The Fertilisers (Sampling and Analysis) Regulations 1991

Urease solution: 4.25 suspend 0.5 g of active urease in 100 ml of distilled water. Using 0.1 N hydrochloric acid (4.24), adjust the pH to 5.4, measured by a pH meter.

• (4.26) 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.

• (4.27) Copper oxide (CuO): 0.3 to 0.4 g per determination or an equivalent quantity of copper sulphate pentahydrate of 0.95 to 1.25 g per determination.

• (4.28) Anti-bump granules washed in hydrochloric acid and ignited.

• (4.29) Indicator solutions:

Mixed indicator solution: 4.29.1 Solution A: dissolve 1 g of methyl red in 37 ml of 0.1 N 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.

Methyl red indicator solution: 4.29.2 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: 4.30 Litmus, bromothymol blue (or other papers sensitive to pH 6 to 8).

• (4.31) Ethanol or methanol: solution 95%.

APPARATUS

5

• (5.1) Distillation apparatus. See Method 2.

• (5.2) Apparatus for the determination of ammoniacal nitrogen according to analytical technique 7.2.5.3. An example of 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.

• (5.3) Apparatus for the estimation of urea nitrogen according to the urease technique (7.2.6.1).

• It consists of a 300 ml Erlenmeyer flask, with a seperating funnel and a small absorber. An example of recommended apparatus is reproduced in Figure 7 in the Appendix.

• (5.4) Rotary shaker, 35-40 turns per minute.

• (5.5) pH meter.

• (5.6) Laboratory oven.

• (5.7) Sintered glass crucibles, diameter of pores 5 to 15 microns.

PREPARATION OF THE SAMPLE

6

See Method 1.

PROCEDURE

7

• (7.1) Total soluble and insoluble nitrogen

• (7.1.1) In the absence of nitrate

• (7.1.1.1) Digestion

Weigh to the nearest 0.001 g, a quantity of the prepared sample containing 100 mg of nitrogen at the most. Place it in the flask of the distillation apparatus (5. 1). Add 10 to 1 5 g of potassium sulphate (4. 1), the prescribed quantity of catalyst (4.27), and a few anti-bump granules (4.28). Then add 50 ml of dilute sulphuric 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 than the dissolution is as complete as possible. Allow to cool and connect the flask to the distillation apparatus (5.1).

• (7.1.1.2) Distillation of ammonia

Transfer with a pipette, into the receiver of the apparatus, 50 ml standard 0.2 N sulphuric acid (4.8). 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 stongly 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 N (4.10) to the end point of the indicator.

• (7.1.1.3) Blank test

Make a blank test under the same conditions (omitting only the sample) and use this value in the calculation of the final result.

• (7.1.1.4) Expression of the result

$$%N(a-A)×0.28M$ where: a= ml of standard solution of sodium or potassium hydroxide (0.2 N) used for the blank, carried out by placing in the receiver of the apparatus (5.1), 50.0 ml of standard solution of sulphuric acid (0.2 N) (4.8). A= ml of standard solution of sodium or potassium hydroxide (0.2 N) used for the analysis. M=weight of the sample in grams.$

• (7.1.2) In the presence of nitrate

• (7.1.2.1) Test sample

Weigh to the nearest 0.001 g, a quantity of the sample containing not more than 40 mg of nitric nitrogen.

• (7.1.2.2) Reduction 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 it to stand for half an hour. During this time shake again after 10 and 20 minutes.

• (7.1.2.3) Kjeldahl digestion

Add 30 ml of sulphuric acid (4.12), 5 g of potassium sulphate (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 shake the solution frequently to keep the mixture suspended; the liquid darkens and then clears with the formation of a yellow-green anhydrous iron sulphate suspension. Continue heating for one hour after obtaining a clear solution, maintaining it at simmering point. 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 filter into a dry receiver.

• (7.1.2.4) Distillation of ammonia

Transfer by pipette, into the flask of the distillation apparatus (5.1), an aliquot part containing 100 mg of nitrogen at the most. 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 estimation as described in paragraph 7.1.1.2.

• (7.1.2.5) Blank test

See 7.1.1.3.

• (7.1.2.6) Expression of the result

$$%N(a-A)×0.28M$ where: a = ml of standard solution of sodium or potassium hydroxide (0.2 N) used for the blank, carried out by placing in the receiver of the apparatus (5.1), 50.0 ml of standard solution of sulphuric acid (0.2 N) (4.8). A = ml of standard solution of sodium or potassium hydroxide (0.2N) used for the analysis. M = weight of the sample, expressed in grams, present in the aliquot part taken for analysis.$

• (7.2) Forms of soluble nitrogen

Preparation of the solution to be analysed 7.2.1 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. 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 acid 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 the 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 part of the filtrate (7.2.1.1 or 7.2.1.2), containing 100 mg of nitrogen at the most. Add 15 ml of concentrated sulphuric acid (4.12), 0.4 g of copper oxide or 1.25 g of copper sulphate (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 distillaton apparatus (5. 1) and continue the distillation as determination 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 part 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 10 ml of 30% sulphuric 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 100 mg of nitrogen at the most. Add 15 ml of concentrated sulphuric acid (4.12), 0.4 g of copper oxide or 1.25 g of copper sulphate (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, quantitatively transfer the solution 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 the result

$$%N(a-A)×0.28M$ where: a= ml of standard solution of sodium or potassium hydroxide (0.2 N) used for the blank, carried out by placing in the receiver of the apparatus (5.1), 50.0 ml of standard solution of sulphuric acid (0.2 N) (4.8). A = ml of standard solution of sodium or potassium hydroxide (0. 2 N) used for analysis. M=weight of the sample, expressed in grams, present in the aliquot part taken for analysis.$

Total soluble nitrogen with the exception of nitric nitrogen 7.2.3 Transfer by pipette into a 300 mi Kjeldahl flask, an aliquot part of the filtrate (7.2.1.1 or 7.2.1.2) containing not more than 50 mg of nitrogen to be determined. Dilute to 100 ml with water, add 5 g of ferrous sulphate (4.16), 20 ml of concentrated sulphuric 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 distillaton apparatus and continue the determinaton 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 $%N(a-A)×0.28M$ where: a = ml of standard solution of sodium or potassium hydroxide (0.2 N) used for the blank, carried out by placing in the receiver of the apparatus (5.1), 50 ml of the standard sulphuric acid solution (0.2N) (4.8). A = ml of standard solution of sodium or potassium hydroxide (0.2 N) used for the analysis. M = weight 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 part of the filtrate (7.2.1.1) containing 100 mg of ammoniacal nitrogen at the most. 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

$$%N (ammoniacal)(a-A)×0.28M$ where: a = ml of standard solution of sodium or potassium hydroxide (0.2 N) used for the blank, carried out by placing in the receiver of the apparatus (5.1), 50 ml of the standard sulphuric acid solution (0.2 N) (4.8). A = ml of standard solution of sodium or potassium hydroxide (0.2 N) used for the analysis. M = weight 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 part of the filtrate (7.2.1.1 or 7.2.1.2) containing 20 mg of ammoniacal nitrogen at the most. Then assemble the apparatus. Transfer by pipette into the 300 ml Erlenmeyer flask 50 ml of the standard sulphuric acid solution 0.1 N (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-o1 (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 sulphuric 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 N) (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 the result

$$%N (ammoniacal)(a-A)×0.14M$ where: a = ml of standard solution of sodium or potassium hydroxide (0.1 N) used for the blank, carried out by placing in the 300 ml Erlenmeyer flask of the apparatus (5.2), 50 ml of the standard solution of sulphuric acid (0.1 N) (4.17). A = ml of standard solution of sodium or potassium hydroxide (0.1 N) used for the analysis. M = weight of the sample, expressed in grams, present in the aliquot part taken for analysis.$

• (7.2.6) Urea nitrogen

• (7.2.6.1) Urease method

Transfer by pipette into a 500 ml graduated flask, an aliquot part of the filtrate (7.2.1.1 or 7.2.1.2) containing not more than 250 mg of urea nitrogen. To remove phosphates add saturated barium hydroxide solution (4.21) until no further precipitaton 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 N hydrochloric acid (4.23), until pH of 3.0 measured by the pH meter (5.5) is obtained. Then raise the pH to 5.4 with 0.1 N sodium hydroxide (4.20).

• To avoid losses of ammonia during decomposition by the 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 N 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 N 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 quantitatively transfer 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 N) (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

$$%N (ureic)(a-A)×0.14M$ where: a = ml of standard solution of sodium of potassium hydroxide (0.1 N) used for the blank, carried out exactly under the same conditions as the analysis. A = ml of standard solution of sodium of potassium hydroxide (0. I N) used for the analysis. M = weight of the sample, expressed in grams, present in the aliquot part taken for analysis.$

• (1) Remarks

After precipitation by the solutions or 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 part 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 (4.26), 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 a 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

$$%ureaN+biuret6.67×mM$ where: m=weight of the precipitate obtained, in grams. M=weight 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 by difference

Urea 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)

Cyanamide Nitrogen 7.2.7 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.

VERIFICATION OF THE RESULTS

8

• (8.1) 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.

• (8.2) 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 sulphate (4.5) and urea (4.6).

