Why is total nitrogen less than ammonia nitrogen in water quality testing?

- Jul 22, 2020-

However, in actual testing, due to the cumbersome detection steps of total nitrogen and the complicated experimental conditions, the detected data often shows abnormal situations where the total nitrogen content is less than the ammonia nitrogen content, which has to be reworked, which increases the workload and reduces Improve work efficiency. Therefore, it is very necessary to conduct an in-depth analysis of the causes of this abnormal phenomenon to ensure the accuracy of water quality testing data.

Q1 Interfering Factor

Factors affecting total nitrogen less than ammonia nitrogen

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 gas, which is easily 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, which leads to the dissolution of external ammonia nitrogen in the water sample, which increases the concentration error of ammonia nitrogen in the water sample.

02 Errors introduced by samples

Nitrogen compounds in the water are constantly changing. The samples sent back to the laboratory for analysis after collection, storage time, storage location, lighting conditions, analysts, sampling order, etc. will cause errors.

03 Errors introduced by drugs

Potassium sulfate should be purified during the experiment. 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.

04 Errors introduced by experimental methods

The analysis of ammonia nitrogen generally adopts Nessler's reagent photometric method. The color development requires alkaline environment, but the pretreatment process is relatively simple, and the result can be calculated after the color development measurement.


The pre-treatment process for total nitrogen analysis is more complicated. If the sealing is not good during the pre-treatment process, ammonia nitrogen will be released under high temperature and high pressure. Few laboratories can use raw material to seal the bottle stopper every time the total nitrogen is digested. Therefore, the conversion cannot be 100% conversion, which will cause the release of ammonia nitrogen in the total nitrogen process and cause errors.

Multi-parameter Meter

05 Error introduced by sample turbidity

The turbidity effect that can be eliminated by the pretreatment of total nitrogen analysis cannot be eliminated in the ammonia nitrogen analysis. In addition, different types of cuvettes are commonly used in colorimetry. The addition of these several influencing factors brings differences in the final results.

06 Errors introduced by different analytical methods and analytical instruments

Almost all analytical experimental methods for measuring samples have certain method errors. The experimental analysis of total nitrogen and ammonia nitrogen is no exception. The Nessler's reagent photometric method for analyzing ammonia nitrogen has errors, and the alkaline persulfate decomposition method for analyzing total nitrogen also has errors. , The errors of the two analysis methods bring great uncertainty to the final measurement results.

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 color comparison, Precision and accuracy may not be the same, and the errors introduced are also different.

Especially for the colorimetric determination of total nitrogen and ammonia nitrogen, light in two different light regions, visible and ultraviolet, is used, which introduces a greater difference in error.

07 Errors introduced by data processing

In data processing, two aspects may introduce errors: one is the error introduced by different calibration curves; the other is the error introduced by the selection of significant figures.

The sum of the errors of the two aspects forms a considerable error between the two analysis items. The smaller the concentration of the sample, the greater the error. This is that in some cases, the diluted water sample will have ammonia nitrogen less than total nitrogen.

08 Errors caused by factors of different personnel

There are differences in experimental methods and error control: there are objective and subjective factors in the presence of errors in ammonia nitrogen and total nitrogen in the test process. Comprehensive errors will cause ammonia nitrogen in the measurement data to exceed total nitrogen.

nitrogen test meter

Q2 Methods Of Resolution

How to prevent incorrect data caused by errors

To sum up, in sewage testing, ammonia nitrogen and total nitrogen will often appear in the test of ammonia nitrogen higher than total nitrogen, which is inevitable, especially in some waters where the proportion of ammonia nitrogen in total nitrogen is relatively large. In this way, due to the existence of multiple causes of error, the probability of this situation is very high.


The testing personnel should maintain the same analysis time for total nitrogen and ammonia nitrogen to eliminate the interference of drug samples and experimental conditions.


In this case, the method of adding standard recovery can be adopted (in a blank sample or a certain background of known content, add a standard product of known content (tested component), and use the established method to detect its content (measured value) Compared with the added value, if the added value is 100, the measured value is 85, the result is the recovery rate of 85%, which is called standard addition recovery), or the test standard sample is affected by artificial subjective factors to correct the data error. When performing process adjustments by comparing in-plant process data, focus on the longitudinal comparison of ammonia nitrogen data and total nitrogen data to avoid process adjustment confusion caused by errors in ammonia nitrogen and total nitrogen in the same water sample.

About Us

Hangzhou Lohand Biological Technology Co., Ltd. is an enterprise integrating design, development, production and sales of rapid water quality diagnostic test papers, reagents and precision testing instruments. It is located in Jianggan Science and Technology Park, and is based on Zhejiang University Food Science and Fermentation Engineering Research Institute. Both Hexihu Science and Technology Park have established R&D centers that focus on technological innovation, have obtained a number of patents, and produce products in strict accordance with the ISO9001 quality management system. The purpose of our service is to provide customers with professional, fast, affordable and simple testing products and services, and create a new course of water quality monitoring.


Over the past ten years of development, we have been working in China's rapid water quality testing industry, constantly improving product functions and testing accuracy, and committed to the research of environmental water quality monitoring and the development of rapid monitoring products. The products developed and produced mainly include COD, ammonia nitrogen, total phosphorus, total nitrogen integrated machine, mini digestion instrument, water quality monitoring electrode, digital display refractometer, etc. Most of the company's products have obtained CE EU certification; widely used in China Local environmental protection bureaus, industrial enterprises, university research institutes, food manufacturing companies; especially electrochemical testing products have been sold at home and abroad. We sincerely welcome domestic and foreign merchants to win-win cooperation, develop together and create brilliance together.