Firstly, the working principle and internal structure of the open residual chlorine sensor are introduced; Secondly, the design of a flow tank based on PVC material is discussed to achieve constant water flow; Then a potentiostat circuit for the three-electrode residual chlorine sensor is designed to ensure the potential stability and control accuracy of the electrode, so that the output of the residual chlorine sensor is more stable; Finally, the sensor calibration method, calibration process, and operating procedures are described, and compared with the DPD method, the test results have a good consistency.
Residual chlorine refers to the total amount of chlorine (active chlorine, hypochlorite and organic chloride) remaining after adding chlorine preparation into the water and interacting with bacteria, microorganisms, organics, and inorganic substances in the water. The disinfection effect of chlorine preparation depends on the concentration of residual chlorine. Therefore, residual chlorine is an important parameter to judge the quality of water after disinfection, and it is also one of the indicators that often need to be measured in water quality monitoring. In order to ensure the safety of water use, the online determination of residual chlorine must be realized.
The detection methods of residual chlorine include chemical analysis, optical method (colorimetry and spectrophotometry), and electrochemical method. The chemical analysis method has a high detection limit and is only suitable for the determination of high-concentration residual chlorine in the water. In the spectrophotometric method, N N-diethyl-p-phenylenediamine (DPD) spectrophotometry is relatively mature for the determination of free residual chlorine in domestic drinking water and source water, but DPD spectrophotometry is not suitable for continuous online measurement of residual chlorine content in water, and is mainly used for on-site analysis of residual chlorine in the water. The residual chlorine sensor based on electrochemistry is a fast, convenient, sensitive, and reliable residual chlorine detection method without pretreatment before measurement. It is suitable for online detection and control of residual chlorine in tap water, swimming pool water, and medical sewage, as well as for routine analysis in laboratories. However, the residual chlorine sensor is expensive and its maintenance cost is high.
There are two types of sensors for online measurement of residual chlorine: open type and membrane type. The surface facial mask of the coated residual chlorine sensor has a great influence on the electrochemical characteristics of the electrode. Although the surface facial mask reduces the interference of other ions, the surface facial mask needs to be replaced frequently, which increases the cost of measurement. Therefore, the design of a long-life surface and high-stability open residual chlorine sensor eliminates the need to replace the membrane and electrolyte and is simple to maintain. It is especially suitable for the detection and control of incoming water and swimming pool water with a single disinfectant.
This paper mainly studies the key technologies of the residual chlorine measurement and control system based on the open sensor, including the structure of the open residual chlorine sensor, the design of the flow cell with constant flow, and the circuit design of the potentiostat for the three-electrode sensor, in order to achieve a residual chlorine monitoring system with high accuracy, low cost and small maintenance, which has a positive significance for the detection and control of residual chlorine.
Structure of open residual chlorine sensor
The open residual chlorine sensor is designed for long life, maintenance free, and low cost to meet the needs of long-term online measurement. The open residual chlorine sensor adopts a three-electrode system. The working electrode (WE) and the counter electrode (CE) use platinum rings nested in the front end of the sensor. The two platinum rings are led out through leads. Since the leads need to pass through the chamber filled with electrolyte, the leads use PTFE-encapsulated fine copper wires. The reference electrode (RE) is an Ag/AgCl electrode with stable potential and small temperature coefficient. The reference electrode is immersed in the electrolyte in the sensor chamber. The electrolyte filled in the sensor is a mixture of 3 mol/L potassium chloride gel electrolyte and liquid electrolyte. The electrolyte is separated from the external environment through conductive rubber so that the internal electrolyte is not easy to lose and does not need to be added again in use. The platinum ring embedded in the front end of the sensor has a thin diameter, while the main chamber of the sensor has a thick diameter. The wide part of the connection between the two is separated from the external environment by conductive adhesive to achieve the balance of internal and external charges.
The characteristics of the open residual chlorine sensor are nonmembrane sensors. Its working electrode (WE) and counter electrode (CE) is open. When measuring, both WE and CE are exposed to the water flow, with strong anti-pollution ability. Platinum is chemically stable, insoluble in strong acid and alkali solutions, and is not easy to be oxidized in water and air, which is convenient for cleaning. The solid potassium chloride gel used in the electrolyte does not need to replace the electrolyte, thus reducing the use cost of the sensor.
The sensor adopts a three-electrode system, and the added counter electrode can counteract the influence of temperature change on the working electrode and improve the selectivity of the sensor. The silver/silver chloride reference electrode is sealed in the sealed electrolyte chamber, the working electrode and the counter electrode directly contact the solution to be measured, and the reference electrode completes the charge exchange with the external solution through the conductive rubber. The residual chlorine in the solution is directly reduced on the working electrode surface to form a current, which is proportional to the residual chlorine concentration in the solution.
Design of flow tank with the constant flow
The open residual chlorine sensor is a membrane-free sensor. The pressure of water flow has no influence on the measurement results, but the flow of water flow has a great influence on the output of the sensor. In the process of measuring residual chlorine, the water flow constantly flows over the electrode surface, and different flow rates affect the redox reaction process on the electrode surface. The flow rate will fluctuate from high to low, which will inevitably cause fluctuations in the measured value of residual chlorine and interfere with the measurement of residual chlorine. The residual chlorine can be measured by using the finished product flow cell on the market, and the price is high. After many experiments, we designed a flow tank with a black PVC pipe and pressure-reducing valve. The test results are very good and it is suitable for domestic use. Its structure is shown in Figure 2. The whole flow cell is composed of black PVC pipes. The water inlet is connected to the main pipe through the adapter, hose, and pressure-reducing valve. The water sample can be recycled back or directly led out from the outlet during measurement. The middle part is used to insert an open residual chlorine sensor, which can ensure that the sensor is immersed in a continuously flowing water sample, and that the platinum ring part has enough space to contact the water flow.
