Method 330.5

Chlorine, Total Residual (Spectrophotometric, DPD)

This method is for the determination of total residual chlorine from natural and treated waters at concentrations from 0.2-4 mg/L. Chlorine (hypochlorite ion, hypochlorous acid) and chloramines stoichiometrically liberate iodine from potassium iodide at pH 4 or less. The liberated iodine reacts with N,N-diethyl-p-phenylene diamine (DPD) to produce a red colored solution. The solution is spectrophotometrically compared to a series of standards, using a graph or a regression analysis calculation. The results are read or calculated into mg/L Cl.

Any oxidizing agent can cause an interference with the method.   The oxidizing agents are usually present at insignificant concentrations compared to the residual chlorine concentrations.  Turbidity and color will prevent the colorimetric analysis.

(EPA: Office of Water)


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Method Data

Hold Times, Preservatives, Preps, Collection, Analytical & Documentation
Holding Time:   The method say to analyze the sample immediately. 15 minutes is usually the cited holding time.
Preservatives:   None noted in method.
Required Preps:   250 mL amber glass or plastic bottle
Collection Method:   Grab sampling
Analytical Methodology:   Spectrophotometer
Documentation:   330.5

Analyte List*

Analyte Formula CAS Number Detection Limit

* 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.