What is the error in the determination of total nitrogen content in water by UV-Vis spectrophotometry?

In the test report data of some sewage plants, it is sometimes found that the total nitrogen of the same water sample is lower than the ammonia nitrogen. Total nitrogen theoretically includes ammonia nitrogen. The total nitrogen contained in the sample must be higher than the ammonia nitrogen, at least the ammonia nitrogen will not be higher than the total nitrogen. Because total nitrogen refers to the total amount of nitrogen (inorganic nitrogen) in various ions such as ammonia, nitrous acid, and nitric acid contained in water and nitrogen (organic nitrogen) in organic nitrogen compounds such as protein. The total nitrogen determination method usually uses potassium persulfate oxidation to convert organic nitrogen and inorganic nitrogen compounds into nitrate, which is then determined by ultraviolet method. The reasons for the errors are summarized as follows: 01 Errors caused by the experimental environment: There are toilets or ammonia storage in the surrounding environment of the laboratory. The air in the laboratory contains a small amount of ammonia, which is very soluble in water, so that the experimental water also contains ammonium ions to varying degrees. 02 Sample introduction error: Due to the constant change of nitrogen compounds in the water, the sample is sent back to the laboratory for experimental analysis after collection. Its storage time, storage location, light conditions, and even the order in which analysts take samples will bring different errors to the experimental analysis of ammonia nitrogen and total nitrogen. 03 Misunderstanding of drug introduction: Potassium persulfate should be purified first in the experiment. The absorbance of the unpurified potassium persulfate solution is much higher than that of the purified potassium persulfate solution, and the standard deviation of the purified potassium persulfate solution is higher, which has little influence on the deviation of the water sample measurement results. 04 Errors introduced by the experimental method: The analysis of ammonia nitrogen usually adopts the more classic Nessler reagent spectrophotometry. During the pre-pressurization process, if the seal is not good, it will also cause the release of ammonia nitrogen under high temperature and high pressure. In general, very few laboratories use raw meal with sealed stoppers for each total nitrogen digestion, so transformation cannot be performed. for 100% conversion. 05 Errors introduced by sample turbidity: The turbidity effect that can be eliminated by the pretreatment of total nitrogen analysis cannot be eliminated in the analysis of ammonia nitrogen. In addition, different kinds of cuvettes are commonly used in colorimetry. The addition of these influencing factors will affect the final result. make change. 06 Different analytical methods and instruments: Almost all analytical experimental methods have certain method errors, and the experimental analysis of total nitrogen and ammonia nitrogen is no exception. The Nessler reagent photometric method for analyzing ammonia nitrogen has errors, and the alkaline persulfate decomposition method for analyzing total nitrogen also has errors. The error of the two analysis methods will bring errors to the final measurement results. There is great uncertainty. 07 Errors introduced by data processing: one is the error caused by different calibration curves; the other is the error caused by the selection of effective figures. 08 Caused by different personnel factors: experimental methods and error control will have different differences In some water samples with a large proportion of ammonia nitrogen in total nitrogen, the probability of this situation is very high due to various induced errors. The testing personnel should keep the analysis time of total nitrogen and ammonia nitrogen consistent to eliminate the interference of drug samples and experimental conditions.