Inflow flow range: 10 L/h~70 L/h. When the flow is greater than 40 L/h, the measurement result is almost independent of the flow, but too large a flow has requirements on the inlet pressure. Connect the water inlet of the constant flow device through the pipe, and the drain pipe connected to the drain outlet shall be vertically downward. Open the water inlet switch and change the water inlet flow by adjusting the pressure-reducing valve to ensure that the flow is at least 20 L/h and not more than 60 L/h; Because the residual chlorine concentration is measured based on the sensor method, the output of the sensor is related to the flow of the water body, so during the measurement process, it is necessary to ensure that the calibrated flow matches the flow during the measurement.
Measuring principle and circuit design of potentiostat
The open residual chlorine sensor is a three-electrode system. When the working electrode and the electrode are applied with a constant voltage of 0.7 V, hypochlorite in the measured water body is reduced on the working electrode, and a current signal is generated. Its current intensity is in direct proportion to the concentration of free residual chlorine. By measuring the current value, the concentration of free residual chlorine in the measured water body can be determined.
The detection circuit for the output signal of the residual chlorine sensor is shown in Figure 3. The circuit consists of a potential generation circuit, a potentiostat circuit, an I-V conversion circuit, and a filter circuit. IC1 and IC2 form a potential generation circuit, the output reference voltage is adjustable, and the input voltage of the U2B positive terminal is changed by adjusting W1 as the reference voltage and supplied to the potentiostat circuit. U2A, U2B, and auxiliary circuits form a potentiostat circuit. The potential of the reference electrode RE is then compared with the reference voltage through U2A to provide a stable polarization voltage for the counter electrode CE. The role of RE is to provide a stable electrode potential in the measurement process to ensure that the potential between the working electrode and the opposite electrode remains stable in the measurement process. Therefore, in the measurement process, the reference electrode must have a known and constant potential, so that the potential of the counter electrode is constant.
Comparison experiment of sensor calibration, use, temperature and pH compensation, and DPD method
The calibration uses two-point linear calibration, and the zero point of the sensor uses ordinary water samples without ozone and chlorine. The slope point shall be calibrated with specific chlorine-containing water sample, and the residual chlorine value of the water sample shall be measured by DPD spectrophotometry or DPD colorimetry as the slope calibration point.
When calibrating the zero point, use a bucket to take about 30 L of ozone-free and chlorine-free water samples, put a small submersible pump into the bucket, connect the water outlet of the pump with the water inlet of the constant current device through a hose, and the water outlet of the constant current device will recycle the water samples back to the bucket through a hose. Cycle the chlorine-free water continuously for more than 1 h, and then start zero calibration. When calibrating the slope, take a bucket of the water sample, first measure the residual chlorine concentration of the water sample with DPD spectrophotometry or DPD colorimetry, then circulate the water sample for more than 1 h with the same method as zero calibration, measure the voltage value output by the sensor, and then combine the actual residual chlorine value to obtain the linear calibration parameters. When the residual chlorine concentration is measured online, it must be matched with the flow during calibration, because the output of the residual chlorine sensor is related to the flow.
Use and maintenance of sensors
The open residual chlorine sensor is suitable for online detection of residual chlorine for a long time to keep the electrode wet and the conductivity of conductive rubber. Because the reference electrode is wrapped in gel and contacted with the measured water sample through a conductive colloid, it must be soaked for a long time before it can be used normally. Therefore, the new sensor should be turned on day and night for a long time before calibration; If the sensor has been out of service for more than 2 hours, run it for more than 3 hours before use.
During the long-term use of the residual chlorine sensor, the surface cleanliness of the platinum ring as the working electrode and counter electrode will affect the measurement accuracy. When the surface of the platinum ring is dirty, the sensor should be taken out of the galvanostat, and the platinum ring should be partially immersed in 5% HCL solution for 30 seconds. After taking out the platinum ring, it can be installed back into the galvanostat after it is washed with tap water.
The residual chlorine sensor is suitable for storage at room temperature. The sensor will not be used in the short term. The sensor should be placed in a bottle containing tap water to ensure that the sensor is wet; If it is not used for a long time, the residual chlorine sensor should be cleaned and stored after the sensor is dry. When it is used again, the residual chlorine sensor needs to be put into the ordinary water sample for a long time before it can be used normally.
The open residual chlorine electrode is used to determine the residual chlorine in water and wastewater. The measurement range of residual chlorine concentration is 0-20 mg/L, and the resolution is 0.01 mg/L. It can be used for the detection of tap water, sewage, swimming pool water, and natural water, but the concentration range of residual chlorine corresponding to different detection objects is different. For example, the content of free residual chlorine in domestic drinking water of chlorinated pipe network shall not be less than 0.3 mg/L after 30 min of chlorination; The standard value of free residual chlorine in artificial swimming pool water is 0.3 mg/L-0.5 mg/L.
The content of free residual chlorine on the surface of tableware disinfected with chlorine-containing detergent shall be less than 0.3 mg/L; The determination range of residual chlorine in industrial circulating cooling water is 0.03 mg/L-2.5 mg/L. Therefore, the appropriate detection range should be set for different detection objects. In addition, the detection of residual chlorine is related to the pH value of the liquid to be measured. When the pH value of the water body to be measured is 5~8, the measured data have a good correlation. If the pH change range of the test liquid is large, the pH electrode shall be used to collect data as compensation. But for the detection of tap water and swimming pool water, the pH is within the appropriate range.