Appendix A: Methods for Total Chlorine Residual Measurement

San Diego State University
Environmental Health and Safety Department

Water Main and Building Water Disinfection and Testing

Methods for determining total chlorine include amperometric titration, colorimetric DPD, titrimetric DPD, and iodimetric titration. The most widely used method for field test siturations is colorimetric DPD. In this method, DPD (N,N-diethyl-p-phenylenediamine) is oxidized by chlorine causing a magenta (red) color. The intensity of the color is directly proportional to the chlorine concentration in the sample. Colorimetric DPD field test kits are available from several suppliers. The basic procedures for the DPD Colorimetric method include the following steps:

Collect a water sample in the sample tube of the DPD test kit.
Add DPD color reagent to the water sample.
Match color sample with a color on the comparator to estimate the chlorine residual in mg/l.

Each chlorine test kit is designed to measure total residual chlorine only within a certain range of concentration. The low range instruments typically measure total residual chlorine in concentrations no higher than 3.5 to 5 mg/l. Some manufacturers have produced high range test kits that are capable of measuring chlorine at the level of doses required for disinfection of water mains, e.g. 25 mg/l. However, a low range test kit can be used to measure a chlorine concentration higher than the kit’s range by dilution. This dilution method can be performed by applying the graduated cylinder method as described below:

Collect a 2 ml sample of highly chlorinated water and pour the sample into an empty graduated cylinder (50 ml or greater). Add chlorine free water (distilled) for a total of 50 ml and shake gently.
From the graduated cylinder, collect a sample for total residual chlorine analysis using the DPD test kit in the quantity required for the particular kit. Add DPD reagent and estimate chlorine residual based on a colorimetric comparison with the standard according to the instructions for the particular kit.
Multiply the estimated chlorine concentration by the dilution factor, which is calculated as follows:

(Vol. of distilled water + chlorinated sample) = Dilution Factor
(Vol. of chlorinated sample)

Where 2 ml of sample is combined with 48 ml of distilled water in graduated cylinder, the dilution factor is 25 as determined below:

(48 ml of distilled water + 2 ml of sample) = Dilution Factor of 25
(2 ml of chlorinated sample)

For example, if it is determined that the diluted sample from the graduated cylinder has a chlorine residual of 1 mg/ml, the undiluted sample from the disinfected main would have a residual of 25 x 1 mg/l or 25 mg/l. If it is not possible to accurately determine the chlorine residual of the undiluted sample, it may be necessary to provide a different dilution to the sample. For example, if it is anticipated that the chlorine residual is around 100 mg/ml (slug method), a more appropriate dilution factor would be 50. This could be obtained by diluting 1 ml of sample with 49 ml of chlorine free water.


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	Environmental Health & Safety Potable (Drinking) Water San Diego State University Environmental Health and Safety Department Water Main and Building Water Disinfection and Testing Appendix A: Methods for Total Chlorine Residual Measurement Methods for determining total chlorine include amperometric titration, colorimetric DPD, titrimetric DPD, and iodimetric titration. The most widely used method for field test siturations is colorimetric DPD. In this method, DPD (N,N-diethyl-p-phenylenediamine) is oxidized by chlorine causing a magenta (red) color. The intensity of the color is directly proportional to the chlorine concentration in the sample. Colorimetric DPD field test kits are available from several suppliers. The basic procedures for the DPD Colorimetric method include the following steps: Collect a water sample in the sample tube of the DPD test kit. Add DPD color reagent to the water sample. Match color sample with a color on the comparator to estimate the chlorine residual in mg/l. Each chlorine test kit is designed to measure total residual chlorine only within a certain range of concentration. The low range instruments typically measure total residual chlorine in concentrations no higher than 3.5 to 5 mg/l. Some manufacturers have produced high range test kits that are capable of measuring chlorine at the level of doses required for disinfection of water mains, e.g. 25 mg/l. However, a low range test kit can be used to measure a chlorine concentration higher than the kit’s range by dilution. This dilution method can be performed by applying the graduated cylinder method as described below: Graduated Cylinder Method Collect a 2 ml sample of highly chlorinated water and pour the sample into an empty graduated cylinder (50 ml or greater). Add chlorine free water (distilled) for a total of 50 ml and shake gently. From the graduated cylinder, collect a sample for total residual chlorine analysis using the DPD test kit in the quantity required for the particular kit. Add DPD reagent and estimate chlorine residual based on a colorimetric comparison with the standard according to the instructions for the particular kit. Multiply the estimated chlorine concentration by the dilution factor, which is calculated as follows: (Vol. of distilled water + chlorinated sample) = Dilution Factor (Vol. of chlorinated sample) Where 2 ml of sample is combined with 48 ml of distilled water in graduated cylinder, the dilution factor is 25 as determined below: (48 ml of distilled water + 2 ml of sample) = Dilution Factor of 25 (2 ml of chlorinated sample) For example, if it is determined that the diluted sample from the graduated cylinder has a chlorine residual of 1 mg/ml, the undiluted sample from the disinfected main would have a residual of 25 x 1 mg/l or 25 mg/l. If it is not possible to accurately determine the chlorine residual of the undiluted sample, it may be necessary to provide a different dilution to the sample. For example, if it is anticipated that the chlorine residual is around 100 mg/ml (slug method), a more appropriate dilution factor would be 50. This could be obtained by diluting 1 ml of sample with 49 ml of chlorine free water.
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