This method is for the determination of any one form of nitrogen in the presence of any other form.
Any fertiliser in Group 1(a) of Section A, and Groups 1, 2 and 3 of Section B of the Table in Schedule 1 of the Fertilisers Regulations 1991 containing nitrogen in various forms.
3.1—3.1.1 In the absence of nitrates, the sample is subjected to direct Kjeldahl digestion.
3.1.2 In the presence of nitrates, the sample is subjected to Kjeldahl digestion after reduction with the aid of metallic iron and stannous chloride. In both cases, the ammonia is determined according to Method 2.
Note:If analysis shows an insoluble nitrogen content of more than 0.5%, it is presumed that the fertiliser contains other forms of insoluble nitrogen not specified for fertilisers covered by the list in paragraph 2.
3.2 The following are determined from different aliquot parts taken from the same solution of the sample:
3.2.1 Total soluble nitrogen
3.2.1.1In the absence of nitrates, by direct Kjeldahl digestion.
3.2.1.2In the presence of nitrates, by Kjeldahl digestion on an aliquot portion taken from the solution after reduction according to Ulsch, the ammonia being determined in both cases as described in Method 2.
3.2.2 Total soluble nitrogen with the exception of nitric nitrogen, by Kjeldahl digestion after elimination in an acid medium of nitric nitrogen with ferrous sulfate, the ammonia being determined as described in Method 2.
3.2.3 Nitric nitrogen by difference
3.2.3.1In the absence of calcium cyanamide, between (3.2.1.2) and (3.2.2) or between total soluble nitrogen (3.2.1.2) and the sum of ammoniacal nitrogen and ureic nitrogen (3.2.4+3.2.5).
3.2.3.2In the presence of calcium cyanamide, between (3.2.1.2) and (3.2.2) and between (3.2.1.2) and the sum of (3.2.4+3.2.5+3.2.6).
3.2.4 Ammoniacal nitrogen
3.2.4.1Solely in the presence of ammoniacal nitrogen and ammoniacal + nitric nitrogen, by applying Method 2.
3.2.4.2In the presence of ureic nitrogen and/or cyanamide nitrogen, by cold distillation after making slightly alkaline, the ammonia being absorbed in a standard solution of sulfuric acid and determined as described in Method 2.
3.2.5 Ureic nitrogen
Either
3.2.5.1By conversion using urease into ammonia which is titrated with a standard solution of hydrochloric acid,
or:
3.2.5.2By gravimetry with xanthydrol, although biuret will also be precipitated by xanthydrol, this should not give rise to a significant error in the determination since its level is generally low in absolute value in compound fertilisers.
or:
3.2.5.3By difference, according to the following table:
| Case | Nitric Nitrogen | Ammoniacal Nitrogen | Cyanamide Nitrogen | Difference |
|---|---|---|---|---|
| 1 | Absent | Present | Present | (3.2.1.1)-(3.2.4.2+3.2.6) |
| 2 | Present | Present | Present | (3.2.2)-(3.2.4.2+3.2.6) |
| 3 | Absent | Present | Absent | (3.2.1.1)-(3.2.4.2) |
| 4 | Present | Present | Absent | (3.2.2)-(3.2.4.2) |
3.2.6 Cyanamide nitrogen, by precipitation as a silver compound, the nitrogen being estimated in the precipitate by the Kjeldahl method.
Potassium sulfate.
Iron powder, reduced with hydrogen (the prescribed quantity of iron must be able to reduce at least 50 mg of nitric nitrogen).
Potassium thiocyanate.
Potassium nitrate.
Ammonium sulfate.
Urea.
Sulfuric acid solution: dilute an appropriate volume of sulfuric acid (p=1.84 g/ml) with an equal volume of water.
Sulfuric acid, 0.1 M solution.
Sodium hydroxide solution, 30 g per 100 ml, ammonia free.
Sodium or potassium hydroxide, 0.2 M solution, free from carbonates.
Stannous chloride solution:
Dissolve 120 g of stannous chloride (SnCl2.2H2O) in 400 ml of concentrated hydrochloric acid (p=1.18 g/ml) and make up to 1 litre with water. The solution must be perfectly clear and prepared immediately before use.
It is essential to check the reducing power of the stannous chloride: dissolve 0.5 g of stannous chloride in 2 ml of concentrated hydrochloric acid (p=1.18 g/ml) and make up to 50 ml with water. Then add 5 g of Rochelle salt (potassium sodium tartrate) and a sufficient quantity of sodium bicarbonate for the solution to be alkaline to litmus paper.
