SCHEDULE 2METHODS OF ANALYSIS
8b.DETERMINATION OF DIFFERENT FORMS OF NITROGEN IN THE SAME SAMPLE — IN THE ABSENCE OF CYANAMIDE NITROGEN
1. SCOPE
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.
2. FIELD OF APPLICATION
This method is applicable to all fertilisers in Group l(a) of Section A and Groups 1, 2 and 3 of Section B of the Table in Schedule 1 of the Fertilisers Regulations (Northern Ireland) 1990(1) which contain exclusively nitric, ammoniacal or ureic nitrogen.
3. PRINCIPLE
The following determinations are made on different portions of a single sample solution.
Total soluble nitrogen
3.1—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 Urea 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) |
4. REAGENTS
Potassium sulphate.
Iron powder, reduced with hydrogen (the prescribed quantity of iron must be able to reduce at least 50 mg nitric nitrogen).
Potassium nitrate.
Ammonium sulphate.
Urea.
Sulphuric acid, 0.2 N solution.
Sodium hydroxide solution, 30 g per 100 ml, ammonia free.
Sodium or potassium hydroxide, 0.2 N solution, free of carbonates.
Sulphuric acid (d = 1.84 g/ml).
Hydrochloric acid solution: dilute an appropriate volume of hydrochloric acid (d = 1.18 g/ml) with an equal volume of water.
Glacial acetic acid.
Sulphuric acid, solution approximately 30% (W/V) H2SO4.
Ferrous sulphate, crystalline FeSO4.7H2O.
Sulphuric acid, 0.1 N solution.
Octal-l-ol.
Potassium carbonate, saturated solution.
Sodium or potassium hydroxide, 0.1 N solution.
Barium hydroxide, saturated solution.
Sodium carbonate solution, 10 g per 100 ml.
Hydrochloric acid, 2 N solution.
Hydrochloric acid, 0.1 N solution.
Urease solution: suspend 0.5 g active unease in 100 ml distilled water. Using 0.1 N hydrochloric acid (4.21), adjust pH to 5.4, measured with pH meter.
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.
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.
Anti-bump granules of pumice stone washed with hydrochloric acid and ignited.
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.
4.26.2 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; 4 — 5 drops of this indicator can be used instead of the previous one.
Indicator papers: litmus, bromothymol blue (or other papers sensitive to pH 6-8).
Ethanol or methanol, 95% (V/V).
5. APPARATUS
Distillation apparatus. See Method 2.
Apparatus for determination of ammoniacal nitrogen. An example of recommended apparatus is reproduced in Figure 6 in the Appendix.
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.
Rotary shaker: 35 — 40 turns per min.
pH meter.
Sintered glass crucibles, diameter of pores 5 to 15 microns.
6. PREPARATION OF SAMPLE
See Method 1.
7. PROCEDURE
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.
Total nitrogen
In the absence of nitrates
7.2—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, 7.1.1.2.
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 stage of the analysis, the total quantity of nitrogen is unimportant. Add 10 ml 30% sulphuric acid (4.12), 5 g reduced iron (1.2), 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 least 3 hours at ambient temperature. Transfer the liquid quantitatively to a 250 ml 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
where:
a = ml of standard solution of sodium or potassium hydroxide (0.2 N) used for the blank, carried out 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.
where:
a = ml of standard solution of sodium or potassium hydroxide (0.2 N) used for the blank, carried out 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 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
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.
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.
7.4 Nitric nitrogen is obtained: by difference between
(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
In the presence of ureic nitrogen
7.5—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-l-o 1 (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 it through washing flasks containing dilute sulphuric acid and dilute sodium hydroxide. Instead of using air under pressure, a vacuum may be used (water pump) provided that the connections between the apparatus are air tight. The liberation 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
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.
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 nitrogen
Urease method
7.6—7.6.1 Transfer by pipette into a 500 ml graduated flask, a portion of filtrate (7.1) containing not more than 250 mg 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). Introduce through the separating funnel, 20 ml urease solution (4.22). Allow to stand for one hour at 20 — 25°C. Place 25 .O ml of the standard 0.1 N hydrochloric acid solution (4.2) in the dropping funnel, allow to run into the L solution, then rinse with a little water. Transfer quantitatively the contents of the bubble trap to the solution contained in the Erlenmeyer. 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
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.
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 a 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
where:
m = weight of the precipitate in grams.
M = weight of the sample, in grams, present in the aliquot part taken for analysis.
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:
| Case | Nitric N | Ammoniacal N | Ureic N |
|---|---|---|---|
| 1 | Absent | Present | (7.2.4)–(7.5.3) |
| 2 | Present | Present | (7.3.3)–(7.5.3) |
8. VERIFICATION OF RESULTS
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 solution of potassium nitrate (4.3), ammonium sulphate (4.4) and urea (4.5).