Statutory Instrument 1996 No. 1342
The Fertilisers (Sampling and Analysis) Regulations 1996 - continued

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SCHEDULE 2
Regulation 6

METHODS OF ANALYSIS


PART I
        1.    General
    When two or more methods are prescribed in this part of this Schedule to determine a component of a fertiliser the choice of the method shall, except where otherwise indicated, be left to the agricultural analyst concerned; the method used must however be indicated in the certificate of analysis.
        2.    Reagents
    Except where otherwise specified in the method of analysis, all reagents shall be of analytical quality. Where trace elements are to be determined, the purity of the reagents used shall be checked by means of a blank test.
        3.    Water

      (a)  Except where otherwise specified, a reference in this Part of this Schedule to water shall be a reference to demineralized or distilled water.

      (b)  For the determination of any form of nitrogen, water shall be free of all nitrogenous compounds and carbon dioxide.

      (c)  Except where the method of analysis specifies a particular solvent or diluent, all dissolution, dilution, rinsing and washing operations mentioned in the methods of analysis shall be carried out using water.
        4.    Apparatus

      (a)  Only special instruments and apparatus and specifically required apparatus and equipment are mentioned in the methods of analysis.

      (b)  Apparatus and equipment shall be clean.

      (c)  The accuracy of graduated glassware shall be assured by reference to the appropriate standards.
        5.    Methods of Analysis
    1. Preparation of the sample for analysis
    2. Determination of ammoniacal nitrogen
    3.a Determination of nitrate and ammoniacal nitrogen — Ulsch method
    b Determination of nitrate and ammoniacal nitrogen — Arnd method
    c Determination of nitrate and ammoniacal nitrogen — Devarda method
    4.a Determination of total nitrogen in calcium cyanamide — in the absence of nitrate
    b Determination of total nitrogen in calcium cyanamide — in the presence of nitrate
    5. Determination of total nitrogen in urea
    6. Determination of cyanamide nitrogen
    7. Determination of biuret in urea
    8.a Determination of different forms of nitrogen in the same sample — in the presence of cyanamide nitrogen
    b Determination of different forms of nitrogen in the same sample — in the absence of cyanamide nitrogen
    9.a Extraction of total phosphorus — by mineral acids
    b Extraction of phosphorus — by 2% formic acid
    c Extraction of phosphorus — by 2% citric acid
    d Extraction of phosphorus — by neutral ammonium citrate
    e Extraction of phosphorus — by alkaline ammonium citrate (Petermann's method) at 65 C
    f Extraction of phosphorus — by alkaline ammonium citrate (Petermann's method) at ambient temperature
    g Extraction of phosphorus — by alkaline ammonium citrate (Joulie's method)
    h Extraction of phosphorus — by water
    10. Determination of extracted phosphorus
    11. Determination of water-soluble potassium
    12. Determination of chlorides in the absence of organic material
    13.a Determination of fineness of grinding — dry method
    b Determination of fineness of grinding of soft natural phosphates
    14. Methods of analysis and test procedures for ammonium nitrate fertilisers containing more than 28% nitrogen by weight
    a Method for the application of thermal cycles
    b Determination of the oil retention value
    c Determination of combustible ingredients
    d Determination of the pH value
    e Determination of the particle size
    f Determination of the chlorine content (as chloride ion)
    g Determination of copper
    15. Extraction of total calcium, total magnesium, total sodium and total sulfur in the form of sulfates
    16. Extraction of total sulfur
    17. Extraction of water-soluble calcium, magnesium, sodium and sulfur (in the form of sulfates)
    18. Extraction of water-soluble sulfur
    19. Extraction and determination of elemental sulfur
    20. Manganimetric determination of extracted calcium following precipitation in the form of oxalate
    21. Determination of magnesium by atomic absorption spectrometry
    22. Determination of magnesium by complexometry
    23. Determination of sulfates
    24. Determination of the sodium extracted
    25. Trace elements at a concentration less than 10%
    a Extraction of total trace elements
    b Extraction of water-soluble trace elements
    c Removal of organic compounds from fertiliser extracts
    d Determination of trace elements in fertiliser extracts by atomic absorption spectrometry (general procedure)
    e Determination of boron in fertiliser extracts by means of spectrometry with azomethine-h
    f Determination of cobalt in fertiliser extracts by atomic absorption spectrometry
    g Determination of copper in fertiliser extracts by atomic absorption spectrometry
    h Determination of iron in fertiliser extracts by atomic absorption spectrometry
    i Determination of manganese in fertiliser extracts by atomic absorption spectrometry
    j Determination of molybdenum in fertiliser extracts by spectrometry of a complex with ammonium thiocyanate
    k Determination of zinc in fertiliser extracts by atomic absorption spectrometry
    26. Trace elements at a concentration greater than 10%
    a Extraction of total trace elements
    b Extraction of water — soluble trace elements
    c Removal of organic compounds from fertiliser extracts
    d Determination of trace elements in fertiliser extracts by atomic absorption spectrometry (general procedure)
    e Determination of boron in fertiliser extracts by means of acidimetric titration
    f Determination of cobalt in fertiliser extracts by the gravimetric method with 1-nitroso-2-naphthol
    g Determination of copper in fertiliser extracts by the titrimetric method
    h Determination of iron in fertiliser extracts by atomic absorption spectrometry
    i Determination of manganese in fertiliser extracts by titration
    j Determination of molybdenum in fertiliser extracts by the gravimetric method with 8-hydroxyquinoline
    k Determination of zinc in fertiliser extracts by atomic absorption spectrometry

1.

PREPARATION OF THE SAMPLE FOR ANALYSIS
        1.    SCOPE
    The following procedure is to be used for the preparation of the sample for analysis, taken from the final sample.
        2.    PRINCIPLE
        2.1    Solid fertilisers: the preparation of a final sample received at the laboratory is a series of operations, usually sieving, grinding and mixing, carried out in such a way that: —

      (a)  the smallest amount weighed out laid down by the methods of analysis is representative of the laboratory sample; and

      (b)  the fineness of the fertiliser has not been changed by the preparation to the extent that its solubility in the various extraction reagents is appreciably affected.
        2.2    Fluid fertilisers: the final sample is mixed by shaking to ensure that any insoluble matter, particularly crystalline material, is thoroughly dispersed before each test portion is taken.
        3.    APPARATUS
        3.1    Sample divider (optional).
        3.2    Sieves with apertures of 0.2 mm and 0.5 mm.
        3.3    250 ml flasks, stoppered.
        3.4    Porcelain pestle and mortar or grinder.
        4.    CHOICE OF TREATMENT TO BE USED
    Preliminary remark: if the product is suitable, only a representative part of the final sample need be kept.
        4.    Final samples which must not be ground
    Calcium nitrate, calcium magnesium nitrate, sodium nitrate, Chile nitrate, calcium cyanamide, nitrogenous calcium cyanamide, ammonium sulfate, ammonium nitrates of over 30% N, urea, basic slag, natural phosphate rendered partially soluble, precipitated dihydrated dicalcium phosphate, calcined phosphate, aluminium calcium phosphate, soft ground rock phosphate.
        4.2    Finals samples which must be divided and part of which must be ground
    These are products in respect of which certain determinations are carried out without previous grinding (fineness of grinding for example) and other determinations after grinding. They include all compound fertilisers containing the following phosphate ingredients: basic slag, aluminium calcium phosphate, calcined phosphate, soft ground rock phosphate and natural phosphate rendered partially soluble. To that end, divide the final sample into two parts, which are as identical as possible, using a sample divider or by quartering.
        4.3    Final samples in respect of which all determinations are carried out on a grounded product
    These are all the other fertilisers on the list which are not to be found under 4.1 and 4.2. The whole final sample shall be ground.
        5.    METHOD
    The part of the final sample referred to under 4.2 and 4.3 is sieved rapidly through a sieve with apertures of 0.5 mm. The residue is ground roughly as to obtain a product in which there is a minimum of fine particles, and it is then sieved. The grinding must be done in conditions such that the substance is not appreciably heated. The operation is repeated as many times as is necessary until there is no residue, and it must be effected as quickly as possible in order to prevent any gain or loss of constituents (water, ammonia). The whole ground and sieved product is placed in a non-corrodable container provided with an air-tight closure.
    Before any weighing is carried out for the analysis, the whole sample must be thoroughly mixed.
        6.    SPECIAL CASES

      (a)  Fertilisers comprising a blend of several categories of crystals
        In this case, separation frequently occurs. It is therefore absolutely essential to crush and pass the sample through a sieve with apertures of 0.2 mm (for example, mixtures of ammonium phosphate and potassium nitrate). The grinding of the whole of the final sample is recommended in the case of these products.

