The Fertilisers (Sampling and Analysis) Regulations 1996

25j. DETERMINATION OF MOLYBDENUM IN FERTILISER EXTRACTS BY SPECTROMETRY OF A COMPLEX WITH AMMONIUM THIOCYANATE

“1.    SCOPE

This method describes a procedure for determining molybdenum in fertiliser extracts.

“2.    FIELD OF APPLICATION

This procedure is applicable to analysing samples of fertilisers extracted by Methods 25a and 25b for which a declaration of total and/or water-soluble molybdenum is required.

“3.    PRINCIPLE

Molybdenum(v) forms a complex [MoO(SCN)5] – – in an acid medium with SCN – ions. The complex is extracted with n-butyl acetate. Interfering ions such as those of iron remain in the aqueous phase. The yellow-orange colour is determined by molecular absorption spectrometry at 470 nm.

“4.    REAGENTS

Dilute hydrochloric acid solution (HCI), about 6 M

See method 25d (4.1).

Copper solution (70 mg/l) in 1.5 M hydrochloric acid

Dissolve 275 mg of copper sulfate (CuSO₄.5H₂O) weighed to within 0.1 mg in 250 ml of the 6 M hydrochloric acid solution (4.1) in a 1,000 ml volumetric flask. Make up to volume with water and mix thoroughly.

Ascorbic acid solution (50 g/l)

Dissolve 50 g of ascorbic acid (C₆H₈O₆) in water in a 1,000 ml volumetric flask. Make up to volume with water, mix thoroughly and keep in a refrigerator.

n-butyl acetate

Ammonium thiocyanate solution, 0.2 M

Dissolve 15.224 g of NH₄SCN in water in a 1,000 ml volumetric flask. Make up to volume with water; mix thoroughly and store in a dark-coloured bottle.

Stannous chloride solution (50 g/l) in 2 M hydrochloric acid

This solution must be perfectly clear and prepared immediately before use. Very pure stannous chloride must be used otherwise the solution will not be clear.

To prepare 100 ml of solution, dissolve 5 g of SnCl₂.2H₂O in 35 ml of 6 M HCI solution (4.1). Add 10 ml of the copper solution (4.2). Make up to volume with water and mix thoroughly.

Molybdenum calibration solutions

4.7.1. Molybdenum stock solution (500 μg/ml)

Dissolve 0.920 g of ammonium molybdate [(NH₄)6 Mo₇O₂4.4H₂O] weighed to within 0.1 mg in the 6 M hydrochloric acid (4.1) in a 1 litre volumetric flask. Make up to volume with that solution and mix thoroughly.

4.7.2. Molybdenum intermediate solution (25 μg/ml)

Place 25 ml of the stock solution (4.7.1) in a 500 ml volumetric flask. Make up to volume with 6 M hydrochloric acid (4.1) and mix thoroughly.

4.7.3. Molybdenum working solution (2.5 μg/ml)

Place 10 ml of the intermediate solution (4.7.2) in a 100 ml volumetric flask. Make up to volume with 6 M hydrochloric acid (4.1) and mix thoroughly.

“5.    APPARATUS

Spectrometer fitted for molecular absorption with cells having a 20 mm optical path and set to a wavelength of 470 nm.

200 or 250 ml separating funnels.

“6.    PREPARATION OF THE SOLUTION TO BE ANALYSED

Molybdenum extract solution

See Methods 25a and/or 25b and, if appropriate, 25c.

Preparation of the test solution

Dilute an aliquot portion of the extract (6.1) with 6 M hydrochloric acid solution (4.1) so as to obtain an appropriate molybdenum concentration. Let D be the dilution factor.

Take an aliquot portion (a) from the extract solution containing 1 to 12 μg molybdenum and place it in the separating funnel (5.2). Make up to 50 ml with the 6 M hydrochloric acid solution (4.1).

“7.    PROCEDURE

Preparation of the blank solution

Prepare a blank solution by repeating the whole procedure from the extraction stage, omitting only the test sample of fertiliser.

Preparation of the series of calibration solutions

Prepare a series of at least six calibration solutions of increasing concentration corresponding to the optimum response range of the spectrometer.

For the interval 0 – 12.5 μg molybdenum, place 0, 1, 2, 3, 4, and 5 ml, respectively, of the working solution (4.7.3) in the separating funnels (5.2). Make up to 50 ml with 6 M hydrochloric acid (4.1). The funnels contain, respectively, 0, 2.5, 5, 7, 5, 10 and 12.5 μg molybdenum.

Development and separation of the complex

To each separating funnel (6.2, 7.1 and 7.2), add in the following order:

• — 10 ml of the copper solution (4.2)

• — 20 ml of the ascorbic acid solution (4.3);

• mix thoroughly and wait for two or three minutes. Then add:

• — 10 ml of n-butyl acetate (4.4), using a precision pipette;

• — 20 ml of the thiocyanate solution (4.5).

Shake for one minute to extract the complex into the organic phase; allow to separate; after the separation of the two phases, draw off the entire aqueous phase and discard it; then wash the organic phase with:

• — 10 ml of the stannous chloride solution (4.6).

Shake for one minute. Allow to separate and draw off the entire aqueous phase. Remove the organic phase in a test tube; this will make it possible to collect the drops of water in suspension.

Determination

Measure the absorbencies of the solutions obtained at 7.3 at a wavelength of 470 nm using the 0 μg/ml molybdenum calibration solution (7.2) as a reference.

“8.    EXPRESSION OF RESULTS

Construct the calibration curve by plotting the corresponding masses of molybdenum in the calibration solutions (7.2) expressed in μg along the abscissa and the corresponding values of the absorbencies (7.4) given by the spectrometer reading along the ordinate.

From this curve determine the mass of molybdenum in the test solution (6.2) and the blank solution (7.1). These masses are designated (x_s) and (x_b) respectively.

The percentage of molybdenum in the fertiliser is:

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If Method 25c has been used:

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where:

• Mo is the quantity of molybdenum expressed as a percentage of the fertiliser;

• a is the volume in ml of the aliquot taken from the last dilute solution (6.2);

• x_s is the mass in μg of Mo in the test solution (6.2);

• x_b is the mass in μg of Mo in the blank solution (7.1) the volume of which corresponds to the volume (a) of the aliquot of the test solution (6.2);

• V is the volume in ml of the extract solution obtained in accordance with Method 25a or 25b;

• D is the factor corresponding to the dilution performed in 6.2;

• M is the mass in g of the test sample taken in accordance with Method 25a or 25b.

Calculation of the dilution factor D: where (a₁), (a₂) are successive aliquot portions and (v₁), (v₂) are the volumes corresponding to their respective dilutions, the dilution factor D will be:

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