8b. — DETERMINATION OF DIFFERENT FORMS OF NITROGEN IN THE SAME SAMPLE—IN THE ABSENCE OF CYANAMIDE NITROGEN

SCOPE

1

This method is for the determination of any one form of nitrogen in the presence of any other form, in the absence of cyanamide nitrogen.

FIELD OF APPLICATION

2

This method is applicable to all fertilisers 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 1990[^f01005] which contain exclusively nitric, ammoniacal or ureic nitrogen.

PRINCIPLE

3

The following determinations are made on different portions of a single sample solution.

• (3.1) Total soluble nitrogen

• (3.1.1) In the absence of nitrates, by direct Kjeldahl digestion of the solution.

• (3.1.2) In the presence of nitrates, by Kjeldahl digestion of a portion of the solution after reduction by the Ulsch method; ammonia is determined in both cases as described in Method 2.

• (3.2) Total soluble nitrogen except nitric nitrogen, by Kjeldahl digestion after elimination of nitric nitrogen in acid medium by means of ferrous sulphate; ammonia is determined as described in Method 2.

• (3.3) Nitric nitrogen, by difference: between 3.1.2 and 3.2 and/or between total soluble nitrogen (3.1.2) and the sum of ammoniacal and ureic nitrogen (3.4 + 3.5).

• (3.4) Ammoniacal nitrogen, by cold distillation of a weak alkaline solution; the ammonia is obtained in a solution of sulphuric acid and determined as described in Method 2.

• (3.5) Ureic nitrogen, either:

• (3.5.1) By conversion using urease, into ammonia, which is determined by titration with a standard solution of hydrochloric acid,

• or:

• (3.5.2) By gravimetry using 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:

• (3.5.3) By difference, according to the following table:

Case	Nitric nitrogen	Ammoniacal nitrogen	Difference
1	Absent	Present	(3.1.1)-(3.4)
2	Present	Present	(3.2)-(3.4)

REAGENTS

4

• (4.1) Potassium sulphate.

• (4.2) Iron powder, reduced with hydrogen (the prescribed quantity of iron must be able to reduce at least 50 mg nitric nitrogen).

• (4.3) Potassium nitrate.

• (4.4) Ammonium sulphate.

• (4.5) Urea.

• (4.6) Sulphuric acid, 0.2 N solution.

• (4.7) Sodium hydroxide solution, 30 g per 100 ml, ammonia free.

• (4.8) Sodium or potassium hydroxide, 0.2 N solution, free of carbonates.

• (4.9) Sulphuric acid (d = 1.84 g/ml).

• (4.10) Hydrochloric acid solution: dilute an appropriate volume of hydrochloric acid (d = 1.18 g/ml) with an equal volume of water.

• (4.11) Glacial acetic acid.

• (4.12) Sulphuric acid, solution approximately 30% (W/V) H₂SO₄.

• (4.13) Ferrous sulphate, crystalline FeSO₄.7H₂O.

• (4.14) Sulphuric acid, 0.1 N solution.

• (4.15) Octan-1-ol.

• (4.16) Potassium carbonate, saturated solution.

• (4.17) Sodium or potassium hydroxide, 0.1 N solution.

• (4.18) Barium hydroxide, saturated solution.

• (4.19) Sodium carbonate solution, 10 g per 100 ml.

• (4.20) Hydrochloric acid, 2 N solution.

• (4.21) Hydrochloric acid, 0.1 N solution.

• (4.22) Urease solution: suspend 0.5 g active urease in 100 ml distilled water. Using 0.1 N hydrochloric acid (4.21), adjust pH to 5.4, measured with pH meter.

• (4.23) Xanthydrol solution, 5 g per 100 ml in ethanol or methanol (4.28) (do not use products giving a high proportion of insoluble material). The solution can be kept for 3 months in a carefully stoppered bottle in darkness.

• (4.24) Catalyst: copper oxide (CuO), 0.3 to 0.4 g per determination, or an equivalent amount of copper sulphate pentahydrate, 0.95 to 1.25 g per determination.

• (4.25) Anti-bump granules of pumice stone washed with hydrochloric acid and ignited.

• (4.26) Indicator solutions:

Mixed indicator: 4.26.1 Solution A: dissolve 1 g methyl red in 37 ml 0.1 N sodium hydroxide solution and make up to 1 litre with water. Solution B: dissolve 1 g methylene blue in water and make up to 1 litre. Mix 1 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.

Methyl red indicator solution: 4.26.2 dissolve 0.1 g methyl red in 50 ml 95% ethanol, make up to 100 ml with water and filter if necessary; 4-5 drops of this indicator can be used instead of the previous one.

• (4.27) Indicator papers: litmus, bromothymol blue (or other papers sensitive to pH 6-8).

• (4.28) Ethanol or methanol, 95% (V/V).

APPARATUS

5

• (5.1) Distillation apparatus. See Method 2.

• (5.2) Apparatus for determination of ammoniacal nitrogen. An example of recommended apparatus is reproduced in Figure 6 in the Appendix.

• (5.3) Apparatus for determination of urea nitrogen by the urease method (7.6.1). An example of recommended apparatus is reproduced in Figure 7 in the Appendix.

• (5.4) Rotary shaker: 35-40 turns per minute.

• (5.5) pH meter.

• (5.6) Sintered glass crucibles, diameter of pores 5 to 15 microns.

PREPARATION OF SAMPLE

6

See Method 1.

PROCEDURE

7

Preparation of solution for analysis 7.1 Weigh to the nearest 0.001 g, 10 g of the prepared sample and transfer to a 500 ml graduated flask. Add 50 ml water and then 20 ml dilute hydrochloric acid (4.10) and shake. Allow to stand until the evolution of carbon dioxide ceases. Add 400 ml water; shake for half an hour; make up to volume with water, mix, filter on a dry filter into a dry container.

• (7.2) Total nitrogen

In the absence of nitrates 7.2.1 Transfer by pipette into a 300 ml Kjeldahl flask a portion of the filtrate (7.1), containing a maximum of 100 mg nitrogen. Add 15 ml concentrated sulphuric acid (4.9), 0.4 g copper oxide or 1.25 g copper sulphate (4.24) and a few glass beads to control boiling. Heat moderately at first in order to initiate the reaction, then more strongly until the liquid becomes colourless or slightly greenish and white fumes appear. After cooling, transfer the solution into the distillation flask, dilute to about 500 ml with water and add a few granules of pumice stone (4.25). Connect the flask to the distillation apparatus (5.1) and carry out the determination as described in Method 8a, 7.1.12.

In the presence of nitrates 7.2.2 Transfer by pipette into a 500 ml Erlenmeyer flask a portion of the filtrate (7.1) containing not more than 40 mg nitric nitrogen. At this this stage of the analysis, the total quantity of nitrogen is unimportant. Add 10 ml 30% sulphuric acid (4.12), 5 g reduced iron (4.2) and immediately cover the Erlenmeyer flask with a watch glass. Heat gently until the reaction becomes strong but not violent. Stop heating and allow to stand for at leqast 3 hours at ambient temperature. Transfer the liquid quantitatively to a 250 nml graduated flask, ignoring undissolved iron. Make up to the mark with water and mix carefully. Transfer by pipette a portion containing a maximum of 100 mg nitrogen into a 300 ml Kjeldahl flask. Add 15 ml concentrated sulphuric acid (4.9), 0.4 g copper oxide or 1.25 g copper sulphate (4.24) and a few glass beads. Heat moderately at first in order to initiate the reaction, then more strongly until the liquid becomes colourless or slightly greenish and white fumes appear. After cooling, transfer the solution quantitatively to the distillation flask, dilute to about 500 ml with water and add a few granules of pumice stone (4.25). Connect the flask to the distillation apparatus (5.1) and continue the determination as described in Method 8a, 7.1.1.2.

Blank test 7.2.3 Make a blank test in the same conditions (omitting only the sample), and use this value in the calculation of the final result.

Expression of result 7.2.4 $%N (total)(a-A)×0.28M$ where: a = ml of standard solution of sodium or potassium hydroxide (0.2 N) used for the blank, carried out under the same conditions as the analysis. A = ml of standard solution of sodium or potassium hydroxide (0.2 N) used for the analysis. M = weight of the sample, in grams, present in the aliquot part taken for analysis.

Total nitrogen excluding nitric nitrogen 7.3 7.3.1 Transfer by pipette into a 300 ml Kjeldahl flask an aliquot part of filtrate (7.1) containing not more than 50 mg of nitrogen to be determined. Dilute to 100 ml with water, add 5 g ferrous sulphate (4.13), 20 ml concentrated sulphuric acid (4.9) and a few glass beads to control boiling (4.29). Heat moderately at first then more strongly until white fumes appear. Continue the reaction for 15 minutes. Stop heating, introduce 0.4 g copper oxide or 1.25 g copper sulphate (4.24) as catalyst, resume heating and maintain production of white fumes for 10-15 minutes. After cooling, transfer the contents of the Kjeldahl flask quantitatively to the distillation flask (5.1). Dilute to about 500 ml with water and add a few granules of pumice stone (4.2). Connect the flask to the distillation apparatus and continue the determination as in Method 8a, 7.1.1.2. Blank test 7.3.2 See 7.2.3. Expression of result 7.3.3 $%N (total)(a-A)×0.28M$ where: a = ml of standard solution of sodium or potassium hydroxide (0.2 N) used for the blank, carried out by placing in the receiver of the apparatus (5.1), 50.0 ml of standard solution of sulphuric acid (0.2 N) (4.6). A = ml of standard solution of sodium or potassium hydroxide (0.2 N) used for the analysis. M = weight of the sample, in grams, present in the aliquot part taken for analysis.