      (b)  Residue which is difficult to grind and does not contain fertilising substances
        Weigh the residue and take account of its mass when calculating the final result.

      (c)  Products which decompose on heating
        Grinding must be carried out in such a way as to avoid any heating. It is preferable in this case to use a mortar for grinding (for example, compound fertilisers containing calcium cyanamide and urea).

      (d)  Products which are abnormally moist or made into a paste by grinding
        To ensure homogeneity, a sieve is to be chosen which has the smallest apertures compatible with the destruction of lumps by hand or with the pestle. This may be the case for mixtures, certain ingredients of which contain water of crystallisation.
        7.    FLUID FERTILISERS
    Mix thoroughly by shaking, ensuring that any insoluble matter, particularly crystalline material, is thoroughly dispersed, immediately before drawing a portion of the sample of analysis.

2.

DETERMINATION OF AMMONIACAL NITROGEN
        1.    SCOPE
    This method is for the determination of ammoniacal nitrogen.
        2.    FIELD OF APPLICATION
    All nitrogenous fertilisers, including compound fertilisers, in which nitrogen is found exclusively either in the form of ammonium salts, or ammonium salts together with nitrates.
    It is not applicable to fertilisers containing urea, cyanamide or other organic nitrogenous compounds.
        3.    PRINCIPLE
    Displacement of ammonia by means of an excess of sodium hydroxide; distillation; determination of the ammonia absorbed by a given volume of a standard sulfuric acid and titration of the excess acid with a standard solution of sodium or potassium hydroxide.
        4.    REAGENTS
        4.1    Hydrochloric acid solution, 50% (V/V): dilute an appropriate volume of hydrochloric acid (p=1.18 g/ml) with an equal volume of water.

          4.2    Sulfuric acid, 0.05 M solution
    		 for variant (a) (see page 16)

          4.3    Sodium or potassium hydroxide, 0.1 M solution, carbonate free

          4.4    Sulfuric acid, 0.1 M solution
    		 for variant (b) (see page 16)

          4.5    Sodium or potassium hydroxide, 0.2 M solution, carbonate free

          4.6    Sulfuric acid, 0.25 M solution
    		 for variant (c) (see page 16)

          4.7    Sodium or potassium hydroxide, 0.5 M solution, carbonate free
        4.8    Sodium hydroxide solution, 30 g per 100 ml, ammonia free
        4.9    Indicator solutions:

        4.9.1  Mixed indicator:
    Solution A: dissolve 1 g methyl red in 37 ml sodium hydroxide solution 0.1 M 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 solution.


        4.9.2  Methyl red indicator solution:
    Dissolve 0.1 g methyl red in 50 ml ethanol (95%) make up to 100 ml with water and filter if necessary. This indicator may be used (4 to 5 drops) instead of the preceding one.

        4.10  Anti-bump granules of pumice stone, washed in hydrochloric acid and ignited.

        4.11  Ammonium sulfate.
        5.    APPARATUS
        5.1    Distillation apparatus consisting of a round-bottomed flask of suitable capacity connected to a condenser by means of a splash head.
    Examples of the different types of equipment recommended for this determination are reproduced in Figures 1, 2, 3 and 4 in the Appendix.
        5.2    Rotary shaker, 35 to 40 turns per minute.
        6.    PREPARATION OF SAMPLE
    See Method 1.
        7.    PROCEDURE

        7.1.1  Solubility test
    Carry out a solubility test on the sample in water at room temperature in the proportion of 2 g per 100 ml.

        7.1.2  Preparation of the solution
    Weigh 5, 7 or 10 g of the sample to the nearest 0.001 g, as shown in the Table, and place in a 500 ml graduated flask. From the result of the solubility test, proceed as follows:

      (a)  Products completely soluble in water
      Add sufficient water to dissolve the sample; shake, and when completely dissolved, make up to volume and mix thoroughly.

      (b)  Products not completely soluble in water
      Add 50 ml water and then 20 ml hydrochloric acid solution (4.1). Swirl and leave undisturbed until the evolution of carbon dioxide has ceased. Add 400 ml water and shake for half an hour on the rotary shaker (5.2). Make up to volume with water, mix and filter through a dry paper into a dry receiver. Discard the first portion of the filtrate.

        7.2    Determination
    According to the variant chosen, place in the collecting flask a measured quantity of standard sulfuric acid as indicated in the Table on page16. Add the appropriate quantity of the chosen indicator solution (4.9.1 to 4.9.2) and, if necessary, water to obtain a volume of at least 50 ml. The condenser outlet must be below the surface of the standard acid in the collecting flask.

    Transfer by pipette, according to the details given in the Table, an aliquot portion of the clear solution into the distillation flask of the apparatus. Add water to obtain a volume of about 350 ml and several grains of pumice to control the boiling.

    Assemble the distillation apparatus and, taking care to avoid any loss of ammonia, add to the contents of the distillation flask 10 ml of concentrated sodium hydroxide solution (4.8) or 20 ml of the reagent in the cases where 20 ml hydrochloric acid (4.1) have been used in order to dissolve the sample. Warm the flask gently and when boiling commences distil at such a rate that about 200 ml are obtained in 30 minutes.

    When no more ammonia is likely to be evolved, lower the receiving flask so that the tip of the condenser is above the surface of the liquid.

    Test the subsequent distillate by means of an appropriate reagent to ensure that all the ammonia has been completely distilled. Wash the condenser with a little water and titrate the excess acid with the standard solution of sodium or potassium hydroxide prescribed for the variant adopted (see Note).

      Note:  Standard solutions of different strengths may be used for the titration provided that the volumes used do not, as far as possible, exceed 40 to 45 ml.
        7.3    Blank
    Carry out a blank test under the same conditions (omitting only the sample) and allow for this in the calculation of the final result.
        7.4    Control test
    Before carrying out analyses, check that the apparatus is working properly and that the correct application of the method is used by taking an aliquot portion of a freshly prepared solution of ammonium sulfate (4.11) containing the maximum quantity of nitrogen prescribed for the chosen variant.
        8.    EXPRESSION OF RESULT
    Express the result of the analysis as the percentage of ammoniacal nitrogen in the fertiliser as received for analysis using the formula
    %N = (50 - A) × F for variants (a) and (b) and
    %N = (35 - A) × F for variant (c)
    where
    50 (or 35 where variant (c) applies)
    = millilitres of standard solution of sulfuric acid in the receiving flask.
    A = millilitres of sodium or potassium hydroxide used for the titration.
    F = factor taking into account the weight of sample, the dilution, the volume of the aliquot portion distilled and the volumetric equivalent.
    TABLE FOR METHOD 2
    Determination of the ammoniacal nitrogen and of the ammoniacal and nitrate nitrogen in fertilisers. Table of the weighing, dilution and calculation to be carried out for each of the variants (a), (b) and (c) of the method.
    Variant (a), — Approximate maximum quantity of nitrogen to be distilled = 50 mg
    Sulfuric acid 0.05 M to be placed in the receiving flask = 50 ml
    Titration with sodium or potassium hydroxide, 0.1 M solution
    Declaration N% Amount to be weighed (g) (Volume) Dilution (ml) Volume of sample solution to be distilled (ml) Factor F
    0 - 5 10 500 50 0.14
    5 - 10 10 500 25 0.28
    10 - 15 7 500 25 0.40
    15 - 20 5 500 25 0.56
    20 - 40 7 500 10 1.00
    Variant (b), — Approximate maximum quantity of nitrogen to be distilled = 100 mg
    Sulfuric acid 0.1 M to be placed in the receiving flask = 50 ml
    Titration with sodium or potassium hydroxide, 0.2 M solution
    Declaration N% Amount to be weighed (g) (Volume) Dilution (ml) Volume of sample solution to be distilled (ml) Factor F
    0 - 5 10 500 100 0.14
    5 - 10 10 500 50 0.28
    10 - 15 7 500 50 0.40
    15 - 20 5 500 50 0.56
    20 - 40 7 500 50 1.00
    Variant (c), — Approximate maximum quantity of nitrogen to be distilled = 200 mg
    Sulfuric acid 0.25 M to be placed in the receiving flask = 35 ml
    Titration with sodium or potassium hydroxide, 0.5 M solution
    Declaration N% Amount to be weighed (g) (Volume) Dilution (ml) Volume of sample solution to be distilled (ml) Factor F
    0 - 5 10 500 200 0.175
    5 - 10 10 500 100 0.350
    10 - 15 7 500 100 0.500
    15 - 20 5 500 100 0.700
    20 - 40 5 500 50 1.400

3a.

