Chlorine is an important chemical element that is widely used to provide clean drinking water around the world. Chlorine plays a key role in water quality monitoring because it can effectively kill bacteria through chemical reactions. Whether it is in swimming pool treatment or water treatment facilities, the appropriate use of chlorine can effectively keep water pure and safe. This article will give readers a comprehensive introduction to chlorine from the definition, types, applications and monitoring methods of chlorine.

1. What is Chlorine?

Chlorine is a highly toxic gas with a strong pungent odor. It is green at room temperature and pressure and is easily soluble in organic solvents. It has a suffocating odor, a pungent odor, and is corrosive. Chlorine can generally be liquefied when cooled to -34.5°C at 1 atmosphere and is usually stored in a cylinder for use. At room temperature, the chlorine cylinder is under pressure and the temperature must not exceed 50°C. When used, liquid chlorine can be dissolved in water after being vaporized. 

In nature, chlorine exists mainly in the form of Cl- ions in minerals or seawater, and a small amount of chlorine exists in the atmosphere in a free state. However, free chlorine gas will be affected by ultraviolet rays and decompose into two chlorine atoms (free radicals). In addition, chlorine gas is one of the main elements that destroy the ozone layer.

2. 3 type of Chlorine

2.1 Free chlorine

Free chlorine refers to the amount of chlorine that has not yet combined with chlorinated water to effectively disinfect pollutants, which means that this chlorine can freely remove harmful microorganisms in swimming pool water.

2.2 Combined chlorine

Combined chlorine refers to compounds formed by the combination of chlorine with other elements, usually including chlorides, chlorates and organic chlorides. Chlorides are the most common combined chlorine, such as sodium chloride (table salt) and calcium chloride. Combined chlorine is widely used in industry, agriculture and households, such as disinfectants, bleaches, pesticides, etc. The existence of combined chlorine is essential for water treatment and environmental protection, but some organic chlorides are potentially harmful and may have an impact on the environment and health, so they need to be strictly managed and monitored.

2.3 Total chlorine

Total chlorine is the total amount of chlorine in water, including free chlorine and combined chlorine. Free chlorine mainly refers to chlorine gas or hypochlorous acid, which is closely related to water treatment and disinfection. Combined chlorine refers to chlorides formed by combining with organic matter or ammonia nitrogen. The determination of total chlorine is crucial for drinking water, swimming pool water and wastewater treatment, and can reflect the disinfection effect and pollution of water. The appropriate use of chlorine can effectively control bacteria and viruses, but excessive use can cause water pollution and harm to health. Therefore, regular monitoring of total chlorine content is an important part of water quality management.

3. Benefit of Chlorine

Compared with other detergents, chlorine has some significant characteristics and can effectively remove pollutants, bacteria, etc. in water

Disinfection and sterilization: Chlorine is widely used for disinfection of drinking water and swimming pools. It can effectively kill bacteria, viruses and other pathogens in water, ensuring water quality safety and reducing the risk of water-borne diseases.

Bleach: Chlorine-based bleach is widely used in homes and industries for cleaning and bleaching, removing stains and enhancing the brightness of white fabrics and papers.

Environmental purification: Chlorine compounds are used in some cases to treat industrial wastewater and sewage to help remove harmful substances and protect the environment.

4. Difference between Free Chlorine and Total Chlorine

The main forms of chlorine are free chlorine and total chlorine. Free chlorine refers to chlorine in a free state in water. It can effectively kill bacteria and other pathogens and is an active disinfectant in the water treatment process. Total chlorine includes free chlorine and combined chlorine. Combined chlorine refers to chlorine compounds formed after reacting with organic matter such as ammonia and urea. It has weak disinfection ability and mainly exists in the form of chloramines, which may cause odor and irritation in water. In water quality monitoring, free chlorine is usually an important indicator for evaluating the disinfection effect of water, while the detection of total chlorine helps to determine the presence of combined chlorine in the water body. Ideal water quality should maintain a reasonable free chlorine concentration to ensure the disinfection effect, while controlling the total chlorine level to avoid potential side effects.

