The Fertilisers (Sampling and Analysis) Regulations 1991

Determination 7.3 Titration in the presence of eriochrome black-T 7.3.1 Transfer by pipette 50 ml of the solution to be analysed into a 300 ml beaker. Neutralise the excess acid with the 5 N sodium hydroxide solution (4.11) using the pH meter (5.2). Dilute with water to 100 ml. Add 5 ml buffer solution (4.9). The pH measured by the meter must be 10.5±0.1. Add 2 ml potassium cyanide solution (4.7) and three drops eriochrome black-T indicator (4.6). Titrate with the EDTA solution (4.2), stirring gently with the stirrer (5.1). Let “b” be the number of ml of 0.05 molar EDTA solution. Note: For titration with eriochrome black-T, the titration must not exceed 25 ml of EDTA otherwise the volume of the aliquot part must be reduced. Titration in the presence of calcein or of calcon carbonic acid 7.3.2 Place an aliquot part of the solution to be analysed equal to that taken for the above titration in a 300 ml beaker. Neutralise the excess acid with 5 N sodium hydroxide solution (4.11) using the pH meter (5.2). Dilute with water to about 100 ml. Add 10 ml potassium hydroxide—potassium cyanide solution (4.8) and the indicator (4.4) or (4.5). Stir gently and titrate with the EDTA solution. Let “a” be the number of ml of 0.05 molar EDTA solution.

EXPRESSION OF RESULTS

8

$%MgO(b-a)×0.2016M$

14. — DETERMINATION OF CHLORIDES IN THE ABSENCE OF ORGANIC MATERIAL

SCOPE

1

This method is for the determination of chloride, in the absence of organic material.

FIELD OF APPLICATION

2

All fertilisers which are free from organic material, except ammonium nitrate fertilisers of a nitrogen content greater than 28% by weight.

PRINCIPLE

3

The chlorides, dissolved in water, are precipitated in an acid medium by an excess of standard solution of silver nitrate. The excess is titrated with a solution of ammonium thiocyanate in the presence of ferric ammonium sulphate (Volhard’s method).

REAGENTS

4

• (4.1) Nitrobenzene or diethyl ether.

• (4.2) Nitric acid, 10 N solution.

• (4.3) Indictaor solution: dissolve 40 g of ferric ammonium sulphate [Fe₂(SO₄)₃.(NH₄)₂SO₄.24H₂O] in water and make up to 1 litre.

• (4.4) Silver nitrate, 0.1 N solution.

• (4.5) Ammonium thiocyanate, 0.1 N solution.

• Preparation: since this salt is hygroscopic and cannot be dried without risk of decomposition, it is advisable to weigh out approximately 9 g, dissolve in water and make up the volume to one litre. Standardise by titration against 0.1 N silver nitrate solution.

APPARATUS

5

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

PREPARATION OF SAMPLE

6

See Method 1.

PROCEDURE

7

Extraction 7.1 Weigh to the nearest 0.001 g, 5 g of the prepared sample and place in a 500 ml graduated flask and add 450 ml water. Mix for half an hour on the shaker (5.1); make up to 500 ml with distilled water, mix and filter into a beaker.

Determination 7.2 Take an aliquot part of the filtrate containing not more than 0.150 g of chloride. If the sample taken is smaller than 50 ml it is necessary to make up the volume to 50 ml with distilled water. Add 5 ml 10 N nitric acid (4.2), 20 ml indicator solution (4.3), and two drops ammonium thiocyanate standard solution (taken from a burette adjusted to zero). From a burette than add silver nitrate solution (4.4) until there is an excess of 2 to 5 ml. Add 5 ml nitrobenzene or 5 ml diethyl ether (4.1) and shake well to agglomerate the precipitate. Titrate the excess silver nitrate with 0.1 N ammonium thiocyanate (4.5) until a red-brown colour appears which remains after the flask has been shaken slightly. Note: Nitrobenzene or diethyl ether (especially the former) prevents the silver chloride from reacting with thiocyanate ions, thus a clear colour change is obtained.

Blank test 7.3 Make a blank test under the same conditions (omitting only the sample) and allow for it when calculating the final result.

Control test 7.4 Carry out the determination on an aliquot part of a freshly prepared solution of potassium chloride, containing 0.100 g as chloride.

EXPRESSION OF RESULT

8

Express the result of the analysis as a percentage of chloride contained in the sample as it has been received for analysis.

15a. — DETERMINATION OF FINENESS OF GRINDING—DRY METHOD

SCOPE

1

This method is for the determination of the fineness of grinding by the dry method.

FIELD OF APPLICATION

2

All fertilisers in Schedule 1 of the Fertilisers Regulations 1990[^f01020] for which requirements are given of fineness of grinding using 0.630 mm and 0.160 mm sieves.

PRINCIPLE

3

By mechanical sieve shaking, the quantities of product with a granule size greater than 0.63 mm and those with a granule size between 0.16 mm and 0.63 mm are determined and the percentage of fineness of grinding are calculated.

APPARATUS

4

• (4.1) Mechanical sieve shaker.

• (4.2) Sieves with apertures of 0.160 mm and 0.630 mm respectively of standard ranges (diameter 20 cm, height 5 cm).

PROCEDURE

5

Weigh to the nearest 0.05 g, 50 g of the sample. Assemble the two sieves and the collecting container on the shaker (4.1), the sieve with the larger apertures being placed on top. Place the sample for analysis on the top. Sieve for ten minutes and remove the part collected on the bottom. Sieve again for one minute and check that the amount collected on the bottom during this time is not more than 250 mg. Repeat the process (for one minute each time) until the amount collected is less than 250 mg. Weigh the residual material on both sieves seperately.

EXPRESSION OF RESULTS

6

Percentage of material passing sieve of 0.630 mm apertures = (50 — M₁) × 2

15b. — DETERMINATION OF THE FINENESS OF GRINDING OF SOFT NATURAL PHOSPHATES

SCOPE

1

This method is for determining the fineness of grinding of soft natural phosphates.

FIELD OF APPLICATION

2

Soft natural phosphates.

PRINCIPLE

3

For samples of fine particle size, agglomeration may occur thus making dry sieving difficult. For this reason, wet sieving is normally used.

REAGENTS

4

Sodium hexametaphosphate solution, 1 g per 100 ml.

APPARATUS

5

• (5.1) Sieves with apertures of 0.063 mm and 0.125 mm respectively of standard ranges (diameter 20 cm, height 5 cm) and collecting containers.

• (5.2) Glass funnel of 20 cm diameter mounted on a stand.

• (5.3) Laboratory oven.

PROCEDURE

6

Wash both sides of the sieves with water and place the sieve with 0.125 mm apertures above the 0.063 mm sieve.

EXPRESSION OF RESULTS

7

Percentage of material passing sieve of 0.125 mm apertures = (50 − M₁) × 2

REMARK

8

If the presence of lumps is observed after sieving the analysis should be carried out again in the following way:

16. — METHODS OF ANALYSIS AND TEST PROCEDURES FOR AMMONIUM NITRATE FERTILISERS CONTAINING MORE THAN 28% NITROGEN BY WEIGHT

SCOPE AND FIELD OF APPLICATION

1

This method defines the procedures for the application of thermal cycles prior to the execution of the oil retention test in straight ammonium nitrate fertilisers containing more than 28% nitrogen by weight.

THERMAL CYCLES

2

Field application 2.1 This procedure is for thermal cycling prior to determining the oil retention of the fertiliser.

Principle and definition 2.2 In an Erlenmeyer flask, heat the sample from ambient temperature to 50°C and maintain at this temperature for a period of two hours (phase at 50°C). Thereupon, cool the sample until a temperature of 25°C is acheived and maintain at that temperature for two hours (phase at 25°C). The combination of the successive phases at 50°C and 25°C forms one thermal cycle. After being subjected to two thermal cycles, the test sample is held at a temperature of 20±3°C for the determination of the oil retention value.

Apparatus 2.3 Normal laboratory apparatus, in particular Water baths thermostated at 25 (±1) and 50 (±1)°C respectively. Erlenmeyer flasks with an individual capacity of 150 ml.

Procedure 2.4 Put each test sample of 70(±5) grams into an Erlenmeyer flask which is then sealed with a stopper. Move each flask every two hours from the 50°C bath to the 25°C bath and vice versa. Maintain the water in each bath at constant temperature and keep in motion by rapid stirring to ensure the water level comes above the level of the sample. Protect the stopper from condensation by a foam rubber cap.

SCOPE AND FIELD OF APPLICATION

1

This method defines the procedure for the determination of oil retention of straight ammonium nitrate fertilisers containing more than 28% nitrogen by weight.

DEFINITION

2

Oil retention of a fertiliser: the quantity of oil retained by the fertiliser determined under the operating conditions specified and expressed as a percentage by mass.

PRINCIPLE

3

Total immersion of the test portion in gas oil for a specified period, followed by the draining away of surplus oil under specified conditions. Measurement of the increase in mass of the test portion.

REAGENT

4

Gas oil
Viscosity max:	5 mPas at 40°C
Density:	0.8 to 0.85 g/ml at 20°C
Sulphur content:	≤1.0% (m/m)
Ash:	≤0.1% (m/m)

APPARATUS

5

Ordinary laboratory apparatus and:

• (5.1) Balance, capable of weighing to the nearest 0.01 gram.

• (5.2) Beakers, of capacity 500 ml.

• (5.3) Funnel, of plastic materials, preferably with a cylindrical wall at the upper end, diameter approximately 200 mm.

• (5.4) Test sieve, aperture 0.5 mm, fitting into the funnel (5.3).

• Note The size of the funnel and sieve is such as to ensure that only a few granules lie one above another and the oil is able to drain away.

• (5.5) Filter paper, rapid filtering grade, creped, soft, weight 150g/m².

• (5.6) Absorbent tissue (laboratory grade).

PROCEDURE

6

• (6.1) Two individual determinations are carried out in quick succession on seperate portions of the same test sample.

• (6.2) Remove particles smaller than 0.5 mm using the test sieve (5.4). Weigh to the nearest 0.01 gram approximately 50 grams of the sample into the beaker (5.2). Add sufficient gas oil (Section 4) to cover the prills completely and stir carefully to ensure that the surfaces of all the prills are fully wetted. Cover the beaker with a watch galss and leave to stand for one hour at 25(±2)°C.

