Definition
The residual chlorine online monitor is an analytical instrument used for continuous and automatic measurement of residual chlorine content in water bodies. Residual chlorine usually refers to the chlorine with oxidizing ability that remains in the water after chlorine disinfection, including free residual chlorine (such as hypochlorous acid, hypochlorite ions) and bound residual chlorine (such as chloramine). This instrument is widely used in situations where real-time monitoring and control of the disinfection process is required to ensure water quality safety and process stability.
Measurement principle
Mainstream online residual chlorine monitoring techniques are based on electrochemical or optical principles. The electrochemical method usually uses membrane ampere sensors, and its working model can be briefly described as follows: residual chlorine molecules selectively permeate the membrane and undergo a reduction reaction at the cathode, generating a diffusion current directly proportional to the residual chlorine concentration. The process can be represented by the following relations:I = nFA(C/δ), where I is the diffusion current, n is the number of electron transfers, F is the Faraday constant, A is the electrode area, C is the residual chlorine concentration, and δ is the thickness of the diffusion layer. The optical law often uses the N,N-diethyl p-phenylenediamine (DPD) colorimetric method, and the residual chlorine reacts with the DPD reagent to produce red compounds, and its absorbance and residual chlorine concentration are in line with the Lamber-Beer's law within a certain range:A = εbc, where A is the absorbance, ε is the molar absorbance coefficient, b is the optical path, and c is the residual chlorine concentration.
Measurement method
The online monitoring system usually consists of a sampling unit, a reaction or sensing unit, a signal processing unit and a control output unit. For electrochemical methods, the instrument measures directly through immersion or flow sensors and has a fast response time. For the colorimetric method, the system will take water samples at regular intervals, automatically add buffers and color developers, and after the mixing reaction, the optical system will detect the color change and calculate the concentration value. Both methods enable periodic or continuous measurements and transmit data to the monitoring system.
Influencing factors
The accuracy of the measurement results is influenced by several factors. Water temperature affects the rate of chemical reaction and sensor permeability. pH significantly affects the presence of free chlorine and bound chlorine, which usually requires pH compensation. Oxidizing substances (e.g., ozone, bromine, manganates) or reducing substances (e.g., ferrous ions, sulfides) present in water can interfere with the determination. The flow rate, pressure, and turbidity of the water sample may affect the sensor contact or optical path. In addition, sensor membrane aging, reagent failure, or optical window contamination can also lead to measurement deviations and require regular maintenance and calibration.
Applications
Residual chlorine online monitors play an important role in water treatment processes in several industries. In drinking water treatment, disinfectant residues used to monitor the end of the pipe network or secondary water supply ensure water supply safety. In industrial circulating cooling water systems, it is used to control the dosing of fungicides to prevent microbial growth and corrosion. In swimming pool water treatment, it is used to maintain the appropriate level of disinfection. It is also commonly used in water quality management in the food and beverage industry, as well as in aquaculture water treatment. These applications help automate dosing, optimize costs, and meet relevant water quality standards.
Selection considerations
When choosing a suitable online residual chlorine monitor, multiple technical parameters and application conditions need to be comprehensively considered. The measurement range should cover the expected concentration for process control. According to the principle of medium characteristics: the electrochemical method is suitable for continuous and rapid monitoring, and the colorimetric method may have strong anti-interference ability. It is necessary to confirm whether the measurement accuracy and response time of the instrument meet the control requirements. Evaluate the installation environment and select the appropriate protection level and material. Consider maintenance needs such as calibration cycles, reagent consumption, or sensor replacement frequency. At the same time, the instrument's output signal type, data interface, compatibility with existing control systems should also be taken into account. It is recommended to refer to the method requirements for residual chlorine determination in relevant international standards (such as ISO, ASTM) or national and industry standards, and perform verification tests in combination with actual water samples.