_Applied_  
_sciences_  
_Review_  
**Review of Chloride Ion Detection Technology in Water**  
**[PERSON] 1\({}^{\copyright}\), [PERSON] 2, [PERSON] 1,3,* and [PERSON] 1**  
1 College of Biosystems and Food Science, Zhejiang University, Hangzhou 310058, China; [EMAIL_ADDRESS] (D.W.); [EMAIL_ADDRESS] (R.Z.) 2 Hanzhou Lohand Biological Technology Co. Ltd., Hangzhou 310018, China; [EMAIL_ADDRESS]  
3 Department of Food Nutrition and Detection, Hangzhou Vocational and Technical College, Hangzhou 310018, China  
* Correspondence: 2016/[EMAIL_ADDRESS]; Tel./Fax: +86-0571-8898-2156  
**Featured Application: Water is the source of life. It is both food for human beings and necessary for the existence of the whole ecological system. With the development of society, there are more and more water pollutants. Water detection technology needs modernization, intelligence, and speed. The chloride ion, an anion that is widespread in nature, will have harmful effects on industrial and agricultural production and ecology when its levels are too high. How to quickly and timely monitor chloride ions is an important question for industrial production, agricultural production, human activities, and ecological health. **  
**Abstract: The chloride ion (Cl\({}^{-}\)) is a type of anion which is commonly found in the environment and has important physiological functions and industrial uses. However, a high content of Cl\({}^{-}\) in water will do harm to the ecological environment, human health and industrial production. It is of great significance to strictly monitor the Cl\({}^{-}\) content in water. Following the recent development of society and industry, large amounts of domestic sewage and industrial sewage are discharged into the environment, which results in the water becoming seriously polluted by Cl\({}^{-}\). The detection of Cl\({}^{-}\) has gradually become a research focus. This paper introduces the harm of Cl\({}^{-}\) pollution in the environment and summarizes various Cl\({}^{-}\) detection methods, including the volumetric method, spectrophotometry method, electrochemical method, ion chromatography, paper-based microfluidic technology, fluorescent molecular probe, and flow injection. The principle and application of each technology are described; their advantages, disadvantages, and applicability are discussed. To goal of this research is to find a more simple, rapid, environmental protection and strong anti-interference detection technology of Cl\({}^{-}\). **  
**Keywords: chloride ion; detection; review; pollution; water**  
**1. Sources and Hazards of Chloride Ion**  
Chloride ions (Cl\({}^{-}\)) are widely distributed in nature. Almost all natural water contains Cl\({}^{-}\), mainly in the form of calcium, magnesium, and sodium salts. Natural water samples from different sources vary in Cl\({}^{-}\) content. The Cl\({}^{-}\) content in seawater can be as high as 19,000 mg/L, while the Cl\({}^{-}\) content in general surface water and ground water is less than 50 mg/L. Although marine organisms have a relatively high tolerance towards Cl\({}^{-}\), the tolerance of terrestrial animals and plants is limited to a certain range. The Cl\({}^{-}\) in natural water comes from natural strata, some salt deposits, and Cl\({}^{-}\)-containing sediments. With the development of society, human activities and industrial production have led to the increase in chlorine ions in the natural environment year by year. For example, road salt has been heavily used in many higher latitude countries in which it usually shows. It is reported that approximately 490,000 and 20,300,000 tons of salt were used in winter on roads in Poland and America, respectively [1], more than 6,000,000 tons of salt used in Canada [2], and 10,000,000 tons of salt used in France [3]. The concentrationsof Cl\({}^{-}\) induced by road salt infiltration have risen from 183 mg/L to 3568 mg/L near Azerailles, France and from 1243 mg/L to 12,086 mg/L in Poland [3,4].