Calcium and carbonate chemistry, help!

[mhgelb]

OK so I am a chem prof at UW and have a decent background in chemistry. I think I got this calcium, alkalanity figured out but I have one key question.

Reef folks out there seem to use a combo of a calcium reactor and something to add kahlwasser (Ca(OH)2) to their tanks. Here is the way I see it.
Your hard corals are consuming CaCO3 and also CaCO3 is supersaturated in your tank (and in the wild) so it is slowly precipitating in the tank. So it is clear that you need to replace soluble Ca2+ and soluble CO3--.

1. option 1. just add solid CaCO3 to the tank. No good cause it is not going to dissolve much, the water is supersaturated in CaCO3 already so the rate of CaCO3 formtion is faster than its rate of dissolution.

2. option 2. add calcium bicarbonate to the tank. No good cause there is no such thing as solid calcium bicarbonate, if you tried to form it, it would break down to CaCO3 + CO2 + water. this is becasue CaCO3 is very stable.

3. Option 3. Add a solution of CaCl2 (highly soluble in water) and a solution of Na2CO3 (also soluble in water) to your tank in equal amts. Bad idea since then you would also increase Na+ and Cl- in the tank (salinity would rise).

So the challenge is to add Ca2+ to your tank, add CO3-- to your tank but nothing else and there is only 1 way to do it. It is to add Ca(OH)2 (aka Kahlwasser, which is pretty soluble in water) and bubble in CO2. The CO2 reacts with water to make carbonic acid H2CO3, and this reacts with the OH- from the Kahlwasser to make water and CO3--. So just setup a system to add Ca(OH)2 to keep your soluble Ca2+ where yout want it, and add in enough CO2 to keep your pH from rising. This will also replace the CO3--. This is what you guys do, and I have only 1 question left. Why do you put aragonite in your calcium reactor??
Why not just bubble in CO2 into your tank?? The only reason I can think of is that maybe aragonite is not pure CaCO3, maybe it has other essential components that your tank needs like Mg2+, etc.. and you are replacing these also by dissolving aragonite by bubbling CO2 into your calcium reactor. Is this the reason you guys use aragonite in your calcium reactor? Or maybe there is no good reason and then we should just add Kahlwasswer and CO2 to our tanks and forget about the aragonite in the calcium reactor.

thanks

 

[Gazbo]

Well I am new at this as far as calcium reactors but it my understanding that the CO2 will lower PH in the reactor which breaks down your aragonite. So it you where to add CO2 directly to your tank it would lower your PH and make it harmful to your livestock and even if things could thrive with low PH wouldn't you just be breaking down your live rock and sand bed slowly which will ruin your landscaping over time might be a long time but thats just my opinion sure someone more expirence will chime in

 

[DTECH07]

Maybe you have maybe you haven't read these but there straight to the point and as you look at the bottom of the page are the addition articles that covers the complete thinking behind a tanks chemistry. This is a go to spot for the serious hobbiest. Hope it helps and let me know how it works for you
Darryl
The

 

[mhgelb]

For Gazbo, yes adding CO2 to the reactor or to any body of water will lower the pH since CO2 reacts with water to form carbonic acid, H2CO3, which dissociates to form H+ and HCO3- and of course pH goes down when the amt of H+ goes up since that is how pH is defined. But what you say is not quite right. Even if you bubble CO2 through a suspension of CaCO3 in water and let that go in to the tank, the pH of the tank will still go down. Here is why.

You have CaCO3 solid in your reactor and a small amt of it is dissolved in the water to form Ca2+ and CO3--. Now you bubble in CO2 which reacts with water to form H2CO3.
The H2CO3 reacts with the CO3-- to form 2 molecules of HCO3-. The overal reaction is as follows:

CO2 + H2O + Ca2+ + CO3-- > 2 HCO3- + Ca2+ and this flows into the tank.

As this happens more Ca2+ and CO3-- dissolves in the water in the reactor.

In your tank the buffering is controlled mostly by HCO3- and CO3--, and as you add more HCO3- to the tank from your calcium reactor you are adding more of the ACID component of the buffer which means the pH has to drop.

OK so lets bubble in CO2 to the tank without aragonite in the reactor. Now we are adding H2CO3 to the tank. For each CO2 added we form 1 H2CO3 and this is equivalent to 1 HCO3- and 1 H+ so we are adding HCO3- and H+ to the tank, whereas before (with aragonite in the reactor) we are adding HCO3- to the tank. In both cases the pH has to drop but you are correct in that it will drop faster if we just bubble in CO2 without aragonite, but it will drop in both cases.

So even if we added CO2 to a reactor with aragnoite the pH will drop and we would eventually have to off set that by adding OH-, we could add NaOH but that would increase Na+ in the tank so we do it by adding Ca(OH)2 since we want to add Ca2+ rather than Na+.

 

[eartaker]

Wow, just the questions asked in this thread have explained a lot... Thanks for starting this.

 

[mhgelb]

OK so I am pretty sure the only reason folks put aragonite in their calcium reactor is because as it dissolves under the action of bubbling in CO2 you liberate calcium but ALSO other elements in the aragonite like magnesium and strontium, etc... Otherwise you could just bubble in CO2 and add Ca(OH)2. So Darryl above has a good idea. rather than use aragonite (which is a crystalline form of CaCO3) better you should use a bunch of dead coral as this would have all the elements in the skeleton of the coral and you would be adding these to the water which seems like the optimal thing to do. Some forms of aragonite that you buy are indeed crushed coral but the word aragonite defines a crystalline form of pure calcium carbonate. I could make it in the lab by adding pure CaCl2 solution in water to a solution of pure Na2CO3 in water. The CaCO3 would drop out of solution leaving NaCl dissolved in the water, decant off the water and wash the solid CaCO3 a few times with pure water and you have pure CaCO3, nothing else. It would be silly to use this in a calcium reactor, it would add nothing beyond adding CO2 and Ca(OH)2 to your tank. I think this is the reason people add natural aragonite (which is CaCO3 + impurities) or crushed coral to their calcium reactors. It is not to control pH cause if you add CO2 alone or CO2 to aragonite, your tank pH is going to go down for sure. So you have to offset this by adding OH- and better you should add Ca(OH)2 rather than NaOH so you don't increase Na+ in your tank.

 

[kpiotrowski]

some use a little vinergar to disolve kalk ( same thing as the Co2 in the end right?) ( plus you get to carbon dose at the same time). I never went the ca reactor way because I can be more precise with the "balling" meathod.

 

[mhgelb]

let's see. vinegar is acetic acid. reaction would be CaCO3 + CH3COOH > Ca2+ + HCO3- + CH3COO-

so yes you will dissolve CaCO3 and release Ca2+ and bicarbonate to the tank but you will also be adding acetate (CH3COO-) to your tank. Probably acetate is OK as it is a naturally occuring compound. But if you don't want acetate you need to use CO2.

 

[kpiotrowski]

You have a great point about using Co2 to disolve kalk ( using coral skeleton I understand dosen't work cus they don't break down as fast) but it falls into that category of why when kalk can be utillized in a more simple fashion. also most people deliver kalk via the ato sytsem which delivers the solution as water evaporates from the system. the kalk resevior is constantly being diluted with freshwater and hence the concentration is less and less. you would be adjusting your Co2 output on a daliy basis. the some is true with ca reactors but for the most part are more stable.

where there is a will there is a way, so I am sure one could design something...and it may be groundbreaking. I may even buy it as kalk is cheap!