In-situ Calibration of the Hydroperoxyl Radical Using an Immobilized TiO2Photocatalyst in the Atmosphere

Abstract

The present study is the first report of utilizing <TEX>$TiO_2$</TEX> photocatalyst to analytically calibrate the hydroperoxyl radical (<TEX>$HO_2\;^{\cdot}$</TEX>). An in-situ calibration method of <TEX>$HO_2\;^{\cdot}$</TEX> is proposed for air monitoring by using an 2-methyl-6-(pmethoxyphenyl)- 3,7-dihydroimidazo-[1,2-a]pyrazin-3-one (MCLA)-chemiluminescence (CL) technique. In this method, <TEX>$HO_2\;^{\cdot}$</TEX>(<TEX>$pK_a$</TEX> = 4.80) is produced by the ultraviolet (UV) photolysis of immobilized <TEX>$TiO_2$</TEX> using a constant flow rate of air equilibrated water, in which <TEX>$HO_2\;^{\cdot}$</TEX> is controlled by using various lengths of knotted tubing reactor (KTR). The principle of the proposed calibration is based on the experimentally determined halflife (<TEX>$t_{1/2}$</TEX>) of <TEX>$HO_2\;^{\cdot}$</TEX> and its empirically observed pH-dependent rate constant, <TEX>$k_{obs}$</TEX>, at a given pH. The concentration of <TEX>$HO_2\;^{\cdot}$</TEX>/<TEX>$O_2\;^{\cdot}$</TEX>− is increased as pH increases. This pH dependence is due to the different disproportionative reactivities between <TEX>$HO_2\;^{\cdot}$</TEX>/<TEX>$O_2\;^{\cdot}$</TEX>− and <TEX>$HO_2\;^{\cdot}$</TEX>/<TEX>$O_2\;^{\cdot}$</TEX>−. Experimental results indicate the practical feasibility of the approach, producing very promising method.