DETERMINATION OF NITRIC AND AMMONIACAL NITROGEN — ULSCH METHOD
        1.    SCOPE
    This method is for the determination of nitric and ammoniacal nitrogen with reduction according to Ulsch.
        2.    FIELD OF APPLICATION
    All nitrogenous fertilisers, including compound fertilisers, in which nitrogen is found exclusively in nitrate form, or in ammoniacal and nitrate form.
        3.    PRINCIPLE
    Reduction of nitrates and nitrites to ammonia by means of metallic iron in an acidic medium and displacement of the ammonia thus formed by the addition of an excess of sodium hydroxide: distillation of the ammonia and determination of the ammonia absorbed in a known volume of standard sulfuric acid solution. Titration of the excess sulfuric acid with a standard solution of sodium or potassium hydroxide.
        4.    REAGENTS
4.1 Hydrochloric acid solution, 50% (V/V): dilute an appropriate volume of hydrochloric acid (p=1.18 g/ml) with an equal volume of water.
4.2 Sulfuric acid, 0.05 M solution.
4.3 Sodium or potassium hydroxide, 0.1 M solution, carbonate free.
4.4 Sulfuric acid solution, approximately 30% H2SO4 (W/V), ammonia free.
4.5 Powdered iron reduced in hydrogen. (The prescribed quantity of iron must be able to reduce at least 0.05 g nitrate nitrogen.)
4.6 Sodium hydroxide solution, 30 g per 100<+>ml, ammonia free.
4.7 Indicator solutions:
4.7.1 Mixed indicator:
Solution A: dissolve 1 g methyl red in 37 ml 0.1 M 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).
4.7.2 Methyl red indicator solution:
Dissolve 0.1 g methyl red in 50 ml 95% ethanol, make up to 100 ml with water and filter if necessary.
This indicator may be used (4 - 5 drops) instead of the preceding one.
4.8 Anti-bump granules of pumice stone, washed in hydrochloric acid and ignited.
4.9 Sodium nitrate.
        5.    APPARATUS
    See Method 2.
        6.    PREPARATION OF SAMPLE
    See Method 1.
        7.    PROCEDURE
        7.1    Preparation of the solution
    See Method 2.
        7.2    Determination
    Place in the receiving flask an exactly measured quantity of standard sulfuric acid (4.2) as indicated in the Table of Method 2 (variant (a)) and add the appropriate quantity of indicator solution (4.7.1 or 4.7.2).

    The end of the extension tube of the condenser must be below the surface of the standard acid in the receiving flask.

    Using a pipette, transfer an aliquot part of the clear solution as indicated in the Table of Method 2 (variant (a)) to the distillation flask of the apparatus. Add 350 ml water, 20 ml 30% sulfuric acid solution (4.4), stir, and add 5 g of reduced iron (4.5). Wash the neck of the flask with several ml of water, and place a small, long-stemmed funnel in the neck of the flask. Heat in a boiling water bath for an hour and then wash the stem of the funnel with a few ml of water. Allow to cool to room temperature.

    Taking care to avoid any loss of ammonia, add 50 ml concentrated sodium hydroxide solution (4.6) to the contents of the distillation flask, or in the cases where 20 ml of hydrochloric acid (4.1) has been used to dissolve the sample, add 60 ml of concentrated sodium hydroxide solution (4.6). Assemble the distillation apparatus. Distil the ammonia according to the procedure given in Method 2. Titrate the excess acid with the standard solution of sodium or potassium hydroxide (4.3).
        7.3    Blank test
    Carry out a blank test (omitting only the sample) under the same conditions and allow for this in the calculation of the final result.
        7.4    Control test
    Before analysis check that the apparatus is working properly and that the correct application of the method is used by taking an aliquot portion of a freshly prepared solution of sodium nitrate (4.9) containing 0.045 g to 0.05 g of nitrogen.
        8.    EXPRESSION OF RESULTS
    Express the results of analysis as a percentage of nitric nitrogen, or combined ammoniacal and nitric nitrogen, contained in the fertiliser as received for analysis.

3b.

DETERMINATION OF NITRIC AND AMMONIACAL NITROGEN — ARND METHOD
        1.    SCOPE
    This method is for the determination of nitric and ammoniacal nitrogen with reduction according to Arnd (modified for each of the variants (a), (b) and (c)).
        2.    FIELD OF APPLICATION
    See Method 3a.
        3.    PRINCIPLE
    Reduction of nitrates and nitrites to ammonia in a neutral aqueous solution by means of a metallic alloy composed of 60% Cu and 40% Mg (Arnd's alloy) in the presence of magnesium chloride.

    Distillation of the ammonia and absorption in a known volume of standard sulfuric acid solution. Titration of the excess acid with a standard solution of sodium or potassium hydroxide.
        4.    REAGENTS
4.1 Hydrochloric acid solution, 50% (V/V): dilute an appropriate volume of hydrochloric acid (p=1.18 g/ml) with an equal volume of water.
4.2 Sulfuric acid, 0.05 M solution for variant (a) (see page16)
4.3 Sodium or potassium hydroxide, 0.1 M solution, carbonate free
4.4 Sulfuric acid, 0.1 M solution for variant (b) (see page 16)
4.5 Sodium or potassium hydroxide, 0.2 M solution, carbonate free
4.6 Sulfuric acid, 0.25 M solution for variant (c) (see page 16)
4.7 Sodium or potassium hydroxide, 0.5 M solution, carbonate free
4.8 Sodium hydroxide solution, approximately 2 M.
4.9 Arnd's alloy — powdered to pass through a sieve with square apertures less than 1.00 mm.
4.10 Magnesium chloride solution, 20% (W/V):

    Dissolve 200 g magnesium chloride (MgCl2.6H2O) in approximately 600 - 700 ml water in a one litre flat bottomed flask. To prevent frothing, add 15 g magnesium sulfate (MgSO4.7H2O). After dissolution add 2 g magnesium oxide and a few anti-bump granules of pumice stone and concentrate the suspension to 200 ml by boiling, thus expelling any trace of ammonia from the reagents. Cool, make up the volume to 1 litre and filter.
4.11 Indicator solutions:
4.11.1 Mixed indicator:
Solution A: dissolve 1 g methyl red in 37 ml 0.1 M 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 A with 2 volumes of B.
This indicator is violet in acid solution, grey in neutral solution and green in alkaline solution. Use 0.5 ml (10 drops).
4.11.2 Methyl red indicator solution:
Dissolve 0.1 g methyl red in 50 ml 95% ethanol, make up to 100 ml with water and filter if necessary. This indicator may be used (4 to 5 drops) instead of the preceding one.
4.11.3 Congo red indicator solution:
Dissolve 3 g Congo red in 1 litre warm water and filter if necessary after cooling. This indicator may be used, instead of the two described above, in the neutralisation of acid extracts before distillation, using 0.5 ml per 100 ml of liquid to be neutralised.
4.12 Anti-bump granules of pumice stone washed in hydrochloric acid and ignited.
4.13 Sodium nitrate.
        5.    APPARATUS
    See Method 2.
        6.    PREPARATION OF SAMPLE
    See Method 1.
        7.    PROCEDURE
        7.1    Preparation of the solution for analysis
    See Method 2.
        7.2    Determination
    According to the chosen variant, place in the receiving flask a measured quantity of standard sulfuric acid as indicated in the Table of Method 2. Add the appropriate quantity of chosen indicator solution (4.11.1 or 4.11.2) and if necessary water to give a volume of a least 50 ml. The end of the extension tube of the condenser must be below the surface of the solution.

