A permeable membrane capacitance sensor for ionogenic gases: Application to the measurement of total organic carbon

We introduce a unique sensor, a tubular membrane capacitance sensor (TMCS). Wire electrodes are wrapped on a porous hydrophobic membrane tube and capacitance is measured with an inexpensive universal serial bus powered capacitance to voltage digital converter. At low solution conductivities, the double layer capacitance of the ionic solution is approximately linearly related to the solution conductance. Porous polypropylene hollow fibers were coated repeatedly with silicone rubber solutions to render the pores highly hydrophobic and hence minimize gradual liquid intrusion into the pores that leads to baseline drift. We applied the sensor to the determination of TOC in water samples where the analyte carbon was oxidized to CO2 by UV/persulfate. The evolved CO2 is sensed by the TMCS as the water slowly passing through it absorbs the CO2 resulting in a change in conductance and capacitance. The system was capable of detecting 800 ng C as glucose and 100 ng C as inorganic carbonate. The sensor combines the dual functions of gas collection and measurement, thus permitting a simple miniature economic device.