Abstract: using the alkaline potassium persulfate digestion ultraviolet spectrophotometric method for determination of total nitrogen in water, examining the factors affect the total nitrogen in the experimental results. Experimental results showed that potassium persulfate reagent selection, configuration of the alkaline potassium persulfate reagent, digestion time, digestion temperature and pressure control, the control of hydrochloric acid to join after the reaction time and so on all can influence the outcome of the determination of total nitrogen.

 

1 topic content and scope of application

1.1 the theme

This standard specifies the alkaline potassium persulfate in 120 ~ 124 ℃ dissolve, ultraviolet spectrophotometric method of determination of total nitrogen in water.

1.2 the applicable scope

This standard applies to the determination of surface water and groundwater. This law can be determined by nitrite nitrogen, nitrate, ammonia water, ammonium salt, dissolved inorganic ammonia and nitrogen combined for most of the organic nitrogen compounds.

The lowest detection concentration of nitrogen is 0.050 mg/L, determination of upper limit of 4 mg/L.

The method of the molar absorption coefficient of 1.47 x 103 l. mo1-1 cm - 1.

If owner and bromine iodine ions in the determination of interference, iodine ions relative to total nitrogen content is more than 2.2 times of bromide ions relative to total nitrogen content of more than 3.4 times the interference.

Some organic matter under the condition of the determination of the provisions of this law cannot be fully into nitrates have influence on measurement.

Definition 2

2.1 the filter can be total nitrogen: refers to the water soluble and contain the filter can be solid (less than 0.45? Nitrogen content of m particulate matter).

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

Three principles

   

n more than 60 ℃ in aqueous solution, potassium persulfate can be decomposed to produce potassium hydrogen sulfate and atomic oxygen, potassium hydrogen sulfate dissociation and produce hydrogen ions in the solution, so in sodium hydroxide alkaline medium decomposition can be completely.

Decomposition of the atomic state oxygen under the condition of 120 ~ 124 ℃, can make the chlorine compounds in water samples of nitrogen into nitrates. And in the process of the organic matter is broken down by oxidation at the same time. Available wavelengths of uv spectrophotometry, 220 and 275 nm, respectively measured absorbance A220 and A275 corrected absorbance by type (1) work out A:

A = 2 a275 A220 -.......................................................................................... (1)

According to the value of A check calibration curve and calculate the content of total nitrogen (NO3 -- N).

4 reagents and materials

Unless otherwise specified (4.1), the analysis using both comply with national standard and professional standard analytical reagent.

4.1 water, without ammonia. According to one of the following methods preparation;

4.4.1 ion exchange method: distilled water through a strong acid type cation exchange resin (hydrogen) column, effluent collected in glass bottle with a sealed glass cover.

4.1.2 distillation: in 1000 mL of distilled water, add 0.10 mL of sulphuric acid (p = 1.84 g/mL). And heavy distillation in the whole glass distiller, abandon to top 50 ml distillate, then put the distillate collected in glass bottle with a glass stopper.

4.2 sodium hydroxide solution, 200 g/L: according to take 20 m sodium hydroxide (NaOH), soluble in water (3.1), diluted to 100 ml.

4.3 the sodium hydroxide solution, 20 g/L: (4.2) solution diluted 10 times.

4.4 alkaline potassium persulfate solution: according to take 40 g potassium persulfate (K2S2OB), the other says 15 g sodium hydroxide (NaOH), soluble in water (4.1), diluted to 1000 ml, solution in the polyethylene bottle, the longest can be stored for a week.

4.5 the hydrochloric acid solution, 1 + 9.

4.6 potassium nitrate standard solution.

4.6.1 potassium nitrate standard reservior fluid, CN = 100 mg/L: potassium nitrate (- KNO3) in 105 ~ 110 ℃ oven dry 3 h, after cooling in the dryer, called 0.7218 g, soluble in water (4.1), moved to 1000 ml volumetric flask, dilute water (4.1) to the line in 0 ~ 10 ℃ the dark save, or add 1 ~ 2 ml trichloromethane preservation, stability of 6 months.

4.6.2 use liquid potassium nitrate standard, CN = 10 mg/L: reserve liquid water (4.1) will be diluted 10 times. When using preparation.

   

.7 the sulfuric acid solution, 1 + 35.

5 instruments and equipment

5.1 commonly used laboratory instruments and the following instruments.

5.2 ultraviolet spectrophotometer and 10 mm quartz color dish.

5.3 medical portable steam sterilizer or household pressure cooker pressure is 1.1 ~ 1.4 kg/cm2), the pot temperature of 120 ~ 124 ℃.

5.4 glass grinding plug colorimetric tube, 25 ml.

Glass can be used in hydrochloric acid (1 + 9) or immerse sulfuric acid (1 + 35), (4.1) rinse with water after cleaning several times.

6 samples

6.1 sampling

Immediately after the water samples collected in the fridge or below 4 ℃ condition in the preservation, but not more than 24 h.