Titrate with 0.1 M iodine solution in the presence of a starch solution as an indicator.
1 ml of 0.1 M iodine solution corresponds to 0.01128 g of SnCl2.2H2O.
At least 80% of the total tin present in the solution thus prepared must be in bivalent form. For the titration, at least 35 ml of 0.1 M iodine solution must therefore be used.
Sulfuric acid, concentrated (p=1.84 g/ml).
Hydrochloric acid solution: dilute an appropriate volume of hydrochloric acid (p=1.18 g/ml) with an equal volume of water.
Glacial acetic acids.
Sulfuric acid solution, approximately 30% (W/V) H2SO4.
Ferrous sulfate, crystalline, FeSO4.7H2O
Sulfuric acid, 0.05 M solution.
Octan-1-ol.
Potassium carbonate, saturated solution.
Sodium or potassium hydroxide, 0.1 M solution, free from carbonate.
Barium hydroxide, saturated solution.
Sodium carbonate solution, 10 g per 100 ml.
Hydrochloric acid, 2 M solution.
Hydrochloric acid, 0.1 M solution.
Urease solution: Suspend 0.5 g of active urease in 100 ml of distilled water. Using 0.1 M hydrochloric acid (4.24), adjust the pH to 5.4, measured by pH meter.
Xanthydrol solution, 5 g per 100 ml in ethanol or methanol (4.31) (do not use products giving a high proportion of insoluble matter). The solution may be kept for three months in a well-stoppered bottle, away from the light.
Copper oxide (CuO): 0.3 to 0.4 g per determination or an equivalent quantity of copper sulfate pentahydrate (0.95 to 1.25 g) per determination.
Anti-bump granules washed in hydrochloric acid and ignited.
Indicator solutions:
4.29.1 Mixed indicator solution: Solution A: dissolve 1 g of methyl red in 37 ml of 0.1 M sodium hydroxide solution and make up to one litre with water. Solution B: dissolve 1 g of methylene blue in water and make up to one litre. Mix one volume of solution A and 2 volumes of solution B. This indicator is violet in acid solution, grey in neutral solution and green in alkaline solution. Use 0.5 ml (10 drops) of this indicator solution.
4.29.2 Methyl red indicator solution: Dissolve 0.1 g of methyl red in 50 ml of 95% ethanol, make up to 100 ml with water and filter if necessary. This indicator (4 to 5 drops) can be used instead of the previous one.
Indicator papers: Litmus, bromothymol blue (or other papers sensitive in the range pH 6 to 8).
Ethanol or methanol: solution 95%.
Distillation apparatus. See Method 2.
Apparatus for the determination of ammoniacal nitrogen 7.2.5.3. An example of the recommended apparatus is reproduced in Figure 6 in the Appendix.
The apparatus is made up of a specially shaped receptacle with a ground glass neck, a side neck, a connecting tube with a splash head and a perpendicular tube for the introduction of air. The tubes can be connected to the receptacle by means of a simple perforated rubber bung. It is important to give a suitable shape to the end of the tubes introducing air, since the bubbles of gas must be evenly distributed throughout the solutions contained in the receptacle and the absorber. The best arrangement consists of small mushroom-shaped pieces with an external diameter of 20 mm and six openings of 1 mm around the periphery.
Apparatus for the estimation of urea nitrogen (7.2.6.1).
It consists of a 300 ml Erlenmeyer flask, with a separating funnel and a small absorber. An example of the recommended apparatus is reproduced in Figure 7 in the Appendix.
Rotary shaker, 35 – 40 turns per minute.
pH meter.
Laboratory oven.
Sintered glass crucibles, diameter of pores 5 to 15 microns.
See Method 1.
7.1—7.1.1 In the absence of nitrate
7.1.1.1Digestion
Weigh to the nearest 0.001 g, a quantity of the prepared sample containing not more than 100 mg of nitrogen. Place in the flask of the distillation apparatus (5.1). Add 10 to 15 g of potassium sulfate (4.1), the prescribed quantity of catalyst (4.27), and a few anti-bump granules (4.28). Then add 50 ml of dilute sulfuric acid (4.7), and mix thoroughly. First heat gently, mixing from time to time, until foaming ceases. Then heat so that the liquid boils steadily and keep it boiling for one hour after the solution has become clear, preventing any organic matter from sticking to the sides of the flask. Allow to cool. Carefully add about 350 ml of water, with mixing. Ensure that the dissolution is as complete as possible. Allow to cool and connect the flask to the distillation apparatus (5.1).