• (6.3) Filter the entire contents of the beaker through the funnel (5.3) containing the test sieve (5.4). Allow the portion retained by the sieve to remain there for one hour so that most of the excess oil can drain away.

• (6.4) Lay two sheets of filter paper (5.5) (about 500 × 500 mm) on top of each other on a smooth surface; fold the four edges of both filter papers upwards to a width of about 40 mm to prevent the prills from rolling away. Place two layers of absorbent tissue (5.6) in the centre of the filter papers. Pour the entire contents of the sieve (5.4) over the absorbent tissues and spread the prills evenly with a sof flat brush. After two minutes lift one side of the tissues to transfer the prills to the filter papers beneath and spread them evenly over these with the brush. Lay another sheet of filter paper, similarly with its edges turned upward, on the sample and roll the prills between the filter papers with circular movements while exerting a little pressure. Pause after every eight circular movements to lift the opposite edges of the filter papers and return to the centre the prills that have rolled to the periphery. Keep to the following procedure: make four complete circular movements first clockwise and then anticlockwise. Then roll the prills back to the centre as described above. This procedure to be carried out three times (24 circular movements, edges lifted twice). Carefully insert a new sheet of filter paper between the bottom sheet and the one above it and allow the prills to roll onto the new sheet by lifting the edges of the upper sheet. Cover the prills with a new sheet of filter paper and repeat the same procedure as described above. Immediately after rolling, pour the prills into a tared dish and reweigh to the nearest 0.01 gram to determine the weight of the quantity of gas oil retained.

Repeating the rolling procedure and reweighing 6.5 If the quantity of gas oil retained in the portion is found to be greater than 2.00 grams, place the portion on a fresh set of filter papers and repeat the rolling procedure, lifting the corners in accordance with Section 6.3 (two times eight circular movements, lifting once). Then reweigh the portion.

EXPRESSION OF RESULTS

7

Method of calculation and formula 7.1 The oil retention, from each determination (6.1) expressed as a percentage by mass of the sieved test portion, is given by the equation: $Oil retention=100×m2-m1m1$ where: m₁ is the mass, in grams, of the sieved test portion (6.2); m₂ is the mass, in grmas, of the test portion accodring to Section 6.4 or 6.5 respectively as the result of the last weighing. Take as the result the arithmetic mean of the two individual determinations.

SCOPE AND FIELD OF APPLICATION

1

This method defines the procedure for the determination of the combustible content of straight ammonium nitrate fertilisers containing more than 28% nitrogen by weight.

PRINCIPLE

2

The carbon dioxide produced by inorganic filters is removed in advance with an acid. The organic compounds are oxidised by means of a chromic acid/sulphuric acid mixture. Carbon dioxide formed is absorbed in a barium hydroxide solution. The precipitate is dissolved in a solution of hydrochloric acid and measured by back-titration with sodium hydroxide solution.

REAGENTS

3

• (3.1) Analytical-grade chromium VI oxide; Cr-(VI)-)3.

• (3.2) Sulphuric acid diluted to 60% by volume:

• pour 360 ml of water into a one-litre beaker and carefully add 640 ml of sulphuric acid, density at 20°C=1.83 g/ml.

• (3.3) Silver nitrate: 0.1 M solution.

• (3.4) Barium hydroxide:

• weigh out 15 grams of barium hydroxide (Ba(OH)₂.8H₂O), and dissolve completely in hot water.

• Allow to cool and transfer to a one-litre flask. Fill up to the mark and mix. Filter through a pleated filter paper.

• (3.5) Hydrochloric acid : 0.1 M standard solution.

• (3.6) Sodium hydroxide: 0.1 M standard solution.

• (3.7) Bromophenol blue: solution of 0.4 grams per litre in water.

• (3.8) Phenolphthalein: solution of 2 grams per litre in 60% by volume ethanol.

• (3.9) Soda lime: particle dimensions, about 1.0 to 1.5 mm.

• (3.10) Demineralised water, freshly boiled to remove carbon dioxide.

APPARATUS

4

• (4.1) Standard laboratory equipment, in particular:

• filter crucible with a plate of sintered glass and a capacity of 15 ml, plate diameter: 20 mm, total height : 50 mm, porosity 4 (pore diameter from 5 to 15μm);

• 600 ml beaker.

• (4.2) Compressed nitrogen supply.

• (4.3) Apparatus made up of the following parts and assembled, if possible, by means of spherical ground joints (see Figure 1).

• (4.3.1) Absorption tube (A) about 200 mm long and 30 mm in diameter filled with soda lime (3.9) kept in place by fibreglass plugs.

• (4.3.2) 500 ml reaction flask (B) with side arm and a round bottom.

• (4.3.3) Vigreux fractionating column about 150 mm long (C’).

• (4.3.4) Double-surface condenser (C), 200 mm long.

• (4.3.5) Drechsel bottle (D) acting as a trap for any excess acid which may distil over.

• (4.3.6) Ice bath (E) to cool the Drechsel bottle.

• (4.3.7) Two absorption vessels (F₁) and (F₂), 32 to 35 mm in diameter, the gas distributor of which comprises a 10 mm disc of low-porosity sintered glass.

• (4.3.8) Suction pump and suction regulating device (G) comprising a T-shaped glass piece inserted into the circuit, the free arm of which is connected to a fine capillary tube by a short rubber tube fitted with a screw clamp.

Caution: the use of boiling chromic acid solution in an apparatus under reduced pressure is a hazardous operation and requires appropriate precautions.

PROCEDURE

5

Sample for analysis 5.1 Weigh approximately 10 grams of ammonium nitrate to the nearest 0.001 grams.

Removal of carbonates 5.2 Place the sample for analysis in the reaction flask (B). Add 100 ml of H₂So₄ (3.2). The prills dissolve in about 10 minutes at ambient temperature. Assemble the apparatus as indicated in the diagram: connect one end of the absorption tube (A) to the nitrogen source (4.2) via a non-return flow device containing 5 to 6 mm of mercury and the other end to the feed tube which enters the reaction flask. Place the Vigreux fractionating column (C’) and the condenser (C) with coolinhg water supply in position. Adjust the nitrogen to provide a moderate flow through the solution, bring the solution to boiling point and heat for two minutes. At the end of this time there should be no more effervesence, if effervesence is seen, continue heating for 30 minutes. Allow solution to cool for at least 20 minutes with the nitrogen flowing through it. Complete assembly of the apparatus as indicated in the diagram by connecting the condenser tube to the Drechsel bottle (D) and the bottle to the absorption vessels F₁ and F₂. The nitrogen must continue to pass through the solution during the assembly operation. Rapidly introduce 50 ml of barium hydroxide solution (3.4) into each of the absorption vessels (F₁ and F₂). Bubble a stream of nitrogen through for about 10 minutes. The solution must remain clear in the absorbers. If this does not happen, the carbonate removal process must be adjusted.

Oxidation and absorption 5.3 After withdrawing the nitrogen feed tube, rapidly introduce 20 grams of chromium trioxide (3.1) and 6 ml of silver nitrate solution (3.3) via the side arm of the reaction flask (B). Connect the apparatus to the suction pump and adjust the nitrogen flow so that a steady stream of gas bubbles passes through the sintered-glass absorbers (F₁) and (F₂). Heat the reaction flask (B) until the liquid boils and keep it boiling for one-and-a-half hours[^f01021]. It may be necessary to adjust the suction-regulating valve (G) to control the nitrogen flow since it is possible that the barium carbonate precipitated during the test may block the sintered glass discs. The operation is satisfactory when the barium hydroxide solution in the absorber (F₂) remains clear. Otherwise repeat the test. Stop heating and dismantle the apparatus. Wash each of the distributors both inside and outside to remove barium hydroxide and collect the washings in the corresponding absorber. Place the distributors one after the other in a 600 ml beaker which will subsequently be used for the determination. Rapidly filter under vacuum firstly the contents of absorber F₂ and then of absorber F₁ using the sintered glass crucible. Collect the precipitate by rinsing the absorbers with water (3.10) and wash the crucible with 50 ml of the same water. Place the crucible in the 600 ml beaker and add about 100 ml of boiled water (3.10). Introduce 50 ml of boiled water into each of the absorbers and pass nitrogen through the distributors for five minutes. Combine the water with that fro the beaker. Repeat the operation once to ensure that the distributors are rinsed thoroughly.

Measurement of the carbonates originating from organic material 5.4 Add five drops of phenolphthalein (3.8) to the contents of the beaker. The solution becomes red in colour. Add hydrochloric acid (3.5) drop by drop until the pink colour just disappears. Stir the solution well in the crucible to check that the pink colour does not reappear. Add five drops of bromophenol blue and titrate with hydrochloric acid until the solution turns yellow. Add a further 10 ml of hydrochloric acid. Heat the solution to boiling point and continue boiling for a maximum of one minute. Check carefully that no precipitate remains in the liquid. Allow to cool and back titrate with the sodium hydroxide solution (3.6).

BLANK TEST

6

Carry out a blank test following the same procedure and using the same quantities of all reagents.

EXPRESSION OF RESULTS

7

The content of combustible ingredients (C), expressed as carbon, as a percentage by mass of the sample, is given by the formula:

$$C%=0.06xV1-V2E$ where: E = the mass in grams of the test portion; V₁ = the total volume in ml of 0.1 M hydrochloric acid added after the change in colour of the phenolphthalein; V₂ = the volume in ml of the 0.1 M sodium hydroxide solution used for back titration.$

SCOPE AND FIELD OF APPLICATION

1

This method defines the procedure for measuring the pH value of a solution of a straight ammonium nitrate fertiliser containing more than 28% nitrogen by weight.

PRINCIPLE

2

Measurement of the pH of an ammonium nitrate solution by means of a pH meter.

REAGENTS

3

Distilled or demineralised water, free from carbon dioxide.

Buffer solution, pH 6.88 at 20°C 3.1 Dissolve 3.40±0.01 grams of potassium dihydrogen orthophosphate (KH₂PO₄) in approximately 400 ml of water. The dissolve 3.55±0.01 gram of disodium hydrogen orthophosphate (Na₂HPO₄) in approximately 400 ml of water. Transfer the two solutions without loss into a 1,00 ml standard flask, make up to the mark and mix. Keep the solution in an airtight vessel.

