In the process of sewage treatment, why the detection value of total nitrogen is less than ammonia nitrogen

In water quality testing, total nitrogen and ammonia nitrogen are the two most common important indicators. Theoretically speaking, the total nitrogen content in the water body should be greater than the ammonia nitrogen content, and their relationship should be: total nitrogen = organic nitrogen + ammonia nitrogen + nitrate nitrogen + nitrite nitrogen + others. However, in the actual detection, due to the cumbersome detection steps of total nitrogen and complicated experimental conditions, the detected data often have abnormal situations where the total nitrogen content is less than the ammonia nitrogen content, so it has to be reworked and redone, which increases the workload and reduces the work efficiency. Therefore, it is very necessary to conduct an in-depth analysis of the reasons for this abnormal phenomenon to ensure the accuracy of water quality testing data. Several influencing factors that total nitrogen is less than ammonia nitrogen 1. Errors caused by the experimental environment. There are toilets or ammonia storage in the surrounding environment of the laboratory, etc. The air in the laboratory contains a small amount of ammonia gas, which is very soluble in water, so that The experimental water also contains ammonium ions to varying degrees. In laboratory analysis, the preparation and storage of ammonia-free water used for diluting water samples are often not taken seriously, resulting in the dissolution of external ammonia nitrogen into the water samples, increasing the concentration error of ammonia nitrogen in water samples. 2. The error of sample introduction is because the nitrogen compounds in the water are constantly changing. After collection, the samples sent back to the laboratory for experimental analysis, their storage time, storage location, lighting conditions, etc., and even the order in which the analysts took samples, etc. , will bring different errors to the experimental analysis of ammonia nitrogen and total nitrogen. 3. In the error experiment of drug introduction, potassium persulfate should first be purified. The absorbance of the unpurified potassium persulfate solution is much greater than that of the purified potassium persulfate solution, and the standard deviation of the purified potassium persulfate solution is smaller. , the impact on the deviation of the water sample measurement results is small. 4. Errors introduced by the experimental method The analysis of ammonia nitrogen usually adopts the more classic Nessler reagent photometry. Although the color development requires an alkaline environment, the pretreatment process is relatively simple. After direct color development and measurement, the results can be calculated. Relatively speaking, the pretreatment process of the analysis of total nitrogen is more complicated, and it needs to undergo 30 minutes of pressurized treatment under alkaline conditions. If the seal is not good during the pretreatment process, it will also lead to the release of ammonia nitrogen under high temperature and high pressure. Few laboratories use raw material with a sealed bottle stopper for each total nitrogen digestion, so the conversion cannot be 100% conversion, which will lead to the release of ammonia nitrogen during the total nitrogen process, which will cause errors. 5. The error introduced by sample turbidity. The influence of turbidity that can be eliminated by the pretreatment of total nitrogen analysis cannot be eliminated in the analysis of ammonia nitrogen. In addition, different types of cuvettes are commonly used for colorimetry. The addition of these several influencing factors will have a significant impact on the final result. come to the difference. Since both test methods are used to measure absorbance, the turbidity caused by suspended matter in the sample is the most difficult factor to eliminate in sample analysis. In the experimental analysis and determination of total nitrogen and ammonia nitrogen, the pretreatment of total nitrogen analysis can eliminate The influence of turbidity cannot be eliminated in the analysis of ammonia nitrogen, which may cause a high level of ammonia nitrogen in the detection of water samples. 6. Errors introduced by different analytical methods and analytical instruments Almost all analytical and experimental methods for measuring samples have certain method errors, and the experimental analysis of total nitrogen and ammonia nitrogen is no exception. The Nessler reagent photometric method for the analysis of ammonia nitrogen has errors. The alkaline persulfate decomposition method of nitrogen also has errors, and the errors of the two analysis methods bring errors to the final measurement results, which have great uncertainty. Various measuring instruments, colorimetric tubes, cuvettes and other instruments used in the entire analysis process of the two projects may introduce errors of different degrees; the sensitivity of the spectrophotometer used in colorimetry, The precision and accuracy may not be the same, and the magnitude of the error introduced is not the same. In particular, the colorimetric determination of total nitrogen and ammonia nitrogen uses light in two different light regions, visible and ultraviolet, and the error introduced is even greater. 7. Errors introduced by data processing In data processing, errors may be introduced from two aspects: one is the errors introduced by different calibration curves. Although the two curves used in these two projects have passed the statistical test, there is a difference between the curves and the curves. The difference, this difference brings errors; the second is the selection of significant figures to introduce errors. The sum of the errors in the two aspects forms a large error between the two analysis items. The smaller the concentration of the sample, the larger the error. In some cases, the diluted water sample will have ammonia nitrogen less than the total nitrogen. 8. There are also various error experimental methods caused by different personnel factors, and there will be different differences in error control: From the above analysis, we can see that the errors of ammonia nitrogen and total nitrogen in the testing process are objective and subjective. Affected by many factors, the comprehensive error will lead to the situation that the ammonia nitrogen may exceed the total nitrogen.