    Using a pipette, take, according to the Table, an aliquot part of the clear solution and place in the distillation flask. Add sufficient water to obtain a total volume of about 350 ml (see Note), 10 g Arnd's alloy (4.8), 50 ml magnesium chloride solution (4.10) and a few fragments of pumice stone (4.12). Rapidly connect the flask to the distillation apparatus. Heat gently for about 30 minutes. Then increase the heating to distil the ammonia. Continue the distillation for about an hour.

    After this time, the residue in the flask ought to have a syrupy consistency. When the distillation has finished, titrate the quantity of excess acid in the receiving flask according to the procedure in Method 2.

      Note:  When the sample solution is acidic (addition of 20 ml hydrochloric acid (4.1) to dissolve the sample) the aliquot part taken for analysis is neutralised in the following way: to the distillation flask containing the aliquot part add about 250 ml water, the necessary quantity of one of the indicators (4.11.1, 4.11.2, 4.11.3) and swirl or mix carefully. Neutralise with 2 M sodium hydroxide solution (4.8) and acidify again with a drop of hydrochloric acid (4.1). Then proceed as indicated in 7.2.
        7.3    Blank test
    Carry out a blank test under the same conditions (omitting only the samples) and allow for this in the calculation of the final result.
        7.4    Control test
    Before analysis, check that the apparatus is working properly and that the correct technique is applied using a freshly prepared solution of sodium nitrate (4.13) containing 0.050 g to 0.150 g nitrogen depending on the variant chosen.
        8.    EXPRESSION OF RESULTS
    Express the results of the analysis as a percentage of nitric nitrogen, or combined ammoniacal and nitric nitrogen, contained in the fertiliser as received for analysis.

3c.

DETERMINATION OF NITRIC AND AMMONIACAL NITROGEN — DEVARDA METHOD
        1.    SCOPE
    This method is for the determination of nitric and ammoniacal nitrogen with reduction according to Devarda (modified for each of the variants (a), (b) and (c)).
        2.    FIELD OF APPLICATION
    See Method 3a.
        3.    PRINCIPLE
    Reduction of nitrates and nitrites to ammonia in a strongly alkaline solution by means of a metallic alloy composed of 45% A1, 5% Zn and 50% Cu (Devarda's alloy). Distillation of the ammonia and absorption in a known volume of standard sulfuric acid; titration of the excess sulfuric acid with a standard solution of sodium or potassium hydroxide.
        4.    REAGENTS
4.1 Hydrochloric acid solution, 50% (V/V): dilute an appropriate volume of hydrochloric acid (p=1.18 g/ml) with an equal volume of water.
4.2 Sulfuric acid, 0.05 M solution for variant (a) (see page 16)
4.3 Sodium or potassium hydroxide, 0.1 M solution, carbonate free
4.4 Sulfuric acid, 0.1 M solution for variant (b) (see page 16)
4.5 Sodium or potassium hydroxide, 0.2 M solution, carbonate free
4.6 Sulfuric acid, 0.25 M solution for variant (c) (see page 16)
4.7 Sodium or potassium hydroxide, 0.5 M solution, carbonate free
4.8 Devarda's alloy — powdered so that 90 to 100% will pass through a sieve with apertures less than 0.25 mm square, 50 to 75% will pass through a sieve with apertures of less than 0.075 mm square. (Pre-packed bottles containing a maximum of 100 g are recommended.)
4.9 Sodium hydroxide solution, 30 g per 100 ml, ammonia free.
4.10 Indicator solutions:
4.10.1 Mixed indicator:
Solution A: dissolve 1 g methyl red in 37 ml 0.1 M 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 A with 2 volumes of B.
This indicator is violet in acid solution, grey in neutral solution and green in alkaline solution. Use 0.5 ml (10 drops).
4.10.2 Methyl red indicator:
Dissolve 0.1 g methyl red in 50 ml 95% ethanol, make up to 100 ml with water and filter if necessary. This indicator (4 to 5 drops) may be used instead of the preceding one.
4.11 Ethanol, 95%.
4.12 Sodium nitrate.
        5.    APPARATUS
        5.1    Distillation apparatus consisting of a round bottomed flask of suitable capacity, connected to a condenser by means of a splash head, equipped, in addition, with a bubble trap on the receiving flask to prevent any loss of ammonia.

    An example of the type of apparatus recommended for this determination is reproduced in Figure 5 in the Appendix.
        6.    PREPARATION OF THE SAMPLE
    See Method 1.
        7.    PROCEDURE
        7.1    Preparation of the solution for analysis
    See Method 2.
        7.2    Determination
    According to the variant chosen, place in the receiving flask an exactly measured quantity of standard sulfuric acid as indicated in the Table. Add the appropriate quantity of the chosen indicator solution (4.10.1 or 4.10.2) and sufficient water to give a volume of 50 ml. The end of the extension tube of the condenser must be below the surface of the solution. Fill the bubble trap with distilled water.

    Using a pipette, take an aliquot part of the clear solution as indicated in the Table and place in the distillation flask. Add sufficient water to the distillation flask to obtain a volume of 250 - 300 ml, then add 5 ml ethanol (4.11) and 4 g Devarda's alloy (4.8).

      Note:  In the presence of calcium salts such as calcium nitrate and calcium ammonium nitrate, it is necessary to add 0.7 g disodium hydrogen phosphate (Na2HPO4.2H2O) before distillation for each gram of sample present in the aliquot part, to prevent the formation of calcium hydroxide.
    Taking the necessary precautions to avoid loss of ammonia, add to the flask about 30 ml of 30% sodium hydroxide solution (4.9) and finally, in the case of acid-soluble samples, an additional quantity sufficient to neutralise the quantity of hydrochloric acid (4.1) present in the aliquot part taken for the analysis. Connect the distillation flask to the apparatus, ensuring the tightness of connections. Carefully swirl the flask to mix the contents.

    Warm gently, so that the release of hydrogen decreases appreciably over about half an hour and the liquid begins to boil. Continue the distillation, increasing the heat so that at least 200 ml of liquid distils in about 30 minutes. (Do not prolong the distillation beyond 45 minutes.)

    When the distillation is complete, disconnect the receiving flask from the apparatus, carefully wash the extension tube and bubble trap, collecting the rinsings in the titration flask. Titrate the excess acid according to the procedure in Method 2.
        7.3    Blank test
    Carry out a blank test under the same conditions omitting only the sample and allow for this in the calculation of the final results.
        7.4    Control test
    Before carrying out the analysis, check that the apparatus is working properly and that the correct application of the method is used, by taking an aliquot portion of a freshly prepared solution of sodium nitrate (4.12) containing, according to the variant chosen, 0.050 g to 0.150 g.
        8.    EXPRESSION OF RESULTS
    Express the results of analysis as a percentage of nitric nitrogen, or combined ammoniacal and nitric nitrogen, contained in the fertiliser as received for analysis.

4a.