Nitric nitrogen is obtained: by difference between 7.4 (7.2.4)-(7.5.3 + 7.6.3) or (7.2.4)-(7.5.3 + 7.6.5) or (7.2.4)-(7.5.3 + 7.6.6)

Ammoniacal nitrogen 7.5 In the presence of ureic nitrogen 7.5.1 Transfer by pipette into the dry flask of the apparatus (5.2) a portion of filtrate (7.1) containing a maximum of 20 mg ammoniacal nitrogen. Connect up the apparatus. Place in the 300 ml Erlenmeyer flask 50.0 ml standard 0.1 N sulphuric acid solution (4.14) and an amount of distilled water such that the level of the liquid is about 5 cm above the opening of the intake tube. Introduce through the side neck of the reaction flask distilled water so as to bring the volume to about 50 ml. Shake. To avoid foaming during aeration add several drops of octan-1-ol (4.15). Add 50 ml saturated potassium carbonate solution (4.16) and immediately begin to expel the ammonia thus released from the cold suspension. A strong current of air is necessary (flow rate of about 3 litres per minute) and should be purified by passing ot through washing flasks containing dilute sulphuric acid and dilute sodium hydroxide. Instead of using air under pressure, a vacuum may be used (wtaer pump) provided that the connections between the apparatus are air tight. The liberration of ammonia is generally complete after three hours. However, it is desirable to make certain of this by changing the Erlenmeyer flask. When the process is finished, disconnect the Erlenmeyer flask from the apparatus, rinse the end of the intake tube and the walls of the Erlenmeyer flask with a little distilled water and titrate the excess acid against standard 0.1 N sodium hydroxide solution (4.17). Blank test 7.5.2 See 7.2.3. Expression of result 7.5.3 $%N (ammoniacal)(a-A)×0.14M$ where: a = ml of standard solution of sodium or potassium hydroxide (0.1 N) (4.17) used for the blank, carried out by placing in the receiver of the apparatus (5.2), 50.0 ml of standard solution of sulphuric acid (0.1 N) (4.14). A = ml of standard solution of sodium or potassium hydroxide (0.1 N) (4.17) used for the analysis. M = weight of the sample, in grams, present in the aliquot part taken for analysis.

Ureic acid 7.6 Urease method 7.6.1 Transfer by pipette into a 500 ml graduated flask, a portion of filtrate (7.1) containing not more than 250 mg of ureic nitrogen. To remove phosphates, add a suitable quantity of saturated barium hydroxide solution (4.18) until further addition does not cause the production of more precipitate. Eliminate excess barium ions (and any dissolved calcium ions) by means of 10% sodium carbonate solution (4.19). Allow to settle and check whether precipitation is complete. Make up to the mark, mix and filter through a fluted filter. Transfer by pipette 50 ml filtrate into the 300 ml Erlenmeyer Flask of the apparatus (5.3). Acidify with 2 N hydrochloric acid (4.20) to pH 3.0, measured by means of the pH meter (5.5). Raise the pH to 5.4 by the addition of 0.1 N sodium hydroxide (4.17). To avoid ammonia losses when hydrolysis by urease occurs, close the Erlenmeyer flask by means of a stopper provided with a dropping funnel and a small bubble trap containing exactly 2 ml standard 0.1 N hydrochloric acid solution (4.21). Intorduce through the seperating funnel, 20 ml urease solution (4.22). Allow to stand for one hour at 20-25°C. Place 25.0 ml of the standard 0.1 N hydrochloric acid solution (4.2) in the dropping funnel, allow to run into the solution, then rinse with a little water. Transfer quantitatively the contents of the bubble trap to the solution contained in the Erlenmeyer flask. Titrate the excess acid using the standard 0.1 N sodium hydroxide solution (4.17), until a pH of 5.4 is obtained, measured on the pH meter.

Remarks

1

After precipitation by the barium hydroxide and sodium carbonate solutions, make up to the mark, filter and neutralise as quickly as possible.

2

The titration may also be carried out using an indicator (4.26), although the change of colour is more difficult to observe.

Blank test 7.6.2 See 7.2.3

Expression of result 7.6.3 $%N (ureic)(a-A)×0.14M$ where: a = ml of standard solution of sodium or potassium hydroxide (0.1 N) (4.17) used for the blank, carried out in exactly the same conditions as the analysis. A = ml of standard solution of sodium or potassium hydroxide (0.1 N) (4.17) used for the analysis. M = weight of the sample, in grams, present in the aliquot part taken for analysis.

Gravimetric method using xanthydrol 7.6.4 Transfer by pipette into a 100 ml beaker a portion of the filtrate (7.1) containing not more than 20 mg urea. Add 40 ml acetic acid (4.11). Stir with a glass rod for one minute. Allow any precipitate to settle for five minutes. Filter, wash with a few ml acetic acid (4.11). Add 10 ml xanthydrol solution (4.23) to the filtrate drop by drop, 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 (5.6) which has been previously dried and weighed, using a slight reduction in pressure. Wash three times with 5 ml ethanol (4.28), without trying to remove all the acetic acid. Place in an oven at a temperature of 130°C for one hour (do not exceed 145°C). Allow to cool in a desiccator and weigh.

Expression of result 7.6.5 $%N (urea)6.67×mM$ where: m = weight of the precipitate in grams. M = weight 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 in absolute value in compound fertilisers.

Difference method 7.6.6 Ureic N can also be calculated as indicated in the following table: CaseNitric NAmmoniacal NUreic N 1AbsentPresent(7.2.4)-(7.5.3)2PresentPresent(7.3.3)-(7.5.3)

EXPRESSION OF RESULTS

8

• (8.1) Before each analysis, check the functioning of the apparatus and the correct application of the methods used with a standard solution containing the different forms of nitrogen in proportions similar to those in the sample. This standard solution is prepared from solutions of potassium nitrate (4.3), ammonium sulphate (4.4) and urea (4.5).

9a. — EXTRACTION OF TOTAL PHOSPHORUS BY MINERAL ACIDS

SCOPE

1

This method is for the determination of phosphorus soluble in mineral acids.

FIELD OF APPLICATION

2

Applicable exclusively to the phosphate fertilisers listed in Group 2(a) of section A and Groups 1, 2 and 4 of Section B of the Table in Schedule 1 of the Fertilisers Regulations 1990[^f01006].

PRINCIPLE

3

Extraction of the phosphorus in the fertiliser with a mixture of nitric acid and sulphuric acid.

REAGENTS

4

• (4.1) Sulphuric acid (d = 1.84 g/ml).

• (4.2) Nitric acid (d = 1.40 g/ml).

APPARATUS

5

• (5.1) A Kjeldahl flask, with a capacity of at least 500 ml, or a 250 ml round-bottomed flask with a glass tube forming a reflux condenser.

PREPARATION OF THE SAMPLE

6

See Method 1.

PROCEDURE

7

Extraction 7.1 Weigh to the nearest 0.001 g, 2.5 g of the prepared sample and place it in a dry Kjeldahl flask. Add 15 ml water and stir so as to suspend the substance. Add 20 ml nitric acid (4.2) and carefully add 30 ml sulphuric acid (4.1). When the initial violent reaction has ceased, slowly bring the contents of the flask to boiling and boil for 30 minutes. Allow to cool and then carefully add with mixing about 150 ml water and boil for 15 minutes. Cool completely and transfer the liquid quantitatively to a 500 ml graduated flask. Make up to volume, mix and filter through a dry fluted filter, discarding the first portion of the filtrate.

Determination 7.2 Determine the phosphorus according to Method 10 on an aliquot part of the clear filtrate.

9b. — EXTRACTION OF PHOSPHORUS BY 2% FORMIC ACID

SCOPE

1

This method is for the determination of phosphorus soluble in 2% formic acid (20 g per litre).

FIELD OF APPLICATION

2

Soft natural phosphate exclusively.

PRINCIPLE

3

To differentiate between hard natural phosphates and soft natural phosphates, phosphorus soluble in formic acid is extracted in specific conditions.

REAGENTS

4

• (4.1) Formic acid, 2% (20 g per litre): dilute 82 ml formic acid (concentration 98-100% ; d = 1.22 g/ml) to 5 litres with distilled water.

APPARATUS

5

• (5.1) 500 ml graduated flask (for example Stohmann).

• (5.2) Rotary shaker, 35-40 turns per minute.

PREPARATION OF THE SAMPLE

6

See Method 1.

PROCEDURE

7

Extraction 7.1 Weigh to the nearest 0.001 g, 5 g of the prepared sample and place it in a dry 500 ml graduated Stohmann flask (5.1) with a wide neck. While continuously rotating the flask by hand, add the formic acid (4.1) (at 20±1°C) until it is approximately 1 cm below the graduation mark and make up to the volume. Close the flask with rubber stopper and shake for 30 minutes at 20±2°C on the rotary shaker (5.2). Filter the solution through a dry fluted filter, into a dry receiver, discarding the first portion of the filtrate.

