Shen ChangHong A large number of domestic sewage, farmland drainage or nitrogen-containing industrial wastewater are discharged into the water body, which increases the content of organic nitrogen and various inorganic nitrogen compounds in the water, and the large-scale reproduction of organisms and microorganisms consumes dissolved oxygen in the water, which worsens the quality of the water body. When there are excessive nitrogen and phosphorus substances in lakes and reservoirs, phytoplankton propagate vigorously and become eutrophic. Therefore, total nitrogen is one of the important indicators to measure water quality. So, what are the measuring methods of total nitrogen?
Measurement of total nitrogen
Ultraviolet method: this method can measure the content of total nitrogen in surface water, domestic sewage and industrial wastewater. Measurement principle: inorganic nitrogen and organic nitrogen in water are oxidized and digested by potassium persulfate and then converted into nitrates. In the medium of sulfuric acid and phosphoric acid, nitrates react with 2,6-xylenol to form 4-nitro-2,6-xylenol and develop color.
Precautions for measurement of total nitrogen by ultraviolet method
1. During the experiment, the glassware and colorimetric tube used shall be cleaned with sulfuric acid if necessary.
2. Before the prepared sample is tested on the machine, ensure that the outer wall of the colorimetric tube is wiped clean without water drops or hand marks.
3. When analyzing the water sample, shake the water sample evenly and then take the sample. The water sample cannot be filtered.
How to eliminate interference substances in determination of total nitrogen
When the organic matter (calculated by COD) in the water sample exceeds 300mg / L, it will interfere with the determination. It is necessary to dilute the water sample or increase the amount of alkaline potassium persulfate to remove.
When the trivalent iron in the water sample exceeds 6mg / L, it interferes with the determination. The water sample can be adjusted to alkalinity with sodium hydroxide, and then filtered to remove iron ions.
How to use reagents in total nitrogen analyzer?
Unless otherwise specified, all reagents of analytical purity or above shall be distilled water
1. Reagent 1: take a small package of total nitrogen "reagent a" in a small beaker, use a pipette to take 20.0ml of total nitrogen "reagent B" for dissolution, refrigerate in a refrigerator at 2-8 ℃ for 2 weeks.
2. Reagent 2: for direct use, keep in refrigerator at 2-8 ℃.
3. Mixed sulfuric acid of sulfur and phosphorus: prepared according to the ratio of "sulfuric acid": "phosphoric acid" = 3:2. (for example, add 300ml concentrated sulfuric acid into 200ml concentrated phosphoric acid slowly and mix evenly)
4、 Standard solution (500mg/L): accurately weigh 1.8045g of potassium nitrate dried at 105 ℃ for 2h, dissolve it in distilled water, dilute it with distilled water to the scale line in a 500mL volumetric flask, and shake it for standby.
5. Standard solution (10.0mg / L): transfer 2.0ml of 500mg / L standard solution into a 100ml volumetric flask, and use distilled water to fix the volume to the mark line, and shake well for standby.