Water as a place for a long time, can add about 0.5 mL to 1000 mL water sample (p = 1.84 g/mL) of sulfuric acid acidification to pH less than 2, and the determination of as soon as possible.

Samples can be stored in glass bottles.

6.2 sample preparation

Samples from laboratory (6.1) (4.3) or a sulfuric acid solution with sodium hydroxide solution (4.7) to adjust pH to 5 ~ 9 sample made.

If do not contain suspended solids in the sample according to the (7.1.2) determination of steps, containing suspended solids in the sample according to the determination (7.1.3) step.

7 analysis steps

7.1 the determination of

7.1.1 in dividing straws in 10.00 mL sample (more than 100 CN? G, take amount and add water (4.1) can reduce the dilution and 10 ml) in the colorimetric tube.

7.1.2 sample without suspended matter, according to the following steps.

A. add 5 ml of alkaline potassium persulfate solution (4.4), plug the plug with cloth and rope tied method such as cork, in case the pop-up.

B. the colorimetric tube put in medical portable steam sterilizer, heating, make the gauge pressure to 1.1 ~ 1.4 kg/cm2, timing starts at this point after the temperature of 120 ~ 124 ℃. Or put colorimetric tube at home in the pressure cooker, heated to top pressure valve timing starts when you blow. Keep the temperature of heating and a half hours.

C. cooling, open vent valve, remove the cover, remove the colorimetric tube well cooling to room temperature.

D. with hydrochloric acid (1 + 9) 1 ml, dilute ammonia to 25 ml line, in the blender.

E. moving part of the solution to 10 mm, quartz color plate, on the ultraviolet spectrophotometer, with no ammonia as reference, respectively in 220 and 275 nm wavelength for determination of absorbance, and absorbance of A type (1) calculate the correction.

7.1.3 sample containing suspended solids, press the 7.1.2 above a to d steps first, and then to clarify back take supernatant on quartz color in the dish. Then according to the above 7.1.2

Zero concentration (blank) solution and other standard using potassium nitrate solution (4.6.2) series of complete analysis of calibration steps, 220 and 275 nm wavelengths after determination of absorbance, press type, respectively, and other series of calibration correction except zero concentration and absorbance As the concentration of zero calibration absorbance Ab and its difference Ar

As = As220-2 as275.......................................................................................... (2)

Ab = Ab220-2 ab275.......................................................................................... (3)

Ar = the As - Ab......................................................................................................... (4)

Type: AS220 - standard solution at 220 nm wavelength absorbance;

AS275 - standard solution at 275 nm wavelength absorbance;

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

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

According to the value of Ar and corresponding NO3 -- N content (MCG) drawing the calibration curve.

The results of 8 said

8.1 calculation method

(1) is calculated according to the type specimen correction absorbance Ar, find out the corresponding total nitrogen on the calibration curve? G number, total nitrogen content (mg/L) press type calculation: type: m - sample measure nitrogen content, MCG;

V - test sample volume, mL.

Precision and accuracy

9.1 repeatability

Zero concentration (blank) solution and other standard using potassium nitrate solution (4.6.2) series of complete analysis of calibration steps, 220 and 275 nm wavelengths after determination of absorbance, press type, respectively, and other series of calibration correction except zero concentration and absorbance As the concentration of zero calibration absorbance Ab and its difference Ar

As = As220-2 as275.......................................................................................... (2)

Ab = Ab220-2 ab275.......................................................................................... (3)

Ar = the As - Ab......................................................................................................... (4)

Type: AS220 - standard solution at 220 nm wavelength absorbance;

AS275 - standard solution at 275 nm wavelength absorbance;

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

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

According to the value of Ar and corresponding NO3 -- N content (MCG) drawing the calibration curve.

The results of 8 said

8.1 calculation

   

Precision and accuracy

9.1 repeatability

21 laboratory determination of sodium nitrite, respectively amino propionic acid and ammonium chloride mixed samples; CW604 ammonia standard sample; L - glutamic acid mixed with glucose. These three works nitrogen content were 1.49, 2.64 and 1.15 mg/L, its analysis results are as follows:

Indoor relative standard deviation of each lab were 2.3, 1.6 and 2.5%. Indoor repeated measurement allows precision were 0.074, 0.092 and 0.063 mg/L.

9.2 reproducibility

The laboratory of the above three kinds of unified synthetic samples determination. Laboratory between relative standard deviation were 3.1%, 1.1% and 4.2%; Reproducibility relative standard deviation were 4.0%, 1.9% and 4.8%; The total relative standard deviation were 3.8, 1.9 and 4.9%.

9.3 accuracy

The laboratory determination of the three unified synthetic samples, the laboratory average relative error was 6.3%, 2.4% and 8.7%.

Indoor single relative error was 7.5%, 3.8% and 9.8%. Laboratory, the average recovery confidence limit were 99.0 + / - 6.4%, 99.0 + / - 5.1% and 101 plus or minus 9.4%.

(source: China national chemical instrument network)