Determination 7.2 Determine the phosphorus according to Method 10 in an aliquot part of the clear filtrate.

9c. — EXTRACTION OF PHOSPHORUS BY 2% CITRIC ACID

SCOPE

1

This method is for the determination of phosphorus soluble in 2% citric acid (20 g per litre).

FIELD OF APPLICATION

2

Only applicable to basic slag fertilisers in Group 2(a) of Section A and Groups 1, 2 and 4 of Section B of the Table in Schedule 1 of the Fertilisers Regulations 1990[^f01007].

PRINCIPLE

3

Extraction of phosphorus from the fertiliser with a 2% citric acid solution (20 g per litre) in given conditions.

REAGENT

4

• (4.1) 2% citric acid solution (20 g per litre), prepared from citrci acid and monohydrate.

• Note: Verify the concentration of this citric acid solution by titrating 10 ml of the latter with a sodium hydroxide standard solution 0.1 N, using phenolpthalein as an indicator. If the solution is correct, the titre should be 28.55 ml.

APPARATUS

5

• (5.1) Rotary shaker: 35-40 turns per minute.

PREPARATION OF THE SAMPLE

6

The analysis is carried out on the product as received after carefully mixing the original sample to ensure it is homogenous. See Method 1.

PROCEDURE

7

Extraction 7.1 Weigh to the nearest 0.001 g, 5 g of the prepared sample and plcae it in a dry flask with a sufficiently wide neck, with a capacity of 600 ml, allowing the liquid to be shaken thoroughly. Add 500 ml±1 ml of the citric acid solution (4.1) at 20±1°C. When adding the first mls of the reagent shake vigorously by hand to stop the formation of lumps and to prevent the substance sticking to the sides. Close the flask with a rubber stopper and shake it on the rotary shaker (5.1) for exactly 30 minutes at a temperature of 20±2°C. Filter immedsiately through a dry fluted filter, into a dry glass receiver and discard the first 20 ml of the filtrate. Continue the filtering until a sufficient quantity of filtrate is obtained to carry out the phosphorus determination.

Determination 7.2 Determine the phosphorus according to Method 10 on an aliquot part of the clear filtrate.

9d. — EXTRACTION OF PHOSPHORUS BY NEUTRAL AMMONIUM CITRATE

SCOPE

1

This method is for the determination of phosphorus soluble in neutral ammonium citrate.

FIELD OF APPLICATION

2

All fertilisers in Group 2(a) of Section A and Groups 1, 2 and 4 of Section B and Group 2 of Section C of the Table in Schedule 1 of the Fertilisers Regulations 1990[^f01008] in respect of which solubility in neutral ammonium citrate is laid down.

PRINCIPLE

3

Extraction of phosphorus at a temperature of 65°C using a neutral ammonium citrate solution (pH=7.0) under specific conditions.

REAGENTS

4

• (4.1) Neutral ammonium citrate solution (pH=7.0).

• This solution must contain per litre 185 g of citric acid monhydrate and must have a specific gravity of 1.09 at 20°C and a pH of 7.0. The reagent is prepared as follows:

• dissolve 370 g citric acid monohydrate in about 1.5 litres of water and make an approximately neutral solution by adding 345 ml of ammonia solution (28-29% of NH₃). If the NH₃ concentration is lower than 28% add a correspondingly larger quantity of ammonia solution and dilute the citric acid in correspondingly smaller quantities of water.

• Cool and make exactly neutral by keeping the electrodes of the pH meter (5.1) immersed in the solution, add the ammonia solution (28-29% of NH₃) drop by drop, stirring continuously (with a mechanical stirrer) until a pH of exactly 7.0 at 20°C is obtained.

• At this point make up the volume to 2 litres and test the pH again. Keep the reagent in a closed container and check the pH at regular intervals.

APPARATUS

5

• (5.1) pH meter.

• (5.2) Water bath which can be set thermostaically at 65°C, equipped with a mechanically operated shaking tray (see Figure 8 in the Appendix).

PREPARATION OF THE SAMPLE

6

See Method 1.

PROCEDURE

7

Extraction 7.1 Transfer 1[^f01009] or 3[^f01010] grams, as appropriate, of the fertiliser to be analysed into a 200 or 250 ml Erlenmeyer flask containing 100 ml of ammonium citrate solution previously heated to 65°C. Stopper the Erlenmeyer flask and shake in order to suspend the fertiliser without forming lumps. Remove the stopper for an instant in order to balance the pressure and close the Erlenmeyer flask again. Place the flask in the water-bath (5.2) set to maintain the contents of the flask at exactly 65°C. Shake mechanically for one hour so as to ensure complete suspension of the sample[^f01011]. The level of suspension in the flask must stay constantly below that of the water in the bath. After exactly one hour remove the Erlenmeyer flask from the water-bath. Cool immediately under running water to ambient temperature and quantitatively transfer the contents from the Erlenmeyer flask into a graduated 500 ml flask with a jet of water. Make up the volume with water. Mix thoroughly and filter through a dry fluted filter (medium speed) into a dry container, discarding the first part of the filtrate (about 50 ml). About 100 ml of clear filtrate should be collected.

Determination 7.2 Determine the phosphorus according to Method 10 in an aliquot part of the clear filtrate.

9e. — EXTRACTION OF PHOSPHORUS BY ALKALINE AMMONIUM CITRATE (PETERMANN'S METHOD) AT 65°C.

SCOPE

1

This method is for the determination of phosphorus soluble in alkaline ammonium citrate.

FIELD OF APPLICATION

2

Exclusively to precipitated dihydrated dicalcium phosphate (CaHPO₄.2H₂O).

PRINCIPLE

3

Extraction of phosphorus at a temperature of 65°C with an alkaline solution of ammonium citrate (Petermann) under specified conditions.

REAGENTS

4

Petermann’s solution 4.1

Preparation from diammonium citrate: Dissolve 941 g diammonium citrate in about 3,500 ml water in a 5 litre graduated flask. Stand the flask in a bath of running water, mix and cool. Add, in small amounts, 430 ml of ammonia solution (d = 0.880 g/ml), from a freshly opened bottle (or an equivalent amount of diluted ammonia, for example if d = 0.906 g/ml then 502 ml are required). Adjust the temperature to 20°C, make up to volume with water and mix.

Preparation from citric acid and ammonia: Dissolve 865 g citric acid monohydrate in about 2,500 ml distilled water in a container of about 5 litres capacity. Place container in an ice bath and add in small amounts, shaking continually, 966 ml of ammonia solution (d = 0.880 g/ml), from a freshly opened bottle (or an equivalent amount of diluted ammonia, for example if d = 0.906 g/ml, then, 1,114 ml are required). Adjust the temperature to 20°C, transfer to a 5 litre graduated flask, make up to the mark with distilled water and mix.

Check the ammoniacal nitrogen content as follows: Transfer 25 ml of the solution into a 250 ml graduated flask, make up to volume with distilled water and mix. Determine the ammoniacal nitrogen content on 25 ml of this solution following Method 2. If the solution is correct, 15 ml 0.5 N H2SO4 are consumed—Calculate the concentration of ammoniacal nitrogen in the reagent solution (1 ml 0.5 N H₂SO₄ = 0.007 g nitrogen). If the concentration of ammoniacal nitrogen is greater than 42 g/litre, ammonia can be expelled by a stream of inert gas or by moderate heating to bring back the pH to 9.7. Carry out a second determination. If the concentration of ammoniacal nitrogen is less than 42 g/litre, calculate the volume of ammonia solution required to achieve this level (1 ml ammonia solution, d = 0.880 g/ml contains approximately 0.22 g ammoniacal nitrogen). For each ml of ammonia solution required add 0.173 g of citric acid. Whenever corrections are made to this reagent solution, it is imperative that the final concentration of both the citric acid and ammoniacal nitrogen are as specified.

APPARATUS

5

• (5.1) Water bath which can be maintained at a temperature of 65°±1deg;C.

• (5.2) 500 ml graduated flask (for example Stohmann flask).

PREPARATION OF SAMPLE

6

See Method 1.

PROCEDURE

7

Extraction 7.1 Weigh to the nearest 0.001 g, 1 g of the prepared sample and transfer to the 500 ml graduated flask (5.2). Add 200 ml alkaline ammonium citrate solution (4.1). Stopper the flask and shake vigorously by hand to avoid the formation of lumps and to prevent any adherence of the substance to the sides. Place the flask in the water bath at 65°C and shake every 5 minutes during the first half an hour. After each shaking, raise the stopper to equilibrate the pressure. The level of water in the water bath should be above the level of solution in the flask. Allow the flask to remain in the water bath a further hour at 65°C and shake every ten minutes. Remove the flask, cool to a temperature of about 20°C, make up to volume of 500 ml with water. Mix and filter through a dry fluted filter paper, rejecting the first portion of filtrate.

Determination 7.2 Determine the phosphate according to Method 10 on an aliquot part of the clear filtrate.

9f. — EXTRACTION OF PHOSPHORUS BY ALKALINE AMMONIUM CITRATE (PETERMANN'S METHOD) AT AMBIENT TEMPERATURE

SCOPE

1

This method is for the determination of phosphorus soluble in cold alkaline ammonium citrate.