Buffer solution, pH 4.00 at 20°C 3.2 Dissolve 10.21±0.01 grams of potassium hydrogen phthalate (KHC₈O₄H₄) in water, transfer without loss into a 1,00 ml standard flask, make up to the mark and mix. Keep this solution in an airtight vessel.

• (3.3) Commercially available pH standard solutions may be used.

APPARATUS

4

pH meter, equipped with glass and calomel electrodes or equivalent, sensitivity 0.05 pH unit.

APPARATUS

5

Calibration of the pH meter 5.1 Calibrate the pH meter (4) at a temperature of 20(±1)°C, using the buffer solutions (3.1), (3.2) or (3.3). Pass a slow stream of nitrogen onto the surface of the solution and maintain this throughout the test.

Determination 5.2 Pour 100.0 ml of water onto 10 (±0.01) grams of the sample in a 250 ml beaker. Remove the insolubles by filtering, decanting or centrifuging the liquid. Measure the pH value of the clear solution at a temperature of 20 (±1)°C according to the same procedure as for the calibration of the meter.

EXPRESSION OF RESULTS

6

Express the result in pH units, to the nearest 0.1 unit and state the temperature used.

SCOPE AND FIELD OF APPLICATION

1

This method defines the procedure for the test sieving of straight ammonium nitrate fertilisers containing more than 28% nitrogen by weight.

PRINCIPLE

2

The test sample is sieved on a nest of three sieves, either by hand or by mechanical means. The mass retained on each sieve is recorded and the percentage of material passing the required sieves are calculated.

APPARATUS

3

• (3.1) 200 mm diameter woven-wire test sieves to BS 410 (1986) with apertures of 2.0 mm, 1.0 mm and 0.5 mm respectively of standard ranges. One lid and one receiver for these sieves.

• (3.2) Balance to weigh to 0.1 gram.

• (3.3) Mechanical sieve shaker (if available) capable of imparting both vertical and horizontal motion to the test sample.

PROCEDURE

4

• (4.1) The sample is divided representatively into portions of approximately 100 grams.

• (4.2) Weigh one of these portions to the nearest 0.1 gram.

• (4.3) Arrange the nest of sieves in ascending order; receiver 0.5 mm, 1 mm, 2 mm and place the weighed test portion on the top sieve. Fit the lid to the top of the nest of sieves.

• (4.4) Shake by hand or machine, imparting both a vertical and horizontal motion and, if by hand, tapping occasionally. Continue this process for 10 minutes or until the quantity passing through each sieve in one minute is less than 0.1 gram.

• (4.5) Remove the sieves from the nest in turn and collect the material retained, brush gently from the reverse side with a soft brush, if necessary.

• (4.6) Weigh the material retained on each sieve and that collected in the receiver, to the nearest 0.1 gram.

EVALUATION OF RESULTS

5

• (5.1) Convert the fraction masses to a percentage of the total of the fraction masses (not of the original charge).

• Calculate the percentage in the receiver (ie <0.5 mm): A%

• Calculate the percentage retained on the 0.5 mm sieve: B%

• Calculate the percentage passing 1.0 mm, ie (A + B)%.

• The sum of the fraction masses should be within 2% of the initial mass taken.

• (5.2) At least two seperate analyses should be carried out and the individual results for A should not differ by more than 1.0% absolute and for B by more than 1.5% absolute. Repeat the test if this is not the case.

EXPRESSION OF RESULTS

6

Report the mean of the two values for A on the one hand and for A + B on the other hand.

SCOPE AND FIELD OF APPLICATION

1

This method defines the procedure for the determination of the chlorine content (as chloride ion) of straight ammonium nitrate fertilisers containing more than 28% nitrogen by weight.

PRINCIPLE

2

Chloride ions dissolved in water are determined by potentiometric titration with silver nitrate in an acidic medium.

REAGENTS

3

Distilled or demineralised water, free from chloride ions.

• (3.1) Acetone AR.

• (3.2) Concentrated nitric acid (density at 20°C = 1.40 g/ml).

• (3.3) Silver nitrate 0.1 M standard solution. Store this solution in a brown glass bottle.

• (3.4) Silver nitrate 0.004 M standard solution—prepare this solution at the time of use.

• (3.5) Potassium chloride 0.1 M standard reference solution. Weigh, to the nearest 0.1 mg, 3.7276 grams of analytical-grade potassium chloride, previously dried for one hour in an oven at 130°C and cooled in a desiccator to ambient temperature. Dissolve in a little water, transfer the solution without loss into a 500 ml standard flask, dilute to the mark and mix.

• (3.6) Potassium chloride 0.004 M standard reference solution—prepare this solution at the time of use.

APPARATUS

4

• (4.1) Potentiometer with silver indicating electrode and calomel reference electrode, sensitivity 2 mV, covering the range −500 to +500 mV, or with silver and mercury (1) sulphate electrodes.

• (4.2) Bridge, containing a saturated potassium nitrate solution, connected to the calomel electrode (4.1), fitted at the ends with porous plugs. This bridge is not necessary if silver and mercury (1) sulphate electrodes are used.

• (4.3) Magnetic stirrer, with a Teflon-coated rod.

• (4.4) Microburette with a fine-pointed tip, graduated in 0.01 ml divisions.

PROCEDURE

5

Standardisation of the silver nitrate solution 5.1 Take 5.00 ml and 10.00 ml of the standard reference potassium chloride solution (3.6) and place in two low-form beakers of convenient capacity (for example 250 ml). Carry out the following titration of the contents of each beaker. Add 5 ml of the nitric acid solution (3.2), 120 ml of the acetone (3.1) and sufficient water to bring the total volume to about 150 ml. Place the rod of the magnetic stirrer (4.3) in the beaker and set the stirrer in motion. Immerse the silver electrode (4.1) and the free end of the bridge (4.2) in the solution. Connect the elctrodes to the potentiometer (4.1) and, after verifying the zero of the apparatus, note the value of the starting potential. Titrate, using the microburette (4.4), adding initially 4 or 9 ml respectively of the silver nitrate solution corresponding to the standard reference potassium chloride solution used. Continue the addition in 0.1 ml portions for the 0.004 M solutions and in 0.05 ml portions for the 0.1 M solutions. After each addition, await the stabilisation of the potential. Record the volumes added and the corresponding values of the potential in the first two columns of a table. In a third column of the table, record the successive increments (Δ₁E) of the potential E. In a fourth column, record the differences (Δ₂E) positive or negative, between the potential increments (Δ₁E). The end of the titration corresponds to the addition of the 0.1 or 0.05 ml portion (V₁) of the silver nitrate solution which gives the maximum value of (Δ₁E). In order to calculate the exact volume (Veq) of the silver nitrate solution corresponding to the end of the reaction, use the formula: $Veq=V0+(V1×bB)$ where: V₀ is the total volume, in ml, of the silver nitrate solution immediately lower than the volume which gives the maximum increment of Δ₁E; V₁ is the volume, in ml, of the last portion of the silver nitrate solution added (0.1 or 0.05 ml); b is the last positive value of Δ₂E; B is the sum of the absolute values of the last positive values of Δ₂E and the first negative value of Δ₂E (see example in Table 1).

Blank test 5.2 Carry out a blank test and take account thereof when calculating the final result. The result V₄ of the blank test and take account thereof when calculating the final result. $V4=2V3-V2$ where: V₂ is the value, in ml, of the exact volume (Veq) of the silver nitrate solution corresponding to the titration of 10 ml of the potassium chloride standard reference solution used; V₃ is the value, in ml, of the exact volume (Veq) of the silver nitrate solution corresponding to the titration of 5 ml of the potassium chloride standard reference solution used.

Check test 5.3 The blank test can at the same time serve as a check that the apparatus is functioning satisfactorily and that the test procedure is being implemented correctly.

Determination 5.4 Take a portion of sample in the range of 10 to 20 grams and weigh to the nearest 0.01 gram. Transfer quantitatively to a 250 ml beaker. Add 20 ml of water, 5 ml of nitric acid solution (3.2), 120 ml of acetone (3. 1) and sufficient water to bring the total volume to about 150 ml. Place the rod of the magnetic stirrer (4.3) in the beaker, place the beaker on the stirrer and set the stirrer in motion. Immerse the silver electrode (4.1) and the free end of the bridge (4.2) in the solution, connect the electrodes to the potentiometler (4. 1) and, after having verified the zero of the apparatus, note the value of the starting potential. Titrate with the silver nitrate solution, by additions from the microburette (4.4) in increments of 0.1 ml. After each addition, await the stabilisation of the potential. Continue the titration as specified in 5. 1, starting from the fourth paragraph: “Record the volumes added and the corresponding values of the potential in the first two columns of a table….”.

EXPRESSION OF RESULTS

6

Express the result of the analysis as the percentage of chlorine contained in the sample as received for analysis.

SCOPE AND FIELD OF APPLICATION

1

This method defines the procedure for the determination of copper content of straight ammonium nitrate fertilisers containing more than 28% nitrogen by weight.

PRINCIPLE

2

The sample is dissolved in dilute hydrochloric acid and the copper content is determined by atomic absorption spectrophotometry.

REAGENTS

3

• (3.1) Hydrochloric acid (density at 20°C=1.18 g/ml).

• (3.2) Hydrochloric acid, 6 M solution.

• (3.3) Hydrochloric acid, 0.5 M solution.

• (3.4) Ammonium nitrate.

• (3.5) Hydrogen peroxide, 30%

• (3.6) Copper solution[^f01022] (stock): weigh, to the nearest 0.001 gram, 1 gram of pure copper, dissolve in 25 ml 6 M hydrochloric acid solution (3.2), add 5 ml of hydrogen peroxide (3.5) in portions and dilute to I litre with water. 1 ml of this solution contains 1,000,μg of copper (Cu).

• (3.6.1) Copper solution (dilute): dilute 10 ml of stock solution (3.6) to 100 ml with water and then dilute 10 ml of the resulting solution to 100 ml with water. 1 ml of the final dilution contains 10μg of copper (Cu).

• Prepare this solution at the time of use.

APPARATUS

4

Atomic absorption spectrophotometer with a copper lamp (324.8 nm).

