The Use of para-Chlorobenzoic Acid (pCBA) as an Ozone/Hydroxyl Radical Probe Compound

Since OH· radicals cannot be measured directly during an ozonation process, para-chlorobenzoic acid (pCBA) has been used recently as an OH· radical probe compound during ozonation based on its very slow direct reaction with ozone and fast reaction with OH· radicals. However, in experiments of this study it was shown that pCBA accelerated ozone decay. Furthermore, the formation of hydrogen peroxide was observed during this process. The formed H2O2 increases the decomposition of aqueous ozone and leads to enhanced formation of OH· radicals. The chain reaction therefore changes to HO2 − ion initiated decay of ozone instead of hydroxide ion, OH−. Thus, an error in applying pCBA as a probe compound in low scavenger containing waters is likely to occur if the scavenging rate of pCBA makes up more than 5% of the total scavenging rate.