Determination of total nitrogen in water by ultraviolet spectrophotometry Basic Potassium persulfate digestion

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Abstract: Potassium chloride and ammonium sulfate can increase their solubility in water, but they are almost insoluble in saturated ammonium sulfate solution. The aqueous solution is neutral and has a pH of approximately 7. Relative density 2.66. Melting point 1067 ℃. The main uses include serum protein biochemical testing, catalysts for Kjeldahl nitrogen determination, preparation of other potassium salts, fertilizers, drugs, preparation of glass, alum, etc.

1.1 Theme Content

This standard specifies the method for the determination of total nitrogen in water by ultraviolet spectrophotometry after digestion with alkaline Potassium persulfate at 120~124 ℃.

1.2 Scope of Application

This standard is applicable to the determination of surface water and groundwater. This method can determine the total nitrogen in nitrite nitrogen, nitrate ammonia, inorganic ammonium salt, dissolved ammonia and most organic nitrogen compounds in water.

The minimum detectable concentration of nitrogen is 0.050mg/L, and the upper limit for determination is 4mg/L.

The molar absorption coefficient of this method is 1.47 × 103L · mo1-1 · cm-1.

The main interfering substances in the determination are iodine ions and bromine ions, with iodine ions interfering more than 2.2 times the total nitrogen content and bromine ions interfering more than 3.4 times the total nitrogen content.

Some organic substances that cannot be completely converted into nitrates under the determination conditions specified in this law have an impact on the determination.

2 Definition

2.1 Filterable total nitrogen: refers to the nitrogen content of soluble and filterable solids (less than 0.45 μ m of particulate matter) in water.

2.2 Total nitrogen: refers to the nitrogen content in soluble and suspended particles.

3 Principles

In the aqueous solution above 60 ℃, Potassium persulfate can be decomposed to produce Potassium bisulfate and atomic oxygen, and Potassium bisulfate can be dissociated in the solution to produce hydrogen ions, so it can promote the decomposition process to be complete in the alkaline medium of sodium hydroxide.

The decomposed atomic oxygen can convert the nitrogen element containing chloride compounds in the water sample into nitrate under conditions of 120-124 ℃. And during this process, organic matter is simultaneously oxidized and decomposed. UV spectrophotometry can be used to measure absorbance A220 and A275 at wavelengths 220 and 275nm, respectively. Calculate the corrected absorbance A according to equation (1):

A=A220-2A275 (1)

Check the calibration curve according to the value of A and calculate the total nitrogen (calculated as NO3-N) content.

4 Reagents and Materials

Unless otherwise specified in (4.1), analytical pure reagents that meet national or professional standards are used for analysis.

4.1 Water, ammonia free. Prepare according to one of the following methods;

4.1.1 Ion exchange method: Distilled water is passed through a strong acid type cation exchange resin (hydrogen type) column, and the effluent is collected in a glass bottle with a sealed glass cap.

4.1.2 Distillation method: Add 0.10mL of sulfuric acid (p=1.84g/mL) to 1000mL of distilled water. And re distill in a full glass still, discard the first 50mL of distillate, and then collect the distillate in a glass bottle with a glass stopper.

4.2 Sodium hydroxide solution, 200g/L: weigh 20m sodium hydroxide (NaOH), dissolve it in water (3.1), and dilute it to 100mL.

4.3 Sodium hydroxide solution, 20g/L: obtained by diluting (4.2) solution 10 times.

4.4 Alkaline Potassium persulfate solution: weigh 40g of Potassium persulfate (K2S2OB) and 15g of sodium hydroxide (NaOH), dissolve in water (4.1), dilute to 1000mL, and store the solution in a polyethylene bottle for up to one week.

4.5 Hydrochloric acid solution, 1+9.

4.6 Potassium nitrate Standard solution.

4.6.1 Potassium nitrate standard stock solution, CN=100mg/L: Potassium nitrate (KNO3) is dried in an oven at 105~110 ℃ for 3h. After cooling in a dryer, 0.7218g is weighed, dissolved in water (4.1), transferred to a 1000mL volumetric flask, diluted with water (4.1) until the marking line is stored in a dark place at 0~10 ℃, or 1~2mL trichloromethane is added for storage, which can be stable for 6 months.

4.6.2 Potassium nitrate standard solution, CN=10mg / L: it is obtained by diluting the stock solution 10 times with water (4.1). Preparation during use.

4.7 Sulfuric acid solution, 1+35.

5 Instruments and equipment

5.1 Commonly used laboratory instruments and the following instruments.

5.2 Ultraviolet spectrophotometer and 10mm quartz colorimetric dish.

5.3 Medical portable steam sterilizer or household Pressure cooking (pressure is 1.1~1.4kg/cm2), the temperature in the cooker is equivalent to 120~124 ℃.

5.4 Glass ground stopper colorimetric tube, 25mL.

The glassware used can be soaked in hydrochloric acid (1+9) or sulfuric acid (1+35), cleaned, and then rinsed several times with water (4.1).

6 samples

6.1 Sampling

Immediately after collecting the water sample, place it in a refrigerator or store it at a temperature below 4 ℃ for no more than 24 hours.