PROCEDURE

5

Preparation of the solution for analysis 5.1 Weigh, to the nearest O.OO1 gram, 25 grams of the sample, place it in a 400 ml beaker, add carefully 20 ml of hydrochloric acid (3.1) (there may be a vigorous reaction due to carbon dioxide formation). Add more hydrochloric acid, if necessary. When effervescence has stopped, evaporate to dryness on a steam bath, stirring occasionally with a glass rod. Add 15 ml 6 M hydrochloric acid solution (3.2) and 120 ml of water. Stir with the glass rod, which should be left in the beaker and cover the beaker with a watch glass. Boil the solution gently until dissolution is complete and then cool. Transfer the solution quantitatively into a 250 ml graduated flask, by washing the beaker with 5 ml 6 M hydrochloric acid (3.2), and twice with 5 ml of boiling water, make up to the mark with 0.5 M hydrochloric acid (3.3) and mix carefully. Filter through a copper-free filter paper[^f01023], discarding the first 50 ml.

Blank solution 5.2 Prepare a blank solution from which only the sample has been omitted and allow for this in the calculation of the final result.

Determination 5.3 Preparation of sample and blank test solutions 5.3.1 Dilute the sample solution (5.1) and the blank test solution (5.2) with 0.5 M hydrochloric acid solution (3.3) to a concentration of copper within the optimal measuring range of the spectrophotometer. Normally no dilution is needed. Preparation of the calibration solutions 5.3.2 By diluting the standard solution (3.6.1) with 0.5 M hydrochloric acid solution (3.3), prepare at least five standard solutions corresponding to the optimal measuring range of the spectrophotometer (0 to 5.0 μ/l Cu). Before making up to the mark, add to every solution ammonium nitrate (3.4) to give a final concentration of 100 mg per ml.

Measurement 5.4 Set up the spectrophotometer (4) at a wavelength of 324.8 nm using an oxidising air-acetylene flame. Spray successively, in triplicate, the calibration solutions (5.3.2), the sample solution and the blank solution (5.3.1), washing the instrument through with distilled water between each spraying. Plot the calibration curve using the mean absorbances of every standard used as the ordinates and the corresponding concentrations of copper in μ/ml as the abscissae. Determine the concentration of copper in the final sample and blank solutions by reference to the calibration curve.

EXPRESSION OF RESULTS

6

Calculate the copper content of the sample taking into account the weight of the test sample, the dilutions carried out in the course of the analysis and the value of the blank. Express the result as mg Cu/kg.

PART II

General

1

• (a) 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 the analysis.

• (b) Any reference to water in this Schedule means purified water as defined in the European Pharmacopeia.

Reagents and Apparatus

2

• (a) All reagents used shall be of analytical quality.

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

• (c) Solutions for which no solvents are prescribed must be aqueous.

• (d) Only special instruments or apparatus requiring special standards are mentioned in the descriptions of the methods of analysis.

Methods of Analysis

3

1

Preparation of the sample for analysis

2

Determination of moisture

3

Determination of total nitrogen—chromium powder reduction method

4

Determination of urea

5.a

Extraction of phosphorus—by mineral acids (total phosphorus)

b

Extraction of phosphorus—by 2% citric acid

6

Determination of extracted phosphorus—spectrophotometric method

7.a

Determination of potassium—gravimetric method

b

Determination of potassium—flame photometric method

8

Determination of total magnesium

9.a

Determination of boron—titrimetric method

b

Determination of boron—spectrophotometric method

10

Determination of cobalt

11

Determination of molybdenum

12

Determination of copper

13

Determination of iron

14

Determination of manganese

15

Determination of the nuetralising value in limiting materials

16

Determination of fineness of products other than potassic bag slag

17

Determination of fineness of potassic basic slag.

1. — PREPARATION OF THE SAMPLE FOR ANALYSIS

INTRODUCTION

1

The preparation of a sample for analysis from the final sample received at the laboratory is a series of operations, usually sieving, grinding and mixing carried out in such a way that the smallest amount weighed, as prescribed by the method of analysis chosen, is representative of the final sample. The sample should be ground to the fineness required by the method of analysis. (Overgrinding must be avoided in cases where this will affect the solubility in various reagents.) With some materials, fine grinding may lead to loss of gain or moisture and allowance for this must be made.

SCOPE AND FIELD OF APPLICATION

2

This method is applicable to fertilisers in Groups 1(b), 1(c), 2(d), 3(b), 3(c), 4(a), 4(b), 4(c) of Section A and Group 5 of Section B of the Table in Schedule 1 of the Fertilisers Regulations 1990[^f01024]. It is also applicable to products in Group 5(a) of Section A in the said table when the determination of total magnesium is required.

PRINCIPLE

3

• (3.1) Solid fertilisers: the whole final sample is ground to the required fineness. All the ground sample is thoroughly mixed before each test portion is taken.

• (3.2) Fluid fertilisers: the final sample is thoroughly mixed before each test portion is taken.

APPARATUS

4

• (4.1) Sample grinder capable of grinding the fertiliser to pass the specified sieve.

• (4.2) Mortar and pestle of suitable material and size.

• (4.3) Sieves having square apertures of 0.18 mm, 0.5 mm and 1.0 mm. Test sieves conforming to British Standard 410; 1976 are suitable.

• (4.4) Sample containers of non-corrodible materials, with air tight closures.

PROCEDURE

5

WARNING

All operations connected with this procedure should be carried out as quickly as possible to minimise absorbtion or loss of water. Care should be taken during grinding that the temperature of the fertiliser does not rise above 45°C to avoid loss of volatile constituents. Grinding beyond the fineness required must in all cases be avoided.

Grinding and sieving 5.1 The procedure in 5.1.1 should be followed except when a grinding machine is not available, in which case 5.1.2 is applicable. 5.1.1 Grind the final sample until all the sample has passed through, or for the specified time, depending on the type of grinder (4.1). To check that the grinding has been adequate sieve a small portion of the ground sample through a 0.5 mm sieve (4.3) and discard it. If the whole of this portion does not pass the sieve, return the remainder of the sample to the grinder and repeat the grinding until satisfactory grinding is achieved. 5.1.2 Sieve the whole final sample through a 0.5 mm sieve (4.3). Grind the residue on the sieve, using the pestle and mortar (4.2), until all the material passes through the sieve. Carefully mix the sample. 5.2 Place the prepared sample in a clean container (4.4) and seal it until required for analysis. 5.3 Before taking each test portion for analysis, the whole sample must be well mixed. Form the material into a flattened cone and using a spatula take the required test portion at random in small increments. 5.4 If the sample contains foreign matter which cannot be ground this shall be removed, weighed and allowed for in the results of the analysis. This material shall be retained and if possible its nature recorded.

SPECIAL CASES

6

Samples not to be ground 6.1 For those samples where the amount of phosphorous pentoxide solubile in 2percnt; citric acid and the fineness of grinding are to be determined, the sample should be well mixed (soft lumps may be disintergrated by lightly crushing) and divided into two parts, which are as identical as possible. The above mentioned determinations shall be carried out on the unground sample. All other determinations shall be carried out on the sample prepared in accordance with the directions in paragraph 5.1.

Products which may be difficult to grind mechanically, including products with abnormal moisture or products which become doughy through grinding 6.2 Some products such as superphosphate may become doughy if ground mechanically. In these case crush the sample in a mortar (4.2) so that all the material passes through a 1.0 mm sieve (4.3). Place the material so crushed in a clean container (4.4) and seal it until required for analysis.

Organic materials 6.3 Some organic materials may be of such a nature that the procedures given above cannot be used (for example fresh guano, leather, wool and animal residues). In these cases the analyst should use the best practicable means to obtain a representative sample.

Fertilisers comprising several different materials 6.4 These fertilisers include materials with marked differences in texture or mechanical properties (hardness, density, etc). They may be difficult to grind entirely (for example mixtures of organic and inorganic materials) or they may segregate during handling (for example “Kalimagnesia”). Special procedures are necessary in these cases: 6.4.1 for mixtures other than those in 6.4.2, follow the procedure in 5.1.1, repacing the 0.5 mm sieve by one with apertures of 0.18 mm. A grinding machine, capable of grinding the whole of the sample to the required fineness in one pass, is strongly recommended; 6.4.2 in the case of mixtures containing one or more very hard components, or mixtures containing organic materials, it may be difficult to grind and homogenise all the components. To avoid overgrinding some of the softer components proceed as follows:— grind the sample as in 5.1.1 or 5.1.2 to pass a 0.5 mm sieve. Re-seive the sample through a 0.18 mm sieve and reduce the residue to a convenient size by further grinding or other practicable means. Thoroughly remix the sample and place in a clean container (4.4).

FLUID FERTILISERS

7

Mix thoroughly by shaking, ensuring that any insoluble matter, particularly crystaline material, is throughly dispersed, immediately before drawing a portion of the sample for analysis.

2. — DETERMINATION OF MOISTURE

SCOPE AND FIELD OF APLICATION

1

This method is applicable to all fertilisers where a correction for moisture is necessary.

PRINCIPLE

2

The sample is dried to constant weight in an oven at 100°C. The loss in weight corresponds to the moisture content of the sample.

APPARATUS

3

• (3.1) Suitable containers with lids ensuring air tight closure; the dimensions should allow the sample to be spread at about 0.3 g per cm₂.

• (3.2) Electrically heated oven, suitably ventilated and capable of being maintained at 100±2°C.

PREPARATION OF SAMPLE

4

See Method 1.

PROCEDURE

5

Weigh to the nearest 0.001 g, 5 g of the prepared sample and transfer to a previously weighed container (3.1). Place the uncovered container and the lid in the oven (3.2) for 2 to 3 hours. Replace the lid on the container, remove from the oven and allow to cool in a desiccator and weigh. Reheat for another hour, cool and reweigh. If the difference in weight exceeds 0.01g continue the heating and cooling procedure until a weight constant within 0.01 g is attained.

EXPRESSION OF RESULT

6

Calculate the total loss of weights and express it as a percentage of the original weight.

3. — DETERMINATION OF TOTAL NITROGEN-CHROMIUM POWDER REDUCTION METHOD

SCOPE AND FIELD OF APPLICATION

1

This method is applicable to fertilisers in Groups 1(b), 1(c), 3(b), 4(a) and 4(c) of Section A, Group 5 of Section B and Groups 1(c) and 1(d) of Section C of the Table in Schedule 1 of the Fertilisers Regulations 1990[^f01025] in respect of which the indication of total nitrogen is required.

PRINCIPLE

2

The nitrate is reduced to ammonia by chromium powder in an acid medium. Organic and ureic nitrogen is converted into ammonium sulphate by digestion with concentrated sulphuric acid using a catalyst. The ammonia is distilled from an alkaline solution and absorbed in a standard acid. The excess acid is tirated with standard alkali.

