GB 7173-87
This standard applies to the determination of soil total nitrogen content.
1 Measurement principle
When the sample is digested with concentrated sulfuric acid under the participation of an accelerator, various nitrogen-containing organic compounds undergo a complex pyrolysis reaction and are converted into ammonium nitrogen. The ammonia distilled after alkalization is absorbed by boric acid and titrated with an acid standard solution to determine the total nitrogen content of the soil (excluding all nitrate nitrogen).
In the determination of total nitrogen including nitrate and nitrite nitrogen, the nitrite nitrogen in the sample is first oxidized to nitrate nitrogen with potassium permanganate before the sample is decocted, and then the whole nitrate state is reduced with the reduced iron powder. Nitrogen is reduced and converted into ammonium nitrogen.
2 Instruments and equipment
2.1 soil sample grinder;
2.2 Agate mortar;
2.3 soil sieve: pore size 1.0mm (18 mesh); 0.25mm (60 mesh);
2.4 analytical balance: the amount of sense is 0.0001g;
2.5 Hard Kelvin flask: volume 50ml, 100ml;
2.6 semi-mineral distillation apparatus;
2.7 semi-micro burette: volume 10ml, 25ml;
2.8 conical flask: volume 150ml;
2.9 Electric Furnace: 300W Variable Temperature Electric Furnace.
3 Reagents
3.1 Sulfuric acid (GB 625-77): Chemically pure;
3.2 Sulfuric acid (GB 625-77) or hydrochloric acid (GB 622-77): analytically pure, 0.005 mol/L sulfuric acid or 0.01 mol/L hydrochloric acid standard solution;
3.3 Sodium hydroxide (GB 629-81): industrial or chemical pure, 10mol/L sodium hydroxide solution;
3.4 Boric acid-indicator mixture;
3.4.1 Boric acid (GB 628-78): analytically pure, 2% solution (W/V);
3.4.2 mixed indicator: 0.5g bromocresol green (HG3-1220-79) and 0.1g methyl red (HG3-958-76) in an agate mortar, add a small amount of 95% ethanol, grind to the indicator all After dissolution, add 95% ethanol to 100 ml. Prior to use, add 20 ml of mixed indicator per liter of boric acid solution and adjust to a reddish purple (pH about 4.5) with dilute base. The liquid should not be stored for too long. If there is a change in pH during use, it must be adjusted with dilute acid or dilute alkali at any time.
3.5 Accelerator: 100g of potassium sulfate (HG 3-920-76, chemically pure), 10g of copper sulfate pentahydrate (GB 665-78, chemically pure), and 1g of selenium powder (HG3-926-76) were studied in the mortar. Must be fully mixed.
3.6 potassium permanganate solution: 25g potassium permanganate (GB 643-77) dissolved in 500ml of ionized water, stored in a brown bottle;
3.7 1∶1 sulfuric acid;
3.8 reduced iron powder: grinding through the aperture 0.15mm (100 mesh) sieve;
3.9 Octyl alcohol.
4 Preparation of soil samples
The soil samples passed through a 1 mm (18 mesh) sieve were laid thin on kraft paper and divided into small squares. Use a small spoon in each square, and take an equal amount of soil sample (not less than 20g in total) in an agate mortar to make it pass through a 0.25mm sieve. After mixing evenly.
5 Measurement procedure
5.1 Weigh out the air-dried soil sample (pass 0.25mm sieve) 1.0 × × × g (containing about 1mg of nitrogen), and determine the soil moisture content.
5.2 Earth Sample Cooking
5.2.1 Excluding the digestion of nitrate and nitrite nitrogen:
The soil sample was sent to the bottom of a dry Kettle flask, and a small amount of non-ionized water (about 0.5 to 1 ml) of wet soil was added. 2 g of accelerator and 5 ml of concentrated sulfuric acid were added and shaken. Place the Kelvin bottle tilted on a 300W variable temperature electric furnace and heat it with a small amount of heat. When the reaction in the bottle is mild (about 10~15min), strengthen the firepower to keep the boiled soil fluid slightly boiled. The heated area does not exceed the temperature of the bottle. The liquid level prevents the ammonium salt from being thermally decomposed due to the excessive wall temperature, resulting in loss of nitrogen. The temperature of the cooking is preferably that the sulfuric vapor condenses at the upper 1/3 of the bottle neck. After the liquid to be decocted and the soil particles all become grayish and greenish, continue digestion for 1 hour. After digestion is complete, cool and wait for distillation. When the soil samples are digested, two blank measurements are made. Except that soil samples are not used, all other operations are the same as those for soil samples.
5.2.2 Cooking including nitrate and nitrite nitrogen:
Put the soil sample into the bottom of a dry 50ml Kelvin bottle, add 1ml of potassium permanganate solution, shake the Kelvin bottle, slowly add 2ml of 1:1 sulfuric acid, continue to turn the Kelvin bottle, then place for 5min, then add 1 drop of Xin alcohol. 0.5g (±0.01g) of reduced iron powder is fed into the bottom of the Kelvin bottle through a long-neck funnel, a small funnel is put on the bottle mouth, and the Kelvin bottle is rotated to contact the iron powder with the acid, and when violent reaction stops (about 5min) , The Kaiser flask is placed on the electric furnace and slowly heated for 45 minutes (the fluid in the flask should be kept slightly boiled so as not to cause a large amount of water loss). Stop the fire. After the bottle has cooled, add 2 g of accelerator and 5 ml of concentrated sulfuric acid through a long-necked funnel and shake. According to step 5.2.1, boil until the soil solution turns yellow-green, and continue digestion for 1 hour. After digestion is complete, cool and wait for distillation. While cooking the soil sample, make two blank determinations.
5.3 Distillation of ammonia
5.3.1 Before distilling, check the distillation unit for leaks and wash the pipes with water distillate.
5.3.2 After the liquid to be decontaminated is cooled, digestive liquid is quantitatively transferred into the distiller with a small amount of non-ionized water, and the kelvin flask is washed with water for 4 to 5 times (the total water consumption does not exceed 30 to 35 ml).
In a 150 ml Erlenmeyer flask, add 5 ml of a 2% boric acid-indicator mixture, place it on the end of the condenser tube, and place the nozzle 3 to 4 cm above the boric acid level. Then, 20 ml of a 10 mol/L sodium hydroxide solution was slowly added to the distillation chamber, and steam distillation was conducted. When the volume of the distilled liquid was about 50 ml, the distillation was completed. The ends of the condenser tube were washed with a small amount of water that had been adjusted to pH 4.5.
5.3.3 Titrate with 0.005 mol/L sulfuric acid (or 0.01 mol/L hydrochloric acid) standard solution from blue-green to fresh red. Record the volume (ml) of acid standard solution used. The volume of the acid standard solution used for the blank measurement is generally not to exceed 0.4 ml.
6 Calculation of measurement results
6.1 Calculation Formula
Soil total nitrogen (%) = [(V-V0) × CH × 0.014/m] × 100
Where: V -
The volume of the acid standard solution used when titrating the test solution, ml;
V0── volume of the acid standard solution used when titrating the blank, ml;
CH── concentration of acid standard solution, mol/L;
0.014── The mass of nitrogen atoms;
M─ Dry soil quality, g.
6.2 Parallel determination results, expressed as an arithmetic average, with three decimal places retained.
6.3 The difference between the parallel determination results: When the soil nitrogen content is more than 0.1%, it shall not exceed 0.005%; when the nitrogen content is 0.1-0.06%, it shall not exceed 0.004%; if the nitrogen content is less than 0.06%, it shall not exceed 0.003%.
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