Development of a new method for direct measurement of pCO sub(2) in natural waters

A new direct measurement method for pCO sub(2) in water samples is presented. The technique is based on the fact that the dissolved CO sub(2) in a water sample is able to diffuse through the wall of a thin silicon rubber tubing, whereas the dissolved ions remain. The CO sub(2) passing through the wall is equilibrated with the deionized water flowing inside the tubing, and its electric conductivity is measured by a conductivity flow cell afterward. Laboratory experiments on 255 CO sub(2) standard samples with pCO sub(2) varying from 300 to 30,000 ppmv show a mean CO sub(2) exchange ratio R across the silicon rubber membrane of 0.95, with SD = plus or minus 0.04. For pCO sub(2) in the 40-300 ppmv range (40 CO sub(2)-standard samples), a mean exchange ratio R = 0.93 plus or minus 0.09 has been determined. The detection limit of this method is 0.6 mu M ( similar to 18 ppmv at 25 degree C).