REAGENTS

3

• (3.1) Sodium hydroxide solution: 40 g per 100 ml, ammonia free.

• (3.2) Sulphuric acid, 0.1 N solution.

• (3.3) Sulphuric acid, 0.2 N solution.

• (3.4) Sulphuric acid, 0.5 N solution.

• (3.5) Sodium hydroxide, 0.2 N solution, carbonate free.

• (3.6) Chronium metal powder, 100 mesh, low nitrogen content.

• (3.7) Anti-bump granules of pumice stone, washed in hydrochloric acid and ignited.

• (3.8) Anti-foaming agent, paraffin wax.

• (3.9) Sulphuric acid (d=1.84 g/ml).

• (3.10) Hydrochloric acid (d=1.18 g/ml).

• (3.11) Catalyst mixture: 1,000 g potassium sulphate and 50 g copper sulphate pentahydrate. The ingredients must be ground and thoroughly mixed.

• (3.12) Indicator solutions:

Mixed indicator: 3.12.1 mix 50 ml of 2 g/litre ethanolic solution of methyl red with 50 ml of 1 g/litre ethanolic solution of methylene blue.

Methyl red indicator: 3.12.2 dissolve 0.1 g methyl red in 50 ml ethanol. This indicator may be used instead of the preceding one.

• (3.13) pH indicator paper, wide range.

APPARATUS

4

Apparatus for mineral acid digestion and distillation according to Kjeldahl’s method.

PREPARATION OF SAMPLE

5

See Method 1.

PROCEDURE

6

Reduction 6.1 Weigh, to the nearest 0.001 g, between 0.5 and 2.0 g of the prepared sample, containing not more than 0.06 g nitric nitrogen and 0.235 g total nitrogen and transfer to a Kjeldahl flask. Add sufficient water to make the total volume 35 ml. Allow the flask to stand for 10 minutes with occasional gentle swirling to ensure solution of all nitrate salts. Add 1.2 g chromium powder (3.6) and 7 ml hydrochloric acid (3.10), mix well and allow the flask to stand for at least 5 minutes but not more than 10 minutes at ambient temperature. Heat the flask gently so that the contents just begin to boil in about 7 minutes. Continue boiling gently for 10 minutes. Remove the flask from the heat and allow to cool.

Hydrolysis, when the fertiliser is known not to contain organic matter 6.2 Place the flask (6.1) in a fume cupboard, add a small quantity of anti-bump granules (3.7) and then carefully add 25 ml sulphuric acid (3.9). Mix the contents of the flask and heat gently until boiling. Continue heating until dense white fumes of sulphuric acid are evolved for at least 15 minutes. Allow the mixture to cool and then carefully add 250 ml water. Allow to cool to room temperature and continue as described in 6.4.

Digestion when the fertiliser is known to contain organic matter 6.3 Add a small quantity of anti-bump granules (3.7), 10 g of the catalyst mixture (3.11) and then carefully add 25 ml sulphuric acid (3.9) (see Note). Add 0.5 g parafin wax (3.8) to reduce foaming and mix. Heat the flask moderately at first, shaking from time to time until frothing ceases and the liquid is practically colourless. Continue the digestion for at least a further 60 minutes. Allow the mixture to cool and then carefully add 250 ml water. Allow to cool to room temperature, and continue as described in 6.4. Note: If organic matter other than urea exceeds 1.0 g add an additional 1.0 ml sulphuric acid for each 0.1 g organic matter in excess of 1.0 g.

Distillation 6.4 Transfer an appropriate volume of 0.1 N, 0.2 N or 0.5 N sulphuric acid (3.2, 3.3, 3.4) to the collecting flask of the distillation apparatus, according to the presumed level of nitrogen; add a few drops of indicator solution (3.12.1 or 3.12.2). Taking precautions against the loss of ammonia, carefully add to the contents of the Kjeldahl flask (6.2 or 6.3) 100 ml sodium hydroxide solution (3.1). Mix well and connect immediately to the distillation apparatus. Heat the flask so that approximately 150 ml of the liquid are distilled in 30 minutes. At the end of this time, lower the collecting flask so that the tip of the condenser is above the surface of the liquid. Test the subsequent distilate by means of the indicator paper (3.13) to ensure that all the ammonia is completely distilled. Remove the source of heat. Titrate the excess acid with 0.2 N sodium hydroxide solution (3.5) to the end point of the indicator.

Blank test 6.5 Carry out a blank test (omitting only the sample) under the same conditions and allow for this in the calculation of the final results.

EXPRESSION OF RESULTS

7

Determine the quantity of sulphuric acid consumed.

4. — DETERMINATION OF UREA

SCOPE AND FIELD OF APPLICATION

1

This method is applicable to fertilisers in Group 1(c) of Section A, Group 5 of Section B and Group 1(d) of Section C of the Table in Schedule 1 of the Fertilisers Regulations 1990[^f01026].

PRINCIPLE

2

The sample is suspended in acid solution with a clarifying agent and filtered. The urea content of the filtrate is determined after the addition of 4-dimethylamino-benzaldehyde (4-DMAB) by measuring the absorbance at 435 nm.

REAGENTS

3

• (3.1) Activated charcoal.

• (3.2) Carrez solution I:

• dissolve 21.9 g zinc acetate dihydrate in water, add 3 ml glacial acetic acid and dilute to 100 ml with water.

• (3.3) Carrez solution II: 10.6 g potassium ferrocyanide per 100 ml.

• (3.4) Hydrochloric acid solution 0.02 N.

• (3.5) Sodium acetate solution: 136 g sodium acetate trihydrate per litre.

• (3.6) 4-dimethylamino-benzaldehyde solution:

• dissolve 1.6 g of 4-dimethylamino-benzaldehyde (4-DMAB) in 100 ml 96percnt; ethanol and add 10 ml of hydrochloric acid (d=1.18 g/ml).

• (3.7) Urea standard solution: 1.0 g per 100 ml (1 ml of this solution=10 mg urea).

APPARATUS

4

Spectrophotometer with 10 mm cells.

PREPARATION OF SAMPLE

5

See Method 1.

PROCEDURE

6

Preparation of the solution for analysis 6.1 Weigh to the nearest 0.001 g, 2 g of the prepared sample, or a suitable amount expected to contain between 50 and 500 mg of urea and transfer it to a 500 ml graduated flask. Add 150 ml 0.02 N hydrochloric acid solution (3.4), shake for 30 minutes then add 10 ml sodium acetate solution (3.5) and mix well. Add 2 g activated charcol (3.1) to the flask, shake well and allow to stand for a further 15 minutes. Add 5 ml Carrez solution I (3.2), followed by 5 ml Carrez solution II (3.3), mixing well between additions. Dilute to volume with water and mix well. Filter a portion of the solution through a dry filter paper into a clean dry filter paper into a clean dry 250 ml beaker.

Determination 6.2 Transfer 10 ml of the filtrate (6.1) to a 50 ml graduated flask, add 10 ml 4-DMAB solution (3.6), dilute to 50 ml with water mix well and allow to stand for 10 minutes. Measure the absorbance of the solution at 435 nm, in a 10 mm cell against a reference solution prepared by diluting 10 ml 4-DMAB solution (3.6) to 50 ml with water.

Calibration curve 6.3 Transfer amounts of standard urea solution (3.7) corresponding to 50, 100, 150 and 250 mg of urea into a series of 250 ml graduated flasks; add 75 ml 0.02 N hydrochloric acid solution (3.4) and proceed as described above (6.1) commencing at “…shake for 30 minutes…”. Measure the absorbance of the solutions and construct a calibration graph relating the absorbances to the amounts of urea present.

EXPRESSION OF RESULTS

7

Determine the amount of urea in the sample by reference to the calibration graph. Express the result in terms of percentage ureic nitrogen of the sample:

5a. — EXTRACTION OF PHOSPHORUS BY MINERAL ACIDS (TOTAL PHOSPHOROUS)

SCOPE AND FIELD OF APPLICATION

1

This method is applicable to fertilisers in Groups 2(b), 2(c), 2(d), 3(b) and 4(c) of Section A and Group 5 of Section B of the Table in Schedule 1 of the Fertilisers Regulations 1990[^f01027] in respect of which the indication of total phosphorus is required.

PRINCIPLE

2

The phosphorous is extracted from the fertiliser with a mixture of nitric acid and sulphuric acid.

REAGENTS

3

• (3.1) Sulphuric acid (d=1.84 g/ml).

• (3.2) Nitric acid (d=1.42 g/ml).

APPARATUS

4

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

PREPARATION OF THE SAMPLE

5

See Method 1.

PROCEDURE

6

Extraction 6.1 Weigh to the nearest 0.001 g, 2.5 g of the prepared sample and place it in a dry Kjeldahl flask (4.1). Add 15 ml water and stir so as to suspend the substance. Add 20 ml nitric acid (3.2) and carefully add 30 ml sulphuric acid (3.1) (see Note). When the initial violent reaction has ceased, slowly bring the contents of the flask to boiling and boil for 30 minutes. Allow to cool and then carefully add with mixing about 150 ml water. Boil for 15 minutes. Cool completely and transfer the liquid quantitively to a 500 ml graduated flask. Make up to volume, mix and filter through a dry fluted filter, discarding the first portion of the filtrate.

Determination 6.2 Determine the phosphorus according to Method 6a or Method 6b on an aliquot part of the clear filtrate.

Note: If the sample contains cellulosic matter, the following procedure is suggested to avoid excessive frothing during digestion: weigh to the nearest 0.001 g, 2.5 g of the prepared sample and place it in a dry Kjeldahl flask. Add 30 ml sulphuric acid (3.1) and carefully boil until most of the organic matter has been destroyed. Allow to cool, add 15 ml water and 20 ml nitric acid (3.2); bring to the boil and continue boiling for 30 minutes. Continue as described in 6.1 from “Allow to cool and then…”.

5b. — EXTRACTION OF PHOSPHORUS BY 2 % CITRIC ACID

SCOPE AND FIELD OF APPLICATION

1

This method is applicable to fertilisers in Groups 2(b), 3(b) and 4(b) of Section A of the Table in Schedule 1 of the Fertilisers Regulations 1990[^f01028] in respect of which an indication of the phosphorus soluble in 2% citric acid is required.

