Total nitrogen (TN) and ammonia nitrogen (NH₃-N) are both key indicators reflecting water pollution levels. TN refers to the total content of inorganic and organic nitrogen in water in various forms, while NH₃-N specifically denotes nitrogen present as free ammonia (NH₃) and ammonium ions (NH₄⁺). During sewage disinfection processes, a phenomenon where NH₃-N is greater than or equal to TN has been observed in some water samples. This paper aims to analyze the factors influencing the accuracy of these two indicators.
1.Impacts of Water Sample Preservation
Nitrogen compounds in water samples are dynamically changing. Therefore, samples should be analyzed immediately after collection or stored in a refrigerator for no longer than 24 hours. Additionally, cross-contamination from ambient air must be avoided during sampling—samples should be sealed promptly after collection. Efforts should also be made to minimize errors caused by light-induced stability changes and indoor environmental fluctuations.
2.Effects of Testing Environment
Analysis of TN and NH₃-N requires an ammonia-free environment, free from petroleum substances and other nitrogen-containing compounds. If such conditions are not met, TN and NH₃-N analyses should be performed using separate equipment. Reagents and glassware must be kept dry and clean to prevent cross-contamination.
3.Influences of Reagent Selection and Preparation
Water is an essential medium for solution preparation. When preparing alkaline potassium persulfate solution, the water bath temperature must be strictly controlled within a specific range; otherwise, potassium persulfate may decompose, leading to inaccurate results.
4.Impacts of Colorimetry and Digestion
Incomplete digestion of potassium persulfate (due to issues such as autoclave temperature or reagent quality) and incomplete sealing of colorimetric tube ground-glass stoppers can cause ammonia volatilization during digestion. When replenishing pure water to the calibration mark, the sample volume increases while TN content decreases, potentially resulting in NH₃-N ≥ TN.
5.Effects of Water Sample Color and Turbidity
Untreated samples or those with incomplete coagulation-sedimentation may exhibit elevated NH₃-N results. In contrast, during TN determination, high-temperature digestion eliminates sample color and turbidity (with white precipitates forming at the bottom of colorimetric tubes, leaving the supernatant colorless and transparent), thus eliminating interference with TN detection.
Mitigation Strategies
1.Timely and Separate Analysis: TN and NH₃-N should be analyzed immediately upon sample receipt using two distinct water quality analyzers.
2.Strict Reagent Control: Ensure high-quality reagents and standardized preparation processes for TN testing to avoid incomplete digestion.
3.Pretreatment for NH₃-N Analysis: Prior to online NH₃-N detection, preprocess samples to remove color and turbidity interference.
