Chlorine sensor: the "sentinel" of water quality safety and its working principle

In the fields of water treatment, drinking water safety, swimming pool management, and industrial circulating water, the content of residual chlorine is a crucial indicator. It represents the continuous disinfection ability of water bodies and is the key line of defense to prevent the growth of microorganisms and ensure the safety of water quality. Accurate and real-time monitoring of residual chlorine concentration cannot be separated from the precise "sentinel" - residual chlorine sensor. This article will deeply analyze the working principle of several mainstream chlorine sensor.

一、Electrochemical Method: The Main Force of Precise Measurement

Electrochemical sensors are currently the most widely used and mature technology for detecting residual chlorine, mainly divided into current type and potential type:

1. Current-type (ampere-type) sensor:

•     Core structure: It contains at least two electrodes (working electrode and counter electrode, and usually a reference electrode), a selective permeation membrane, and an internal electrolyte.
•     Principle: The front end of the sensor is covered with a selective gas permeation membrane (such as PTFE). The free residual chlorine (mainly HOCl) in the water diffuses through this membrane and  enters the electrolyte inside the sensor. A reduction reaction occurs on the surface of the working electrode (usually an inert metal such as gold or platinum, to which a specific negative voltage is          applied):

       HOCl + H₂⁺ + 4e⁻ → 2Cl⁻ + 2H₂O

•     Signal conversion: The current produced by this reduction reaction is proportional to the number of HOCl molecules diffusing into the sensor, and the number of molecules is proportional to the concentration of free chlorine in the water sample. By measuring this weak current signal (called the limiting diffusion current), the free chlorine concentration in water can be accurately calculated.
•   Features: Fast response, high measurement accuracy, good stability, suitable for online continuous monitoring. Regular maintenance (replacing the membrane and electrolyte) is required.

2. Potentiometric sensors:

•     Core structure: Similar to a pH glass electrode, it usually contains a measuring electrode (an electrode sensitive to residual chlorine) and a reference electrode.
•     Principle: The sensitive membrane of the measuring electrode (such as metal/metal oxide membrane) reacts with the residual chlorine in the water sample, causing the potential (voltage) on the electrode surface to change. This potential change follows the Nernst equation and is linearly related to the logarithm of the residual chlorine concentration within a certain range. The concentration of residual chlorine can be calculated by measuring the potential difference between the measuring electrode and the reference electrode.
•     Feature:  The structure is relatively simple, and the maintenance quantity may be lower than that of current-type. The response speed is generally slower than that of the current type, and may be slightly affected by factors such as the ionic strength of the water body.

二、Optical Method: A New Star with No Reagents and Rapid Response

Optical sensors are used to detect changes in optical properties caused by the reaction of residual chlorine with specific chemicals, mainly represented by the instrumentalization of colorimetric methods (especially DPD methods):

1. DPD colorimetric sensor:

•     Principle:  This is the automation and online implementation of the laboratory standard method (such as GB / T 5750 . 11 -2006). The core is to automatically add N, N-diethyl-p-phenylenediamine (DPD) reagent to the water sample.
•     Color reaction:  free residual chlorine (HOCl) immediately oxidizes the colorless DPD to form a stable  red compound .
•     Optical detection: The optical system inside the sensor (usually LED light source and photodetector) measures the absorbance (or the intensity of transmitted light) of the red solution at a specific wavelength (e.g., 515 nm).
•     Concentration conversion:  The size of the absorbance is proportional to the amount of red substance generated, and thus is proportional to the residual chlorine concentration in the water sample. The concentration of residual chlorine can be calculated by the standard curve embedded in the instrument.
•     Features: High measurement accuracy, in line with the principle of standard methods. However, it requires the consumption of chemical reagents (DPD), there are operating costs and waste liquid treatment problems, and the maintenance frequency is relatively high.

2. Reflection method (test strip / reagent strip reader):

•     Principle:  Although it is mainly used for portable or intermittent measurement, its principle can also be used in some online or semi-online devices. Place the colorimetric test strip or reagent strip** with residual chlorine in the water sample in the optical reader.
•     Optical detection: The reader measures the color depth (or the change in reflectance) of the colorimetric area of the test strip/reagent strip through reflected light.
•     Concentration conversion:  The color depth is related to the residual chlorine concentration, and the concentration value is obtained through pre-calibration or built-in algorithms.
•     Features:  Relatively simple operation (for portable), lower cost. Accuracy and stability are generally lower than those of current-type or online DPD sensors, making them more suitable for rapid screening or low-accuracy requirements.

三、Other methods

Constant voltage method:  It can be regarded as a variant of the current type. By controlling the voltage between the electrodes to be constant and measuring the change in current required to maintain this voltage to reflect the residual chlorine concentration.

Chemiluminescence method: The principle of using residual chlorine to participate in a specific reaction and produce a light signal is used for detection. It has high sensitivity, but the instrument is complex and is rarely used.

四、Selection and Application Points

The choice of the principle of the residual chlorine sensor depends on the specific application requirements:

• Accuracy and Reliability Requirements: Current-type electrochemistry and online DPD methods usually have the highest accuracy.
• Response speed: The current-type electrochemical response is the fastest.
• Maintenance cost and convenience: Optical method (DPD) requires reagents and maintenance of reaction pool; Current-type electrochemistry requires periodic replacement of the membrane/electrolyte; Potentiometric electrochemistry and reflection methods are relatively simple to maintain.
• Operating cost: The DPD method has continuous reagent consumption costs.
• Application scenario: Online continuous monitoring, preferred current-type electrochemical or DPD method; Portable and fast detection, optional reflective or small electrochemical sensor.

五、 Summary

From the weak current captured from the electrochemical electrode, to the vivid color change produced in chemical reactions, to the quantitative analysis of precision optical devices, the chlorine sensor converts the invisible disinfectant "chlorine" in water into readable and controllable digital signals through different physical and chemical principles. Whether it is the online sensor that silently guards the water quality of the pipeline network, or the handheld device that quickly judges the water quality on the spot, these precise "sentinels" are indispensable key tools for ensuring water quality safety, achieving precise chlorine control, and optimizing water treatment processes. With the advancement of technology, the chlorine sensor is developing towards higher precision, lower maintenance, and smarter integration (such as the Internet of Things), providing solid technical support for safer and more efficient water resource management.

Although the residual chlorine is small, the protection is heavy. Every precise measurement in the sensor is a silent and firm commitment to human water safety.