E-mail: michael@shboqu.com   |   Expert in Water Quality Measurement and Water Treatment Project

Points to note when measuring total nitrogen

by:BOQU     2023-04-12
1. Reagent preparation 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. Regarding the preparation of alkaline potassium persulfate, it simply said that potassium persulfate and sodium hydroxide were dissolved in water, and no other requirements were made. In fact, the dissolution rate of potassium persulfate is very slow. If you want to speed up the dissolution, you can’t heat it blindly. Even if you heat it, use the water bath heating method, and the temperature of the water bath must be lower than 60°C, otherwise the potassium persulfate will decompose and fail. When preparing the solution, potassium persulfate and sodium hydroxide can be weighed separately, the two are prepared separately, and then mixed to constant volume, or the sodium hydroxide solution is prepared first, and potassium persulfate is added after the temperature drops to room temperature to dissolve. If the two are soluble in water in a beaker, add water slowly and stir at the same time to prevent the sodium hydroxide from exothermic and causing the solution temperature to be too high to cause local potassium persulfate to fail. 2. Washing of glassware The glassware used should be soaked in (1+9) hydrochloric acid first, and then rinsed several times with ammonia-free water before use, otherwise, it will also cause high blank value or poor parallelism. 3. Control of digestion temperature and pressure In a laboratory using a medical portable steam sterilizer, since the measured pressure is 1.1~1.4kg/cm2 and the temperature is 120°C~124°C, it is required to start timing when the specified temperature and pressure are reached during digestion. , and my general experience is to directly open the vent valve for heating for a period of time, and then close the vent valve for digestion after the cold air in the steam sterilizer is completely driven out and hot steam is released, and the digestion temperature is controlled at 123°C , so the measurement result is ideal. 4. Matters needing attention in colorimetry The measurement of this project involves two wavelengths (220nm and 275nm). It is recommended to adjust to another wavelength after measuring the same wavelength of a group of samples, and make a unified measurement. Do not finish measuring the wavelength of a sample. Change another sample after two absorbances, so repeated adjustment of the wavelength will cause certain measurement errors. 5. Reagent selection Potassium persulfate is a crucial reagent in the process of alkaline potassium persulfate digestion for the determination of total nitrogen by ultraviolet spectrophotometry. First of all, the purity of the reagent is related to the level of the blank value and the accuracy of the measurement results. Generally, the total nitrogen content of ordinary analytically pure potassium persulfate does not exceed 0.005%. However, due to differences in reagent quality, the nitrogen content of some manufacturers and batches of reagents often does not meet this requirement, resulting in high blank values. Therefore, if conditions permit, it is recommended to use superior-grade pure reagents to minimize the nitrogen content in the reagents, thereby reducing the experimental blank value.
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