Expert: Dr. Henry Boyter, Jr. - 10/19/2005

Question
I am interested in measuring the productivity of the aquatic
plant Elodea. The plant absorbs CO2 and HCO3 and increase pH
during photosynthesis.

Is there a way to relate a change in pH of say 8.0 to 8.4 into
milligrams of CO2 removed from a fixed volume of 0.25%
NaHCO3 solution?

I am trying to eliminate destructive sampling and need for
analytical balance for short term studies.
Thanks

WES

Answer
It would be tough.  You would have to assume that the buffering capacity of the water is always the same and that just isn't true.  You would be better off measuring the true alkalinity of the water which is a standard test that measures that buffering capacity.  It is a measure of OH(-) and the carbonates in the water.  It also measures other buffering compounds such as ammonia and phosphates.  If all you have is plant material, these should be fairly constant after a period of time (with fish I wouldn't say that).

So you need to measure alkalinity before and after with no other changes in the system OR you are going to need to keep the CO2 and carbonate additions constant during the time period.