Method SM 4500-Cl G

Total Residual Chlorine

This method is for the determination of total residual chlorine from water.  This is a colorimetric version of the DPD (N,N-diethyl-p-phenylenediamine) method (SM 4500-Cl F) and is based on the same principles.  ANy chlorine in the sample existing as hypochlorous acid or hypochlorite ion (free chlorine or free available chlorine) immediately reacts with the DPD indicator to form a pink color which is proportional to the chlorine concentration.

This method is simpler for determining free chlorine in water than the amperometric titration used in SM 4500-Cl D and SM 4500-Cl E and is appropriate for samples containing significant amounts of organic matter.

Chlorine is not stable in aqueous solutions and the chlorine content, particularly weak solutions, will decrease rapidly. Keep samples protected from light and agitation as exposure to sunlight or other strong light or agitation will accelerate the reduction of chlorine.


(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:   Spectrophotometer or Filter Photometer

Analyte List*

Analyte Formula CAS Number Detection Limit
Chlorine, Residual

* 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.