I'm not a chemist and this is only my second post around here, but I'm curious about this. Why are you bothering with membranes? Couldn't you mount the sensor in a container above the water's surface but open to the water. The concentration of CO2 in the air space should reach equilibrium with the concentration in the tank water. Then you could use Henry's Law to calculate the concentration of CO2 in solution.
Caq/Cgas = 0.8317 for CO2 @ 25 C
Last edited by coonass; 12-16-2012 at 12:37 AM.
Reason: added formula