Development of a Low-Cost Spectrophotometric Sensor for ClO$_{2}$ Gas

ClO$_{2}$ is of interest because of it's capability to kill biological hazards such as E. coli and mold. However, ClO$_{2}$ is a toxic, reactive gas that must be generated at the point-of-use. Gas storage is not possible due to the possibility of an explosion. The need to detect the amount of ClO$_{2}$ at the point-of-use necessitates a low cost sensor. A low-cost spectrophotometric sensor based on a broad-band light source, a photodiode detector and a band-pass filter is proposed. To verify the design, precise determinations of the gas-phase cross-section and characterization of the optical components are necessary. Known concentrations of ClO$_{2}$(g) are prepared using the equilibrium relationship between an aqueous solution and the gas phase. The aqueous solutions are obtained by generating the gas via a chemical reaction and passing it through water. The concentrations of the aqueous solutions are then determined by chemical titration and UV-visible absorption measurements. For the solutions, a maximum absorption is observed at 359 nm, and the cross section at this wavelength is determined to be 4.79x10$^{-18}$cm$^{2}$, in agreement with previous observations. Using a broad-band source, the absorption of ClO$_{2}$ gas is successfully analyzed and concentrations are determined as low as 100 ppm. A more recent prototype based on an UV LED can measure down to concentrations as low as one ppm.