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Determination of Differential Carbon Dioxide Concentration by Conductimetric Analysis

USDA-ARS and Dep. of Soil, Water, & Climate, Univ. of Minnesota, 1991 Upper Buford Circle, St. Paul, MN 55108

^* Corresponding author (jbaker{at}soils.umn.edu).

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 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.

Received for publication September 12, 1997.