DETERMINATION OF TOTAL NITROGEN IN CALCIUM CYANAMIDE — IN THE ABSENCE OF NITRATE
        1.    SCOPE
    This method is for the determination of total nitrogen in nitrate-free calcium cyanamide.
        2.    FIELD OF APPLICATION
    Exclusively to calcium cyanamide (nitrate free).
        3.    PRINCIPLE
    After digestion using the Kjeldahl method, the ammoniacal nitrogen formed is displaced by sodium hydroxide and collected in a standard solution of sulfuric acid. The excess sulfuric acid is titrated with a standard solution of sodium or potassium hydroxide.
        4.    REAGENTS
4.1 Sulfuric acid solution 50% (V/V): dilute an appropriate volume of sulfuric acid (p=1.84 g/ml) with an equal volume of water.
4.2 Potassium sulfate.
4.3 Copper oxide (CuO), 0.3 - 0.4 g for each determination or an equivalent quantity of copper sulfate pentahydrate (0.95 to 1.25g) for each determination.
4.4 Sodium hydroxide solution 30g per 100 ml, ammonia free.
4.5 Sulfuric acid, 0.05 M solution for variant (a) (see page 16)
4.6 Sodium or potassium hydroxide, 0.1 M solution, carbonate free
4.7 Sulfuric acid, 0.1 M solution for variant (b) (see page 16)
4.8 Sodium or potassium hydroxide, 0.2 M solution, carbonate free
4.9 Sulfuric acid, 0.25 M solution for variant (c) (see page 16)
4.10 Sodium or potassium hydroxide, 0.5 M solution, carbonate free
4.11 Indicator solutions:
4.11.1 Mixed indicator:
Solution A: dissolve 1 g methyl red in 37 ml 0.1 M 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 A with 2 volumes of B.
This indicator is violet in acid solution, grey in neutral solution and green in alkaline solution. Use 0.5 ml (10 drops).
4.11.2 Methyl red indicator:
Dissolve 0.1 g methyl red in 50 ml 95% ethanol, make up to 100 ml with water and filter if necessary. This indicator (4 to 5 drops) may be used instead of the preceding one.
4.12 Anti-bump granules of pumice stone, washed in hydrochloric acid and ignited.
4.13 Potassium thiocyanate.
        5.    APPARATUS
        5.1    Distillation apparatus. See Method 2.
        6.    PREPARATION OF SAMPLE
    See Method 1.
        7.    PROCEDURE
        7.1    Preparation of the solution
    Weigh to the nearest 0.001 g, 1 g of the prepared sample and place it in the Kjeldahl flask. Add 50 ml 50% sulfuric acid (4.1), 10-15 g potassium sulfate (4.2) and one of the prescribed catalysts (4.3). Heat slowly to drive off the water, boil gently for two hours, allow to cool, and dilute with 100-150 ml water.

    Cool again, transfer the suspension quantitatively to a 250 ml graduated flask, make up to volume with water, shake and filter through a dry filter into a dry flask. Discard the first portion of the filtrate.
        7.2    Determination
    According to the variant chosen (see Method 2) transfer with a pipette 50, 100 or 200 ml of the solution to the distillation apparatus and add sufficient sodium hydroxide solution (4.4) to ensure a considerable excess. Distil the ammonia and titrate the excess acid as described in Method 2.
        7.3    Blank test
    Make a blank test (omitting only the sample) under the same conditions and allow for this in the calculation of the final result.
        7.4    Control test
    Before carrying out the analysis, check that the apparatus is working properly and that the correct application of the method is used, by taking an aliquot portion of a standard solution of potassium thiocyanate (4.13), approximating to the concentration of nitrogen in the sample.
        8.    EXPRESSION OF RESULT
    The result of the analysis must be expressed as the percentage of nitrogen (N) contained in the fertiliser as received for analysis.
    Variant (a): N%=(50 - A) × 0.7
    Variant (b): N%=(50 - A) × 0.7
    Variant (c): N%=(35 - A) × 0.875
    Where A = millilitres of sodium or potassium hydroxide used for the titration.

4b.

DETERMINATION OF TOTAL NITROGEN IN CALCIUM CYANAMIDE — IN THE PRESENCE OF NITRATE
        1.    SCOPE
    This method is for the determination of total nitrogen in calcium cyanamide.
        2.    FIELD OF APPLICATION
    The method is applicable to calcium cyanamide containing nitrates.
        3.    PRINCIPLE
    The direct application of Kjeldahl's method cannot be applied to calcium cyanamides containing nitrates. For this reason the nitric nitrogen is reduced to ammonia with metallic iron and stannous chloride before Kjeldahl digestion. The ammoniacal nitrogen is then determined as in Method 4a.
        4.    REAGENTS
4.1 Sulfuric acid (p=1.84 pg/ml).
4.2 Powdered iron reduced in hydrogen.
4.3 Potassium sulfate, finely pulverised.
4.4 Sulfuric acid, 0.05 M solution for variant (a) (see page 16)
4.5 Sodium or potassium hydroxide, 0.1 M solution, carbonate free
4.6 Sulfuric acid, 0.1 M solution for variant (b) (see page16)
4.7 Sodium or potassium hydroxide, 0.2 M solution, carbonate free
4.8 Sulfuric acid, 0.25 M solution for variant (c) (see page 16)
4.9 Sodium or potassium hydroxide, 0.5 M solution, carbonate free
4.10 Indicator solutions:
4.10.1 Mixed indicator:
Solution A: dissolve 1 g methyl red in 37 ml 0.1 M 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 A with 2 volumes of B.
This indicator is violet in acid solution, grey in neutral solution and green in alkaline solution. Use 0.5 ml (10 drops) of this indicator solution.
4.10.2 Methyl red indicator: Dissolve 0.1 g methyl red in 50 ml 95% ethanol, make up to 100 ml with water and filter if necessary. This indicator (4 to 5 drops) may be used instead of the preceding one.
4.11 Solution of stannous chloride:
Dissolve 120 g of stannous chloride (SnCl2.2H2O) in 400 ml concentrated hydrochloric acid (p=1.18 g/ml) and make up to 1 litre with water. The solution must be completely clear and prepared immediately before use.
It is essential to check the reducing power of the stannous chloride. Dissolve 0.5 g of stannous chloride in 2 ml concentrated hydrochloric acid (p=1.18 g/ml) and make up to 50 ml with water. Then add 5 g of Rochelle salt (potassium sodium tartrate) and a sufficient quantity of sodium bicarbonate for the solution to show an alkaline reaction to a litmus paper test.
Titrate with 0.1 M iodine solution in the presence of a starch solution as an indicator.
1 ml of 0.1 M iodine solution corresponds to 0.01128 g SnCl2.2H2O.
At least 80% of the total tin present in the solution thus prepared must be in the bivalent form. For the titration at least 35 ml of 0.1 M iodine solution should be used.
4.12 Sodium hydroxide solution, 30 g per 100 ml, ammonia free.
4.13 Standard nitrate-ammoniacal solution:
Weigh out 2.500 g of potassium nitrate and 10.160 g of ammonium sulfate into a 250 ml graduated flask. Dissolve in water and make up to 250 ml. 1 ml of this solution contains 0.010 g of nitrogen.
4.14 Anti-bump granules of pumice stone, washed in hydrochloric acid and ignited.
        5.    APPARATUS
    Distillation apparatus. See Method 2.
        6.    PREPARATION OF THE SAMPLE
    See Method 1.
        7.    PROCEDURE
        7.1    Preparation of the solution
    Weigh to the nearest 0.001 g,1 g of the prepared sample into the Kjeldahl flask. Add 0.5 g of powdered iron (4.2) and 50 ml of the stannous chloride solution (4.11), stir and leave standing for half an hour. During the time it is left standing, stir again after 10 and 20 minutes. Then add 10 g of potassium sulfate (4.3) and 30 ml of sulfuric acid (4.1). Boil and continue for an hour after the appearance of white fumes. Leave to cool and dilute with 100-150 ml of water. Transfer the suspension quantitatively into a 250 ml graduated flask, cool and make up to volume with water, mix and filter through a dry paper into a dry container. Discard the first portion of the filtrate.
        7.2    Determination
    According to the variant chosen (see Method 2) transfer with a pipette 50, 100 or 200 ml of the solution to the distillation apparatus and add sufficient sodium hydroxide solution (4.12) to ensure a considerable excess. Distil the ammonia and titrate the excess acid as described in Method 2.
        7.3    Blank test
    Make a blank test (omitting only the sample) under the same conditions and allow for this in the calculation of the final result.
        7.4    Control test
    Before carrying out the analysis, check that the apparatus is working properly and that the correct application of the method is used with a standard solution containing quantities of ammoniacal and nitrate nitrogen comparable to the quantities of cyanamide and nitrate nitrogen contained in nitrated calcium cyanamide.
        8.    EXPRESSION OF RESULT
    The result of the analysis must be expressed as the percentage of total nitrogen (N) contained in the fertiliser as received for analysis.

    Variant (a): N%=(50 - A) × 0.7

    Variant (b): N%=(50 - A) × 0.7

    Variant (c): N%=(35 - A) × 0.875

    Where A = millilitres of sodium or potassium hydroxide used for the titration.

5.