5. Application of Chlorine

5.1 How does a water plant use chlorine as a disinfectant

Water plants use chlorine as a disinfectant mainly to effectively kill pathogenic microorganisms in water and ensure the safety and hygiene of drinking water.

The chlorine disinfection process is usually divided into several steps.

First, during the water treatment process, the water plant will add chlorides such as chlorine gas, sodium chloride or liquid chlorine to the treated water. While ensuring that the chlorine concentration in the water is appropriate, the chlorine molecules will release chloride ions, which can quickly react with bacteria, viruses and other microorganisms in the water to destroy their cell structure, thereby achieving a disinfection effect.

Second, the chlorine dosage and contact time will be adjusted according to water quality and demand to ensure the thoroughness of disinfection. At the same time, the water plant will regularly test the residual chlorine in the water to ensure that the disinfection effect is sustained and effective.

Finally, after chlorine disinfection, the chloride in the water will be removed through clear precipitation and filtration steps to ensure that the final water supply will not cause harm to the human body. In short, chlorine as a disinfectant is an important part of water treatment in water plants and helps to ensure the safety of drinking water.

5.2 How chlorine purifies swimming pool water

Chlorine is a commonly used disinfectant in swimming pool water purification. Its main function is to kill bacteria, viruses and other harmful microorganisms in the water, thereby ensuring the safety of water quality. After chlorine is added, it reacts with water molecules in the water to form chlorine gas and hypochlorous acid, the latter of which is a powerful disinfectant that can effectively oxidize and destroy the cell structure of pathogens.

As chlorine is continuously released, it reacts with organic matter and dirt in the water to form chloramines. Although these substances have certain disinfection functions, their long-term presence may cause water quality to deteriorate and cause odor. Therefore, it is very important to maintain an appropriate chlorine concentration and change the water regularly.

In addition, in order to improve the disinfection effect of chlorine, the pH value of the swimming pool needs to be controlled in an appropriate range (generally 7.2-7.6), which helps the activity of chlorine and makes it work efficiently. In these ways, chlorine can effectively purify and maintain the cleanliness and safety of swimming pool water.

6. How to monitor Free Chlorine and Total Chlorine

The methods for residual chlorine monitoring mainly include residual chlorine test strip method, DPD method (dichlorophenylarsine diisocyanate method), residual chlorine sensor, etc.

Among them, the test strip method is a simple and quick method for residual chlorine detection. When used, the test strip containing a specific chemical reagent is immersed in the water sample, and the concentration of residual chlorine is evaluated by observing the color change on the test strip and comparing it with the color standard.

The DPD method is a commonly used residual chlorine determination technology. This method uses dichlorophenylarsine diisocyanate as an indicator to react with the residual chlorine in the water sample to produce a specific color change. The residual chlorine concentration can be accurately determined by measuring the degree of color change by colorimetry or photometry.

Yosemitech Y535-A Free Chlorine Sensor uses a negative pulse with a specific voltage amplitude applied between the polarized electrode and the reference electrode to selectively select free chlorine to participate in the reaction, so that it undergoes an electrochemical reaction at the cathode of the electrode, thereby forming a current signal proportional to the free chlorine concentration. The sensor calculates the concentration of free chlorine by collecting and analyzing the current signal.

Y535-A Free Chlorine Sensor-Water Quality Sensor Manufacturer-Yosemitech (yosemitesensors.com)

In addition, the methods for determining total chlorine also include DPD method, titration method and chloride electrode method. These methods can simultaneously determine the total chlorine concentration in the water sample, including free residual chlorine and chlorine disinfection by-products.

Titration is a quantitative analysis technique. Its principle is to gradually add chemical reagents of known concentration to the water sample until it reacts completely with the total chlorine in the water sample. By recording the volume of the reagent used, the concentration of total chlorine can be calculated.

7. Conclusion

Free chlorine and total chlorine content are important monitoring parameters in water treatment and sanitation facilities. Among them, free chlorine is a key indicator to ensure that water quality is safe for drinking, while total chlorine is used to evaluate the effect of chlorine disinfection and its impact on the environment.

Here at Yosemitech, we can provide you with the sensors and monitoring equipment that you need to consistently monitor chlorine. Contact us right now, if you need help identifying which products are right for you.