I. Basic Concepts of Ammonia Nitrogen and Total Nitrogen
In water quality monitoring, nitrogen-containing compounds are important indicators for evaluating water eutrophication. Organic nitrogen includes nitrogen-containing organic matter such as proteins, amino acids, and urea, which can gradually decompose into forms such as NH₃, NH₄⁺, NO₂⁻, NO₃⁻ through biological oxidation under aerobic conditions.
Among them, NH₃ and NH₄⁺ are called ammonia nitrogen, NO₂⁻ is called nitrite nitrogen, and NO₃⁻ is called nitrate nitrogen. These forms respectively represent different stages of the conversion of organic nitrogen to inorganic substances, and their contents can all serve as water quality indicators.
Total nitrogen (TN) is a comprehensive water quality indicator that includes all contents from organic nitrogen to nitrate nitrogen. The contents of all nitrogen-containing organic and inorganic substances are calculated as mg/L of nitrogen (N₂).
II. Measurement Methods for Ammonia Nitrogen
The measurement of ammonia nitrogen mainly uses the spectrophotometric method or the ammonia gas-sensing electrode method. The two methods have their own characteristics and are suitable for different scenarios.
2.1 Spectrophotometric Method
The principle of the spectrophotometric method in online ammonia nitrogen analyzers is as follows:
- In the presence of sodium dichloroisocyanurate, ammonium ions react with sodium salicylate to form a blue compound;
- This compound has maximum absorption at a wavelength of 660nm and can be measured by spectrophotometry;
- To prevent interference from water sample turbidity, the measurement light is compared with a scattered reference light beam at a wavelength of 880nm.
2.2 Ammonia Gas-Sensing Electrode Method
The ammonia gas-sensing electrode method is based on the principle of ion-selective electrodes; for details, please refer to the relevant ion-selective electrode section. This method is suitable for specific water quality conditions, and actual working conditions need to be considered during selection.
III. Automatic Monitoring Methods for Total Nitrogen
Automatic monitoring of total nitrogen mainly uses the ultraviolet absorption method and the chemiluminescence method. The two methods differ in principle, anti-interference capability, and applicability.
3.1 Ultraviolet Absorption Method
The ultraviolet absorption method is based on the national standard GB 11894-89, in which nitrogen-containing compounds are decomposed and oxidized to NO₃⁻ with potassium persulfate, and then total nitrogen is measured by the ultraviolet method.
Note: This method is susceptible to interference from bromide ions and should be used with caution in water with high bromide content.
3.2 Chemiluminescence Method
The chemiluminescence method has no interference and is considered the preferred method for automatic online monitoring. Its workflow is as follows:
- The carrier gas brings the water sample into a reaction tube equipped with a catalyst;
- Through high-temperature (700～950℃) low-temperature closed combustion, nitrogen-containing compounds are oxidized to NO;
- NO then reacts with O₃ generated by an ozone generator, and the emitted chemiluminescence intensity is measured.
IV. Method Comparison and Selection Recommendations
To facilitate selection by engineering technicians and procurement personnel, the following table compares the characteristics of the main measurement methods for ammonia nitrogen and total nitrogen.
| Measurement Item | Method | Key Principle | Interference Factors |
|---|---|---|---|
| Ammonia nitrogen | Spectrophotometric method | Reaction with sodium salicylate to form a blue compound, absorption at 660nm, reference at 880nm | Water sample turbidity (compensated by reference light) |
| Ammonia nitrogen | Ammonia gas-sensing electrode method | Ion-selective electrode | See electrode section |
| Total nitrogen | Ultraviolet absorption method | Potassium persulfate oxidation to NO₃⁻, ultraviolet measurement | Bromide ions |
| Total nitrogen | Chemiluminescence method | High-temperature combustion oxidation to NO, reaction with O₃ to measure luminescence intensity | No interference |
In summary, the chemiluminescence method has become the preferred method for online monitoring of total nitrogen because it has no interference, while ammonia nitrogen measurement can choose either the spectrophotometric method or the ammonia gas-sensing electrode method according to on-site conditions.