DETERMINATION OF TOTAL NITROGEN IN UREA
        1.    SCOPE
    This method is for the determination of total nitrogen in urea.
        2.    FIELD OF APPLICATION
    The method is applicable exclusively to urea fertilisers which are nitrate free.
        3.    PRINCIPLE
    Urea is transformed quantitatively into ammonia by boiling in the presence of sulfuric acid. The ammonia thus obtained is distilled from an alkaline medium and collected in an excess of standard sulfuric acid. The excess acid is titrated by means of a standard alkaline solution.
        4.    REAGENTS
4.1 Sulfuric acid, concentrated (p = 1.84 g/ml).
4.2 Sodium hydroxide solution, 30 g per 100 ml, ammonia free.
4.3 Sulfuric acid, 0.05 M solution for variant (a) (see page 16)
4.4 Sodium or potassium hydroxide, 0.1 M solution, carbonate free
4.5 Sulfuric acid, 0.1 M solution for variant (b) (see page 16)
4.6 Sodium or potassium hydroxide, 0.2 M solution, carbonate free
4.7 Sulfuric acid, 0.25 M solution for variant (c) (see page 16)
4.8 Sodium or potassium hydroxide, 0.5 M solution, carbonate free
4.9 Indicator solutions:
4.9.1 Mixed indicator:
Solution A: dissolve 1 g methyl red in 37 ml 0.1 M 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 A with 2 volumes of B.
This indicator is violet in acid solution, grey in neutral solution and green in alkaline solution. Use 0.5 ml (10 drops).
4.9.2 Methyl red indicator:
Dissolve 0.1 g methyl red in 50 ml 95% ethanol, make up to 100 ml with water. Filter if necessary. This indicator (4 to 5 drops) may be used instead of the preceding one.
4.10 Anti-bump granules of pumice stone, washed in hydrochloric acid and ignited.
4.11 Urea.
        5.    APPARATUS
        5.1    Distillation apparatus. See Method 2.
        6.    PREPARATION OF THE SAMPLE
    See Method 1.
        7.    PROCEDURE
        7.1    Preparation of the solution
    Weigh to the nearest 0.001 g, 2.5 g of the prepared sample into a 300 ml Kjeldahl flask and moisten with 20 ml water. Add with care 20 ml concentrated sulfuric acid (4.1) and a few anti-bump granules (4.10). To prevent splashing, place a long-stemmed glass funnel in the neck of the flask. Heat slowly at first, then increase the heat until white fumes are observed (30 - 40 minutes).
    Cool and dilute with 100 - 150 ml water. Transfer quantitatively to a 500 ml graduated flask, discarding any sediment. Allow to cool to room temperature. Make up to volume with water, mix and, if necessary, filter through a dry paper into a dry receptacle. Discard the first portion of the filtrate.
        7.2    Determination
    According to the variant chosen (see Method 2) transfer with a pipette 25, 50 or 100 ml of the solution to the distillation apparatus and add sufficient sodium hydroxide solution (4.2) to ensure a considerable excess. Distil the ammonia and titrate the excess acid as described in Method 2.
        7.3    Blank test
    Carry out a blank test (omitting only the sample) under the same conditions and allow for this in the calculation of the final result.
        7.4    Control test
    Before carrying out the analysis, check that the apparatus is working properly and that the correct application of the method is used, with an aliquot portion of a freshly prepared solution of urea (4.11).
        8.    EXPRESSION OF RESULT
    Express the result as the percentage of total nitrogen (N) contained in the fertiliser as received for analysis.

    Variant (a): N%=(50 - A) × 1.12

    Variant (b): N%=(50 - A) × 1.12

    Variant (c): N%=(35 - A) × 1.40

    Where A = millilitres of sodium or potassium hydroxide used for the titration.

6.

DETERMINATION OF CYANAMIDE NITROGEN
        1.    SCOPE
    This method is for the determination of cyanamide nitrogen.
        2.    FIELD OF APPLICATION
    Calcium cyanamide and calcium cyanamide/nitrate mixtures.
        3.    PRINCIPLE
    Cyanamide nitrogen is precipitated as a silver complex and estimated in the precipitate by Kjeldahl's method.
        4.    REAGENTS
4.1 Glacial acetic acid.
4.2 Ammonia solution: dilute one volume of ammonia (p=0.88 pg/ml) with 3 volumes of water.p
4.3 Ammoniacal silver solution, according to Tollens, freshly prepared: mix 500 ml silver nitrate solution (10 g per 100 ml) with 500 ml ammonia solution (4.2).
Do not expose unnecessarily to light, heat or air.
Safety precaution: when handling ammoniacal silver nitrate solution, safety goggles must be worn.
4.4 Concentrated sulfuric acid (p=1.84 g/ml).
4.5 Potassium sulfate.
4.6 Copper oxide (CuO), 0.3 - 0.4 g for each determination or an equivalent quantity of copper sulfate pentahydrate (0.95 - 1.25 g) for each determination.
4.7 Sodium hydroxide solution, 30 g per 100 ml, ammonia free.
4.8 Sulfuric acid, 0.05 M solution.
4.9 Sodium or potassium hydroxide, 0.1 M solution.
4.10 Indicator solutions:
4.10.1 Mixed indicator:
Solution A: dissolve 1 g methyl red in 37 ml 0.1 M 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 A with 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).
4.10.2 Methyl red indicator:
Dissolve 0.1 g methyl red in 50 ml 95% ethanol, make up to 100 ml with water. Filter if necessary. This indicator (4 to 5 drops) may be used instead of the preceding one.
4.11 Anti-bump granules of pumice stone, washed in hydrochloric acid and ignited.
4.12 Potassium thiocyanate.
        5.    APPARATUS
        5.1    Distillation apparatus. See Method 2.
        5.2    
    500 ml graduated flask (e.g. Stohmann).
        5.3    
    Rotary shaker, 35 - 40 turns per minute.
        6.    PREPARATION OF THE SAMPLE
    See Method 1.
        7.    PROCEDURE
        7.1    Preparation of the solution for analysis
    Weigh, to the nearest 0.001 g, 2.5 g of the prepared sample into a small glass mortar. Grind the sample three times with water, pouring off the water after each grinding into the 500 ml graduated flask (5.2). Transfer the sample quantitatively into the flask, washing the mortar, pestle and funnel with water. Make up with water to approximately 400 ml. Add 15 ml acetic acid (4.1). Shake on the rotary shaker (5.3) for two hours.

    Make up to 500 ml with water, mix and filter. Discard the first portion of the filtrate.

    Proceed immediately to 7.2.
        7.2    Determination
    Transfer 50.0 ml of the filtrate to a 250 ml beaker. Add ammonia solution (4.2) until slightly alkaline and add 30 ml warm ammoniacal silver nitrate (4.3) to precipitate the yellow silver complex of cyanamide. Leave overnight, filter and wash the precipitate with cold water until completely free of ammonia.

    Place the filter paper and the precipitate, still moist, in a Kjeldahl flask, add 10 - 15 g potassium sulfate (4.5), the catalyst (4.6) in the prescribed proportion, then 50 ml water and 25 ml concentrated sulfuric acid (4.4). Warm the flask slowly, whilst shaking it gently until the contents come to the boil. Increase the heat, boil until the contents of the flask become either colourless or pale green. Continue boiling for one hour, then leave to cool.

    Transfer the liquid quantitatively from the Kjeldahl flask to the distillation flask, add a few anti-bump granules of pumice stone (4.11) and make up with water to a total volume of approximately 350 ml. Mix and cool. Add sufficient sodium hydroxide solution (4.7) to ensure a considerable excess.

    Distil the ammonia and titrate the excess acid as described in Method 2 (variant (a)).
        7.3    Blank test
    Make a blank test (omitting only the sample) under the same conditions and allow for this in the calculation of the final result.
        7.4    Control test
    Before carrying out the analysis, check that the apparatus is working properly and that the correct application of the method is used, with an aliquot portion of a standard solution of potassium thiocyanate (4.12), corresponding to 0.05 g of nitrogen.
        8.    EXPRESSION OF RESULT
    Express the result as the percentage of cyanamide nitrogen contained in the fertiliser as received for analysis.

    N%=(50 - A) × 0.56

    Where A = millilitres of sodium or potassium hydroxide used for the titration.

7.

