Alkaline potassium persulfate digestion photometry
Method summary
In aqueous solutions above 60°C, potassium persulfate can decompose to produce potassium hydrogen sulfate and atoms As oxygen, potassium hydrogen sulfate dissociates in the solution to produce hydrogen ions, so the decomposition process becomes complete in the alkaline medium of sodium hydroxide.
The decomposed atomic oxygen can convert nitrogen elements containing nitrogen compounds in water samples into nitrates at 120-124°C. And during this process, organic matter is oxidized and decomposed at the same time. UV spectrophotometry can be used to measure the absorbance A220 and A275 at wavelengths of 220nm and 275nm, respectively, to correct the interference of absorbance of organic matter at 220nm.
This method can determine the total nitrogen in nitrite nitrogen, nitrate nitrogen, inorganic ammonium salts, dissolved ammonia, and most organic nitrogen-containing compounds in water. The detection range is 0.05~4mg/L.
The molar absorption coefficient of this method is 1.47×103L·mol-1·cm-1.
The main interferences in the measurement are iodide ions and bromide ions. The iodide ion interferes with more than 2.2 times the total nitrogen content, and the bromide ion interferes with more than 3.4 times the total nitrogen content.
Some organic substances may have an impact on the determination if they cannot be completely converted into nitrates under the determination conditions specified in this method.
Filterable total nitrogen: refers to the nitrogen content of soluble and filterable solids (particulate matter less than 0.45 μm) in water.
Total nitrogen: refers to the nitrogen content in soluble and suspended particles.
Instruments and Devices
UV spectrophotometer 10mm quartz cuvette.
The pressure of a medical lift-type steam sterilizer or household pressure cooker is 0.11~0.14MPa, and the temperature in the pot is equivalent to 120~124℃.
25mL colorimetric tube with glass ground stopper.
The glassware used can be soaked in (1+9) HCl or (1+35) H2SO4, and then rinsed several times with ammonia-free water.
Reagents
The distilled water used is ammonia-free.
Hydrochloric acid.
Sulfuric acid.
Sodium hydroxide solution (200g/L).
Sodium hydroxide solution (20g/L).
Weigh 40g of potassium persulfate (K2S2O8) for alkaline potassium persulfate solution (40g/L), and dissolve 15g of sodium hydroxide (NaOH) in water, dilute it to 1000mL, and store the solution in a polyethylene In vinyl bottles, it can be stored for up to one week.
Potassium nitrate standard stock solution ρ (TN) = 100 mg/L. Dry potassium nitrate (KNO3) in an oven at 105~110°C for 3 hours. After cooling in a desiccator, weigh 0.7218g and dissolve it in distilled water. , transfer to a 1000mL volumetric flask, dilute with water to the mark and store in a dark place at 0 to 10°C, or add 1 to 2 mL of chloroform for storage. This solution is stable for 6 months.
Potassium nitrate standard solution ρ(TN)=10.0mg/L is prepared by diluting the potassium nitrate standard stock solution with water. Prepare it when used.
Calibration curve
Add 0.0mL, 0.25mL, 0.50mL, 1.00mL, 3.00mL, 5.00mL, 10.00mL potassium nitrate standard solution ( 10mg/L), add ammonia-free distilled water to dilute to 10.00mL.
Add 5 mL of alkaline K2S2O8 solution, plug the bottle stopper tightly, and tie the bottle stopper with cloth or rope to prevent it from popping out. Place the ground plug colorimetric tube in a medical portable steam sterilizer or household pressure cooker, and heat it until the pressure gauge pointer reaches 0.11~0.14MPa. At this time, the timer starts when the temperature reaches 120~124°C. Keep this temperature for half an hour. Cool, open the valve to release air, remove the outer cover, take out the colorimetric tube and cool it to room temperature. Add 1mL (1+9) HCl, dilute to 25mL with ammonia-free water, and mix well. Use a 10mm quartz cuvette, use ammonia-free distilled water as a reference on a UV spectrophotometer, measure the absorbance at wavelengths of 220nm and 275nm, and calculate the corrected absorbance As and zero of other calibration series except zero concentration using the following formulas: The corrected absorbance of concentration Ab from and its difference Ar.
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Where: As220 is the absorbance of the standard solution at a wavelength of 220nm; As275 is the absorbance of the standard solution at a wavelength of 275nm; Ab220 It is the absorbance of the 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 the Ar value as the ordinate of the curve and the NO3-N content as the abscissa (μg).
Analysis steps
Immediately after collection, water samples should be stored in the refrigerator or below 4°C, but not for more than 24 hours. When the water sample is left for a long time, about 0.5 mL H2SO4 can be added to 1000 mL of water sample, acidified to a pH less than 2, and measured as soon as possible.
Add 200g/L NaOH solution and (1+35)H2SO4 to adjust the pH to 5~9 during analysis.
Take 10.0mL of sample and place it in a colorimetric tube with a ground stopper.
When ρ(TN)>100μg, the sampling volume can be reduced and diluted to 10.0mL with ammonia-free water.
When the test solution does not contain suspended solids, follow the steps of the calibration curve. Measure the absorbance at wavelengths of 220nm and 275nm, and calculate the corrected absorbance A using formula (81.21).
When the test solution contains suspended solids, follow the steps of the calibration curve. After the test solution is clarified, take the supernatant and measure the absorbance at wavelengths of 220nm and 275nm, and use formula (81.21) to calculate the corrected absorbance A .
The blank test uses ammonia-free water instead of the sample, uses exactly the same reagents and dosage as the sample analysis, and follows the steps of the calibration curve.
For the calculation of the mass concentration of total nitrogen in water samples, refer to formula (81.9).
Notes
When the measurement is near the detection limit, the absorbance Ab of the blank test must be controlled not to exceed 0.03; if it exceeds this value, the water, reagents, utensils and household items used must be checked. Pressure cooker or medical portable sterilizer.