Potentiometric Free Chlorine Detection without Using Conventional Reference Electrodes

Abstract

Introduction Water chlorination is one of key process for supplying safe drinking water. The concentration of fee chlorine has to be monitored as an important index to make the process efficient and is mainly measured at water treatment plant. For realizing smart water supply system based on various IoT technologies, low cost and maintenance-free type sensor which can be easily applied to whole pipeline network system including water outlet on every home would be required. Various kinds of analytical techniques for detection of free chlorine concentration have been proposed and practically utilized. Comparing with regent-type colorimetric methods, electrochemical methods are suitable for in-line monitoring with simplicity and low cost. So far, many amperometric sensors have been proposed [1, 2], Recently, the authors reported a simple sensor device which is composed of two dissimilar electrodes [3]. The open circuit potential of each electrode has a different sensitivity to free chlorine and then the differential signal is extracted as sensor output. In this study, sensing characteristics of this sensor device in flowing-tap-water system were evaluated. Principle The schematic diagram of simple two-electrode system to enable in-line monitoring of free chlorine concentration is represented in Fig.1. Ideally, the sensor device consists of detecting and compensating metal electrodes that are active and inactive to free chlorine as target compound, respectively. Furthermore, sensitivities to interferences, such as temperature, pressure, pH, conductivity, and so on, have to be almost same in order to self-compensate the fluctuation of various environmental factors. In our previous study [3], it was found that platinum and SUS316 electrode are suitable for detecting and compensating electrode, respectively. The difference in open circuit potential could be explained by mixed potential theory. This passive-type sensor which utilize its electromotive force between dissimilar electrodes would have many advantages of low cost, low power consumption and these features enable long-term use of sensor device without periodical maintenance. Method Pt and SUS316 were used as test electrodes. These metal rods were embedded in center of a Teflon mold. Prior to use, each electrode surface was polished with SiC paper and ultrasonically rinsed with ethanol. In order to validate the sensor performances of Pt-SUS316 pair of electrodes in flowing-tap-water system, these electrodes were directly inserted in pipeline, as shown in Fig. 1. Practical tap water with or without free chlorine was fed into this test pipeline at various flow rate by using activated carbon filter which can selectively remove free chlorine. The differential open circuit potentials of Pt-SUS316 pair were measured using a high-input impedance electrometer. The concentration of residual chlorine was checked by free chlorine portable photometer based on DPD method. Results and Conclusions As a first step, the baseline drift behavior of Pt-SUS316 electrode pair was checked, as shown in Fig. 2. Baseline drift was defined as the variation in sensor output from that at the start time of each experiment. Although a slight baseline drift was observed, the value of the drifts were within the range of ±20 mV. However, trend of the drift could not be clarified. It would be caused by tiny fluctuation of environmental conditions such as temperature, pH, etc. Typical sensor performance in flowing-tap-water system was described in Fig. 3. Potential difference (sensor output) between Pt and SUS316 was around 200 mV in practical tap water without free chlorine. It indicates that open circuit potential of Pt was higher than that of SUS316 under this condition. When the flow experiment started, the baseline level was slightly fluctuated. After the level was stabilized, tap water containing free chlorine (0.6 mg/L) flow thorough the test pipeline. The senor output sharply increased and reached to steady state within 10 min. This large response was mainly attributed to high sensitivity of pt to free chlorine. Therefore, concentration of free chlorine could be monitored under in-line condition without using conventional reference electrodes. On the other hand, recovery rate was very slow and seem to become slower. However, reproducibility of the sensor response with repeated exposure to free chlorine was good since the steady state values almost unchanged.