7.1.1.2Distillation of ammonia
Transfer with a pipette 50 ml of standard 0.1 M sulfuric acid (4.8) into the receiver of the apparatus. Add the indicator (4.29.1 or 4.29.2). Ensure that the tip of the condenser is at least 1 cm below the level of the solution.
Taking the necessary precautions to avoid any loss of ammonia, carefully add to the distillation flask enough of the concentrated sodium hydroxide solution (4.9) to make the liquid strongly alkaline (120 ml is generally sufficient: check by adding a few drops of phenolphthalein. At the end of the distillation the solution in the flask must still be clearly alkaline). Adjust the heating of the flask so as to distil 150 ml in half an hour. Test with indicator paper (4.30) that the distillation has been completed. If it has not, distil a further 50 ml and repeat the test until the supplementary distillate reacts neutrally to the indicator paper (4.30). Then lower the receiver, distil a few ml more and rinse the tip of the condenser. Titrate the excess acid with a standard solution of potassium or sodium hydroxide 0.2 M (4.10) to the end point of the indicator.
7.1.1.3Blank test
Make a blank test under the same conditions (omitting only the sample) and use this value in the calculation of the final result.
7.1.14 Expression of the result
No math image to display
where:
a = ml of standard solution of sodium or potassium hydroxide (0.2 M) used for the blank.
A = ml of standard solution of sodium or potassium hydroxide (0.2 M) used for the analysis.
M = mass of the sample in grams.
7.1.2 In the presence of nitrate
7.1.2.1Test sample
Weigh to the nearest 0.001g, a quantity of the sample containing not more than 40 mg of nitric nitrogen.
7.1.2.2Reduction of the nitrate
Mix the sample in a small mortar with 50 ml of water. Transfer with the minimum amount of distilled water into a 500 ml Kjeldahl flask. Add 5 g of reduced iron (4.2) and 50 ml of stannous chloride solution (4.11). Shake and leave to stand for half an hour. During this time shake again after 10 and 20 minutes.
7.1.2.3Kjeldahl digestion
Add 30 ml of sulfuric acid (4.12), 5 g of potassium sulfate (4.1), the prescribed quantity of catalyst (4.27) and some anti-bump granules (4.28). Heat gently with the flask slightly tilted. Increase the heat slowly and swirl the solution frequently to keep the mixture suspended; the liquid darkens and then clears with the formation of a yellow-green anhydrous iron sulfate suspension. After obtaining a clear solution simmer for one hour. Leave to cool. Cautiously take up the contents of the flask in a little water and add little by little 100 ml of water. Mix and transfer the contents of the flask into a 500 ml graduated flask. Rinse the flask several times with distilled water. Make up the volume with water and mix. Filter through a dry paper into a dry receiver. Discard the first portion of the filtrate.
7.1.2.4Distillation of ammonia
Transfer into the flask of the distillation apparatus (5.1), an aliquot part containing not more than 100 mg of nitrogen. Dilute to about 350 ml with distilled water, add a few anti-bump granules (4.28), connect the flask to the distillation apparatus and continue the determination as described in paragraph 7.1.1.2.
7.1.2.5Blank test
See 7.1.1.3.
7.1.2.6Expression of the result
No math image to display
where:
a = ml of standard solution of sodium or potassium hydroxide (0.2 M) used for the blank.
A = ml of standard solution of sodium or potassium hydroxide (0.2 M) used for the analysis.
M = mass of the sample, expressed in grams, present in the aliquot part taken for a analysis.
7.2—7.2.1 Preparation of the solution to be analysed
Weigh to the nearest 0.001 g, 10 g of the sample and place it in a 500 ml graduated flask.
7.2.1.1In the case of fertilisers not containing cyanamide nitrogen
Add to the flask 50 ml of water and then 20 ml of dilute hydrochloric acid (4.13). Shake and leave it to stand until the evolution of carbon dioxide ceases. Then add 400 ml of water and shake for half an hour on the rotary shaker (5.4).
Make up to the volume with water, mix and filter through a dry filter into a dry receiver. Discard the first portion of the filtrate.
