farley · Mar 12, 2015

redfishbluefish said:
Jim, I'm at a loss. After reading the link provided, it would appear that D would be the correct answer. My little pea-brain never saw the dissociation of the calcium carbonate to be caused by the reduction in the carbonate present. I always looked at it as the presence of the acid....the protons....that reacted with the calcium carbonate to knock it apart. I can't wrap my little brain around that.

If instead of calcium carbonate we were talking about the solubility of sodium chloride in fresh water, would it then make sense that it is the concentration of sodium and/or chloride in the water that impacts the possible net dissolution or precipitation of sodium chloride?

redfishbluefish · Mar 12, 2015

Randy Holmes-Farley said:
If instead of calcium carbonate we were talking about the solubility of sodium chloride in fresh water, would it then make sense that it is the concentration of sodium and/or chloride in the water that impacts the possible net dissolution or precipitation of sodium chloride?

No! Because sodium chloride quickly dissociates (dissolves) in water while calcium carbonate does not. Yes, eventually you hit saturation with the salt where no more will dissolve, but with calcium carbonate you'd still be looking at that rock with hardly any of it dissolved. It needs a bit of a chemical nudge by the presence of some protons to get moving into solution. I don't see this as being "low" in carbonate, but see it as being high in protons (acid), to get it to dissociate. By simply putting calcium carbonate in a low concentration of carbonate is not going to get it to dissociate.....you need the acid!

In addition, and please correct this if I'm wrong, but the dissociation that does occur with the calcium carbonate is predominately bicarbonate within the calc reactor (at the lower pH), and not carbonate.

H2O · Mar 12, 2015

Sounds like C from what i read about calcium reactor is more co2 will melt the media faster and that will drop the ph

Randy Holmes-Farley · Mar 12, 2015

redfishbluefish said:
No! Because sodium chloride quickly dissociates (dissolves) in water while calcium carbonate does not. Yes, eventually you hit saturation with the salt where no more will dissolve, but with calcium carbonate you'd still be looking at that rock with hardly any of it dissolved. It needs a bit of a chemical nudge by the presence of some protons to get moving into solution. I don't see this as being "low" in carbonate, but see it as being high in protons (acid), to get it to dissociate. By simply putting calcium carbonate in a low concentration of carbonate is not going to get it to dissociate.....you need the acid!

In addition, and please correct this if I'm wrong, but the dissociation that does occur with the calcium carbonate is predominately bicarbonate within the calc reactor (at the lower pH), and not carbonate.

I'm not sure that NaCl reaches equilibrium faster than does calcium carbonate (although it might, I've only measured the solubility of calcium carbonate, not really the rate of attaining it, but it is a straightforward experiment using something like conductivity). It may just be that so little calcium carbonate is needed to reach saturation of the nearby fresh water that you hardly notice it happening.

But, if you want, use my analogy but pick something with very low solubility, comparable to calcium carbonate or even lower. Say, silver chloride. Its dissolution is limited by silver and chloride in solution, but you wouldn't even notice the tiny bit that dissolves into fresh water.

So there is an interesting question of whether when you put calcium carbonate into low pH solutions, whether the carbonate at the crystal surface becomes protonated into bicarbonate and then desorbs or if desorbs as carbonate and instantly becomes bicarbonate.

Remarkably, I found a free online paper that addresses this question, and the answer seems to depend on pH. Calcite in pure fresh water above pH 6.5 desorbs carbonate that converts into bicarbonate. But at pH below 3.5, acid protonates the carbonate while still on the solid. In between there is a mixed mechanism that depends on both pH and the amount of carbonate (the PCO2 in the article).

http://www.mdpi.com/1420-3049/15/1/258/pdf

Randy Holmes-Farley · Mar 18, 2015

And the answer is...D. It reduces the concentration of carbonate, thereby increasing the solubility of the solid media

The equilibrium solubility of calcium carbonate (CaCO₃) in any solution is determined by the calcium concentration times the carbonate concentration. Exactly at saturation, that is called the solubility product, Ksp:

K_sp = [Ca⁺⁺] x [CO₃^--]

Ksp is a number, like 0.003, and it depends on the salinity, temperature, and other factors.

If the calcium concentration times the carbonate concentration is below the Ksp for the solution in question, any solid calcium carbonate will dissolve. If it is above that concentration, the calcium carbonate will precipitate. In normal seawater, that product exceeds the Ksp, and calcium carbonate can very slowly precipitate, but magnesium tends to get onto the growing crystal surface and prevents it from doing so in a reasonable time frame. That is called being supersaturated.

In a CaCO₃/CO₂ reactor, the added CO₂ lowers the pH, converting much of the carbonate into bicarbonate. Each lowering of the solution pH by 1 pH unit drops the carbonate concentration by about a factor of 10. So going from pH 8.2 to 6.5 drops carbonate by about a factor of 50, taking it from being supersaturated in seawater to being below saturation and able to dissolve in the reactor.

Happy Reefing.