The Fertilisers (Sampling and Analysis) Regulations 1996

26j. DETERMINATION OF MOLYBDENUM IN FERTILISER EXTRACTS BY THE GRAVIMETRIC METHOD WITH 8-HYDROXYQUINOLINE

1.    SCOPE

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

2.    FIELD OF APPLICATION

This procedure is applicable to extracts from samples of fertilisers obtained by Methods 26a and 26b for which a declaration of molybdenum is required.

3.    PRINCIPLE

The molybdenum level is determined by precipitation as molybdenyl oxinate under specific conditions.

4.    REAGENTS

Sulfuric acid solution, approximately 1 M

Carefully pour 55 ml of sulfuric acid (H₂SO₄, ρ =1.84 g/ml) into a 1-litre volumetric flask containing 800 ml of water. Mix. After cooling, make up to one litre. Mix again.

Diluted ammonia solution (1:3)

Mix 1 volume of concentrated ammonia solution (NH₄OH, ρ = 0.9 g/ml) with 3 volumes of water.

Diluted acetic acid solution (1:3)

Mix 1 volume of concentrated acetic acid (99.7% CH₃COOH, ρ = 1.049 g/ml) with 3 volumes of water.

Solution of disodium salt of ethylene diamine tetraacetic acid (EDTA)

Dissolve 5 g of Na₂EDTA in water in a 100 ml volumetric flask. Make up to the calibration mark and mix.

Buffer solution

In a 100 ml volumetric flask, dissolve 15 ml of concentrated acetic acid and 30 g of ammonium acetate in water. Make up to 100 ml.

8-hydroxyquinoline (oxine) solution

In a 100 ml volumetric flask dissolve 3 g of 8-hydroxyquinoline in 5 ml of concentrated acetic acid. Add 80 ml of water. Add the ammonia solution (4.2) drop by drop until the solution becomes cloudy and then add the acetic acid (4.3) until the solution becomes clear again.

Make up to 100 ml with water.

5.    APPARATUS

Filter crucible P₁6/ISO 4793, porosity 4, capacity 30 ml.

pH meter with glass electrode.

Drying oven at 130 to 135°C.

6.    PREPARATION OF THE SOLUTION TO BE ANALYSED

Preparation of the molybdenum solution. See Methods 26a and 26b.

7.    PROCEDURE

Preparation of the test solution

Place an aliquot portion containing 25 to 100 mg Mo in a 250 ml beaker. Make up the volume to 50 ml with water.

Adjust this solution to pH 5 by adding the sulfuric acid solution (4.1) drop by drop. Add 15 ml of EDTA solution (4.4) and then 5 ml of buffer solution (4.5). Make up to about 80 ml with water.

Obtaining and washing the precipitate

Obtaining the precipitate

Heat the solution slightly. Stirring constantly, add the oxine solution (4.6). Continue the precipitation until formation of a deposit is no longer observed. Add further reagent until the supernatant solution turns slightly yellow. A quantity of 20 ml should normally be sufficient. Continue to heat the precipitate slightly for two to three minutes.

Filtration and washing

Filter through a filter crucible (5.1). Rinse several times with 20 ml of hot water. The rinse water should gradually become colourless indicating that oxine is no longer present.

Weighing the precipitate

Dry the precipitate at 130 to 135°C to constant weight (at least one hour).

Allow to cool in a desiccator and then weigh.

8.    EXPRESSION OF RESULTS

1 mg of molybdenyl oxinate, MoO₂(C₉H₆ON)₂, corresponds to 0.02305 mg Mo.

The percentage of molybdenum in the fertiliser is given by:

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where:

• X is the mass in mg of the molybdenyl oxinate precipitate;

• V is the volume in ml of the extract solution in accordance with Methods 26a or 26b;

• a is the volume in ml of the aliquot portion taken from the last dilution;

• D is the dilution factor of the aliquot portion;

• M is the mass in g of the test sample.