7.2.1.2In the case of fertilisers containing cyanamide nitrogen
Add to the flask 400 ml of water and a few drops of methyl red (4.29.2). If necessary make the solution acidic by using acetic acid (4.14). Add 15 ml of acetic acid (4.14). Shake on the rotary shaker (5.4) for 2 hours. If necessary, re-acidify the solution during the operation, using acetic acid (4.14). Make up to the volume with water, mix, filter immediately through a dry filter into a dry receiver and immediately determine the cyanamide nitrogen.
In both cases, determine the various soluble forms of nitrogen the same day the solution is made up, starting with cyanamide nitrogen and urea nitrogen, if they are present.
7.2.2 Total soluble nitrogen
7.2.2.1In the absence of nitrate
Transfer by pipette into a 300 ml Kjeldahl flask, an aliquot portion of the filtrate (7.2.1.1 or 7.2.1.2), containing not more than 100 mg of nitrogen. Add 15 ml of concentrated sulfuric acid (4.12), 0.4 g of copper oxide or 1.25 g of copper sulfate (4.27) and a few anti-bump granules (4.28). First heat gently to begin the digestion and then at a higher temperature until the liquid becomes colourless or slightly greenish and white fumes are clearly apparent. After cooling, quantitatively transfer the solution into the distillation flask, dilute to about 500 ml with water and add a few anti-bump granules (4.28). Connect the flask to the distillation apparatus (5.1) and continue the distillation as described in paragraph 7.1.1.2.
7.2.2.2In the presence of nitrate
Transfer by pipette into a 500 ml Erlenmeyer flask, an aliquot portion of the filtrate (7.2.1.1 or 7.2.1.2) containing not more than 40 mg of nitric nitrogen. At this stage of the analysis the total quantity of nitrogen is not important. Add 100 ml of 30% sulfuric acid (4.15), 5 g of reduced iron (4.2) and immediately cover the Erlenmeyer flask with a watch glass. Heat gently until the reaction is steady but not vigorous. At this juncture stop the heating and allow the flask to stand for at least three hours at ambient temperature. With water, quantitatively transfer the liquid into a 250 ml graduated flask, leaving behind the undissolved iron and make up to the mark with water. Mix thoroughly, and transfer by pipette into a 300 ml Kjeldahl flask, an aliquot part containing not more than 100 mg of nitrogen. Add 15 ml of concentrated sulfuric acid (4.12), 0.4 g of copper oxide or 1.25 g of copper sulfate (4.27) and some anti-bump granules (4.28). First heat gently to begin the digestion and then at a higher temperature until the liquid becomes colourless or slightly greenish and white fumes are clearly apparent. After cooling transfer the solution quantitatively into the distillation flask, dilute to approximately 500 ml with water and add some anti-bump granules (4.28). Connect the flask to the distillation apparatus (5.1) and continue the determination as described in paragraph 7.1.1.2.
7.2.2.3Blank test
See 7.1.1.3.
7.2.2.4Expression of result
No math image to display
where:
a = ml of standard solution of sodium or potassium hydroxide (0.2 M) used for the blank.
A = ml of standard solution of sodium or potassium hydroxide (0.2 M) used for analysis.
M = mass of the sample, expressed in grams, present in the aliquot part taken for analysis.
7.2.3 Total soluble nitrogen with the exception of nitric nitrogen
Transfer by pipette into a 300 ml Kjeldahl flask, an aliquot portion of the filtrate (7.2.1.1 or 7.2.1.2) containing not more than 50 mg of nitrogen. Dilute to 100 ml with water, add 5 g of ferrous sulfate (4.16), 20 ml of concentrated sulfuric acid (4.1) and some anti-bump granules (4.28). First heat gently and then increase the heat until white fumes appear. Continue the digestion for 15 minutes. Stop the heating, introduce the copper oxide (4.27) as a catalyst and keep it at a temperature such that white fumes are emitted for a further 10 to 15 minutes. After cooling, quantitatively transfer the contents of the Kjeldahl flask into the distillation flask of the apparatus (5.1). Dilute to approximately 500 ml with water and add a few anti-bump granules (4.28). Connect the flask to the distillation apparatus and continue the determination as described in paragraph 7.1.1.2.
7.2.3.1Blank test
See 7.1.1.3
7.2.3.2Expression of result
No math image to display
where:
a = ml of standard solution of sodium or potassium hydroxide (0.2 M) used for the blank.
A = ml of standard solution of sodium or potassium hydroxide (0.2 M) used for analysis.
M = mass of the sample, expressed in grams, present in the aliquot part taken for analysis.
