role of alkalinity and carbon dioxide in pH stability

[brandon4291]

Randy,


Which ion or compound in these two part additives has the affinity for CO2 gas? carbonate? What effect does CO2 production have on these buffering agents, is it the change of carbonate to bicarbonate (due to the addition of CO2 gas) that reduces buffering capacity?

I am also thinking plant growth in the form of vigorous macroalgae will lend pH support to some degree as decalcification takes place, as they extract their carbon needs from the bicarbonate liberating carbonate once again. Additionally, what preference would these types of plants have for their carbon source, the dissolved form as carbonic acid or bound as in bicarbonate?I would like to be clear in my notions on this subject, appreciate your input for sure to this lengthy query.

Brandon M

 

[randy holmes-farley]

The different two part additives have very different pH's. There is a realtionship between pH, CO2, and alkalinity. In general, the higher the pH and the higher the alkalinity, the more CO2 will be held in the water (as carbonate and bicarbonate).

Those two part additives with a high pH (like the original B-ion ic) will pull in a lot of CO2 from the air. Those with a low pH (like bicarbonate B-ionic) will not pull in any.


As to pH stability, it is provided by carbonate/bicarbonae, and borate/boric acid. Co2 itself does little to impact pH stability (unless you add or subtract a lot) but it does itself lower pH when it forms carbonic acid

This article goes into great detail on pH buffering and stability:

http://www.advancedaquarist.com/issues/dec2002/chem.htm

This article describes the realtionship between pH and alkalinity (and CO2):

http://www.advancedaquarist.com/issues/may2002/chem.htm

This one describes issues around CO2 in general:

http://www.animalnetwork.com/fish/libra ... ordNo=2074

 

[brandon4291]

Great article referrals! They all answer my questions in great detail. I had looked around for some time on the internet for articles about marine photosythesis and relationships with CO2, but none were focused on CO2 like the last article. those are going into the favorites column on the toolbar for future readings...

Thanks Randy

Brandon

 

[brandon4291]

one last follow up question: does the pH enhancing effects of photosynthetic activity primarily come from the uptake of CO2 from the water column (less carbonic acid) or from the splitting of the bicarbonate ion (as in decalcification) which I thought yielded carbonate? Since carbonate is a component of total alkalinity, I thought it might be liberated back into solution and provide more buffering than when compared to the dark phase of photosynthesis. In the article it seemed the major benefit was the uptake of CO2 from the water column by the plants, I wasnt sure how photosynthesis affects carbonate/bicarbonate balance.

As I re-read that article on CO2 interaction, the mechanics of carbonate-bicarbonate balances are slowly sinking in but Im still a little unclear on how photosynthesis can enhance buffering capability in our reef tanks.

Thanks

Brandon

 

[randy holmes-farley]

does the pH enhancing effects of photosynthetic activity primarily come from the uptake of CO2 from the water column (less carbonic acid) or from the splitting of the bicarbonate ion (as in decalcification)

The pH raising effect comes because photosynthesis results in a net removal of CO2 from the system. Exactly how that is accomplished on a molecular scale is not really important for the effect, though it may be interesting.

For example:

1. If a photosynthesizing organism takes up CO2 itself, then the CO2 level int eh water declines, and the pH can rise. The system will immediately reequilibrate itself according to LeChatliers principle:

CO2 + H2O <---> H2CO3 <----> H+ + HCO3- <-----> 2H+ + CO3--

If you remove the CO2 from the left hand side, the reactions shifts to the right, taking up H+ and raising the pH.

2. If a photosynthesizing organism takes up HCO3- and takes up H+ (or equivalently, releases OH-) to form H2CO3 inside of it that is used, then the CO2 level in the water declines, and the pH can rise.



but Im still a little unclear on how photosynthesis can enhance buffering capability in our reef tanks.

I don't think that it does. It raises pH by lowering the CO2 in the water, but it doesn't increase buffering directly.

Now, if you have a refugium on a reverse light cycle, that will serve to "buffer" the main tank by providing CO2 during the refugium nighttime and limiting the pH rise of the main tank's daytime, and taking up CO2 during the main tanks nighttime will limit the pH drop because that is the refugium's daytime when photosynthesis is sucking up a lot of CO2 in the refugium.

 

[brandon4291]

Now I begin to understand. I have resolved to read that article on carbon dioxide once a day for the rest of the week, then it should sink in nicely by the time the weekend rolls around. Each part is not tough to understand, but to see it all as the big picture is the challenge for someone learning marine chemistry! Your threads and articles are -primo-information for lack of a better descriptive.

Thanks

Brandon

 

[randy holmes-farley]

Your threads and articles are -primo-information for lack of a better descriptive.

Thanks.
FWIW, I 'm happy to answer any other questions that might come up on these topics.