## 3 The Development of Detection Technology
### Advances in Spectral Method  
Spectral methods include ultraviolet- and visible-spectrophotometer spectroscopy, resonance light scattering, Raman spectroscopy, atomic absorption spectroscopy, near-infrared spectroscopy, etc.  
Compared with the volumetric method, the cyanomercuric sulfate photometric method has the characteristics of higher sensitivity and efficiency, but the interference of halogen elements is high, and the test uses mercury-containing reagents, which is not friendly to the environment. Therefore, many scholars have studied silver nitrate turbidimetry, which uses the positive correlation between the absorbance value of the suspension generated by the reaction of Cl\({}^{-}\) and silver nitrate to determine Cl\({}^{-}\) concentration [57; 58; 59]. However,the silver chloride precipitation is unstable, and stabilizers such as polyethylene glycol, glycerin, macrodextrin, and Tween-80 are usually added to the reaction to stabilize the suspension; the precipitation generation is greatly affected by temperature, so controlling a certain reaction temperature is also key [60].  
[PERSON] and [PERSON] [61] found that silver chloride aggregates produce strong resonance-scattering light, and the resonance-scattering light intensity with a maximum peak of 571 nm had a linear relationship with Cl\({}^{-}\) concentration in the range of 1.42-8.52 ng/mL; the detection limit was 0.71 ng/mL. [PERSON] et al. [62] established a detection method combining Raman spectroscopy and silver nitrate turbidimetry. Under acidic conditions, a silver chloride colloid was obtained by reaction with acetone as stabilizer, and then the Raman spectra of silver chloride colloid were collected by a small Raman spectrometer. The Gaussian fitting peaks of the Raman spectrum at 1050 and 1635 cm\({}^{-}\) had a good linear relationship with Cl\({}^{-}\) concentration. The detection range was 0.10-4.00 mg/L, and the detection limit was 0.015 mg/L. This method has been successfully applied to the detection of Cl\({}^{-}\) in sulfur-containing sewage. Atomic absorption spectrometry was an indirect method for the determination of Cl\({}^{-}\). After the known amount of silver ions reacted with the Cl\({}^{-}\) in the sample, the silver chloride was filtered to obtain the supernatant, and the remaining silver ions in the supernatant were measured to obtain the content of Cl\({}^{-}\) indirectly. [PERSON] et al. [63] indirectly measured the Cl\({}^{-}\) content in the dry base of molecular sieve powder using an atomic absorption spectrometer. By using a nitric acid digestion sample, the Cl\({}^{-}\) recovery rate was more than 95%, and the minimum detection limit was 0.024 mg/L. Laser-induced breakdown spectroscopy has been reported to be used in the detection of Cl\({}^{-}\) concentration on steel surfaces [64]. Due to the high ionization energy of Cl\({}^{-}\), the sensitivity of direct laser-induced breakdown spectroscopy to Cl\({}^{-}\) element detection was poor. [PERSON] et al. [65] proposed a new technique of indirect laser-induced breakdown spectroscopy to improve detection sensitivity. The method indirectly detected the excess silver after the precipitation reaction of silver ions and Cl\({}^{-}\) to detect Cl\({}^{-}\) in water, and the limit of quantification was 5 mg/L. Near-infrared spectroscopy had the advantages of simple sample pretreatment and environmental friendliness. [PERSON] et al. [66] used NIR spectroscopy to non-destructively detect the content of Cl\({}^{-}\) in cement paste. The near-infrared spectra of different cement pastes containing different Cl\({}^{-}\) concentrations (ordinary Portland cement, high fast-setting cement, b-type slag cement) were compared. It was found that there was a correlation between Cl\({}^{-}\) concentration and NIR absorption value in cement, and the correlation between NIR absorption intensity and the measured value of Cl\({}^{-}\) content in ordinary Portland cement was the best. Therefore, this method is more suitable for the detection of Cl\({}^{-}\) in ordinary Portland cement.