PRINCIPLE

2

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

REAGENT

3

• (3.1) 2% citric acid solution (20 g per litre), prepared from citric acid monohydrate.

APPARATUS

4

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

PREPARATION OF THE SAMPLE

5

The analysis is carried out on the product as received after carefully mixing the original sample to ensure it is homogeneous.

PROCEDURE

6

Extraction 6.1 Weigh to the nearest 0.001 g, 5 g of the prepared sample, and place it in a dry flask with a sufficiently wide neck, with a capacity of 600 ml, allowing the liquid to be shaken thoroughly. Add 500 ml of the citric acid solution (3.1) at 20 ± 1°C. When adding the first few ml of the reagent, shake vigorously by hand to stop the formation of lumps and to prevent the substance sticking to the sides of the flask. Close the flask with a rubber stopper and shake it in the rotary shaker (4.1) for exactly 30 minutes at a temperature of 20 ± 2°C. Filter immediately through a dry fluted filter into a dry glass receiver and discard the first 20 ml of the filtrate. Continue the filtering until a sufficient quantity of filtrate is obtained to carry out the phosphorous determination.

Determination 6.2 Determine the phosphorus according to Method 6a or Method 6b on an aliquot part of the clear filtrate.

6. — DETERMINATION OF EXTRACTED PHOSPHORUS—SPECTROPHOTOMETRIC METHOD

SCOPE AND FIELD OF APPLICATION

1

This method is for the determination of the phosphorus extracted in Methods 5a and 5b.

PRINCIPLE

2

An acidic solution of the extracted phosphorus is treated with molybdovanadate reagent and the absorbance of the yellow solution is measured at 430 nm.

REAGENTS

3

• (3.1) Nitric acid (d = 1.42 g/ml).

• (3.2) Molybdovanadate reagent: dissolve seperately 20 g ammonium molybdate and 0.47 g ammonium vanadate in water, mix, acidify with 140 ml nitric acid (3.1) and dilute to 1 litre with water.

• (3.3) Phosphorus standard solution: dissolve 4.387 g of potassium dihydrogen phosphate, previously dried at 105°C for 1 hour, in water and dilute to 1 litre.

• 1 ml of this solution contains 1 mg phosphorus (P) or 2.29 mg phosphorus pentoxide (P₂O₅).

• (3.4) Sodium hydroxide, approximately 5 N solution.

APPARATUS

4

Spectrophotometer with 10 mm cells.

PROCEDURE

5

Determination 5.1 For Total Phosphorus 5.1.1 Dilute, if necessary, the prepared extract to obtain a phosphorus concentration of about 20 μg/ml. Transfer 10 ml of this solution to a glass stoppered test tube, add 10 ml freshly prepared molybdovanadate reagent (3.2) and mix. Allow to stand for 10 minutes at 20°C and then measure the absorbance of the solution at 430 nm against a freshly prepared reference solution made by adding 10 ml molybdovanadate reagent (3.2) to 10 ml water. For Water Soluble Phosphorus and Citric Acid Soluble Phosphorus 5.1.2 Dilute, if necessary, the prepared extract to obtain a phosphorus concentration of about 80 μg/ml. Transfer 25 ml of this solution to a 100 ml conical flask, add 5 ml nitric acid (3.1) and boil gently for 30 minutes. Cool the solution and neutralise with sodium hydroxide solution (3.4). Cool the solution to 20°C, transfer quantitatively to a 100 ml graduated flask and make up to the mark with water. Transfer 10 ml of this solution to a glass stoppered test tube, add 10 ml freshly prepared molybdovanadate reagent (3.2) and mix. Proceed as described in 5.1.1 from “Allow to stand . . . ”.

Calibration 5.2 From the standard solution (3.3) prepare a series of solutions containing respectively 5, 10, 20, 30 and 40μg/ml of phosphorus (P). Transfer 10 ml of each solution into a glass-stoppered test tubes, add 10 ml molybdovanadate reagent (3.2), mix and proceed as described in 5.1.1 from “Allow to stand . . .”. Construct a graph relating the absorbance to the amount of phosphorus present. The calibration curve should be prepared at the same time as the determination is carried out.

EXPRESSION OF RESULTS

6

Determine the amount of phosphorus in the sample by reference to the calibration curve. Express the result in terms of percentage phosphorus (P) or percentage phosphorus pentoxide (P₂O₅) of the sample:

7a. — DETERMINATION OF POTASSIUM-GRAVIMETRIC METHOD

SCOPE AND FIELD OF APPLICATION

1

This method is applicable to fertilisers in Groups 3(b), 3(c), 3(d) and 4(c) of Section A and Group 5 of Section B of the Table in Schedule 1 of the Fertilisers Regulations 1990[^f01029] in respect of which an indication of total potassium is required.

PRINCIPLE

2

The sample is ashed and dissolved in dilute hydrochloric acid or, if it contains no organic substances, it is dissolved directly in dilute hydrochloric acid. After the removal of interfering substances the potassium is precipitated in a slightly alkaline medium in the form of potassium tetraphenylborate (KTPB).

REAGENTS

3

• (3.1) Formaldehyde, 25-35% solution, filtered if necessary before use.

• (3.2) Potassium chloride.

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

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

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

• (3.6) STPB solution: dissolve 32.5 g sodium tetraphenylborate in 480 ml of water, add 2 ml of sodium hydroxide solution (3.3) and 20 ml of a magnesium chloride solution (100 g of MgCl₂.6H₂O per litre). Stir for fifteen minutes and filter through a fine, ashless filter. Store this reagent in a plastic container.

• (3.7) Liquid for washing: dilute 20 ml of the STPB solution (3.6) to 1 litre with water.

• (3.8) Hydrochloricacid(d=1.18g/ml).

APPARATUS

4

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

• (4.2) Oven regulated at 120°C±1O°C.

PREPARATION OF SAMPLE

5

See Method 1.

PROCEDURE

6

Preparation of the solution for analysis 6.1 Fertilisers containing little or no organic matter 6.1.1 Weigh to the nearest 0.001 g, 2.5 g of the prepared sample and transfer to a 400 ml beaker. Add 50 ml water and 5 ml hydrochloric acid (3.8) and evaporate to dryness on a steam bath. Add 5 ml hydrochloric acid (3.8) and 50 ml water. Bring the contents to the boiling point, breaking down any crystals or lumps with a glass rod. Dilute the solution with water to about 100 ml and boil gently for a few minutes. Allow to cool, transfer to a 250 ml graduated flask, dilute to the mark with water and mix; filter through a dry paper. Fertilisers containing organic matter 6.1.2 Weigh to the nearest 0.01 g, 10 g of the prepared sample into a suitable crucible and place in a cold muffle furnace. Gradually raise the temperature to about 475°C (not to exceed 500°C). Maintain at this temperature for at least 16 hours and then open the furnace and allow the crucible to cool. Grind the residue to eliminate any lumps, add 50 ml water and 10 ml hydrochloric acid (3.8) and evaporate to dryness on a steam bath. Proceed as in 6.1.1, commencing “Add 5 ml hydrochloric acid (3.8) and 50 ml water.”.

Deterimination 6.2 6.2.1 Transfer by pipette an aliquot part of the filtrate (6.1.1 or 6.1.2), containing 25-50 mg of potassium (30-60 mg K₂O) into a 250 ml beaker; make up to 50 ml with water. 6.2.2 To remove interferences, add 10 ml of the EDTA solution (3.5), several drops of the phenolphthalein solution (3.4) and stir in, drop by drop, sodium hydroxide solution (3.3) until it turns red, then finally add a few more drops of sodium hydroxide to ensure an excess (usually I ml of sodium hydroxide is sufficient to neutralise the sample and ensure an excess). 6.2.3 To eliminate most of the ammonia boil gently for 15 minutes. Add water to make the volume up to 60 ml. Bring the solution to the boil, remove the beaker from the heat and add 10 ml formaldehyde (3.1). Add several drops of phenolphthalein solution (3.4), and if necessary, more sodium hydroxide solution until a distinct red colour appears. Cover the beaker with a watch glass and place it on a steam bath for fifteen minutes.

Weighing the crucible 6.3 Dry the filter crucible (4.1) to constant weight in the oven at 120°C (4.2) (about 15 minutes). Allow the crucible to cool in a desiccator and then weigh it.

Precipitation 6.4 Remove the beaker from the steam bath and stir in drop by drop 10 ml of the STPB solution (3.6). This addition should take about 2 minutes; allow to stand for at least 10 minutes before filtering.

Fitlering and washing 6.5 Filter under vacuum into the weighed crucible, rinse the beaker with the liquid for washing (3.7), wash the precipitate three times with the liquid for washing (60 ml in all of the liquid for washing) and twice with 5 to 10 ml of water.

Drying and weighing 6.6 Wipe the outside of the crucible with a filter paper and place in the oven (4.2) for one and a half hours at a temperature of 120°C. Allow the crucible to cool in a desiccator to ambient temperature and weigh rapidly.

Blank test 6.7 Make a blank test under the same conditions (omitting only the sample) and allow for this in the calculation of the final result.

Control test 6.8 Carry out the determination on an aliquot part of an aqueous solution of potassium chloride, containing at the most 40 mg of K₂O.

EXPRESSION OF RESULTS

7

Calculate the percentage potassium content of the sample as K₂O, taking into account the weight of the test sample, the volume of the aliquot part taken for the determination and the value of the blank determination. (Conversion factor, KTPB to K₂O = 0.1314.)

7b. — DETERMINATION OF POTASSIUM-FLAME PHOTOMETRIC METHOD

SCOPE AND FIELD OF APPLICATION

1

This method is applicable to fertilisers in Groups 3(b), 3(c), 3(d) and 4(c) of Section A and Group 5 of Section B of the Table in Schedule 1 of the Fertilisers Regulations 1990[^f01030]in respect of which an indication of total potassium is required.

PRINCIPLE

2

The sample is ashed and dissolved in dilute hydrochloric acid or, if it contains no organic substances, it is dissolved directly in dilute hydrochloric acid. The solution is diluted and the potassium content of the extract is determined by flame photometry.

REAGENTS

3

• (3.1) Ammonia solution (30%V/V): dilute 30 ml concentrated ammonia solution (d=0.88g/ml) to 100 ml.

• (3.2) Ammonium oxalate solution: saturated aqueous solution.

