CO2 Testing

[Dana Riddle]

Anyone testing for CO2 in a reef aquarium? I added some phenolphthalein to a sample and the result suggests CO2 is present. Awaiting NaOH titrant to quantify.

 

[Sallstrom]

We had a researcher put in a DIY CO2-measuring device in one of our tanks many years ago. Can't remember the results, but the idea was simple and cheap. He measured CO2 in the air inside a small container, which was put in the water. The container had semipermeable sides which allowed the air and water CO2 to "even" out.
That way he could use meter for air instead of water.
Just a tip if anyone want to build their own

 

[Jay Z]

I believe the folks with the mind streams can help with this. It does monitor co2 in the water.

I do keep a close eye on incoming co2 levels and standing levels in my house. Nothing for in the water yet, but I am interested in it.

 

[Dkeller_nc]

The way one typically does this is to measure the total alkalinity of a sample of seawater and the pH. One can then calculate the amount of dissolved CO2 in the water based on the reaction coefficients for the carbonic acid/bicarbonate/carbonate equilibrium. I would suspect that's exactly what the Mindstream does.

If you're interested in how this is done by research institutions with respect to the inorganic carbon cycle in the world's oceans, here's a best practices guide that goes into detail about how to do the measurements and calculations.

 

[Randy Holmes-Farley Verified Community Expert]

CO2 is always present, proportional to the alk and pH, as noted above. As you acidify seawater in an all test, all of the bicarbonate and carbonate gets converted into CO2.

 

[taricha]

Dana, I did something like this recently.
I compared room CO2 to tank pH at constant alkalinity. And calculated the "CO2" in the water.
I used a calculation like what @Dkeller_nc described in post above to calculate CO2 and generate the graphs with indoor vs outdoor air skimming.

Tank pH effect with large indoor CO2 concentration

My tank is in a classroom, and I realized this means that the CO2 in the room may go quite high and this might make for an interesting demonstration of how the CO2 affects the tank pH. So here we go. This is what the pH swing looks like when the skimmer intake line is run out the window. The...

www.reef2reef.com

(Jim Welsh pointed out I was using the calculation the with wrong parameters and actually the CO2 in the water may have been even 50% higher than what I reported, but the trend is interesting regardless.)

 

[Bleigh]

Wouldn’t you expect some dissolved co2? I think Gibbs feee energy states it should tend towards forms of carbonate... but I haven’t looked at those equations in forever. But there has to be some level for it to reach equilibrium.

 

[vetteguy53081]

I will be placing a CO2 scrubber at the Skimmer

 

[taricha]

Wouldn’t you expect some dissolved co2? I think Gibbs feee energy states it should tend towards forms of carbonate... but I haven’t looked at those equations in forever. But there has to be some level for it to reach equilibrium.

Yep. The "CO2" in the seawater is mostly of form HCO3 bicarbonate with lesser amounts of CO3 carbonate ...

This online calculator is fun to play with and enlightening -Alk, pH, CO2, speciation in seawater etc. https://biocycle.atmos.colostate.edu/shiny/carbonate/

 

[Dkeller_nc]

Wouldn’t you expect some dissolved co2? I think Gibbs feee energy states it should tend towards forms of carbonate... but I haven’t looked at those equations in forever. But there has to be some level for it to reach equilibrium.

Yep, the equilibria at reef-tank pH is strongly shifted towards bicarbonate/carbonate. If you want to get in deep into the chemistry, this section of a course's notes might help.

 

[lexinverts]

Anyone testing for CO2 in a reef aquarium? I added some phenolphthalein to a sample and the result suggests CO2 is present. Awaiting NaOH titrant to quantify.

Here's my daily cycle of CO2 and O2 according to Mindstream:

 

[Bleigh]

Yep, the equilibria at reef-tank pH is strongly shifted towards bicarbonate/carbonate. If you want to get in deep into the chemistry, this section of a course's notes might help.

One of my minors was geophysics... heavy emphasis on ocean chemistry. Lol. I learned it all about a decade ago. Thanks for the link!