FIELD OF APPLICATION

2

Disintegrated phosphates exclusively.

PRINCIPLE

3

Extraction of phosphorus at a temperature about 20°C with an alkaline solution of ammonium citrate (Petermann’s solution) in specific conditions.

REAGENT

4

See Method 9e.

APPARATUS

5

• (5.1) 250 ml graduated flask (for example Stohmann).

• (5.2) Rotary shaker, 35-40 turns per minute.

PREPARATION OF THE SAMPLE

6

See Method 1.

PROCEDURE

7

Extraction 7.1 Weigh to the nearest 0.001 g, 2.5 g of the prepared sample and put it in a 250 ml graduated flask (5.1). Add a little of Petermann’s solution (4) at 20°C, shake very hard in order to stop the formation of lumps and to prevent any of the substance adhering to the side of the flask. Make up to the mark with Petermann’s solution and close the flask with a rubber stopper. Shake for two hours on the rotary shaker (5.2). Filter immediately through a dry fluted filter, into a dry container, discarding the first portion of the filtrate.

Determination 7.2 Determine the phosphorus according to Method 10 on an aliquot part of the clear filtrate.

9g. — EXTRACTION OF PHOSPHORUS BY JOULIE'S ALKALINE AMMONIUM CITRATE

SCOPE

1

This method is for the determination of phosphorus soluble in Joulie’s alkaline ammonium citrate.

FIELD OF APPLICATION

2

All the straight and compound phosphate fertilisers, in which the phosphate occurs in an aluminocalcic form.

PRINCIPLE

3

Extraction by shaking vigorously with an alkaline solution of ammonium citrate of defined specification (and where appropriate in the presence of oxine), at about 20°C.

REAGENTS

4

Joulie’s alkaline solution of ammonium citrate: 4.1 this solution contains 400 g of citric acid monohydrate and 153 g of NH₃ per litre. Its free ammonia content is approximately 55 g per litre. It can be prepared by one of the methods described below: 4.1.1 In a litre graduated flask, dissolve 400 g of citric acid monohydrate in approximately 600 ml ammonia solution (d = 0.925 g/ml), containing 200 g NH₃ per litre; this may be prepared by diluting 760 ml ammonia solution (d = 0.880 g/ml) from a freshly opened bottle with water to 1 litre. The citric acid is added successively in quantities of 50 to 80 g maintaining the temperature below 50°C. Make up the volume to 1 litre with ammonia. 4.1.2 In a 1 litre graduated flask, dissolve 400 g of diammonium citrate in 300 ml of water. Add 440 ml of ammonia solution (d = 0.925 g/ml) (see 4.1.1 above). Make up the volume to 1 litre with water. Verification of the total ammonia content: take a 10 ml sample of the citrate solution and place it in a 250 ml flask. Make up to the volume with distilled water. Determine the ammoniacal nitrogen content on 25 ml of this solution according to Method 2. In these conditions the reagent is considered to be correct when the volume of 0.5 N sulphuric acid consumed is between 17.7 and 18.0 ml (1 ml 0.5 N H₂SO₄-0.008516 g NH₃). If this is not so add 4.25 ml of ammonia (d = 0.925 g/ml) per 0.1 ml below the 18 ml indicated above.

• (4.2) 8-Hydroxyquinoline (oxine), powdered.

APPARATUS

5

• (5.1) Rotary shaker, 35-40 turns per minute.

PREPARATION OF THE SAMPLE

6

See Method 1.

PROCEDURE

7

Extraction 7.1 Weigh to the nearest 0.0005 g, 1 g of the prepared sample and place in a small mortar (glass or porcelain). Add about ten drops of ammonium citrate solution (4.1) to moisten it and break it up very carefully with a pestle. Add 20 ml ammonium citrate solution (4.1), mix to a paste and leave it to settle for about 1 minute. Decant the liquid in to a 500 ml graduated flask straining off particles which might have escaped the preceding moist disintegration. Add 20 ml ammonium citrate solution (4.1) to the residue, grind as above and decant the liquid into the graduated flask. Repeat the process four times, so that by the end of the fifth time all the product can be poured into the flask. The total quantity of ammonium citrate solution used for these processes must be approximately 100 ml. Rinse the pestle and mortar above the graduated flask with 40 ml of distilled water. Stopper the flask and shake for three hours on the rotary shaker (5.1). Leave the flask standing for fifteen to sixteen hours and then shake it again under the same conditions for three hours. The temperature during the whole process should be kept at 20°±2°C. Make up to volume with distilled water and mix. Filter through a dry filter, discard the first portion of the filtrate and collect the clear filtrate in a dry flask.

Determination 7.2 Determine the phosphorus according to Method 10 on an aliquot part of the clear filtrate.

APPENDIX

8

The use of oxine makes it possible to apply this method to fertilisers containing magnesium. This is recommended when the ratio of magnesium and phosphoric anhydride contents is higher than 0.03 (Mg/P₂O₅>0.03). If this is the case, add 3 g of oxine to the moistened sample for analysis. The use of oxine in the absence of magnesium is not, moreover, likely to interfere subsequently with the determination. In the known absence of magnesium, oxine may be omitted.

9h. — EXTRACTION OF PHOSPHORUS BY WATER

SCOPE

1

This method is for the determination of water-soluble phosphorus.

FIELD OF APPLICATION

2

All fertilisers, including compound fertilisers, where water-soluble phosphorus is to be determined.

PRINCIPLE

3

Extraction in water by shaking under specific conditions.

APPARATUS

4

• (4.1) 500 ml graduated flask (for example Stohmann).

• (4.2) Rotary shaker, 35-40 turns per minute.

PREPARATION OF THE SAMPLE

5

See Method 1.

PROCEDURE

6

Extraction 6.1 Weigh to the nearest 0.001 g, 5 g of the prepared sample and place it in a 500 ml graduated flask (4.1). Add to the flask 450 ml of water, the temperature of which must be between 20° and 25°C. Shake on the rotary shaker (4.2) for 30 minutes. Then make up to the mark with water, mix thoroughly by shaking the filter through a dry fluted filter, into a dry container.

Determination 6.2 Determine the phosphorus according to Method 10, on an aliquot part of the clear filtrate.

10. — DETERMINATION OF EXTRACTED PHOSPHORUS (Gravimetric method using quinoline phosphomolybdate)

SCOPE

1

This method is for the determination of phosphorus in the extracts from fertilisers.

FIELD OF APPLICATION

2

The method is applicable to all extracts of fertilisers[^f01012], for the determination of the different forms of phosphorus.

PRINCIPLE

3

After hydrolysis, phosphorus is precipitated in an acid solution in the form of quinoline phosphomolybdate. The precipitate is collected, washed, dried at 250°C and weighed.

REAGENTS

4

• (4.1) Concentrated nitric acid (d = 1.40 g/ml).

• (4.2) Molybdate reagent:

• (4.2.1) Preparation of the reagent based on sodium molybdate:

• Solution A: dissolve 70 g sodium molybdate dihydrate in 100 ml water.

• Solution B: dissolve 60 g citric acid monohydrate in 100 ml water and add 85 ml concentrated nitric acid (4.1).

• Solution C: stir solution A into solution B to obtain solution C.

• Solution D: to 50 ml water add 35 ml concentrated nitric acid (4.1), add 5 ml freshly distilled quinoline. Add this solution to solution C, mix thoroughly and leave standing overnight in the dark. Make up to 500 ml with water, mix again and filter through a sintered glass funnel (5.3).

• (4.2.2) Preparation of the reagent based on ammonium molybdate:

• Solution A: dissolve 100 g ammonium molybdate in 300 ml water, heating gently and stirring from time to time.

• Solution B: dissolve 120 g citric acid monohydrate in 200 ml water and add 170 ml of concentrated nitric acid (4.1).

• Solution C: add 10 ml freshly distilled quinoline to 70 ml of concentrated nitric acid (4.1).

• Solution D: slowly pour, stirring well, solution A into solution B. After thoroughly mixing, add solution C to this mixture and make up to 1 litre with water. Leave standing for two days in a dark place and filter through a sintered glass funnel (5.3).

• The reagents 4.2.1 and 4.2.2 can be used in the same way; both must be kept in the dark in stoppered polyethylene bottles.

APPARATUS

5

• (5.1) Filter crucible with porosity of 5 to 20 microns.

• (5.2) Drying oven regulated at 250°C±10°C.

• (5.3) Sintered glass funnel with porosity of 5 to 20 microns.

PROCEDURE

6

Treatment of the solution 6.1 With a pipette take an aliquot part of fertiliser extract (see the Table) containing about 0.01 g of P₂O₅ and put it in a 500 ml Erlenmeyer flask. Add 15 ml concentrated nitric acid[^f01013] (4.1) and dilute with water to about 100 ml.

Hydrolysis 6.2 Bring the contents of the Erlenmeyer flask to the boil slowly and keep at this temperature until hydrolysis is completed (this usually takes 1 hour). Care must be taken to avoid losses by splashing and excessive evaporation which would reduce the initial volume by more than half, by fitting a reflux condenser. After hydrolysis make up to the initial volume with distilled water.

Weighing the crucible 6.3 Dry the filter crucible (5.1) for at least 15 minutes in the drying oven (5.2). Cool the crucible in a desiccator and weigh.

