Total nitrogen is an important indicator of water quality testing, but the determination process is complex and prone to errors. The water sample pretreatment, digestion and determination methods of each determination method have similarities, differences and advantages and disadvantages. At present, the mainstream total nitrogen detection method in China is still alkaline potassium persulfate digestion ultraviolet spectrophotometry, but this method takes a long time for water sample pretreatment, and the cleanliness of water and glassware used in the experiment process is also relatively high. High, with limitations in various aspects, and cannot meet the needs of water body monitoring staff. 1. Comparison of water sample pretreatment methods During the determination of total nitrogen in surface water, it is generally necessary to put the collected water samples in polyethylene bottles and add concentrated sulfuric acid as a fixative to adjust the pH value to 1-2. Store in refrigerator at 4°C. Alkaline potassium persulfate ultraviolet spectrophotometry needs to adjust the pH value of the water sample to 5-9 before the measurement; if the water sample contains Fe3+, Cr+VI and other coexisting ions, add hydroxylamine hydrochloride to eliminate interference; for turbidity High water samples can be centrifuged after digestion to eliminate the interference of solid particles on the determination of total nitrogen content. Basic potassium persulfate ion chromatography needs to add an appropriate amount of Ba(OH)2 powder to the water sample after sampling to make the SO42- ion in the water into a precipitated state, and then filter it with a 0.45μm microporous membrane. The filtered water sample is used Store clean glass bottles in a refrigerator at 4°C. The high-temperature catalytic oxidation method needs to add acid to acidify the water sample before the measurement; if the water sample contains large suspended particles, it can be measured after 20 minutes of ultrasonic pulverization; if the halogen content in the water sample is high, the water sample needs to be diluted After the measurement to avoid damage to the instrument. 2. Comparison of digestion methods of alkaline potassium persulfate digestion UV spectrophotometry and alkaline potassium persulfate ion chromatography need to use an autoclave to digest water samples at 120-124°C and alkaline conditions. During the digestion process, potassium persulfate oxidizes nitrogen-containing compounds in the water sample to be tested into nitrates. In addition, COD
digestion instrument, microwave oven and electric heating constant temperature drying oven can also be used instead of autoclave for simple digestion of water samples. When the ultraviolet on-line digestion-gas phase molecular absorption spectrometry is used to determine total nitrogen, after the water sample is digested by ultraviolet on-line, nitrogen-containing compounds are converted into nitrates, titanium trichloride reduces NO3- to NO gas, and NO gas is used as total nitrogen Characterization of content. The high-temperature catalytic oxidation method uses high-temperature combustion tubes or high-temperature combustion reaction furnaces to digest water samples. Under the combined action of high temperature of 850°C, high-purity oxygen and catalysts, the nitrogen-containing compounds in the samples are converted into NO gas. When measuring total nitrogen by continuous flow analysis, the sample in alkaline medium is oxidized to NO3- by potassium persulfate under the condition of 107-110°C and ultraviolet irradiation. In the process of determination of total nitrogen by ozone ultraviolet combination-spectrophotometry, the free oxygen radicals produced by ozone under the irradiation of ultraviolet light react with water to form hydroxyl radicals, which have strong oxidation ability to organic matter, and can make water samples The nitrogen-containing organic matter is oxidized to NO3-. 3. Comparison of detection methods Alkaline potassium persulfate digestion UV spectrophotometry uses quartz cuvettes, adopts UV spectrophotometer to measure absorbance values A220 and A275 at 220nm and 275nm wavelengths, total nitrogen concentration value and absorbance value (A220— A275) proportional to. Alkaline potassium persulfate ion chromatography adopts ion chromatography to detect the content of NO3- in the water sample after digestion, and uses 4.5mmol/L Na2CO3 and 0.8mmol/L NaHCO3 mixed solution as eluent, and the eluent flow rate is 1.0mL/min; set the suppression current of the suppressor to 25mA; each injection volume is 25μL. The high-temperature catalytic oxidation method passes the NO gas obtained after digestion into the electrochemical detector, and an electrode reaction will occur on the surface of the inner electrode of the detector, and the amount of NO gas will be characterized in the form of current, so as to calculate the total amount of NO gas in the water sample. nitrogen concentration. When the continuous flow analysis method is used to measure total nitrogen, the nitrate is first reduced to nitrite through a cadmium column. In acidic medium, NO2- is diazotized with sulfonamide and then coupled with naphthaleneethylenediamine hydrochloride to form a purple compound. Absorbance was measured at a wavelength of 540nm to obtain the total nitrogen concentration of the sample. The combination of ozone ultraviolet-spectrophotometry uses titanium trichloride to reduce NO3- to NO gas, and detects the concentration of NO gas by gas-phase molecular absorption spectrometer, so as to obtain the concentration of total nitrogen.
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