7.2.4 Nitric nitrogen is obtained:
7.2.4.1In the absence of calcium cyanamide
By the difference between the results obtained in paragraphs 7.2.2.4 and 7.2.3.2 and/or the result obtained in paragraph 7.2.2.4 and the sum of the results obtained in paragraphs 7.2.5.2 or 7.2.5.5 and 7.2.6.3 or 7.2.6.5 or 7.2.6.6.
7.2.4.2In the presence of calcium cyanamide
By the difference between the results obtained in paragraphs 7.2.2.4 and 7.2.3.2 and between the result obtained in paragraph 7.2.2.4 and the sum of the results obtained in paragraphs 7.2.5.5 and 7.2.6.3 or 7.2.6.5 or 7.2.6.6 and 7.2.7.
7.2.5 Ammoniacal nitrogen
7.2.5.1Solely in the presence of ammoniacal nitrogen and ammoniacal + nitric nitrogen
Transfer by pipette into the flask of the distillation apparatus (5.1) an aliquot portion of the filtrate (7.2.1.1) containing not more than 100 mg of ammoniacal nitrogen. Add water to obtain a total volume of about 350 ml and some anti-bump granules (4.28) to facilitate boiling. Connect the flask to the distillation apparatus, add 20 ml of sodium hydroxide solution (4.9) and distil as described in paragraph 7.1.1.2.
7.2.5.2Expression of result
No math image to display
where:
a = ml of standard solution or potassium hydroxide (0.2 M) used for the blank.
A = ml of standard solution of sodium or potassium hydroxide (0.2 M) used for the analysis.
M = mass of the sample, expressed in grams, present in the aliquot part taken for analysis.
7.2.5.3In the presence of urea and/or cyanamide nitrogen
Transfer by pipette into the dry flask of the apparatus (5.2), an aliquot portion of the filtrate (7.2.1.1 or 7.2.1.2) containing not more than 20 mg of ammoniacal nitrogen. Then assemble the apparatus. Transfer by pipette into the 300 ml Erlenmeyer flask 50 ml of the standard sulfuric acid solution 0.1 M (4.17) and enough distilled water for the level of the liquid to be approximately 5 cm above the opening of the delivery tube; add the indicator (4.29.1). Introduce, through the side neck of the reaction flask, distilled water to make up the volume to about 50 ml and mix. To avoid foaming during aeration, add a few drops of octan-1-ol (4.18). Make the solution alkaline by adding 50 ml of saturated potassium carbonate solution (4.19) and immediately begin to expel the ammonia thus liberated from the cold suspension. A strong current of air is necessary (flow of approximately 3 litres per minute) and should be purified beforehand by passing it through washing flasks containing dilute sulfuric acid and dilute sodium hydroxide. Instead of using pressurised air, it is also possible to use a vacuum (water pump) provided that the inflow tube is connected in a sufficiently airtight manner to the receiver used to collect the ammonia. The liberation of the ammonia is generally complete after three hours. It is nevertheless advisable to verify this by changing the receiving flask. When the operation is finished, disconnect the flask from the apparatus, rinse the tip of the tube and the sides of the flask with a little distilled water. Titrate the excess acid with standard sodium hydroxide solution (0.1 M) (4.20) to the end point of the indicator (4.29.1).
7.2.5.4Blank test
See 7.1.1.3.
7.2.5.5Expression of result
No math image to display
where:
a = ml of standard solution of sodium or potassium hydroxide (0.1 M) used for the blank.
A = ml of standard solution of sodium or potassium hydroxide (0.1 M) used for the analysis.
M = mass of the sample, expressed in grams, present in the aliquot part taken for analysis.
7.2.6 Ureic nitrogen
7.2.6.1Urease method
Transfer by pipette into a 500 ml graduated flask, an aliquot portion of the filtrate (7.2.1.1 or 7.2.1.2) containing not more than 250 mg of ureic nitrogen. To remove phosphates add saturated barium hydroxide solution (4.21) until no further precipitation occurs. Eliminate the excess of barium ions and any dissolved calcium ions by adding 10% sodium carbonate solution (4.22). Allow the precipitate to settle and check whether total precipitation has occurred. Make up to the mark, mix and filter through a pleated filter. Transfer by pipette 50 ml of the filtrate into the 300 ml Erlenmeyer flask of the apparatus (5.3). Acidify the filtrate with 2 M hydrochloric acid (4.23), until a pH of 3.0 measured by the pH meter (5.5) is obtained. Then raise the pH to 5.4 with 0.1 M sodium hydroxide solution (4.20).
