Redox Reactions and the Analytical Chemistry of Chlorine Dioxide and Ozone

Abstract

Ozone and chlorine dioxide are strong oxidants which are being given considerable attention as alternative oxidants for the disinfection of drinking water. The details of the inorganic oxidation-reduction (redox) reactions of both ozone and chlorine dioxide have been described in detail in the literature for many years. Only recently has the mechanism of the self-decomposition of ozone in aqueous solution become reasonably well understood. It is becoming clear that ozone can be involved in both one and two electron transfer redox processes. On the other hand, chlorine dioxide is most frequently encountered as a one-electron oxidant. The formation of highly reactive, complex intermediates is commonplace in the reactions of chlorine dioxide and chlorite ion. Their role is also of primary importance in understanding the microscopic details of most oscillating reactions where one- and two-electron reactions play a pivotal role. This paper will review the inorganic chemistry of ozone and chlorine dioxide along with the associated underlying by-product chemistry of both species -- with emphasis on the role of highly reactive non-metal species in these processes. Existing analytical methods for ozone and chlorine dioxide also will be reviewed. Some of the methods reviewed require only modest additional field/laboratory evaluation before they could be designated as recommended methods. On the other hand some of the newly published methods are in such a preliminary state of development that they will require many months/years before they can be demonstrated and recommended as advantageous methods. Specifically, guidance and recommendations in selecting residual monitoring techniques for ozone, chlorine dioxide, chlorine/hypochlorous (free chlorine), chlorite ion and chlorate ion will be provided.