• (3.3) Hydrochloric acid(d=1.18g/ml).

• (3.4) Potassium dihydrogen phosphate: dried for one hour at 105°C.

• (3.5) Potassium solution (stock): dissolve 3.4807 g potassium dihydrogen phosphate (3.4) in water and dilute to 1 litre.

• (3.6) Potassium solution (dilute): dilute 50 ml stock solution (3.5) to 1 litre with water. 1 ml contains 50 potassium (K).

APPARATUS

4

• (4.1) Flame photometer.

PREPARATION OF SAMPLE

5

See Method 1.

PROCEDURE

6

Preparation of the solution for analysis 6.1 Fertilisers containing little or no organic matter 6.1.1 Weigh to the nearest 0.001 g, 2.5 g of the prepared sample and transfer to a 400 ml beaker. Add 50 ml water and 5 ml hydrochloric acid (3.3) and evaporate to dryness on a steam bath. Add to the residue 125 ml water and 50 ml ammonium oxalate solution (3.2) and boil for 30 minutes. If necessary, a small quantity of potassium-free antifoaming agent may be added. Cool the mixture, add a slight excess of ammonia solution (3.1) and allow to cool. Transfer to a 250 ml graduated flask, dilute to the mark with water, mix and filter through a dry paper. Fertilisers containing organic matter 6.1.2 Weigh to the nearest 0.01 g, 10 g of the prepared sample into a suitable crucible and place in a cold muffle furnace. Gradually raise the temperature to about 475°C (not to exceed 500°C). Maintain at this temperature for at least 16 hours and then open the furnace and allow the crucible to cool. Grind the residue to eliminate any lumps, add 50 ml water and 10 ml hydrochloric acid (3.3) and evaporate to dryness on a steam bath. Add to the residue 125 ml water and 50 ml ammonium oxalate solution (3.2) and boil for 30 minutes. Cool the mixture, add a slight excess of ammonia solution (3.1) and allow to cool. Transfer to a 500 ml graduated flask, dilute to the mark with water, mix and filter through a dry paper.

Blank solution 6.2 Prepare a blank solution from which only the sample has been omitted and allow for this in the calculation of the final results.

Determination 6.3 Preparation of sample and blank test solutions 6.3.1 Dilute sample solutions (6.1.1 or 6.1.2) and the blank solution (6.2) to a concentration within the optimal measuring range of the flame photometer. Preparation of the calibration solutions 6.3.2 By diluting the standard solution (3.6), prepare at least five standard solutions of increasing concentration corresponding to the optimal measuring range of the flame photometer.

Measurement 6.4 Set the flame photometer to measure the potassium emission according to the manufacturer’s instructions. Spray successively, in triplicate, the standard solutions (6.3.2), the sample solution and the blank solution (6.3. 1), washing the instrument through with distilled water between each spraying. Plot the calibration curve using the median emissions as the ordinates and the corresponding concentrations of potassium in μ/ml as the abscissae. Determine the concentration of potassium in the final sample solution by reference to the calibration curve. The concentration of potassium in the final solution may be confirmed as follows: prepare two further dilutions of the standard potassium solution to contain respectively 1 mg/litre more and I mg/litre less than the estimated potassium content of the diluted solution of the sample. Successively spray the low standard solution, the diluted solution of the sample and the high standard solution. Take the median result of each of the three readings and calculate the potassium content of the sample solution.

EXPRESSION OF RESULTS

7

Calculate the percentage potassium content of the sample as K taking into account the weight of the test sample and the dilutions carried out in the course of the analysis. (Conversion factor K to K₂O=1.204.)

8. — DETERMINATION OF TOTAL MAGNESIUM

SCOPE AND FIELD OF APPLICATION

1

• (1.1) This method is applicable to all fertilisers.

PRINCIPLE

2

• (2.1) Solubilisation by boiling in dilute hydrochloric acid.

REAGENTS

3

• (3.1) Diluted hydrochloric acid:

• one volume of hydrochloric acid (d=1.18) plus one of water.

APPARATUS

4

• (4.1) Electric hot plate with adjustable temperature.

PREPARATION OF THE SAMPLE

5

• (5.1) See method 1.

PROCEDURE

6

Test sample 6.1 Magnesium is extracted from a test sample of five grams weighed to within one milligram.

Preparation of the solution 6.2 Add approximately 400 millilitres of water and, taking care when the sample contains a significant quantity of carbonates, 50 millilitres of dilute hydrochloric acid (4. 1) a small amount at a time. Bring to the boil and maintain for 30 minutes. Allow to cool, stirring occasionally. Decant quantitatively into a 500 millilitre graduated flask. Make up to volume with water and mix. Pass through a dry filter into a dry container, discarding the initial portion. The extract must be completely transparent. Stopper if the filtrate is not used immediately.

DETERMINATION OF MAGNESIUM BY ATOMIC ABSORPTION SPECTRO-PHOTOMETRY 8.2

SCOPE AND FIELD OF APPLICATION

1

This method applies to all fertiliser extracts obtained by method 8.1.

PRINCIPLE

2

• (2.1) Determination of magnesium by atomic absorption spectrophotometry after appropriate dilution of the extract.

REAGENTS

3

• (3.1) Hydrochloric acid, I M solution.

• (3.2) Hydrochloric acid, 0.5 M solution.

• (3.3) Standard solution of magnesium, 1.00 mg/ml.

• (3.3.1) Dissolve 1.013 grams of magnesium sulphate (MgSO₄.7H₂O) in the 0.5 M hydro-chloric acid solution (3.2).

• (3.3.2) Weigh out 1.658 grams of magnesium oxide (MgO), previously calcined to remove all traces of carbonation. Place in a beaker with 100 ml of water and 120 ml of I M hydrochloric acid (3. 1). When it has dissolved, decant quantitatively into a 1,000 ml graduated flask. Make up the volume by adding and mix

• or

Commercial standard solution 3.3.3 The laboratory is responsible for testing such solutions.

Strontium chloride solution 3.4 Dissolve 75 grams of strontium chloride (SrCl₂. 6H₂O) in a hydrochloric acid solution (3.2) and make up to 500 ml with the same acid solution.

APPARATUS

4

• (4.1) Spectrophotometer fitted for atomic absorption, with a magnesium lamp, set at 285.2 nm.

• (4.2) Air-acetylene flame.

PREPARATION OF THE SAMPLE

5

• (5.1) See Method 8.1.

PROCEDURE

6

• (6.1) If the fertiliser has a declared magnesium (Mg) content of more than 6% (ie 10% as MgO), take 25 ml (V1) of the extraction solution (5). Transfer into a 100 ml graduated flask, and make up to volume with water and mix. The dilution factor is D₁=1OO/V₁.

• (6.2) Using a pipette, take 10 millilitres of the extraction solution (5) or the solution (6.1). Transfer into a 200 ml graduated flask. Make up to volume with the 0.5 M hydrochloric acid solution (3.2) and mix. The dilution factor is 200/10.

• (6.3) Dilute this solution (6.2) with the 0.5 M hydrochloric acid solution (3.2) so as to obtain a concentration in the optimum working field of the spectrophotometer (4.1). V₂ is the volume of the sample in 100 ml. The dilution factor is D₂= 100/V ₂.

• The final solution should contain 10% v/v of the strontium chloride solution (3.4).

Preparation of the blank solution 6.4 Prepare a blank solution by repeating the whole procedure from the extraction (method 8.1), omitting only the test sample of fertiliser.

Preparation of calibration solutions 6.5 By diluting the standard solution (3.3) with the 0.5 M hydrochloric acid, prepare at least five calibration solutions of increasing concentration within the optimum measuring range of the apparatus (4.1). These solutions should contain 10% v/v of the strontium chloride solution (3.4).

Measurement 6.6 Set up the spectrophotometer (4.1) at a wavelength of 285.2 nm. Spray, successively, the calibration solutions (6.5), the sample solution (6.3) and the blank solution (6.4), washing the instrument through with the solution to be measured next. Repeat this operation three times. Plot the calibration curve using the mean absorbances of each of the calibrations (6.5) as the ordinates and the corresponding concentration of magnesium in μ/ml as the abscissae. Determine the concentration of magnesium in the sample (6.3), Xs, and blank (6.4), Xb, by reference to the calibration curve.

EXPRESSION OF RESULTS

7

Calculate the amount of magnesium (Mg) or magnesium oxide (MgO) in the sample by reference to the calibration solutions and taking into consideration the blank.

9a. — DETERMINATION OF BORON-TITRIMETRIC METHOD

SCOPE AND FIELD OF APPLICATION

1

This method is applicable to all fertilisers where the levels of boron are greater than 1,000 mg/kg.

PRINCIPLE

2

The sample is dissolved in acid, the solution treated with lead nitrate in order to remove phosphate and the borate in the filtrate is titrated potentiometrically in the presence of mannitol.

REAGENTS

3

• (3.1) Calcium oxide.

• (3.2) Mannitol.

• (3.3) Sodium carbonate.

• (3.4) Hydrochloric acid solution 50% (V/V): dilute 50 ml concentrated hydrochloric acid (d = 1.181 g/ml) with water to 100 ml.

• (3.5) Hydrochloric acid, 0.5 N solution.

• (3.6) Lead nitrate solution, 10 g per 100 ml.

• (3.7) Sodium hydroxide, 0.5 N solution.

• (3.8) Sodium hydroxide, 0.05 N solution, carbonate free.

• (3.9) Methyl red indicator solution: dissolve 0. 1 g of methyl red in 50 ml 95% ethanol, make up to 100 ml with water and filter if necessary.

• (3.10) Phenolphthalein indicator solution: dissolve 0.25 g phenolphthalein in 150 ml 95% alcohol and dilute with water to 250 ml.

APPARATUS

4

• (4.1) pH meter.

PREPARATION OF SAMPLE

5

See Method 1.