When the water is left for a long time, approximately 0.5mL of sulfuric acid (p=1.84g/mL) can be added to 1000mL of water sample, acidified to a pH less than 2, and determined as soon as possible.

Samples can be stored in glass bottles.

6.2 Preparation of test specimens

Take the laboratory sample (6.1) and adjust the pH to 5-9 with sodium hydroxide solution (4.3) or sulfuric acid solution (4.7) to prepare the sample.

If the sample does not contain suspended solids, measure according to step (7.1.2), and if the sample contains suspended solids, measure according to step (7.1.3).

7 Analysis steps

7.1 Determination

7.1.1 Take 10.00mL of the sample using an undifferentiated pipette (when CN exceeds 100 μ g, the amount of sample can be reduced and diluted to 10mL with water (4.1)) and place it in a colorimetric tube.

When the sample does not contain suspended solids, follow the following steps.

a. Add 5mL alkaline Potassium persulfate solution (4.4), plug the grinding plug and tie the bottle plug with cloth and rope to prevent it from popping out.

b. Place the colorimetric tube in a medical portable steam sterilizer and heat it until the pressure gauge pointer reaches 1.1-1.4kg/cm2. At this point, the temperature reaches 120-124 ℃ and the timing begins. Or place the colorimetric tube in a household Pressure cooking and heat it until the top pressure valve blows air. Heat at this temperature for half an hour.

c. Cool down, open the valve to release air, remove the outer cover, take out the colorimetric tube and cool it to room temperature.

d. Add 1mL of hydrochloric acid (1+9), dilute with ammonia free water to the 25mL mark, and mix well.

e. Transfer a portion of the solution to a 10mm quartz colorimetric dish and measure the absorbance at wavelengths 220 and 275 nm using ammonia free water as a reference on a UV spectrophotometer. Calculate the corrected absorbance A using equation (1).

When the sample contains suspended solids, first follow steps a to d in 7.1.2 above, and then transfer the supernatant to a quartz colorimetric dish after clarification. Continue the measurement according to step e in 7.1.2 above.

7.2 Blank test

The blank test is divided by 10mL of water (4.1) instead of the test portion, and parallel operations are carried out using the same reagents, dosage, and analytical steps as the determination.

Note: When the measurement is close to the detection limit, the absorbance Ab of the blank test must be controlled to not exceed 0.03. If it exceeds this value, the pressure of water, reagents, utensils, household Pressure cooking or medical portable sterilizer should be checked.

7.3 Calibration

7.3.1 Preparation of calibration series:

a. Using a graduated pipette, add 0.0, 0.10, 0.30, 0.50, 0.70, 1.00, 3.00, 5.00, 7.00, and 10.00mL of nitrate nitrogen standard solution (4.6.2) to a group of (10) colorimetric tubes (5.4). Dilute to 10.00mL with water (4.1).

b. Perform the measurement according to steps a to e in 7.1.2.

7.3.2 Drawing of calibration curve:

The calibration series prepared from zero concentration (blank) solution and other Potassium nitrate standard solution (4.6.2) have completed all the analysis steps. After the absorbance is measured at the wavelength of 220 nm and 275 nm, the corrected absorbance As of other calibration series except zero concentration, the corrected absorbance Ab of zero concentration and the difference Ar are calculated respectively according to the following formula

As=As220-2As275 (2)

Ab=Ab220-2AB275 (3)

Ar=As Ab (4)

Where: AS220-- absorbance of Standard solution at 220nm wavelength;

AS275- absorbance of Standard solution at 275 nm wavelength;

Ab220- absorbance of zero concentration (blank) solution at a wavelength of 220nm;

Ab275- absorbance of zero concentration (blank) solution at a wavelength of 275nm.

Draw a calibration curve based on the Ar value and the corresponding NO3-N content (micrograms).

8 Expression of Results

8.1 Calculation method

Calculate the corrected absorbance Ar of the sample according to equation (1), and find the corresponding total nitrogen on the calibration curve? The g number and total nitrogen content (mg/L) are calculated using the following formula:

In the formula: m - nitrogen content measured by the sample, micrograms;

V - Volume of the sample used for determination, mL.

9 Precision and Accuracy

9.1 Repeatability

The mixed samples of Sodium nitrite, aminopropionic acid and ammonium chloride were determined in 21 laboratories; CW604 ammonia nitrogen standard sample; L-glutamic acid and glucose mixed works. The nitrogen content of the above three works is 1.49, 2.64, and 1.15mg/L, respectively. The analysis results are as follows:

The indoor relative standard deviations of each laboratory are 2.3, 1.6, and 2.5%, respectively. The allowable precision for indoor repeated measurements is 0.074, 0.092, and 0.063mg/L, respectively.

9.2 Reproducibility

The above laboratory tested the three unified synthetic samples mentioned above. The relative standard deviations between laboratories are 3.1%, 1.1%, and 4.2%, respectively; The relative standard deviations of reproducibility are 4.0%, 1.9%, and 4.8%, respectively; The total relative standard deviations were 3.8, 1.9, and 4.9%, respectively.

9.3 Accuracy

The above laboratory measured the three unified synthetic samples, and the relative errors of the mean values within the laboratory were 6.3%, 2.4%, and 8.7%, respectively.

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