# Research on On-Line Detection Method and System for Nitrate in Groundwater
## 1 Introduce  
With the rapid development of industry and agriculture and the expansion of urban population, nitrate (NO\({}_{3}\)\({}^{-}\)) has become one of the most common pollutants in surface water and groundwater worldwide. The application of large amounts of nitrogen fertilizers in agricultural production and the direct discharge of domestic sewage and nitrogen-containing industrial wastewater all lead to an overall upward trend in nitrate concentration in natural water bodies, which has become an important environmental problem [1, 2]. At the same time, nitrate has strong solubility; its migration rate in water or soil is very fast, so once it enters the natural environment, it is widely dispersed through various circulation activities in nature and thus nitrate widely exists in the natural environment and ecosystem. Many studies have proved that excessive nitrate content in water can cause serious environmental and physiological problems [3, 4, 5]. When the nitrate content in water exceeds the standard, it will accelerate the eutrophication of water and poison aquatic animals and plants. Long-term consumption of drinking water with excessive nitrate will also significantly increase the risk of muscle cramps, methemoglobin disease and stomach cancer [6]. Excessive nitrate in the water will seriously affect drinking water safety andfishery production. Therefore, the detection of nitrate concentration in water is of great practical significance. At present, nitrate detection methods mainly include spectrophotometry, chromatography, luminescence analysis, capillary electrophoresis, electrochemical detection, etc. [7; 8; 9; 10]. Among them, gas-phase molecular absorption spectrometry, ultraviolet spectrophotometry and phenol disulfide acid spectrophotometry are standard methods for nitrate detection. Such detection methods based on optical principles have the advantages of low detection limit, good accuracy and high sensitivity [11; 12; 13; 14; 15; 16], but expensive optical analytical instruments are required for detection. The commercial testing equipment developed based on this principle usually has the problems of large volume, stringent water sample pretreatment requirements and high price, which cannot fully meet the practical needs of on-site, rapid and multi-point testing in China's vast waters. The nitrate detection method based on electrochemical principle has the advantages of simple operation, fast response speed, high sensitivity and less required reagents, which is conducive to the realization of on-site, rapid and portable detection. Therefore, in recent years, researchers have paid attention to this issue, and a variety of electrochemical electrodes for nitrate detection have been developed [17; 18; 19; 20; 21; 22; 23; 24]. Most of these studies focused on the optimization of modification methods and the selection of sensitive materials to improve the detection sensitivity of the electrode to nitrate, but rarely considered the service life and maintenance cycle of the electrode [25; 26]. When the electrochemical electrode is applied in the aqueous environment, the analyte to be measured will have an electrochemical reaction on the surface of the sensitive material, and the concentration of the analyte can be measured by examining the response signal. This detection method is efficient: it requires simple equipment, is low cost and has high sensitivity. However, in specific applications, for the same electrode, with the increase in detection times, the reaction product will gradually adhere to the surface of the sensing material so that its effective area is gradually reduced and the response signal of the modified electrode is also reduced, which is called the electrode passivation effect. Due to the existence of the passivation effect, when the electrochemical sensor is used for detection, the sensor probe must be calibrated regularly or a disposable probe must be used, and this frequent manual maintenance work greatly limits the promotion and application of electrochemical sensors [27; 28; 29].