Chlorine Disinfectant Applications, Residual Chlorine Control & Detection Technology Evolution

I. Chlorine Disinfectants: Safety Considerations Behind Widespread Applications

With their high-efficiency bactericidal ability, chlorine disinfectants have become a "conventional weapon" in scenarios such as tap water purification, swimming pool water quality maintenance, and tableware disinfection. They play an irreplaceable role both in safeguarding residents' daily drinking water safety and in blocking microbial transmission in public places. However, while killing harmful microorganisms, chlorine disinfectants react with organic substances in water to produce a variety of disinfection by-products. As health awareness improves, the safety of water quality after chlorination disinfection has gradually become a core focus of the public and the industry.

II. Residual Chlorine: A Double-Edged Sword for Water Quality Safety

Residual chlorine content is a key indicator for evaluating water disinfection efficacy, and its level is directly related to water quality safety:

1. Necessity of Appropriate Residual Chlorine: After water is treated with chlorine-containing disinfectants, a certain amount of residual chlorine (existing in forms such as hypochlorous acid and hypochlorite ions) must be retained to continuously inhibit the reproduction of residual bacteria and viruses in water, ensure stable water quality during transportation, and avoid "secondary pollution".

2. Hazards of Excessive Residual Chlorine: Excessive residual chlorine not only may cause secondary pollution of water quality, but also promote the formation of carcinogens (such as trihalomethanes). Long-term consumption of such water may induce health problems such as hemolytic anemia. Therefore, precise control and detection of residual chlorine content throughout the water supply treatment process is a core link to ensure drinking water safety.

III. Iteration of Residual Chlorine Detection Methods: From "Inefficient and Harmful" to "Rapid and Safe"

In the past, the detection of residual chlorine and total chlorine in water mainly relied on the o-tolidine method and iodometric method, but these two methods have obvious limitations:

• The operation process is cumbersome, the analysis cycle is as long as several hours, and professional technicians are required for the entire operation, which cannot meet the needs of emergency testing and on-site real-time testing;

• The o-tolidine reagent has definite carcinogenicity, posing health risks to laboratory personnel.

For this reason, the Ministry of Health of the People's Republic of China officially eliminated the o-tolidine method and recommended the DPD spectrophotometry as a new-generation residual chlorine detection technology in the Sanitary Standards for Drinking Water issued in June 2001, solving the dual pain points of "inefficiency" and "harmfulness" of traditional methods.

IV. Three Core Existing Forms of Chlorine in Water

To understand the significance of residual chlorine detection, it is necessary to first clarify the different existing forms of chlorine in water:

V. Detection Principle of DPD Spectrophotometry

DPD (namely N,N-diethyl-p-phenylenediamine) is the core reagent of this method: when DPD comes into contact with free residual chlorine in water, it rapidly undergoes a redox reaction to form a stable red compound. The depth of the red color produced by the reaction is linearly correlated with the content of free residual chlorine in water — by measuring the absorbance of the red solution with a spectrophotometer and comparing it with a standard curve, the concentration of free residual chlorine in water can be calculated quickly and accurately. This method not only features simple operation (no complex pretreatment required) and a detection cycle shortened to a few minutes, but also avoids the use of harmful reagents, making it particularly suitable for on-site real-time detection scenarios.

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