Chlorine dioxide redox potential

When there is a peak flow of chlorine in the feed-stream the redox potential decreases and the amount of caustic that is dosed increases automatically. Caustic is fed to the jet-loop and its flow is regulated by the measurement of the redox potential. When the lead time is long there is a shortage of caustic, which results in a lower level of free caustic that leads to a higher chlorine concentration. This chlorine subsequently reacts with the chlorite in the hypochlorite solution to form chlorine dioxide. The absorption of chlorine dioxide by the caustic is then very limited in step one and is hardly absorbed at all in the caustic present in step two of the production and is emitted with the inert gas stream. 

Chlorine dioxide is a bent (O-Cl-0 bond angle 117°), relatively stable free-radical species with a solubility in water about an order of magnitude greater than chlorine gas. Both the O and Cl atoms have some radical density and may enter into one-electron redox reactions. The redox potential for addition of an electron to aqueous CIO2 is 0.936 V, very close to that for OCl (Merenyi et al., 1988), but lower than that of HOCl. The compound is stable for long periods in acidic solution, but decomposes in alkali by disproportionation to chlorite (CIO2 ) and chlorate (CIO3 ) ... 

The electrochemical behaviour of chlorine dioxide/chlorine redox couple in acidic media depends upon the cathode material. Low overpotential materials (e.g. platinum) will reduce chlorine close to the thermodynamic potential of 1.36 V vs. NHE ... 

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