Chlorine Testing: Protecting Your Family’s Health One Drop at a Time

Maintaining your family’s health and safety requires routine chlorine testing. Find out what options you have and how to take action.

Are you in charge of ensuring the safety of the water supply within your business or home? In such a case, regular chlorine testing is crucial.

Water containing chlorine is typically used to clean laundry, dishes, and pool sanitation. Chlorine kills harmful bacteria in water, thus making it an integral part of effective water treatments. The presence of chlorine helps to protect individuals from many diseases due to the consumption of contaminated water. Therefore, testing the chlorine levels in an extensive-scale water supply system is essential.

This article presents a complete procedure to accurately and effectively test for chlorine levels. It will cover topics such as testing tools, pre-test preparations, and different sample collection techniques, providing you with well-researched information about how to carry out chlorine testing procedures appropriately.

Introduction to Chlorine and Its Uses

Chlorine, represented by the chemical symbol Cl2, is one of the top ten highest-volume chemicals produced in the United States. It is manufactured commercially through saltwater electrolysis (sodium chloride brine). It has various applications in the industry and household cleaning products.

Chlorine has a wide range of applications. It is utilized for disinfecting water and plays a crucial role in the sanitation of sewage and industrial waste. In producing paper and cloth, chlorine is employed as a bleaching agent. It is also found in cleaning products, including household bleach and chlorine dissolved in water. Additionally, chlorine produces chlorides, chlorinated solvents, pesticides, polymers, synthetic rubbers, and refrigerants.

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Why Is Chlorine Used in Water Globally?

Chlorine is added in small amounts as a disinfectant because of its multiple benefits. It effectively oxidizes natural substances and is an effective disinfecting agent.

In recent decades, the shift to chlorination of water has been linked to a significant improvement in public health. Treating water with chlorine-based disinfectants eliminates pathogens such as bacteria, viruses, and protozoa, which can cause illnesses if consumed.

You might ask the rationale behind adding chlorine to our water despite its potential health risks. Why not consider alternative methods for eliminating microorganisms?

The reason for this is that chlorine is relatively inexpensive and easily accessible. Other disinfection methods, such as reverse osmosis and UV purification, can be costly on a large scale. Therefore, public water systems often opt for using total chlorine to disinfect water as it is cost-effective and readily available.

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Testing Water for Chlorine: What Are Your Options?

Here are the main chlorine testing available on the market.

1. DPD Colorimetric Method

The DPD method is traditionally the most widely used method for measuring chlorine levels in water samples. It is a colorimetric method used to determine the concentration of free and total chlorine.

DPD is a colorless compound that turns magenta pink in the presence of oxidizing agents such as chlorine. The addition of potassium iodide in a second step can determine the combined chlorine concentration. The DPD method is the only colorimetric method that allows the differentiation of the different forms of combined chlorine. It can be used for both visual and photometric testing.

How to use:

  1. Collect a water sample: Obtain a sample from the source you wish to test. 
  2. Add DPD reagent: Add a measured amount of DPD reagent to the water sample. 
  3. Check color change: Observe the color change in the water sample. The color should vary to magenta-pink if chlorine is present. 
  4. Compare to standard: Compare the color of the water sample to a standard color chart to determine the chlorine concentration. To determine the combined chlorine add potassium iodide reagent, observe the color change, and compare it with a classic color chart. 
  5. Record the results: Record the test results, including the chlorine concentration and other relevant information. 

Note: The specific procedure may vary depending on the type of DPD reagent and equipment used, but these are the basic steps for using the DPD colorimetric method.

Main Points:

✅ Free and total chlorine measurements

✅ Economic and effective method for water testing

❌ Not suitable for sizeable water-scale chlorine measurement

2. Potassium Iodide Colorimetric Method

Higher-range chlorine levels in water samples can be measured using potassium iodide reagents. When chlorine reacts with potassium iodide in an acidic solution, iodine is released, and the solution turns brown. 

The chlorine concentration can be determined by analyzing the color of the solution through visual or photometric methods. This method can measure chlorine concentrations as high as 0.5 mg/L and above, but it should be noted that it does not differentiate between free chlorine and total chlorine in the sample.

How to use: 

  1. Prepare a water sample for testing. 
  2. Add a known amount of potassium iodide reagent to the water sample. 
  3. Add an acidic solution to the sample to start the chlorine and potassium iodide reaction. 
  4. Observe the color of the solution and compare it to a color chart or use a photometer to measure the intensity of the color. 
  5. Use the color intensity measurement or the color chart to calculate chlorine concentration in the sample. 
  6. Record the results and repeat the process as necessary.

Main Points

✅ Large-scale water chlorine measurement

✅Economical and effective water testing method

❌Total chlorine measurement only

3. Test Strips

Test strips are a visual method of semi-quantitatively measuring free and total chlorine. They work by using an indicator applied onto the strip, which will change color in the presence of chlorine. Some common indicators used on free chlorine test strips include tetramethylbenzidine (TMB) and syringaldazine (FACTS).

Test Strips

How to use:

  1. Obtain a test strip specifically designed to measure chlorine levels. 
  2. Dip the strip into the water sample for a specific period per the manufacturer’s instructions. 
  3. Remove the strip from the water and wait for a certain amount of time as per the manufacturer’s instructions. 
  4. Compare the indicator’s color on the strip to the color chart provided by the manufacturer. 
  5. The indicator’s color will indicate the chlorine concentration in the water sample. 

Main Points 

✅Free and total chlorine measurements

✅High range chlorine measurements, upto 750mg/L

❌Estimate chlorine result

❌Subjective test method

❌Light conditions affect the perception of color

4. Digital Colorimeters

A more accurate method for measuring chlorine levels is using a digital colorimeter. This device employs two different techniques. One such technique is the addition of a DPD tablet to the water sample, which causes a color change, indicating the presence of chlorine.

Another approach used by digital colorimeters is the insertion of a vial into the device. The instrument then reads the intensity of the color change by emitting specific wavelengths. This method gives quick and precise measurements of chlorine levels.

How to use:

  1. Obtain a digital colorimeter that is specifically designed to measure chlorine levels. 
  2. Prepare a water sample for testing. 
  3. Follow the manufacturer’s instructions to calibrate the colorimeter properly. 
  4. Insert the DPD tablet or a vial as per the method used by the colorimeter.
  5. Dip the colorimeter into the water sample for a specific period per the manufacturer’s instructions.
  6. Read the results displayed on the colorimeter and record the results.

Main Points:

✅Easy to use

✅Digital reading

✅Fast and accurate reading

❌Low sensitivity

❌ Can’t detect colorless compounds

5. Laboratory Testing

They can obtain accurate and precise measurements of chlorine levels in water through certified laboratory testing. This process involves sending a water sample to a laboratory for analysis. The lab will perform the necessary tests and provide the results to the client within a few days.

Laboratory Testing


In conclusion, chlorine testing is a necessary procedure to ensure safe water. It involves measuring the amount of chlorine in a sample of water and comparing it to standards set by the government. Chlorine testing is necessary to protect people from dangerous bacteria and other contaminants found in water. It is a simple process that can help keep us healthy and safe. Happy testing!

Related: 10 Best Testing Kits, How to Drain Water From Pool After Rain, Chlorine-Free Pools

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