 

[Bleigh]

Here's my daily cycle of CO2 and O2 according to Mindstream:

Oh my god! This makes me sooo excited. I can’t wait to have all of this for my tank.

 

[Dana Riddle]

Thanks all for the replies. I was using a Seneye device to measure pH in the tank. When the sensor expired, I began using an American Marine pH monitor with a new (but not temperature compensated) probe. The pH with the Seneye was 8.1 but was reported to be less than 8 with the American Marine unit. I confirmed the low pH with a second non-ATC probe. I ordered a new pH probe (ATC) for my Hach data logger and the pH was a little higher but still below 8. I could take the easy way out and put a soda lime CO2 scrubber on the intake to the skimmer. But that's never been me. And I wonder why I constantly feel overloaded. LOL. The tank management practices changed about the time I began to see pH issues (heavy live green water additions, skimmer running only at night, etc.) As soon as the CO2 titrant arrives, I'll get busy looking at CO2, dissolved oxygen, total N & P, acid hydrolyzable P, nutrients, etc. In the meantime, most corals are showing good growth rates (some of the Cyphastreas were growing over their epoxy bases within days.)

 

[Bleigh]

Thanks all for the replies. I was using a Seneye device to measure pH in the tank. When the sensor expired, I began using an American Marine pH monitor with a new (but not temperature compensated) probe. The pH with the Seneye was 8.1 but was reported to be less than 8 with the American Marine unit. I confirmed the low pH with a second non-ATC probe. I ordered a new pH probe (ATC) for my Hach data logger and the pH was a little higher but still below 8. I could take the easy way out and put a soda lime CO2 scrubber on the intake to the skimmer. But that's never been me. And I wonder why I constantly feel overloaded. LOL. The tank management practices changed about the time I began to see pH issues (heavy live green water additions, skimmer running only at night, etc.) As soon as the CO2 titrant arrives, I'll get busy looking at CO2, dissolved oxygen, total N & P, acid hydrolyzable P, nutrients, etc. In the meantime, most corals are showing good growth rates (some of the Cyphastreas were growing over their epoxy bases within days.)

That’s awesome! Can’t wait to see what your conclusions are! I’ll sit in for that lecture too! ;Happy

 

[Randy Holmes-Farley Verified Community Expert]

Wouldn’t you expect some dissolved co2? I think Gibbs feee energy states it should tend towards forms of carbonate... but I haven’t looked at those equations in forever. But there has to be some level for it to reach equilibrium.

Always, if you have a sensitive enough technique.

 

[JimWelsh]

Awaiting NaOH titrant to quantify.

As soon as the CO2 titrant arrives

Dana, I'm confused about how you intend to titrate CO2 using NaOH. Isn't a strong acid usually used for titrating the carbonate alkalinity in seawater down to the carbonic acid endpoint, rather than titrating up to the phenolphthalein endpoint? Am I missing something? I'm also not sure I understand your phenolphthalein test comment. Did the sample fail to turn pink, indicating a pH lower than the phenolphthelein endpoint due to excess CO2? Is your thought process that you can titrate the excess carbonic acid with the NaOH to the phenolphthalein endpoint as a measure of the excess carbonic acid? Again, I'm confused, and not at all sure what your thought process is here. Please elaborate, so I can better understand.

 

[Dana Riddle]

Dana, I'm confused about how you intend to titrate CO2 using NaOH. Isn't a strong acid usually used for titrating the carbonate alkalinity in seawater down to the carbonic acid endpoint, rather than titrating up to the phenolphthalein endpoint? Am I missing something? I'm also not sure I understand your phenolphthalein test comment. Did the sample fail to turn pink, indicating a pH lower than the phenolphthelein endpoint due to excess CO2? Is your thought process that you can titrate the excess carbonic acid with the NaOH to the phenolphthalein endpoint as a measure of the excess carbonic acid? Again, I'm confused, and not at all sure what your thought process is here. Please elaborate, so I can better understand.

The chemists at Hach can explain better than I. Go to hach.com, search for 'CO2 test digital titrator' and look in 'Downloads.' There the procedure (applicable to seawater) is outlined. I'm not a chemist by trade, so I'm relying on their veracity.