GB/T 22923-2008Determination of nitrogen phosphorus potassium for fertilizers by auto analyzer (English PDF)

3.1 Determination of nitrogen content - Nitrogen analyzer method

3.1.1 Principle

The nitrate in the acidic medium is reduced into the ammonium salt, which is digested by concentrated sulfuric acid in the presence of a mixed catalyst to convert the amide nitrogen into an ammonium salt. Ammonia is distilled from the alkaline solution, AND the ammonia is absorbed with boric acid absorption solution AND then titrated with the sulfuric acid standard titration solution.

Automatic nitrogen analyzer can integrate such functions as distillation, titration, result display, or calculation, etc., to finish determination automatically and quickly.

WEIGH the sample of total nitrogen content of about 100 mg and nitrate nitrogen content of less than 25 mg (accurate to 0.0002 g) in the digestion tube;

ADD 0.5 g of chromium powder; USE a small amount of water to wash the tube wall; PLACE the digestion tube in a ventilation cabinet; ADD 5 mL of hydrochloric acid in the digestion tube; LET it stand at room temperature for at least 5 min, (BUT not exceeding 10 min). PLACE the digestion tube in the digestion furnace which has been heated to 300 °C in advance; INSERT in the pear-shaped glass funnel; HEAT to boil it for 1 min ~ 2 min; PAY attention to avoid drying it; COOL it down; RINSE the funnel and digestion tube wall.

3.1.4.3 Treatment of sample containing amide nitrogen and ammonium nitrogen

WEIGH the sample containing nitrogen of about 100 mg (accurate to 0.0002 g) into the digestion tube; USE a small amount of water to wash the tube wall;

PLACE the digestion tube in a ventilation cabinet; ADD 10 mL of sulfuric acid and 0.5 g of mixed catalyst; INSERT the pear-shaped glass funnel; HEAT it for 1 h in the digestion furnace at 340 °C; PAY attention to avoid drying it; COOL it down; RINSE the funnel and digestion tube wall.

3.1.4.4 Treatment of sample containing nitrate nitrogen and amide nitrogen

WEIGH the sample of total nitrogen content of about 100 mg and nitrate nitrogen content of less than 25 mg (accurate to 0.0002 g) in the digestion tube;

ADD 0.5 g of chromium powder; USE a small amount of water to wash the tube wall; PLACE the digestion tube in a ventilation cabinet; ADD 5 mL of hydrochloric acid in the digestion tube; LET it stand at room temperature for at least 5 min, (BUT not exceeding 10 min). PLACE the digestion tube in the digestion furnace which has been heated to 300 °C in advance; INSERT in the pear-shaped glass funnel; HEAT to boil it for 1 min ~ 2 min; PAY attention to avoid drying it; COOL it down; carefully ADD 10 mL sulfuric acid and 0.5 g of mixed catalyst; PLACE it into the digestion furnace again; INCREASE the temperature to 340 °C; HEAT it until the solution color changes into purple;

PAY attention to avoid drying it; COOL it down; RINSE the funnel and digestion tube wall.

3.1.4.5 Treatment of organic-inorganic compound fertilizer sample containing no nitrate nitrogen

WEIGH the sample containing nitrogen of about 100 mg (accurate to 0.0002 g) into the digestion tube which had been dried in advance; PLACE the digestion tube into the ventilation cabinet; ADD 10 mL of sulfuric acid and 0.5 g of mixed catalyst; INSERT the pear-shaped glass funnel; LET it stand for 10 min;

PLACE the digestion tube over the digestion furnace to slowly heat it (if the reaction is intense to produce too much foam, MOVE it away from the digestion tube to cool it down for a moment); after the intense reaction finishes, INCREASE the temperature of the digestion furnace to 340 °C to continue digestion, until the solution is colorless or in light color. PAY attention to avoid drying it; COOL it down; RINSE the funnel and digestion tube wall.

3.2.1.3.2

Automatic digestion furnace, the temperature can be controlled within the range of 340 °C ± 5 °C;

3.2.1.3.5

Flow analyzer, it has the ammonium nitrogen channel and automatic injection device, with the ammonium nitrogen detection lower limit of 0.2 mg/L.

3.2.1.4 Preparation of sample solution

MAKE parallel determinations against two set of materials.

PREPARE laboratory samples in accordance with the corresponding product standards.

3.2.1.4.1 Treatment of sample containing ammonium nitrogen only. WEIGH the sample containing about 100 mg of nitrogen (accurate to 0.0002 g) into a 250-mL conical flask; ADD about 100 mL of water and 5 mL of sulfuric acid; HEAT to boil it for 15 min; COOL it down; quantitatively TRANSFER it to a 250-mL volumetric flask; USE water to dilute it to the mark; MIX it uniformly; dry FILTER it; DISCARD the initial filtrate.

3.2.1.4.2 Treatment of sampling containing amide nitrogen and ammonium nitrogen. Treatment of sample containing ammonium nitrogen only. WEIGH the sample containing about 100 mg of nitrogen (accurate to 0.0002 g) into a digestion tube; USE a small amount of water to rinse the tube wall; PLACE the digestion tube into the ventilation cabinet; ADD 5 mL of sulfuric acid; PLACE the digestion tube on the digestion furnace; HEAT it at 340 °C for 1 h; quantitatively TRANSFER it to a 250 mL volumetric flask; USE water to dilute it to the mark; MIX it uniformly; dry FILTER it; DISCARD the initial filtrate.

3.2.1.4.3 Preparation of blank solution. except for not adding sample, the rest procedures are same as the preparation of sample solution.

3.2.1.5 Analytical procedures

3.2.1.5.1 Drawing of working curves

PIPETTE 0 mL, 0.5 mL, 1.0 mL, 2.0 mL and 4.0 mL of ammonium nitrogen (NH4+-N) standard solution respectively into five 100 mL volumetric flask; USE the corresponding blank solution to dilute it to the mark; MIX it uniformly. AND the concentrations of this standard solution are 0, 50 mg/L, 100 mg/L, 200 mg/L, and 400 mg/L, respectively.

Before making the determination, with reference to the instrument instruction manual, SELECT the best working parameters.

Then, PLACE the solution sucker of the flow analyzer into the buffer solution, sodium salicylate solution, and dichloroisocyanic acid solution respectively. At

PREPARE laboratory samples in accordance with the corresponding product standards.

WEIGH the sample contain the nitrate nitrogen of about 50 mg (accurate to 0.0002 g); PLACE it into a 250mL-conical flask; ADD about 100 mL of water;

HEAT to boil it for 15 min; COOL it down; quantitatively TRANSFER it to a 250-mL volumetric flask; USE water to dilute it to the mark; MIX uniformly; dry FILTER it; DISCARD the initial part of the filtrate.

Preparation of blank solution. except for not adding sample, the rest procedures are same as the preparation of sample solution. […]

3.2.2.5 Analytical procedures

3.2.2.5.1 Drawing of working curves

PIPETTE 0 mL, 1.0 mL, 2.0 mL, 4.0 mL, and 8.0 mL of nitrate nitrogen... […]