Determination of differential carbon dioxide concentration by conductimetric analysis

There are many applications in such disciplines as agronomy, plant physiology, and ecology where it is necessary to measure the difference in atmospheric CO2 concentration [CO2] between two points. This is commonly done with an infrared gas analyzer, but such instruments are expensive, representing a substantial and sometimes prohibitive share of the cost of CO2-related research. The goal of this project was to explore a simple inexpensive alternative, in which the difference in [CO2] between two air streams is determined by bubbling the air through cells containing deionized water while measuring the ratio of their conductivities with a half-bridge measurement of electrode pairs suspended in each cell. The underlying principles are presented, and it is shown that (i) differential [CO2] is directly proportional to the conductivity ratio minus its inverse and (ii) the coefficient of proportionality is equal to the mean [CO2] of the two air streams. A system was designed and constructed to test these principles, and the results confirmed them. Dynamic response was characterized, and shown to be proportional to Q/V, where Q is the air flow rate and V is the water-filled volume of the cell. Differential resolution was found to be in the range of 0.4 to 0.8 μmol mol-1, but better resolution is theoretically possible. Differential measurement of [CO2] by conductimetry shows considerable promise, particularly considering the straightforward nature of the relationship and the relatively low cost of the required components.