DETERMINATION OF BIURET IN UREA
        1.    SCOPE
    This method is for the determination of biuret in urea.
        2.    FIELD OF APPLICATION
    The method is applied exclusively to urea.
        3.    PRINCIPLE
    In an alkaline medium, in the presence of potassium sodium tartrate, biuret and bivalent copper form a violet cupric compound, the absorbance of which is measured at 546 nm.
        4.    REAGENTS
4.1 Methanol.
4.2 Sulfuric acid solution, approximately 0.05 M.
4.3 Sodium hydroxide solution, approximately 0.1 M.
4.4 Alkaline solution of potassium sodium tartrate:
In a 1 litre graduated flask dissolve 40 g of sodium hydroxide in 500 ml of water and leave to cool. Add 50 g of potassium sodium tartrate (KNaC4H4O6.4H2O). Make up to the mark and mix. Leave standing 24 hours before use.
4.5 Copper sulfate solution:
In a 1 litre graduated flask dissolve 15 g of copper sulfate (CuSO4.5H2O) in 500 ml of water. Make up to the mark and mix.
4.6 Biuret standard solution:
In a 250 ml graduated flask, dissolve 0.250 g of pure biuret[8] in water. Make up to the mark and mix. 1 ml of this solution contains 0.001 g of biuret. This solution should be freshly prepared.
4.7 Methyl red indicator solution:
Dissolve 0.1 g methyl red in 50 ml 95% ethanol and make up to 100 ml with water. Filter if necessary.
        5.    APPARATUS
        5.1    Spectrophotometer.
        6.    PREPARATION OF SAMPLE
    See Method 1.
        7.    PROCEDURE
        7.1    Preparation of the standard curve
    Transfer 2, 5, 10, 20, 25 and 50 ml aliquot portions of biuret standard solution (4.6) into a series of six 100 ml graduated flasks. Make up the volumes to about 50 ml with water, add one drop of indicator solution (4.7) and neutralise, if necessary, with 0.05 M sulfuric acid (4.2). Add the swirling 20.0 ml of the alkaline tartrate solution (4.4) and then 20.0 ml copper sulfate solution (4.5). Make up to the mark with water, mix and allow to stand at 30+ 2 C for fifteen minutes.

    At the same time prepare a reagent blank as follows. Place 50 ml water in a 100 ml graduated flask and proceed as described above from "... add one drop of indicator solution ...".

    Measure the absorbance of each solution at 546 nm against the reagent blank as reference, using cells of suitable path length. Plot the calibration curve, using the absorbances as the ordinates and the corresponding quantities of biuret in milligrams, as the abscissae.
        7.2    Preparation of solution for analysis
    Weigh to the nearest 0.001g, 10 g of the prepared sample; dissolve in about 150 ml of water in a 250 ml graduated flask and make up to the mark and mix. Filter if necessary.

      Note 1:  If the sample for analysis contains more than 0.015 g of ammoniacal nitrogen, dissolve in 50 ml methanol (4.1) in a 250 ml beaker. Reduce by evaporation to a volume of about 25 ml. Transfer quantitatively to a graduated 250 ml flask. Make up to the mark with water. Filter, if necessary, through a dry fluted paper into a dry receiver.

      Note 2:  Elimination of the opalescence: if any colloidal substance is present difficulties may arise during filtration. In that case the solution for analysis is prepared as follows: dissolve the sample in 150 ml of water, add 2 ml 1 M hydrochloric acid, and filter the solution into a 250 ml graduated flask. Wash the filters with water and make up to volume. Continue the process according to the method described in 7.3.
        7.3    Determination
    According to the presumed biuret content, transfer with a pipette 25 or 50 ml from the solution prepared in 7.2, to a 100 ml graduated flask and neutralise if necessary with 0.05 M sulfuric acid or sodium hydroxide solution (4.2 or 4.3) as required, using methyl red indicator (4.7). Add 20.0 ml of the alkaline solution of potassium sodium tartrate (4.4) and 20.0 ml of the copper solution (4.5). Make up to volume, mix thoroughly and leave standing for 15 minutes at 30 C+2. Measure the absorbance of the solution as described in 7.1.
        8.    EXPRESSION OF RESULTS
    % biuret =
    C × 2.5
    V
    where:

      C = mass, in mg, of biuret read from the standard curve;
      V = volume of the aliquot used for the determination.

8a.

DETERMINATION OF DIFFERENT FORMS OF NITROGEN IN THE SAME SAMPLE — IN THE PRESENCE OF CYANAMIDE NITROGEN
        1.    SCOPE
    This method is for the determination of any one form of nitrogen in the presence of any other form.
        2.    FIELD OF APPLICATION
    Any fertiliser in Group 1(a) of Section A, and Groups 1, 2 and 3 of Section B of the Table in Schedule 1 of the Fertilisers Regulations 1991 containing nitrogen in various forms.
        3.    PRINCIPLE
        3.1    Total soluble and insoluble nitrogen

        3.1.1  In the absence of nitrates, the sample is subjected to direct Kjeldahl digestion.

        3.1.2  In the presence of nitrates, the sample is subjected to Kjeldahl digestion after reduction with the aid of metallic iron and stannous chloride. In both cases, the ammonia is determined according to Method 2.

      Note:  If analysis shows an insoluble nitrogen content of more than 0.5%, it is presumed that the fertiliser contains other forms of insoluble nitrogen not specified for fertilisers covered by the list in paragraph 2.

        3.2    Forms of soluble nitrogen
    The following are determined from different aliquot parts taken from the same solution of the sample:

        3.2.1  Total soluble nitrogen

      3.2.1.1  In the absence of nitrates, by direct Kjeldahl digestion.

      3.2.1.2  In the presence of nitrates, by Kjeldahl digestion on an aliquot portion taken from the solution after reduction according to Ulsch, the ammonia being determined in both cases as described in Method 2.

        3.2.2  Total soluble nitrogen with the exception of nitric nitrogen, by Kjeldahl digestion after elimination in an acid medium of nitric nitrogen with ferrous sulfate, the ammonia being determined as described in Method 2.

        3.2.3  Nitric nitrogen by difference

      3.2.3.1  In the absence of calcium cyanamide, between (3.2.1.2) and (3.2.2) or between total soluble nitrogen (3.2.1.2) and the sum of ammoniacal nitrogen and ureic nitrogen (3.2.4+3.2.5).

      3.2.3.2  In the presence of calcium cyanamide, between (3.2.1.2) and (3.2.2) and between (3.2.1.2) and the sum of (3.2.4+3.2.5+3.2.6).

        3.2.4  Ammoniacal nitrogen

      3.2.4.1  Solely in the presence of ammoniacal nitrogen and ammoniacal + nitric nitrogen, by applying Method 2.

      3.2.4.2  In the presence of ureic nitrogen and/or cyanamide nitrogen, by cold distillation after making slightly alkaline, the ammonia being absorbed in a standard solution of sulfuric acid and determined as described in Method 2.

        3.2.5  Ureic nitrogen
      Either

        3.2.5.1  By conversion using urease into ammonia which is titrated with a standard solution of hydrochloric acid,
          or:

        3.2.5.2  By gravimetry with xanthydrol, although biuret will also be precipitated by xanthydrol, this should not give rise to a significant error in the determination since its level is generally low in absolute value in compound fertilisers.
          or:

        3.2.5.3  By difference, according to the following table:
        Case Nitric Nitrogen Ammoniacal Nitrogen Cyanamide Nitrogen Difference
        1 Absent Present Present (3.2.1.1)-(3.2.4.2+3.2.6)
        2 Present Present Present (3.2.2)-(3.2.4.2+3.2.6)
        3 Absent Present Absent (3.2.1.1)-(3.2.4.2)
        4 Present Present Absent (3.2.2)-(3.2.4.2)

        3.2.6  Cyanamide nitrogen, by precipitation as a silver compound, the nitrogen being estimated in the precipitate by the Kjeldahl method.
        4.    REAGENTS
        4.1    Potassium sulfate.
        4.2    Iron powder, reduced with hydrogen (the prescribed quantity of iron must be able to reduce at least 50 mg of nitric nitrogen).
        4.3    Potassium thiocyanate.
        4.4    Potassium nitrate.
        4.5    Ammonium sulfate.
        4.6    Urea.
        4.7    Sulfuric acid solution: dilute an appropriate volume of sulfuric acid (p=1.84 g/ml) with an equal volume of water.
        4.8    Sulfuric acid, 0.1 M solution.
        4.9    Sodium hydroxide solution, 30 g per 100 ml, ammonia free.
        4.10    Sodium or potassium hydroxide, 0.2 M solution, free from carbonates.
        4.11    Stannous chloride solution:

    Dissolve 120 g of stannous chloride (SnCl2.2H2O) in 400 ml of concentrated hydrochloric acid (p=1.18 g/ml) and make up to 1 litre with water. The solution must be perfectly clear and prepared immediately before use.

