The generation of chlorine dioxide can be performed in several ways. Typical solutions marketed as “chlorine dioxide” solutions are generated on-site as acidification of a chlorite solution. Chlorine dioxide generated for analytical purposes is done so as described as acidification, separation and absorption. Selective Micro® products are generated using membrane technology as described in diffusion of chlorine dioxide.
Acidification of a Chlorite Solution. Most "chlorine dioxide" solutions are acidified sodium chlorite solutions. These solutions of sodium chlorite require activation by the addition of granular acid or an acid solution, which results in very small yields of chlorine dioxide, high residuals (such as chlorites and chlorates), and low pH.
Acidification, Separation, and Absorption. Recommended by the "Standard Methods for Examination of Water and Wastewater," this typically utilizes an aspirator bottle, a reaction chamber, a scrubber bottle (sometimes two), and a collecting bottle. This method produces small volumes of high purity, usually for analytical purposes.
Acidification of Chlorine Gas. Some large-scale generators make chlorine dioxide gas by exposing chlorine gas to strong acids such as sulfuric acid. The resulting chlorine dioxide gas is captured and absorbed into solution, giving a high purity solution of chlorine dioxide.
Diffusion of Chlorine Dioxide. This method uses membrane technology to selectively allow the diffusion of chlorine dioxide gas into solution without allowing passage of reaction residuals such as chlorates, chlorites and remaining acids used in the generation process. The reaction takes place inside an envelope constructed of membranes which have the secondary role of containing the residuals during and after generation.
Detecting ClO2
Chlorine dioxide can also be detected in several ways. Some of these methods (such as DPD, Amperometric, and Iodometric) are standardized, widely accepted and used. Colorimetric describes the method developed and used by Selective Micro Technologies on a regular basis along with the DPD method. While the Colorimetric method is not widely accepted, it is much more efficient in daily use (at market-dictated concentrations of interest) and is easily calibrated to the accepted methods described below.
DPD Method. DPD (N,N-diethyl-p-phenylenediamine), along with several other chemicals, are added to a chlorine dioxide solution to produce a color that is absorbed at a specific wavelength. This measurement is taken with a colorimeter. This method is excellent for low concentrations of chlorine dioxide (0.04-5 ppm).
Amperometric Method. This is a complicated process of successive titrations that is actually an extension of the titration procedure for free chlorine. Four groups of chlorine species are titrated separately, of which chlorine dioxide is in the last group. This method is best suited for use if a solution is suspected to contain chlorine species other than chlorine dioxide in it.
Iodometric Method. This method of titration can be used only if a solution is known to be a pure chlorine dioxide solution (i.e. no other chlorine species present). It is a simple titration of iodine liberated from potassium iodide by chlorine dioxide.
Colorimetric. This method measures chlorine dioxide solution directly using a colorimeter at a specific wavelength within a specific concentration range. For a concentration range of 5-50 ppm, a 360nm wavelength is used, while at a concentration range of 50-250 ppm, a 420nm wavelength is used. The 360nm wavelength is the lambda-max of chlorine dioxide and therefore offers the highest resolution, but the 420nm offers very acceptable resolution for higher concentrations and a much broader range.
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