Precipitation 6.4 Heat the acid solution in the Erlenmeyer flask until it begins to boil and then precipitate the quinoline phosphomolybdate by adding 40 ml of the precipitating reagent (4.2.1 or 4.2.2)[^f01014] drop by drop, stirring continuously. Place the Erlenmeyer flask in a steam bath, leave it there for 15 minutes, shaking it from time to time. The solution can be filtered immediately or after it has cooled down.

Filtering and Washing 6.5 Filter the solution under vacuum by decantation. Wash the precipitate in the Erlenmeyer flask with 30 ml water. Decant and filter the solution. Repeat this process five times. Quantitatively transfer the rest of the precipitate into the crucible washing it with water. Wash four times with 20 ml water, allowing the liquid to drain from the crucible before each addition.

Drying and weighing 6.6 Wipe the outside of the crucible with a filter paper. Place the crucible in the drying oven (5.5) and keep it there until its weight remains constant at a temperature of 250°C (usually 15 minutes); leave it to cool in a desiccator to ambient temperature and weigh rapidly.

Blank test 6.7 For each series of determination, make a blank test under the same conditions (omitting only the sample) and allow for this in the calculation of the final result.

Control test 6.8 Carry out the determination using an aliquot part of a potassium dihydrogen phosphate solution containing 0.01 g of P₂O₅. TABLE FOR METHOD 10 %P205 in the fertiliser%P in the fertiliserSample for analysis gDilution to mlSampl mlDilution to mlSample to be precipitated mlQuinoline phosphomolybdate conversion factor (F) in percentage P₂0₅Quinoline phosphomolybdate conversion factor (F) in percentage P 1500——10016.0376.9921-50.44-2.22.5500——5012.8305.5945500——2512.8305.5941500——5032.07413.9845-102.2-4.42.5500——2525.66011.1883500——2521.3839.3235500——1032.07413.9841500——2564.14827.96810-254.4-11.02.5500——1064.14827.9683500——1053.45723.3075500——5064.14827.9681500——10160.37069.921+25+112.55005050050128.29655.93735005050050106.91346.61455005050025128.29655.937

EXPRESSION OF RESULTS

7

If the samples for analysis and diltutions shoen in the Table are used the following formulae apply:

• % P in the fertiliser = (A − a) × F’

• % P₂O₅ in the fertiliser = (A − a) × F

• where:

• A = weight in g of the quinoline phosphomolybdate

• a = weight in g of the quinoline phosphomolybdate obtained in the blank test.

• F and F’ = factors given in the last two columns of the Table.

• With samples for analysis and dilutions which differ from those of the Table the following formulae apply: $%P2O5 in the fertiliser(A-a)×f×D×100M$

• $%P in the fertiliser(A-a)×f’×D×100M$ where: f = conversion factor, quinoline phosphomolybdate into P₂O₅ = 0.032074 f’ = conversion factor, quinoline phosphomolybdate into P = 0.013998 D = dilution factor M = weighing of the sample analysed.

11. — DETERMINATION OF WATER-SOLUBLE POTASSIUM

SCOPE

1

This method is for the determination of water-soluble potassium.

FIELD OF APPLICATION

2

All the potassium fertilisers listed in Group 3(a) of Section A and Groups 1, 3 and 4 of Section B and Group 2 of Section C of the Table in Schedule 1 of the Fertilisers Regulations 1990[^f01015].

PRINCIPLE

3

The potassium is extracted with water and after the removal of interfering substances, the potassium is precipitated in a slightly alkaline medium in the form of potassium tetraphenylborate (KTPB).

REAGENTS

4

• (4.1) Formaldehyde, 25-30% solution, filter if necessary before use.

• (4.2) Potassium chloride.

• (4.3) Sodium hydroxide, 10 N solution. Care should be taken to ensure that the sodium hydroxide is free from potassium.

• (4.4) Indicator solution: dissolve 0.5 g phenolphtalein in 100 ml 90% ethanol.

• (4.5) EDTA solution: 4 g of the dihydrated disodium salt of ethylenediaminetetra-acetic acid (EDTA) per 100 ml. Store this reagent in a plastic container.

• (4.6) STPB solution: dissolve 32.5 g sodium tetraphenylborate in 480 ml of water, add 2 ml sodium hydroxide solution (4.3) and 20 ml of a magnesium chloride solution (100 g of MgCL₂.6H₂O per litre). Stir for fifteen minutes and filter through a fine, ashless filter. Store this reagent in a plastic container.

• (4.7) Liquid for washing: dilute 20 ml of the STPB solution (4.6) to 1 litre with water.

• (4.8) Bromine water: saturated bromine solution in water.

APPARATUS

5

• (5.1) Filter crucibles with a porosity of 5 to 20 microns.

• (5.2) Oven regulated at 120±10°C.

PREPARATION OF THE SAMPLE

6

See Method 1.

PROCEDURE

7

Extraction 7.1 Weigh to the nearest 0.001g, 10 g of the prepared sample (5 g for potassium salts containing more than 50% of potassium oxide or 20 g for fetilisers containing less than 5% of potassium oxide) and place in a 600 ml beaker with approximately 400 ml of water. Bring to the boil and allow it to boil for 30 minutes. Cool, transfer quantitatively into a 1 litre graduated flask, make up the volume, mix and filter into a dry receiver. Discard the first 50 ml of the filtrate. Note: If the filtrate is dark in colour, transfer by pipette, an aliquot part containing at the most 100 mg of K₂O and place in a 100 ml graduated flask, add bromine water and bring to the boil to eliminate any surplus bromine. After cooling make up the volume, filter and quantitatively determine the potassium in an aliquot part of the filtrate.

Determination 7.2 Transfer by pipette an aliquot part of the filtrate containing 25-50 mg of potassium (see Table on page 53) into a 250 ml beaker; make up to 50 ml with water. To remove interferences, add 10 ml of the EDTA solution (4.5), several drops of the phenolphthalein solution (4.4) and stir in, drop by drop, sdoium hydroxide solution (4.3) until it turns red, then finally add a few more drops of sodium hydroxide to ensure an excess (usually 1 ml of sodium hydroxide is sufficient to neutralise the sample and ensure an excess). To eliminate most of the ammonia boil gently for 15 minutes. Add water to make the volume up to 60 ml. Bring the solution to the boil, remove the beaker from the heat and add 10 ml formaldehyde (4.1). Add several drops of phenolphthalein solution (4.4) and if necessary, more sodium hydroxide solution until a distinct red colour appears. Cover the beaker with a watch glass and place it on a steam bath for fifteen minutes.

Weighing the crucible 7.3 Dry the filter crucible (5.1) to constant weight in the oven at 120°C (5.2) (about 15 minutes). Allow the crucible to cool in a desiccator and then weigh it.

Precipitation 7.4 Remove the beaker from the steam bath, stir in drop by drop 10 ml of the STPB solution (4.6). This addition should take about 2 minutes; allow to stand for at least 10 minutes before filtering.

Filtering and washing 7.5 Filter under vacuum into the weighed crucible, rinse the beaker with the liquid for washing (4.7), wash the precipitate three times with the liquid for washing (60 ml in all of the liquid for washing) and twice with 5 to 10 ml of water.

Drying and weighing 7.6 Wipe the outside of the crucible with a filter paper and place in the oven (5.2) for one and a half hours at temperature of 120°C. Allow the crucible to cool in a desiccator to ambient temperature and weigh rapidly.

Blank test 7.7 Make a blank test under the same conditions (omitting only the sample) and allow for this in the calculation of the final result.

Control test 7.8 Carry out the determination of an aliquot part of an aqueous solution of potassium chloride, containing at the most 40 mg of K₂O.

EXPRESSION OF RESULTS

8

Method of calculation and formulae 8.1 If the quantities and the dilutions shown in the Table are used, the following formulae apply: % K₂O in the fertiliser = (a − a) × F or % K in the fertiliser = (A − a) × F’ where: A = weight in grams of the precipitate from the sample a = weight in grams of the precipitate from the blank F and F’ = factors—see Table. % of K₂O in the fertiliser% of K in the fertiliserSample for analysis (g)Aliquot part to be taken as a sample for precipitation (ml)Conversion factor F % K₂O g KTPBConversion factor F % K g KTPB 1 — 50.8 — 4.2205013.14010.9065 — 104.2 — 8.3105026.28021.81210 — 208.3 — 16.6102552.56043.62420 — 5016.6 — 41.51010131.400109.060more than 50more than 41.5510262.800218.120 With samples and dilutions which differ from those shown in the Table use the following formulae: $%K2O(A-a)×f×D×100M$ or $%K2O(A-a)×f’×D×100M$ where: f = conversion factor, KTPB into K₂O = 0.1314 f’ = conversion factor, KTPB into K = 0.109 D = dilution factor M = weight in grams of sample for analysis

12a. — DETERMINATION OF WATER-SOLUBLE MAGNESIUM—ATOMIC ABSORPTION SPECTROPHOTOMETRIC METHOD

SCOPE

1

This method is for the determination of water-soluble magnesium.

FIELD OF APPLICATION

2

Exclusively to fertilisers in Groups 1(a) and 3(a) of Section A of the Table in Schedule 1 of the Fertilisers Regulations 1990[^f01016], in respect of which the declaration of water-soluble magnesium is required.