    It is essential to check the reducing power of the stannous chloride: dissolve 0.5 g of stannous chloride in 2 ml of concentrated hydrochloric acid (p=1.18 g/ml) and make up to 50 ml with water. Then add 5 g of Rochelle salt (potassium sodium tartrate) and a sufficient quantity of sodium bicarbonate for the solution to be alkaline to litmus paper.

    Titrate with 0.1 M iodine solution in the presence of a starch solution as an indicator.

    1 ml of 0.1 M iodine solution corresponds to 0.01128 g of SnCl2.2H2O.

    At least 80% of the total tin present in the solution thus prepared must be in bivalent form. For the titration, at least 35 ml of 0.1 M iodine solution must therefore be used.
        4.12    Sulfuric acid, concentrated (p=1.84 g/ml).
        4.13    Hydrochloric acid solution: dilute an appropriate volume of hydrochloric acid (p=1.18 g/ml) with an equal volume of water.
        4.14    Glacial acetic acids.
        4.15    Sulfuric acid solution, approximately 30% (W/V) H2SO4.
        4.16    Ferrous sulfate, crystalline, FeSO4.7H2O
        4.17    Sulfuric acid, 0.05 M solution.
        4.18    Octan-1-ol.
        4.19    Potassium carbonate, saturated solution.
        4.20    Sodium or potassium hydroxide, 0.1 M solution, free from carbonate.
        4.21    Barium hydroxide, saturated solution.
        4.22    Sodium carbonate solution, 10 g per 100 ml.
        4.23    Hydrochloric acid, 2 M solution.
        4.24    Hydrochloric acid, 0.1 M solution.
        4.25    Urease solution: Suspend 0.5 g of active urease in 100 ml of distilled water. Using 0.1 M hydrochloric acid (4.24), adjust the pH to 5.4, measured by pH meter.
        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 sulfate pentahydrate (0.95 to 1.25 g) per determination.
        4.28    Anti-bump granules washed in hydrochloric acid and ignited.
        4.29    Indicator solutions:

        4.29.1  Mixed indicator solution: Solution A: dissolve 1 g of methyl red in 37 ml of 0.1 M sodium hydroxide solution and make up to one litre with water. Solution B: dissolve 1 g of methylene blue in water and make up to one litre. Mix one volume of solution A and 2 volumes of solution B. This indicator is violet in acid solution, grey in neutral solution and green in alkaline solution. Use 0.5 ml (10 drops) of this indicator solution.

        4.29.2  Methyl red indicator solution: Dissolve 0.1 g of methyl red in 50 ml of 95% ethanol, make up to 100 ml with water and filter if necessary. This indicator (4 to 5 drops) can be used instead of the previous one.
        4.30    Indicator papers: Litmus, bromothymol blue (or other papers sensitive in the range pH 6 to 8).
        4.31    Ethanol or methanol: solution 95%.
        5.    APPARATUS
        5.1    Distillation apparatus. See Method 2.
        5.2    Apparatus for the determination of ammoniacal nitrogen 7.2.5.3. An example of the recommended apparatus is reproduced in Figure 6 in the Appendix.

    The apparatus is made up of a specially shaped receptacle with a ground glass neck, a side neck, a connecting tube with a splash head and a perpendicular tube for the introduction of air. The tubes can be connected to the receptacle by means of a simple perforated rubber bung. It is important to give a suitable shape to the end of the tubes introducing air, since the bubbles of gas must be evenly distributed throughout the solutions contained in the receptacle and the absorber. The best arrangement consists of small mushroom-shaped pieces with an external diameter of 20 mm and six openings of 1 mm around the periphery.
        5.3    Apparatus for the estimation of urea nitrogen (7.2.6.1).

    It consists of a 300 ml Erlenmeyer flask, with a separating funnel and a small absorber. An example of the recommended apparatus is reproduced in Figure 7 in the Appendix.
        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.
        6.    PREPARATION OF THE SAMPLE
    See Method 1.
        7.    PROCEDURE
        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 not more than 100 mg of nitrogen. Place in the flask of the distillation apparatus (5.1). Add 10 to 15 g of potassium sulfate (4.1), the prescribed quantity of catalyst (4.27), and a few anti-bump granules (4.28). Then add 50 ml of dilute sulfuric acid (4.7), and mix thoroughly. First heat gently, mixing from time to time, until foaming ceases. Then heat so that the liquid boils steadily and keep it boiling for one hour after the solution has become clear, preventing any organic matter from sticking to the sides of the flask. Allow to cool. Carefully add about 350 ml of water, with mixing. Ensure that the dissolution is as complete as possible. Allow to cool and connect the flask to the distillation apparatus (5.1).

      7.1.1.2  Distillation of ammonia
        Transfer with a pipette 50 ml of standard 0.1 M sulfuric acid (4.8) into the receiver of the apparatus. Add the indicator (4.29.1 or 4.29.2). Ensure that the tip of the condenser is at least 1 cm below the level of the solution.
        Taking the necessary precautions to avoid any loss of ammonia, carefully add to the distillation flask enough of the concentrated sodium hydroxide solution (4.9) to make the liquid strongly alkaline (120 ml is generally sufficient: check by adding a few drops of phenolphthalein. At the end of the distillation the solution in the flask must still be clearly alkaline). Adjust the heating of the flask so as to distil 150 ml in half an hour. Test with indicator paper (4.30) that the distillation has been completed. If it has not, distil a further 50 ml and repeat the test until the supplementary distillate reacts neutrally to the indicator paper (4.30). Then lower the receiver, distil a few ml more and rinse the tip of the condenser. Titrate the excess acid with a standard solution of potassium or sodium hydroxide 0.2 M (4.10) to the end point of the indicator.

      7.1.1.3  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.

        7.1.14  Expression of the result
    % N =
    (a - A) × 0.28
    M
    where:

      a = ml of standard solution of sodium or potassium hydroxide (0.2 M) used for the blank.
      A = ml of standard solution of sodium or potassium hydroxide (0.2 M) used for the analysis.
      M = mass of the sample in grams.


        7.1.2  In the presence of nitrate

      7.1.2.1  Test sample
        Weigh to the nearest 0.001g, 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 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 sulfuric acid (4.12), 5 g of potassium sulfate (4.1), the prescribed quantity of catalyst (4.27) and some anti-bump granules (4.28). Heat gently with the flask slightly tilted. Increase the heat slowly and swirl the solution frequently to keep the mixture suspended; the liquid darkens and then clears with the formation of a yellow-green anhydrous iron sulfate suspension. After obtaining a clear solution simmer for one hour. Leave to cool. Cautiously take up the contents of the flask in a little water and add little by little 100 ml of water. Mix and transfer the contents of the flask into a 500 ml graduated flask. Rinse the flask several times with distilled water. Make up the volume with water and mix. Filter through a dry paper into a dry receiver. Discard the first portion of the filtrate.

      7.1.2.4  Distillation of ammonia
        Transfer into the flask of the distillation apparatus (5.1), an aliquot part containing not more than 100 mg of nitrogen. Dilute to about 350 ml with distilled water, add a few anti-bump granules (4.28), connect the flask to the distillation apparatus and continue the determination as described in paragraph 7.1.1.2.

      7.1.2.5  Blank test
        See 7.1.1.3.

      7.1.2.6  Expression of the result
      % N =
      (a - A) × 0.28
      M
      where:

        a = ml of standard solution of sodium or potassium hydroxide (0.2 M) used for the blank.
        A = ml of standard solution of sodium or potassium hydroxide (0.2 M) used for the analysis.
        M = mass of the sample, expressed in grams, present in the aliquot part taken for a analysis.

        7.2    Forms of soluble nitrogen

        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
      % N =
      (a - A) × 0.28
      M
      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
        % N =
        (a - A) × 0.28
        M
        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
      %N (ammoniacal) =
      (a - A) × 0.28
      M
      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
      % N =
      (a - A) × 0.14