The Fertilisers (Sampling and Analysis) Regulations 1996

Forms of soluble nitrogen

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.

7.2.1.1 In 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.

7.2.1.2 In 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

7.2.2.1 In 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.

7.2.2.2 In 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.

7.2.2.3 Blank test

See 7.1.1.3.

7.2.2.4 Expression 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.

7.2.3.1 Blank test

See 7.1.1.3

7.2.3.2 Expression 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:

7.2.4.1 In 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.

7.2.4.2 In 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

7.2.5.1 Solely 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.

7.2.5.2 Expression 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.

7.2.5.3 In 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).

7.2.5.4 Blank test

See 7.1.1.3.

7.2.5.5 Expression 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

7.2.6.1 Urease 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.

7.2.6.2 Blank test

See 7.1.1.3.

7.2.6.3 Expression 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.

7.2.6.4 Gravimetric 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.

7.2.6.5 Expression 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.

7.2.6.6 Method 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.