# Nitrite: From Application to Detection and Development
## 3 Optical-Based Detection Methods
### Spectrophotometry
For example, based on the redox reaction with iodide ions under acidic conditions of nitrite, the developed method combined with microtiter plate detection can achieve the low consumption of reagents required, greatly saving manpower, financial and material resources [29]. In addition, the use of p-aminobenzene sulfonamide and \(\alpha\)-amino-\(\beta\)-imidazolyl propionic acid in water experiments avoids the use of toxic substances [30]. In an acidic solution, sulfathiazole is first treated with sodium nitrite. Then, a phosphate buffer is added. Coupling reagents like N-(1-naphthalenyl)ethylenediamine are not needed [31] (Figure 4b). Additionally, butane dioic acid, tartaric acid, and malic acid can replace concentrated hydrochloric acid as the acidic medium. The detection time can be shortened by 5-14 min, which improves safety and efficiency. In the acidic medium, nitrite reacts with serna to generate a blue dye. Then, the solution is checked for this blue dye, which is a simple and low-cost method [26]. In the specific detection steps of the spectrophotometer, the influence of interfering substances often occurs, which may cause some errors. Therefore, if the error is large, it is undoubtedly fatal to the detection. To eliminate matrix interference, the standard addition method is widely used. However, it is time-consuming and laborious; thus, researchers have developed a feedback addition method coupled with flow injection analysis. The advantage of it is that it can automatically prepare the spiked samples, which greatly improves the efficiency of the operation [32]. In terms of sensitivity, the design of the flow cell as a liquid-core waveguide can effectively improve the sensitivity of the spectrophotometer [33]. Often, the absorbance is used to reflect the concentration; however, some studies have used the inflection point in the derived absorbance for the corresponding determination, which can also serve the purpose well [34]. While achieving simplicity, speed, sensitivity, and environmental protection, the spectrophotometer and smartphone are closely integrated to achieve on-site analysis [35] (Figure 4c). In conclusion, spectrophotometry has high sensitivity, high accuracy, high efficiency and wide applicability. The principle of it has been improved to further simplify the operation steps and reduce the manpower and material resources. New detection instruments have been researched, and a series of sensors based on the principles of traditional spectrophotometry have been made to ensure the stability. In terms of environmental protection, the combination of micro-detection techniques reduces the use of reagents and avoids the generation and exposure of some unnecessary toxic substances, which is conducive to sustainable development. In the future, more efficient and accurate detection methods and technologies should be developed to meet the needs of nitrite detection in different fields. For example, we can combine new technologies such as mass spectrometry and nanomaterials to improve detection sensitivity and selectivity. In addition, standardization work should be strengthened. Furthermore, unified standard system and technical specifications should be established to ensure the comparability and reliability of detection results.