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Points not to be ignored in total nitrogen detection
The definition of total nitrogen
Total nitrogen, abbreviated as TN, the total nitrogen content in water is one of the important indicators to measure water quality.
The definition of total nitrogen is the content of various forms of inorganic and organic nitrogen in the water, including NO3-, NO2- and NH4+ Organic nitrogen such as nitrogen and protein, amino acids,
and organic amines are calculated in milligrams of nitrogen per liter of water.
It is often used to indicate the degree of water pollution by nutrients. The total nitrogen content in water is also one of the important indicators to measure water quality.
It is helpful to evaluate water pollution and self-purification. When the nitrogen and phosphorus substances in surface water exceed the standard, microorganisms multiply, plankton grows vigorously, and eutrophication appears.
Determination of total nitrogen in water quality At present, the domestic mainstream total nitrogen detection method is alkaline potassium persulfate digestion ultraviolet spectrophotometry (HJ636-2012), and there are some relatively less used detection methods-gas phase molecular absorption Spectrophotometry (HJ/T199-2005).
Flow Injection-Naphthaleneethylenediamine Hydrochloride Spectrophotometry (HJ688-2013), Continuous Flow Injection-Naphthaleneethylenediamine Hydrochloride Spectrophotometry (HJ667-2013)
Attention should be paid when detecting total nitrogen:
1. Reagent configuration The preparation process of alkaline potassium persulfate is very important. If it is not well mastered, it will affect the digestion effect and have a certain impact on the measurement results.
The dissolution rate of potassium persulfate is very slow. If you want to speed up the dissolution, Do not heat blindly. Even if heating, it is better to use the water bath heating method, and the temperature of the water bath must be lower than 60℃, otherwise potassium persulfate will decompose and fail.
When preparing this solution, potassium persulfate and sodium hydroxide can be weighed separately, The two are prepared separately, and then mixed to a constant volume, or the sodium hydroxide solution is prepared first, and potassium persulfate is added to dissolve after the temperature drops to room temperature.
If the two are dissolved in water in a beaker, add water slowly while stirring to prevent the sodium hydroxide from exothermic and the solution temperature is too high, causing local potassium persulfate to fail.
2. Glassware used for washing glassware should be soaked in (1+9) hydrochloric acid first, and then rinsed with ammonia-free water for several times before it can be used. Otherwise, the blank value will be high or the parallelism will be poor. Case.
3. It should be noted that two wavelengths (220nm and 275nm) are involved in the determination of total nitrogen during color comparison.
It is recommended to adjust to another wavelength after the same wavelength of a group of samples is measured, and the measurement should be performed uniformly.
After the two absorbances of one sample, change to another sample, so that repeated adjustment of the wavelength will cause a certain measurement error.
4 Selection of reagents Potassium persulfate is an essential reagent in the determination of total nitrogen by alkaline potassium persulfate digestion and UV spectrophotometry.
The purity of the reagent is related to the level of the blank value and the accuracy of the measurement result.
Therefore, when selecting reagents, try to reduce the nitrogen content in the reagents, thereby reducing the experimental blank value.