PROCEDURE

6

Preparation of the solution for analysis 6.1 In the absence of organic matter 6.1.1 Weigh to the nearest 0.001 g, 2 g of the prepared sample if the boron content is 0.5% or less, or 1 g if the boron content is from 0.5 — 1.0%, and place in a 400 ml beaker. Add I 00 ml water, a few drops of phenolphthalein indicator solution (3.10) and sufficient sodium carbonate (3.3) to make the solution slightly alkaline. Boil gently and keep the boiling solution alkaline, adding more sodium carbonate (3.3) as necessary until all the ammonia which may be present has been evolved. Cool the solution and add 12 ml hydrochloric acid solution (3.4). In the presence of organic matter 6.1.2 Weigh to the nearest 0.001 g, 2 g of the prepared sample if the boron content is 0.5% or less, or 1 g if the boron content is from 0.5 — 1.0%, and place it in a silica dish. Add 0.2 g calcium oxide (3. 1) for each 1 g of sample, moisten with water, mix thoroughly, evaporate the mixture to dryness and transfer the crucible to a cold muffle furnace. Raise the temperature slowly to 450±10°C and then ignite for about 3 hours. Remove the crucible from the furnace, cool and moisten the ash with 10 ml of hydrochloric acid solution (3.4). Warm the solution on a steam bath for 15 minutes, covering the dish with a watch glass. Transfer the contents of the dish quantatively into a 400 ml beaker, add a few drops of phenolphthalein indicator solution (3.10) and dilute to about 120 ml with water.

Determination 6.2 To the prepared solution (6. 1.1 or 6.1.2), add 20 ml lead nitrate solution (3.6) for each 12% P₂O, in the sample if 2 g of the sample has been used; add I 0 ml lead nitrate solution for each 12% P₂O, in the sample if 1 g of the sample has been used. Heat to boiling, remove from the source of heat and make slightly alkaline by addition of sodium carbonate (3.3). Warm the solution on a steam bath for five minutes, cool and transfer the solution quantitatively into a 200 ml graduated flask. Make up to the mark with water, mix and filter through a 24 cm filter paper[^f00007], rejecting the first 10-21 ml of the filtrate. Transfer 100 ml of the filtrate into a 250 ml beaker, add a few drops of methyl red indicator (3.9) and acidify the solution with 0.5 N hydrochloric acid solution (3.5). Heat almost to boiling, stir vigorously to remove carbon dioxide, keeping the solution acidic, by adding if necessary more 0.5 N hydrochloric acid solution (3.5). Neutralise the solution with 0.5 N sodium hydroxide solution (3.7), and then make just acid by addition of 0.5 N hydrochloric acid solution (3.5). Cover the beaker with a watch glass and boil the solution gently for 5 minutes in order to expel any remaining carbon dioxide. Cool the solution rapidly and using the pH meter (4.1), adjust the pH of the solution to 6.3 by the addition of 0.05 N sodium hydroxide solution (3.8). Add 10 g mannitol (3.2) and titrate the solution with 0.05 N sodium hydroxide solution to a pH of 6.3. Continue to add further 10 g portions of mannitol (3.2) and to re-adjust the pH to 6.3 until after the final addition of mannitol the pH remains constant at 6.3. The total amount of 0.05 N sodium hydroxide solution used after the first addition of mannitol corresponds to the amount of boron present in the sample solution. Allow a standard value of 0.1 ml 0.05 N sodium hydroxide solution as ’blank’ value.

EXPRESSION OF RESULT

7

The percentage boron content of the sample is given by the formula

$$0.1082×(T-0.1)M$ where: T = ml of 0.05 N sodium hydroxide solution used after the addition of the mannitol M = weight of the sample in grams.$

9b. — DETERMINATION OF BORON-SPECTROPHOTOMETRIC METHOD

SCOPE AND FIELD OF APPLICATION

1

This method is applicable to all fertilisers for levels of boron up to 1,000 mg/kg.

PRINCIPLE

2

The sample is ashed in the presence of calcium oxide and the residue is dissolved in hydrochloric acid. The resulting solution is treated with carmine to form a coloured complex with boron, the absorption of which is measured at 625 nm.

REAGENTS

3

• (3.1) Calcium oxide.

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

• (3.3) Carminic acid solution: dissolve 0.025 g carminic acid in sulphuric acid (3.2) and dilute to 100 ml with sulphuric acid (3.2).

• (3.4) Hydrochloric acid solution 20% (V/V): dilute 20 ml hydrochloric acid (d = 1. 1 8 g/ml) with water to 100 ml.

• (3.5.1) Boron solution (stock):

• weigh to the nearest 0.001 g, 1.905 g boric acid, dissolve in water and dilute to 1 litre with water.

• 1 ml of this solution = 0.333 mg boron.

• (3.5.2) Boron solution (working standard):

• dilute 10 ml of boric acid stock solution (3.5. 1) with water to 100 ml.

• Transfer 5, 10, 15, 20 and 25 ml respectively into separate 100 ml graduated flasks and dilute to the marks with water. These solutions contain 5, 10, 15, 20 and 25ug of boron per 3 ml of solution.

• (3.6) Hydrazine hydrate (approximately 60% W/W solution).

• WARNING: Hydrazine hydrate is toxic and corrosive, causing burns; avoid contact with eyes and skin.

APPARATUS

4

• (4.1) Spectrophotometer with 10 mm cells.

PREPARATION OF THE SAMPLE

5

See Method 1.

PROCEDURE

6

Preparation of the solution for analysis 6.1 Weigh to the nearest 0.001 g, 5 g of the prepared sample and place it in a silica dish. Add 1 g calcium oxide (3.1), moisten with water, mix thoroughly, evaporate the mixture to dryness and then transfer the crucible to a cold muffle furnace. Raise the temperature slowly to 450±10°C and ignite for about 3 hours. Remove the crucible from the furnace, cool and add hydrochloric acid solution (3.4) until the resulting mixture is acid, then add 5 ml hydrochloric acid solution (3.4) in excess. Heat the mixture at 70°C for 15 minutes, cool and filter through a filter paper[^f00008] into a suitable graduated flask washing both the dish and the filter with water. Make up to the mark with water and mix. Dilute an aliquot of this solution so that 3 ml contains between 5 and 25 μ of boron.

Blank test 6.2 Carry out a blank test omitting only the sample.

Determination 6.3 Transfer 3 ml of the prepared solution (6.1) to a small conical flask, add cautiously 15 ml sulphuric acid (3.2), swirl the flask and add 10 ml carminic acid solution (3.3). Cool the flask rapidly to room temperature, mix well and allow to stand for 2 hours. Measure the absorbance in the spectrophotometer (4.1) at 625 nm with water as reference. Determine the quantity of boron in the solution by reference to the calibration curve (6.4).

Calibration curve 6.4 Transfer 3 ml of each working standard solution (3.5.2) into a series of small conical flasks and proceed as described in 6.3 commencing at “ . . add cautiously 15 ml sulphuric acid (3.2). . .”. Plot a calibration curve of the absorbance of the solution against the corresponding quantities of boron, μg.

EXPRESSION OF RESULTS

7

The boron content in mg/kg is given by the formula:

$$A×V×F3×M$ where: A = weight of boron in the 3 ml aliquot taken for colour development after allowing for the blank reading (μ) V = volume of prepared solution before dilution F = factor allowing for dilution under 6.1 M = weight of the sample in grams.$

10. — DETERMINATION OF COBALT

SCOPE AND FIELD OF APPLICATION

1

This method is applicable to all fertilisers.

PRINCIPLE

2

The sample is dissolved in hydrochloric acid (after ashing if necessary) and the solution is treated with citric acid in order to prevent precipitation of iron and phosphate. Cobalt is extracted as its 2-nitroso-1-naphthol complex into toluene. The cobalt content is measured at 367 nm, by reference to a calibration curve.

REAGENTS

3

• (3.1) Sodium sulphate, anhydrous.

• (3.2) Toluene.

• (3.3) Hydrochloric acid, 2 N solution.

• (3.4) Hydrochloric acid solution, 50— (V/V): dilute 50 ml concentrated hydrochloric acid solution (d = 1.18 g/ml) to 100 ml with water.

• (3.5) Hydrogen peroxide solution, 3% (10 volume).

• (3.6) Nitric acid solution, 30% (V/V): dilute 30 ml nitric acid (d = 1.42 g/ml) with water to 1OOml.

• (3.7) 2-nitroso-1-naphthol solution:

• dissolve 1 g of 2-nitroso-l-naphthol in 100 ml glacial acetic acid and add 1 g activated carbon.

• Shake the solution before use and filter off the required amount.

• (3.8) Sodium citrate solution: 40 g per 100 ml.

• (3.9) Sodium hydroxide, 2 N solution.

• (3.10) Cobalt solution (stock):

• weigh to the nearest 0.001 g, 0.670 g ammonium cobaltous sulphate, [(NH₁)₂CO(SO₄)₂.6H₂O]

• dissolve in water and make up to 100 ml with water. 1 ml of this solution contains 1,000μ cobalt.

• (3.11) Cobalt solution (working standard):

• dilute the stock colbalt solution (3.10) as required so that 1 ml contains 1μ cobalt. Prepare the solution freshly before use.

APPARATUS

4

• (4.1) Spectrophotometer with 10 mm cells.

PREPARATION OF THE SAMPLE

5

See Method 1.

PROCEDURE

6

Preparation of the solution for analysis 6.1 In the absence of organic matter 6.1.1 Weigh to the nearest 0.001 g, 5 g of the prepared sample, place in a 100 ml beaker, add 10 ml hydrochloric acid solution (3.4) and evaporate to dryness on a steam bath. Extract the soluble salts with three successive 10 ml portions of boiling 2 N hydrochloric acid solution (3.3), decanting the solution each time through the same filter paper[^f00009] into a 50 ml graduated flask. Wash the filter paper with a little water, cool the solution to room temperature and make up to the mark with water. In the presence of organic matter 6.1.2 Weigh to the nearest 0.001 g, 5 g of the prepared sample into a silica dish and place a silica cover on top. Transfer the dish to a cold mute furnace, raise the temperature to about 475°C (do not exceed 500°C). Maintain at this temperature for at least 16 hours and then open the furnace and allow the crucible to cool. Add 10 ml hydrochloric acid solution (3.4) and evaporate to dryness on a steam bath. Extract the soluble salts with two successive 10 ml portions of boiling 2 N hydrochloric acid solution (3.3), decanting the solution each time through the same filter paper[^f00010] into a 50 ml graduated flask. Add 5 ml hydrochloric acid solution (3.4) and 5 ml nitric acid solution (3.6) to the residue in the dish and evaporate the mixture to dryness on a hot plate at low heat. Add 10 ml boiling hydrochloric acid solution (3.3) to the residue and filter the solution through the same filter paper into the 50 ml graduated flask. Wash the filter paper with water, cool the solution to room temperature and make up to the mark with water.