Reef Chemistry Question of the Day #219 How Does a Calcium Carbonate/Carbon Dioxide Reactor Work?

[Randy Holmes-Farley]

Reef Chemistry Question of the Day #219

Which of the following cannot be a mechanistic explanation of how a calcium carbonate/carbon dioxide reactor provides calcium and alkalinity to a reef aquarium. In other words, all of these processes (except one) happen to some extent, and when they happen, the dissolution of the solid calcium carbonate is encouraged Which one is not an explanation of why these devices work?

A. The carbon dioxide hydrates to carbonic acid, the carbonic acid releases H+, the H+ combines with a carbonate on the solid calcium carbonate surface surface, and it pops off as bicarbonate.

B. The carbon dioxide hydrates to carbonic acid, the carbonic acid directly combines with a carbonate on the solid calcium carbonate surface surface, and it pops off to form two bicarbonates.

C. The carbon dioxide hydrates to carbonic acid, the carbonic acid releases H+, the H+ combines with a carbonate in the liquid, forming bicarbonate and thereby preventing it from attaching to the solid calcium carbonate surface.

D. The carbon dioxide hydrates to carbonic acid, the carbonic acid releases H+, the H+ combines with a bicarbonate in the liquid, forming carbonic acid and thereby preventing it from attaching to the solid calcium carbonate surface.

Good luck!































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[Ghxst]

B!

 

[reeferfoxx]

D

 

[SDReefer]

I'm going to go with D.

 

[beaslbob]

I though it was because the ph went down.

 

[Randy Holmes-Farley]

I though it was because the ph went down.

lol

That would be the "releases H+" part.

 

[Randy Holmes-Farley]

And the answer is...

Which of the following cannot be a mechanistic explanation of how a calcium carbonate/carbon dioxide reactor provides calcium and alkalinity to a reef aquarium. In other words, all of these processes (except one) happen to some extent, and when they happen, the dissolution of the solid calcium carbonate is encouraged Which one is not an explanation of why these devices work?

D. The carbon dioxide hydrates to carbonic acid, the carbonic acid releases H+, the H+ combines with a bicarbonate in the liquid, forming carbonic acid and thereby preventing it from attaching to the solid calcium carbonate surface.

While part of D does happen (the part up to 'thereby"), it cannot explain how the reactor allows calcium carbonate to dissolve. It is just H+ moving back and forth between carbonic acid and bicarbonate:

H2CO3 <---> H+ + HCO3-

A round trip of this process doesn't change anything with respect to dissolution of calcium carbonate.

All of the others are mechanistic steps that happen to some extent in a reactor. While I have seen published studies in lab settings (see articles below), I've not seen such a study in seawater at the pH of a reactor to know which mechanisms (A = k1 in article below; B = k2 in article below; or C = k3) predominate.

http://www.sciencedirect.com/science/article/pii/0009254189900636

see also

http://pubs.rsc.org/is/content/articlelanding/2016/fd/c6fd00048g/unauth#!divAbstract