Overview of methods
In aqueous solution above 60℃, potassium persulfate can be decomposed into potassium bisulfate and atomic oxygen, and potassium bisulfate dissociates in solution to generate hydrogen ions, so the decomposition process tends to be complete in alkaline medium of sodium hydroxide.
In the temperature range of 120 ~ 124℃, decomposed atomic oxygen can transform nitrogen-containing nitrides in water samples into nitrates. And in this process, organic matter is simultaneously oxidized and decomposed. The absorbance A220 and A275 at the wavelengths of 220nm and 275nm can be determined by ultraviolet spectrophotometry to correct the interference of absorbance of organic matter at 220nm.
This method can determine the sum of nitrite nitrogen, nitrate nitrogen, inorganic ammonium salt, dissolved ammonia and most organic nitrogen compounds in water. The detection range is 0.05 ~ 4 mg/L.
The molar absorption coefficient of this method is1.47×103 l mol-1cm-1.
The interferents in the determination are mainly iodine ion and bromine ion, with iodine ion being more than 2.2 times of the total nitrogen content and bromine ion being more than 3.4 times of the total nitrogen content.
Some organic substances can not be completely converted into nitrate under the determination conditions stipulated in this law, which will affect the determination.
Filterable total nitrogen: refers to the nitrogen content of soluble and filterable solids (particles smaller than 0.45μm) in water.
Total nitrogen: refers to the nitrogen content in soluble and suspended particles.
Instruments and equipment
Ultraviolet spectrophotometer10mm time cuvette.
The pressure of medical portable steam sterilizer or domestic pressure cooker is 0.11~ 0.14 MPa, and the temperature in the cooker is equivalent to 120 ~ 124℃.
25mL colorimetric tube with glass grinding plug.
The glassware used can be soaked in (1+9)HCl or (1+35)H2SO4, and then washed several times with ammonia-free water.
reagent
The distilled water used does not contain ammonia.
muriatic acid
Sulfuric acid.
Sodium hydroxide solution (200g/L).
Sodium hydroxide solution (20g/L).
Alkaline potassium persulfate solution (40g/L) Weigh 40g of potassium persulfate (K2S2O8) and dissolve 15g of sodium hydroxide (NaOH) in water, dilute to 1000mL, and store the solution in a polyethylene bottle for one week.
Potassium nitrate standard stock solution ρ(TN)= 100mg/L potassium nitrate (KNO3) was dried in an oven at105 ~10℃ for 3 hours, cooled in a dryer, and then dissolved in 0.72 18g. This solution can be stable for 6 months.
Potassium nitrate standard solution ρ(TN)= 10.0mg/L is diluted by adding water to potassium nitrate standard reserve solution.
calibration curve
Add 0.0mL, 0.25mL, 0.50mL, 1.00mL, 3.00mL, 5.00mL, 10.00mL potassium nitrate standard solution (10mg/L) into seven colorimetric tubes with stoppers, and add ammonia-free distilled water to dilute to/kloc.
Add 5mL of alkaline K2S2O8 solution, plug the grinding plug, and tie the cork with cloth and rope to prevent it from popping out. Put the grinding plug colorimetric tube into a medical lifting steam sterilizer or household pressure cooker, heat it until the pressure gauge pointer reaches 0.11~ 0.14 MPa, and the temperature reaches 120 ~ 124℃, and then start timing. Keep this temperature and heat for half an hour. Cooling, opening the valve to deflate, taking off the outer cover, taking out the colorimetric tube, and cooling to room temperature. Add 1mL( 1+9)HCl, dilute to 25mL with ammonia-free water, and mix well. Use 10mm time cuvette to meAsure absorbance at 220nm and 275nm with ammonia-free distilled water as reference on an ultraviolet spectrophotometer, and use the following formula to calculate the corrected absorbance as of other calibration series except zero concentration, corrected absorbance ab of zero concentration and their difference Ar respectively.
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Where: As220 is the absorbance of standard solution at the wavelength of 220nm; As275 is the absorbance of standard solution at 275nm wavelength; Ab220 is the absorbance of zero concentration (blank) solution at the wavelength of 220nm; Ab275 is the absorbance of the zero concentration (blank) solution at the wavelength of 275nm.
Draw a calibration curve with Ar value as the ordinate of the curve and NO3-N content as the abscissa (μg).
Analytical method
Water samples should be stored in the refrigerator or below 4℃ immediately after collection, but not more than 24 hours. When the water sample is left for a long time, about 0.5mLH2SO4 can be added to the 1000mL water sample to acidify it to pH less than 2, and it should be determined as soon as possible.
During the analysis, the pH value was adjusted to 5 ~ 9 with 200g/LNaOH solution and (1+35)H2SO4.
Take a sample of 10.0mL and put it in a plug colorimetric tube. ρ(TN)& gt； 100μg, the sampling amount can be reduced and diluted to 10.0mL without adding ammonia water.
When the test solution contains no suspended matter, measure the absorbance at the wavelengths of 220nm and 275nm according to the operation steps of the calibration curve, and calculate the corrected absorbance a with the formula (8 1.2 1).
When the test solution contains suspended matter, follow the operation steps of calibration curve. After the test solution is clarified, take the supernatant and measure the absorbance at the wavelengths of 220nm and 275nm, and calculate the corrected absorbance a with the formula (8 1.2 1).
In the blank test, ammonia-free water is used to replace the sample, and the same reagent and dosage are used to calibrate the curve.
See formula (8 1.9) for the calculation of the mass concentration of total nitrogen in water samples.
Matters needing attention
When the determination is close to the detection limit, the absorbance ab of the blank test must be controlled not to exceed 0.03; If it exceeds this value, check the pressure of water, reagents, utensils and household pressure cooker or medical hand sterilizer.