Reef Central Online Community

Home Forum Here you can view your subscribed threads, work with private messages and edit your profile and preferences View New Posts View Today's Posts

Find other members Frequently Asked Questions Search Reefkeeping ...an online magazine for marine aquarists Support our sponsors and mention Reef Central

Go Back   Reef Central Online Community Archives > General Interest Forums > The Reef Chemistry Forum

 
 
Thread Tools Search this Thread Display Modes
  #1  
Old 08/28/2002, 05:00 AM
simonh simonh is offline
Premium Member
 
Join Date: Sep 2000
Location: England, UK
Posts: 1,521
Calcium carbonate saturation

Hi Randy,

I have another question on what seems to becoming a geeky interest in calcium reactors. In a calcium reactor we must lower the omega for aragonite to below 1 for the calcium carbonate to dissolve. Ignoring kinetics for now, but will dissoloution of the media stop when we get to omega = 1 (I presume stuff still dissolves but also precipitates at the same rate?).

For example at pH 6.8 I calculate that omega=1 when alkalinity=11mEq/L (30 dKH), would this be the theoretical saturation we could acheive or could alkalinity be driven higher within the reactor or second chamber? I know often it is talked of driving the pH to higher levels with a second chamber as while the pH is less than 7.6 there is still CO2 in solution. But, isn't the aragonite saturation more of a limit than the fact there is still CO2 in solution?

Last edited by simonh; 08/28/2002 at 07:07 AM.
  #2  
Old 08/28/2002, 07:34 AM
Randy Holmes-Farley Randy Holmes-Farley is offline
Reef Chemist
 
Join Date: Apr 2001
Location: Arlington, Massachusetts
Posts: 52,068
Ignoring kinetics for now, but will dissoloution of the media stop when we get to omega = 1 (I presume stuff still dissolves but also precipitates at the same rate?).

Yes, you cannot dissolve anything more if omega is one or higher. Then allowing kinetics back into the discussion, you likely won't even be able to reach 1.

Yes, the amount of "CO2" in solution is irrelevant. It is the pH, alkalinity, calcium level, and a few other minor factors (temperature, magnesium, etc) that impact the dissolution.

I didn't do a detailed calculation, but pH = 6.8 sounds about right for omega = 1 for that situation.
__________________
Randy Holmes-Farley
  #3  
Old 08/28/2002, 07:59 AM
simonh simonh is offline
Premium Member
 
Join Date: Sep 2000
Location: England, UK
Posts: 1,521
I used the CO2sys program with the following S=35, t=25, Alk=11.5 mEq/L, pH(NIST)=6.8 and it gave me an omega aragonite as 0.99 and omega calcite as 1.5.

One thing with the program is the calcium concentration is fixed at 420ppm for S=35 so I would expect with the increased calcium level the omega saturation to be > 0.99?
  #4  
Old 08/28/2002, 12:21 PM
Randy Holmes-Farley Randy Holmes-Farley is offline
Reef Chemist
 
Join Date: Apr 2001
Location: Arlington, Massachusetts
Posts: 52,068
Yes, the increased calcium will raise the supersaturation some, but not nearly as much as the alkalinity did (because the 420 ppm only goes up to about 600 ppm in such a situation). That would make the omega for pH 6.8 about 1.4. So the real omega = 1 point will be a bit lower in pH (6.5-6.6 probably).
__________________
Randy Holmes-Farley
 

Thread Tools Search this Thread
Search this Thread:

Advanced Search
Display Modes

Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is On

Forum Jump


All times are GMT -5. The time now is 05:22 PM.


Powered by vBulletin® Version 3.8.4
Copyright ©2000 - 2024, Jelsoft Enterprises Ltd.
Use of this web site is subject to the terms and conditions described in the user agreement.
Reef Central™ Reef Central, LLC. Copyright ©1999-2009