Science: Analytics

The analytical chemistry of chlorine dioxide, especially in solution, is complicated by chlorine dioxide’s volatility, by its sensitivity to light and instability over time, and by interference from related redox species. Various analytical methods are in use, or have been proposed, with varying degrees of success.

Spectrophotometry

Chlorine dioxide can be measured spectrophotometrically, either in solution or in the gas phase. The molar absorptivity of chlorine dioxide at its maximum absorbance wavelength (~359 nm) can be used to calculate directly the concentration of “pure” chlorine dioxide given the absorbance of the sample in a spectrophotometric cell of known pathlength.

In solutions containing both chlorine dioxide and chlorite ion, absorbance measurements are subject to interference from intermediate oxychlorine species. At wavelengths longer than 359 nm, such interference can become dominant and the calculated chlorine dioxide concentration may differ from the true value by as much as a factor of two. Spectrophotometric analysis also suffers from an overlap in the absorbance of chlorine and chlorine dioxide. Unless the sample is chlorine-free, the technique is not accurate.

Amperometric Titration

Amperometric titration is an electrochemical technique that measures the current (amperes) at a constant voltage. As titrant is added to the titration cell, the current will change based on a series of chemical reactions. When complete reaction is attained, a constant current is recorded and the endpoint is reached.

Chlorine dioxide, free available chlorine (FAC) and chlorite ion can be readily measured by amperometric titration at concentrations (in solution) above 1 mg/L. Below this level, accuracy of this method is highly dependent on the skill of the technician. Attempts at amperometric titration to measure chlorate ion, especially at levels less than 1 mg/L, are subject to large errors. There are two amperometric methods approved by the USEPA.

Ion Chromatography (IC)

IC is the method of choice for measuring low levels of chlorite, chlorate, bromide and bromate ions. The USEPA requires that IC be used for these measurements. This method may be used to measure chlorite and chlorate ions from 0.05 to 1.0 mg/L.

DPD

DPD reagent, in wide use for chlorine measurement in drinking water, suffers from serious limitations in measurement of chlorine dioxide, chlorite ion or chlorate ion because it is subject to severe interferences.