PRINCIPLE

3

Solution of magnesium by boiling the test sample in water, and determination by atomic absorption spectrophotometry.

REAGENTS

4

• (4.1) Hydrochloric acid, N solution (approximately).

• (4.2) Hydrochloric acid, 0.5 N solution.

• (4.3) Magnesium standard solution: dissolve 1.013 g magnesium sulphate (MgSO₄.7H₂O) in 0.5 N hydrochloric acid solution (4.2) and dilute to 100 ml with this acid.

• 1 ml of this solution contains 1 mg of magnesium (Mg).

• or

• weigh out 1.658 g of magnesium oxide, previously calcined at 600° for 2 hours, place in a beaker with 100 ml of water and 120 ml of approximately N hydrochloric acid (4.1). After dissolution, transfer quantitively into a one litre graduated flask, make up the volume with water and mix.

• 1 ml of this solution contains 1 mg of magnesium (Mg).

• (4.4) Strontium chloride solution: dissolve 15 g strontium chloride (SrCl₂.6H₂O) in 0.5 N hydrochloric acid solution (4.2) and dilute to 100 ml with the same solvent.

APPARATUS

5

• (5.1) Atomic absorption spectrophotometer with a magnesium lamp (285.2 nm).

PREPARATION OF SAMPLE

6

See Method 1.

PROCEDURE

7

Extraction 7.1 Weigh to the nearest 0.001 g, 5 g of the prepared sample and place in a 500 ml graduated flask. Add about 300 ml water and boil for half an hour. Allow to cool, dilute to the mark with water, mix and filter.

Preparation of the sample solution 7.2 7.2.1 If the fertiliser has a declared magnesium oxide (MgO) content greater than 10%, transfer by pipette 25 ml of the filtrate (7.1) into a 100 ml graduated flask, make up to the mark with water and mix. 7.2.2 Transfer by pipette 10 ml of the filtrate (7.1) or the diluted filtrate (7.2.1) into a 200 ml graduated flask and make up to the mark with 0.5 N hydrochloric acid solution (4.2). 7.2.3 Dilute solution (7.2.2) with 0.5 N hydrochloric acid solution (4.2) to a concentration within the working range of the spectrophotometer. The final solution must contain 10% V/V of the strontium chloride solution (4.4).

Blank solution 7.3 Prepare a blank solution from which only the sample has been omitted.

Standard solutions for calibration 7.4 By diluting the standard solution (4.3) with 0.5 N hydrochloric acid solution (4.2), prepare at least 5 standard solutions of increasing concentration corresponding to the optimal measuring range of the spectrophotometer. The final solutions must contain 10% V/V of the strontium chloride solution (4.4).

Measurement 7.5 Set up the spectrophotometer (5.1), at a wavelength of 28.25 nm using an oxidising air-acetylene flame. Spray successively, in triplicate, the standard solutions (7.4), the sample solution (7.2) and the blank solution (7.3), washing the instrument through with distilled water between each spraying. Plot the calibration curve using the mean absorbances as the ordinates and the corresponding concentrations of magnesium in Ö as the abscissae. Determine the concentration of magnesium in the sample and blank by reference to the claibration curve.

EXPRESSION OF RESULTS

8

Calculate the quantity of magnesium (Mg) or magnesium oxide (MgO) (conversion factor Mg to MgO = 1.66) in the sample taking into consideration the blank. Express the result as a percentage of the sample.

12b. — DETERMINATION OF WATER-SOLUBLE MAGNESIUM—EDTA METHOD

SCOPE

1

This method is for the determination of water-soluble magnesium.

FIELD OF APPLICATION

2

Exclusively to straight fertilisers in Groups 1(a) and 3(a) of Section A of the Table in Schedule 1 of the Fertilisers Regulations 1990[^f01017] in respect of which the indication of water-soluble magnesium, expressed as magnesium oxide, is required.

PRINCIPLE

3

Solution of magnesium by boiling a test sample in water. Titration with EDTA of calcium and magnesium in the presence of eriochrome black-T, followed by titration with EDTA of calcium in the presence of calcein or of calcon carbonic acid. Determination of magnesium by difference.

REAGENTS

4

• (4.1) Magnesium solution, 0.05 M: weigh out 2.016 g magnesium oxide, previously calcined at 600°C for 2 hours, place in a beaker with 100 ml water and stir in 120 ml of approximately N hydrochloric acid. After dissolution, transfer quantitatively into a 1 litre graduated flask, make up the volume with water and mix. Check the strength of the solution gravimetrically by precipitation as magnesium ammonium phosphate.

• 1 ml of the solution should contain 1.216 mg of Mg (= 2.016 mg of MgO).

• (4.2) EDTA solution, 0.05 M: dissolve 18.61 g of the dihydrated disodium salt of ethylenediaminetetra-acetic acid in 600-800 ml water contained in a 1 litre beaker. Transfer the solution quantitatively to a 1 litre graduated flask, make up to volume with water and mix. Check this solution (4.1) by taking a sample of 20 ml of the latter and titrating as described under 7.3.1.

• 1 ml of the EDTA solution should correspond to 1.216 mg of Mg or 2.016 mg of MGO and to 2.004 mg of Ca or 2.804 mg of CaO.

• (4.3) Calcium solution 0.05 M: weigh out 5.004 g of dry calcium carbonate and place in a beaker with 100 ml water. Progressively stir in 120 ml approximately N hydrochloric acid. Bring to the boil in order to drive off the carbon dioxide, cool, transfer quantitatively into a 1 litre graduated flask, make up to volume with water and mix. Check the solution against the EDTA solution (4.2) following analytical procedure 7.3.2. One ml of this solution should contain 2.004 mg of Ca (= 2.804 mg of CaO) and should correspond to 1 ml of the 0.05 molar EDTA solution.

• (4.4) Calcein indicator: carefully mix in a mortar 1 g calcein with 100 g sodium chloride. Use 10 mg of this mixture. The indicator changes from green to orange. Titration must be carried out until an orange colour is obtained which is free from green tinges.

• (4.5) Calcon carbonic acid indicator: dissolve 400 mg calcon carbonic acid in 100 ml methanol. Use three drops of this solution. The indicator changes from red to blue. Titration must be carried out until a blue colour is obtained which is free from red tinges.

• (4.6) Eriochrome black-T indicator: dissolve 300 mg eriochrome black-T in a mixture of 25 ml propan-1-ol and 15 ml triethanolamine. Use three drops of this solution. This indicator turns from red to blue and titration must be carried out until a blue colour is obtained which is free from rred tinges. It changes colour only when magnesium is present. If necessary add 0.1 ml of standard solution 4.1.

• (4.7) Potassium cyanide solution, 2 g per 100 ml.

• (4.8) Solution of potassium hydroxide and potassium cyanide: dissolve 280 g potassium hydroxide and 66 g potassium cyanide in water, make up the volume to 1 litre and mix.

• (4.9) pH 10 buffer solution: dissolve 33 g ammonium chloride in 200 ml water, add 207 ml ammonia solution (d = 0.880 g/ml) from a freshly opened bottle (or an equivalent amount of diluted ammonia, for example if d = 0.91 g/ml, use 250 ml). Make up the volume to 500 ml with water and mix. Check the pH of this solution regularly.

APPARATUS

5

• (5.1) Magnetic or mechanical stirrer.

• (5.2) pH meter.

PREPARATION OF THE SAMPLE

6

See Method 1.

PROCEDURE

7

Extraction 7.1 Weigh to the nearest 0.001 g, 5 g of the prepared sample and place in a 500 ml graduated flask. Add about 300 ml water and boil for half an hour. Cool, make up the volume, mix and filter.

Control test 7.2 Carry out a determination on aliquot parts of solutions (4.1) and (4.3) such that the Ca/Mg ratio is equal to that expected from the sample. For this purpose take (a) ml of standard solution (4.3) and (b — a) ml standard solution (4.1), where (a) and (b) are the numbers of ml EDTA solution used in the two titrations when analysing the sample. This procedure is correct only if the standard solutions of EDTA, calcium and magnesium are exactly equivalent. If this is not the case, it is necessary to make the appropriate corrections.

Determination 7.3 Titration in the presence of eriochrome black-T 7.3.1 Place an aliquot part of the solution to be analysed (see the Table) in a 300 ml beaker and dilute with water to about 100 ml. Add 5 ml buffer solution (4.9). The pH measured by the meter (5.2) must be 10.5±0.1. Add 2 ml potassium cyanide solution (4.7) and 3 drops of the eriochrome black-T indicator (4.6). Stir gently and titrate with the EDTA solution (4.2). Let “b” be the number of ml of 0.05 molar EDTA solution. Note: For titration with eriochrome black-T, the titration must not exceed 25 ml of EDTA, otherwise the volume of the aliquot part must be reduced. Titration in the presence of calcein or of calcon carbonic acid 7.3.2 Place an aliquot part of the solution to be analysed equal to that taken for the above titration in a beaker. Dilute with water to about 100 ml. Add 10 ml potassium hydroxide-potassium cyanide solution (4.8) and the indicator (4.4) or (4.5). Stir gently and titrate with the EDTA solution (4.2). Let “a” be the number of ml of 0.05 molar EDTA solution.