To avoid losses of ammonia during reaction with urease, close the Erlenmeyer flask with a stopper provided with a separating funnel and a small bubble trap containing exactly 2 ml of standard 0.1 M hydrochloric acid (4.24). Introduce through the separating funnel 20 ml of urease solution (4.25), and allow to stand for one hour at 20 – 25 C. Transfer by pipette 25 ml of standard 0.1 M hydrochloric acid (4.24) into the separating funnel, allow it to run through into the solution and then rinse with a little water. In the same way transfer quantitatively the contents of the bubble trap into the solution contained in the Erlenmeyer flask. Titrate the excess acid with the standard solution of sodium hydroxide (0.1 M) (4.20), until a pH of 5.4 is obtained, measured by the pH meter.
7.2.6.2Blank test
See 7.1.1.3.
7.2.6.3Expression of result
No math image to display
where:
a = ml of standard solution of sodium or potassium hydroxide (0.1 M) used for the blank, carried out exactly under the same conditions as the analysis.
A = ml of standard solution of sodium or potassium hydroxide (0.1 M) used for the analysis.
M = mass of the sample, expressed in grams, present in the aliquot part taken for analysis.
Remarks
| (1) | After precipitation by the solutions of barium hydroxide and sodium carbonate, make up to the mark, filter and neutralise as rapidly as possible. |
| (2) | The titration may also be carried out with the indicator (4.29.2), but the end point is then more difficult to observe. |
7.2.6.4Gravimetric method with xanthydrol
Transfer by pipette into a 250 ml beaker, an aliquot portion of the filtrate (7.2.1.1 or 7.2.1.2) containing not more than 20 mg of urea. Add 40 ml of acetic acid (4.14). Stir with a glass rod for one minute, allow any precipitate to settle for five minutes. Filter into a 100 ml beaker, wash with several ml of acetic acid (4.14), then add to the filtrate drop by drop, 10 ml of xanthydrol solution (4.16), stirring continuously with a glass rod. Allow to stand until the precipitate appears, then stir again for one or two minutes. Allow to stand for one and half hours. Filter through a sintered glass crucible (5.7) which has been previously dried and weighed, using a slight reduction in pressure. Wash three times with 5 ml ethanol (4.31) without trying to remove all the acetic acid. Place it in the oven (5.6) at a temperature of 130 C for one hour (do not exceed 145 C). Allow to cool in a desiccator and weigh.
7.2.6.5Expression of result
No math image to display
where:
m = mass of the precipitate obtained, in grams.
M = mass of the sample, in grams, present in the aliquot part taken for analysis. Correct for the blank.
Note:although biuret will also be precipitated by xanthydrol, this should not give rise to a significant error in the determination since its level is generally low.
7.2.6.6Method of difference
Ureic nitrogen may also be calculated according to the following table: —
| Case | Nitric Nitrogen | Ammoniacal Nitrogen | Cyanamide Nitrogen | Ureic Nitrogen |
|---|---|---|---|---|
| 1 | Absent | Present | Present | (7.2.2.4) – (7.2.5.5+7.2.7) |
| 2 | Present | Present | Present | (7.2.3.2) – (7.2.5.5+7.2.7) |
| 3 | Absent | Present | Absent | (7.2.2.4) – (7.2.5.5) |
| 4 | Present | Present | Absent | (7.2.3.2) – (7.2.5.5) |
7.2.7 Cyanamide Nitrogen
Take an aliquot part of the filtrate (7.2.1.2), containing 10 to 30 mg of cyanamide nitrogen and place it in a 250 ml beaker. Continue the analysis according to Method 6.
In certain cases, a difference may be found between the total nitrogen obtained directly from a weighed out sample (paragraph 7.1) and total soluble nitrogen (paragraph 7.2.2). Nevertheless, the difference should not be greater than 0.5%. If this is not the case, the fertiliser contains forms of insoluble nitrogen not specified for fertilisers covered by the list in paragraph 2.
Before each analysis, check that the apparatus is working properly and that the correct application of the method is used, with a standard solution including the various forms of nitrogen in proportions similar to those of the test sample. This standard solution is prepared from solutions of potassium thiocyanate (4.3), potassium nitrate (4.4), ammonium sulfate (4.5) and urea (4.6).