Re: How do you measure accurately carbon dioxide levels in a closed container?

There are several ways to measure the carbon dioxide content in the 
atmosphere of closed systems.  However, for each method some gas or liquid 
needs to be sampled to make the measurement.

A common method analyzes the gas phase of the system by subjecting a gas 
sample to gas chromatography.  Here we can measure carbon dioxide and 
oxygen.  There are sensors that one can buy that also measures the 
concentration of carbon dioxide in the gas phase.  For your project you 
will want something a bit less sophisticated.

You pointed to a method that involves alkali trapping of the carbon 
dioxide.  Here carbon dioxide is trapped in a basic (alkali) solution and 
then the amount of carbon dioxide trapped is measured by titrimetric, 
gravimetric, or conductimetric analysis.    Carbon dioxide is weakly acidic 
and the acid-base reaction produces carbonate.  When sodium hydroxide 
(NaOH) is used as the absorber:

CO2 + 2 Na+ +2 OH- goes to CO3-2 + 2 Na+ +H2O
carbon dioxide plus two sodium ions plus two hydroxide ions goes to 
carbonate (CO3 with a -2 charge) plus 2 sodium ions plus water

As long as there is an excess of hydroxide ions, carbon dioxide will be 
absorbed.  After the carbonate is formed you can titrate the amount of 
hydroxide left using a standardized acid solution.  You can use NaOH 
solution as a trap, but you can also use soda lime, a solid.  When using 
the soda lime you can measure the weight gain caused by absorption of CO2.  
Here the soda lime needs to be fully dried in an oven before weighing.  
Then after exposure to CO2, the soda lime is again dried and weighed.

For your particular application, you may need to set up a flow through 
system where the gas in your chamber is bubbled through the alkali 
solution.  Or remove a known volume of gas from the chamber using a syringe 
and inject that into the alkali solution.  You will need access to a pH 
meter to perform the titrations; pH paper will not be suitable.  

More details are avialable from many sources.  The one I have in front of 
me now is:
Methods of Soil Analysis, Part 2, Bigham et al. editors, Number 5 in the 
Soil Science Society of America Book Series, 1994, Chapter 38.  

If you absolutely cannot locate this item or a substitute yourself, I can 
photocopy the chapter for you and send it via USPS if you send me a 
personal email.

Hope this helps.

Cheers,

Dr. Mark Schneegurt
University of Notre Dame
mschneeg@nd.edu


PS  For your experiment be sure to run some controls.  Prove that the 
system is sealed by measuring carbon dioxide concentrations over several 
days or weeks without plants and soil.  Also be sure to run a control with 
just the soil and no plants.  The soil will likely produce more carbon 
dioxide than it consumes but could also absorb carbon dioxide over time.