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how to calculate residual chlorine

by:BOQU     2023-04-25


Residual chlorine refers to the amount of chlorine that remains in the water after the initial disinfection process. It is a critical factor in ensuring that the water remains safe for consumption. As such, it is essential to measure the residual chlorine in the water regularly. This article seeks to provide an overview of how to calculate residual chlorine and the importance of doing so.

Why is residual chlorine important?

Residual chlorine is critical in ensuring that water is safe for consumption. Chlorine is used as a disinfection agent to kill bacteria, viruses, and other harmful microorganisms that may be present in the water. However, if the chlorine concentration is too high, it can be harmful to human health. On the other hand, if the concentration is too low, then the water may not be adequately disinfected, and harmful pathogens may survive. Therefore, it is essential to measure and maintain the residual chlorine concentration at an adequate level.

How to measure residual chlorine?

There are several ways to measure the residual chlorine concentration in water. Some of the most common methods include colorimetric analysis, titration, and electrochemical analysis. One of the most straightforward methods is using test strips.

Using test strips:

1. Obtain a test strip.

2. Immerse the test strip into the water sample and swirl gently for a few seconds.

3. Remove the test strip from the water and shake off any excess water.

4. Match the color of the test strip to the chart provided to determine the residual chlorine concentration.

Note: Different test strips may have different color charts, so be sure to refer to the instructions provided.

Using titration:

Titration is a more precise method of measuring residual chlorine concentration. It involves adding a reagent to the water sample that reacts with the residual chlorine to form a color change. The amount of reagent required to produce the color change is proportional to the residual chlorine concentration.

1. Obtain a sample of water.

2. Add a measured amount of reagent to the water sample.

3. Titrate the solution with a standardized solution until the desired color change is achieved.

4. Note the amount of standardized solution required.

5. Calculate the residual chlorine concentration using the following formula:

Residual chlorine concentration = (volume of standardized solution × concentration of standardized solution × dilution factor) / (volume of sample)

Using electrochemical analysis:

Electrochemical analysis involves using an electrode to measure the potential difference between the water sample and a reference electrode. The potential difference is proportional to the residual chlorine concentration.

1. Obtain a sample of water.

2. Immerse the electrode into the water sample.

3. Wait for a stable reading to be obtained.

4. Note the reading on the electrode display.

5. Calculate the residual chlorine concentration using the following formula:

Residual chlorine concentration = (reading on electrode display × probe factor × dilution factor) / (volume of sample)

Note: The probe factor is the calibration factor provided by the manufacturer of the electrode.


In conclusion, measuring residual chlorine concentration in water is critical in ensuring safe drinking water. There are several methods of measuring residual chlorine concentration, including test strips, titration, and electrochemical analysis, among others. It is essential to follow the instructions carefully and ensure that the measurement is accurate. Regular monitoring of residual chlorine concentration ensures that the water remains safe for consumption.

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