MINIATURIZED DETECTOR OF SULFUR-DIOXIDE BASED ON THE FLOW CONDUCTOMETRY OF AN ABSORBING SOLUTION

A miniaturized detector based on absorbing conductometry was construct ed in order to measure the SO2 concentration in air. This detector is not only small, but is also capable of continuous measurement of SO2, unlike the conventional detectors for intermittent SO2 measurements. T wo bilateral pairs of Pt electrodes were fabricated on a glass-epoxy s ubstrate through conventional photolithography and sputtering. On this substrate, a cavity for the flow of an absorbing solution was formed with a Teflon sheet and a gas-permeable membrane. The absorbing soluti on, H2SO4-H2O2, was made to flow through this cavity of 2 mm in width and 0.5 mm in thickness by applying pressure to a reservoir of the sol ution. The sample gas was introduced into the other side of the membra ne. Gaseous SO2 permeated through the membrane and dissolved into the absorbing solution, resulting in an increase in the electrical conduct ivity of the solution. Any change in the conductivity was monitored us ing Pt electrodes located both up- and downstream of the absorbing zon e, and thus the SO2 concentrations were determined. The sensitivity an d response time largely depended on the flow rate of the absorbing sol ution. Since the standard flow rate employed was 20 mu l min(-1), the volume of the waste solution, even over long-time operation, was suffi ciently small to be carried. When the absorbing solution was flowed at this rate, from 10 ppb to 1 ppm of SO2 could be measured and the chan ge ratio in the conductivity was 0.971 ppm(-1). The response time (T-9 0%) was 82 sec under the same conditions. Other gases interfered sligh tly; e.g. the sensitivity to CO2 was about 1/30000 of that of SO2, so the selectivity; of this method was sufficiently good for facile use. This study furnished a step for constructing a miniaturized total chem ical analysis system (mu TAS) for SO2 measurements.