EXPRESSION OF RESULTS

8

$%MgO(b-a)×0.2016M$

13a. — DETERMINATION OF TOTAL MAGNESIUM—ATOMIC ABSORPTION SPECTROPHOTOMETRIC METHOD

SCOPE

1

This method is for the determination of total magnesium.

FIELD OF APPLICATION

2

Exclusively to the fertiliser magnesium ammonium nitrate in Group 1(a) of Section A of the Table in Schedule 1 of the Fertilisers Regulations 1990[^f01018] in respect of which the declaration of total magnesium is required.

PRINCIPLE

3

Solution of magnesium by boiling the test sample in dilute acid, and determination by atomic absorption spectrophotometry.

REAGENTS

4

• (4.1) Hydrochloric acid solution 50% (V/V): dilute an appropriate volume of hydrochloric acid (d = 1.18 g/ml) with an equal volume of water.

• (4.2) Hydrochloric acid, N solution (approximately).

• (4.3) Hydrochloric acid, 0.5 N solution.

• (4.4) Magnesium solution: dissolve 1.013 g magnesium sulphate (MgSO₄.7H₂O) in 0.5 N hydrochloric acid solution (4.3) and dilute to 100 ml with this acid.

• 1 ml of this solution contains 1 mg of magnesium (Mg).

• or

• weigh out 1.658 g of magnesium oxide, previously calcined at 600°C for 2 hours, place in a beaker with 100 ml of water and 120 ml of approximately N hydrochloric acid (4.2). After dissolution, transfer quantitatively into a one litre graduated flask, make up the volume with water and mix.

• 1 ml of this solution contains 1 mg of magnesium (Mg).

• (4.5) Strontium chloride solution: dissolve 75 g strontium chloride (SrCl₂.6H₂O) in 0.5 N hydrochloric acid solution (4.3) and dilute to 500 ml with this acid.

APPARATUS

5

• (5.1) Atomic absorption spectrophotometer with a magnesium lamp (285.2 nm).

PREPARATION OF SAMPLE

6

See Method 1.

PROCEDURE

7

Extraction 7.1 Weigh to the nearest 0.001 g, 5 g of the prepared sample and place in a 500 ml graduated flask. Add about 200 ml water, 20 ml hydrochloric acid solution (4.1) and boil for half an hour. Allow to cool, dilute to the mark with water, mix and filter.

Preparation of the sample solution 7.2 7.2.1 If the fertiliser has a declared magnesium oxide (MgO) content greater than 10%, transfer by pipette 25 ml of the filtrate (7.1) into a 100 ml graduated flask, make up to the mark with water and mix. 7.2.2 Transfer by pipette 10 ml of the filtrate (7.1) or the diluted filtrate (7.2.1), into a 200 ml graduated flask and make up to the mark with 0.5 N hydrochloric acid solution (4.3). 7.2.3 Dilute solution (7.2.2) with 0.5 N hydrochloric acid solution (4.3) to a concentration within working range of the spectrophotometer. The final solution must contain 10% V/V strontium chloride solution (4.5).

Blank solution 7.3 Prepare a blank solution from which only the sample has been omitted.

Standard solutions for calibration 7.4 By diluting the standard solution (4.4) with 0.5 hydrochloric acid solution (4.3), prepare at least 5 standard solutions of increasing concentration corresponding to the optimal measuring range of the spectrophotometer. The final solutions must contain 10% V/V of the strontium chloride solution (4.5).

Measurement 7.5 Set up the spectrophotometer (5.1), at a wavelength of 285.2 nm using an oxidising air-acetylene flame. Spray successively, in triplicate, the standard solutions (7.4), the sample solution (7.2) and the blank solution (7.3), washing the instrument through with distilled water between each spraying. Plot the calibration curve using the mean absorbances as the ordinates and the corresponding concentrations of magnesium in Ö as the abscissae. Determine the concetration of magnesium in the sample and blank by reference to the calibration curve.

EXPRESSION OF RESULTS

8

Calculate the quantity of magnesium (Mg) or magnesium oxide (MgO) (conversion factor Mg to MgO = 1.66) in the sample, taking into consideration the blank. Express the result as a percentage of the sample.

13b. — DETERMINATION OF TOTAL MAGNESIUM—EDTA METHOD

SCOPE

1

This method is for the determination of total magnesium.

FIELD OF APPLICATION

2

Exclusively to the fertiliser magnesium ammonium nitrate in Group 1(a) of Section A of the Table in Schedule 1 of the Fertilisers Regulations 1990[^f01019] in respect of which the indication of total magnesium is required.

PRINCIPLE

3

Solution of magnesium by boiling a test sample in dilute acid. Titration of calcium and magnesium with EDTA in the presence of eriochrome black-T, followed by titration with EDTA of calcium in the presence of calcein or of calcon carbonic acid. Determination of magnesium by difference.

REAGENTS

4

• (4.1) Magnesium solution, 0.05 M: weigh out 2.016 g of magnesium oxide previously calcined at 600°C for 2 hours, place in a beaker with 100 ml of water and stir in 120 ml of approximately 1 N hydrochloric acid. After dissolution, transfer quantitatively into a 1 litre graduated flask, make up the volume with water and mix. Check the strength of the solution gravimetrically by precipitation as ammonium-magnesium phosphate.

• 1 ml of the solution should contain 1.216 mg of magnesium (Mg) (= 2.016 mg of magnesium oxide (MgO)).

• (4.2) EDTA solution 0.05 M: dissolve 18.61 g of the dihydrated disodium salt of ethylenediaminetetra-acetic acid in 600-800 ml water contained in a 1 litre beaker. Transfer the solution quantitatively into a 1 litre graduated flask, make up to volume with water and mix. Check this solution with solution (4.1) by taking a sample of 20 ml of the latter and titrating following analytical procedure 7.3.1.

• 1 ml of the EDTA solution should correspond to 1.216 mg of Mg or 2.016 mg of MgO and to 2.004 mg of Ca or 2.804 mg of CaO.

• (4.3) Calcium solution 0.05 M: weigh out 5.004 g of dry calcium carbonate and place in a beaker with 100 ml of water. Progressively stir in 120 ml of approximately N hydrochloric acid. Bring to the boil in order to drive off the carbon dioxide, cool, transfer quantitatively into a 1 litre graduated flask, make up to volume with water and mix. Check this solution against the EDTA solution (4.2) following analytical procedure 7.3.2.

• One ml of this solution should contain 2.004 mg of Ca (= 2.804 mg of CaO) and should correspond to 1 ml of the 0.05 molar EDTA solution.

• (4.4) Calcein indicator: carefully mix in a mortar 1 g of calcein with 100 g of sodium chloride. Use 10 mg of this mixture. The indicator changes from green to orange. Titration must be carried out until an orange colour is obtained which is free from green tinges.

• (4.5) Calcon carbonic acid indicator: dissolve 400 mg of calcon carbonic acid in 100 ml of methanol. Use three drops of this solution. The indicator changes from red to blue. Titration must be carried out until a blue colour is obtained which is free from red tinges.

• (4.6) Eriochrome black-T indicator: dissolve 300 mg of eriochrome black-T in a mixture of 25 ml of propan-1-ol and 15 ml of triethanolamine. Use three drops of this solution. This indicator turns from red to blue and titration must be carried out until a blue colour is obtained which is free from red tinges. It changes colour only when magnesium is present. If necessary add 0.1 ml of standard solution (4.1).

• (4.7) Potassium cyanide solution, 2 g per 100 ml.

• (4.8) Solution of potassium hydroxide and potassium cyanide: dissolve 280 g potassium hydroxide and 66 g potassium cyanide in water, make up the volume to one litre and mix.

• (4.9) pH 10.5 buffer solution: dissolve 33 g ammonium chloride in 200 ml of water, add 207 ml ammonia solution (d = 0.880 g/ml) from a freshly opened bottle (or an equivalent amount of diluted ammonia, for example if d = 0.91 g/ml, use 250 ml). Make up to volume to 500 ml with water and mix. Check the pH of this solution regularly.

• (4.10) Hydrochloric acid solution, 50% (V/V): dilute an appropriate volume of hydrochloric acid (d = 1.18 g/ml) with an equal volume of water.

• (4.11) Sodium hydroxide solution, 5N.

APPARATUS

5

• (5.1) Magnetic or mechanical stirrer.

• (5.2) pH meter.

PREPARATION OF THE SAMPLE

6

See Method 1.

PROCEDURE

7

Extraction 7.1 Weigh to the nearest 0.001 g, 5 g of the prepared sample and place in a 500 ml graduated flask. Add about 200 ml water and 20 ml hydrochloric acid (4.10) and boil for half an hour. Cool, make up to volume with water, mix and filter.

Control test 7.2 Carry out a determination on aliquot parts of solutions (4.1) and (4.3) such that the Ca/Mg ratio is equal to that expected from the sample. For this purpose take (a) ml of standard solution (4.3) and (b — a) ml standard solution (4.1), where (a) and (b) are the numbers of ml EDTA solution used in the two titrations when analysiing the sample. This procedure is correct only if the standard solutions of EDTA, calcium and magnesium are exactly equivalent. If this is not the case, it is necessary to make the appropriate corrections.