Method SM 4500-ClO2 C

Chlorine Dioxide by Amperometry

This method is for the determination of chlorine dioxide (ClO2) from water.  It's also capable of determining chlorite, chlorine and chloramines in water.  This method is useful when it is necessary to know the various chlorine fractions in a water sample. 

There are four titrations performed using phenylarsine oxide.  The first titration step consists of conversion of ClO2 to chlorite and chlorate through addition of sufficient NaOH to produce a pH of 12, followed by neutralization to a pH of 7 and titration of free chlorine.  In the second titration KI (potassium iodide) is added to a sample  treated similarly with alkali to a pH of 7.  This titration yields free chlorine and monochloramine. The third titration involves addition of KI and pH adjustment to 7, followed by titration of free chlorine, monochloramine, and one-fifth of the available ClO2.  In the fourth titration, sufficient H2SO4 (sulfuric acid) is added to lower the pH to 2 which enables all available ClO2 and chlorite, as well as the total free chlorine, to liberate an equivalent amount of iodine from the added KI and be titrated.

Keep samples protected from light and agitation as exposure to sunlight


(Standard Methods)


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Method Data

Hold Times, Preservatives, Preps, Collection, Analytical & Documentation
Holding Time:   Analyze samples within 15 minutes of collection.
Preservatives:   None specified in the method.
Required Preps:   125mL Amber glass or polyethylene bottle with Teflon-lined lid
Collection Method:   Grab sampling
Analytical Methodology:   Amperometric titration

Analyte List*

Analyte Formula CAS Number Detection Limit
Chlorine dioxide

* The analytes and detection limits listed for each method represent the typical detection limits and analytes reported for that particular method. Keep in mind that analyte lists may vary from laboratory to laboratory. Detection limits may also vary from lab to lab and are dependent upon the sample size, matrix, and any interferences that may be present in the sample.