Theoretically, total nitrogen (TN) is the sum of ammonia nitrogen (NH₃-N), nitrate nitrogen, and organic nitrogen. However, discrepancies often exist between theory and practice. In actual scenarios, ammonia nitrogen in biochemical effluent samples may be greater than or equal to total nitrogen. This article provides a brief analysis of the causes for this phenomenon.
The Following Factors May Result in Ammonia Nitrogen Exceeding Total Nitrogen in Biochemical Effluent:
1.Improper sample preservation
Nitrogen compounds in samples are unstable and undergo continuous transformation. Therefore, water samples should be analyzed immediately after collection, or stored in a refrigerator at a temperature below 4 °C for short-term preservation (no longer than 24 hours). In addition, cross-contamination with ambient air must be considered during sample aliquoting. Samples must be properly sealed to avoid the impact of light-induced temperature changes on the laboratory environment, which may cause analytical deviations.
2.Influence of laboratory environment
For online water quality monitoring in laboratories, analyses must be conducted in an ammonia-free environment. The environment must be free of petroleum hydrocarbons and other nitrogen-containing compounds. Total nitrogen analysis must not be performed in the same laboratory where ammonia nitrogen and other nitrogen-related parameters are analyzed. Reagents must be stored separately, kept dry and clean to prevent cross-contamination.
3.Selection and preparation of reagents
The preparation of all solutions uses water as a medium. During the preparation of ammonia-free water, ammonia or ammonium salts in the air are highly likely to dissolve into the water, causing contamination of the reagent water. This introduces irreparable errors to the reagent blank, particularly increasing the TN reagent blank and leading to underestimated TN detection values. Ammonia-free pure water must be properly stored after preparation, and prepared fresh immediately before use as much as possible.
4.Other influencing factors
Other factors include digestion and colorimetric timing, water sample colority and turbidity interference, and high ammonium salt content in potassium sodium tartrate, all of which can cause ammonia nitrogen to exceed total nitrogen. These are not elaborated in detail here, but laboratory technicians should pay close attention during water quality testing.
In summary, ammonia nitrogen and total nitrogen should be analyzed promptly upon sample receipt, preferably by two analysts simultaneously to avoid errors caused by testing at different times. Efforts should be made to control reagent quality and preparation procedures for total nitrogen determination, minimizing errors from incomplete TN digestion. Furthermore, pre-treatment of colority and turbidity interference prior to ammonia nitrogen analysis may lead to overestimated NH₃-N values, which should also be taken into account.
