Measure CO2

[gnikoli]

Is there an accuarate way to measure CO2 of saltwater in reef aquarium? I tried the LaMotte kit, but the indicator turns the sample pink right from the start and the instructions say that means no free CO2 is present. Don't think that is the case and I now understand that the LaMotte kit does not work for saltwater. I realize there are other ways to get at gross CO2 status, but need a relatively low cost method (<$100) with decent resolution for purposes of a science project.

[Randy Holmes-Farley]

There is no benefit to measuring CO2 in marine aquaria that I know of. Nearly all of the CO2 is actually held as bicarbonate and carbonate in seawater. There is little free CO2 at pH 8.2 in seawater.

Gross CO2 status would be best determined, IMO, by knowing the pH and the alkalinity, as shown in this article:

Low pH: Causes and Cures
http://reefkeeping.com/issues/2004-09/rhf/index.htm

[gnikoli]

Thanks Randy. My interest is academic, not practical. Trying to point my son in the right direction for a science project. He can infer increased CO2 at night because of what he knows about photosynthesis and water chemistry, but it would be useful to measure it directly. Is there an indirect way to calculate actual CO2 increase?

[Randy Holmes-Farley]

I don't believe that you will be able to measure it directly. One has to even define what you mean by CO2 in seawater.

You should check the article tha tI linked. It has the theory as well as some practical answers. The theory is discussed in great detail in the book "Aquatic Chemistry Concepts" by Pankow.

There is also a computer program that you can use to get CO2 from alkalinity and pH (I believe):

http://cdiac.esd.ornl.gov/oceans/co2rprt.html

[gnikoli]

Great reference, thanks again!

[Randy Holmes-Farley]

You're welcome.

There are a few threads in this forum where folks have used the program, if you want to track them down. There are also some regulars that have used it a lot more than I have.

Good luck!