The Fertilisers (Sampling and Analysis) Regulations 1996

26k. DETERMINATION OF ZINC IN FERTILISER EXTRACTS BY ATOMIC ABSORPTION SPECTROMETRY

1.    SCOPE

This method describes a procedure for determining zinc 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 zinc is required.

3.    PRINCIPLE

After suitable treatment and dilution of the extracts, the zinc level is determined by atomic absorption spectrometry.

4.    REAGENTS

Hydrochloric acid solution, about 6 M

See Method 26d (4.1).

Hydrochloric acid solution, about 0.5 M

See Method 26d (4.2).

Lanthanum salt solutions (10 g of La per litre)

See Method 26d (4.3).

Zinc calibration solutions

4.4.1. Zinc stock solution (1000 μg/ml)

In a 1 litre volumetric flask dissolve 1 g of zinc powder or flakes weighed to within 0.1 mg in 25 ml of 6 M hydrochloric acid (4.1). When completely dissolved, make up to volume with water and mix thoroughly.

4.4.2. Zinc working solution (100 μg/ml)

In a 200 ml volumetric flask, dilute 20 ml of the stock solution (4.4.1) in 0.5 M hydrochloric acid solution (4.2). Make up to volume with the 0.5 M hydrochloric acid solution and mix thoroughly.

5.    APPARATUS

Atomic absorption spectrometer

See Method 26d (5). The apparatus must be fitted with a source of rays characteristic of zinc (213.8 nm). The spectrometer must allow background correction to be made.

6.    PREPARATION OF THE SOLUTION TO BE ANALYSED

Zinc extract solution

See Methods 25a and/or 26b.

Preparation of the test solution

See Method 26d, (6.2). The test solution must contain 10% by volume of lanthanum salt solution (4.3).

7.    PROCEDURE

Preparation of the blank solution.

See Method 26d. (7.1). The blank solution must contain 10 % by volume of the lanthanum salt solution used in 6. 2.

Preparation of the calibration solutions

See Method 26d (7.2). For an optimum interval of 0 to 5 μg/ml of zinc, place 0, 0.5, 1, 2, 3, 4 and 5 ml, respectively, of the working solution (4.4.2) in a series of 100 ml volumetric flasks. Where necessary, adjust the concentration of hydrochloric acid to bring it as close as possible to that of the test solution. Add 10 ml of the lanthanum salt solution used in (6.2) to each volumetric flask. Make up to 100 ml with the 0.5 M hydrochloric acid solution (4.2) and mix thoroughly.

These solutions contain, respectively: 0, 0.5, 1, 2, 3, 4 and 5 μg/ml of zinc.

Determination

See Method 26d (7.3). Prepare the spectrometer (5) for measurements at a wavelength of 213.8 nm.

8.    EXPRESSION OF RESULTS

See Method 26d (8).

The percentage of zinc in the fertiliser is given by:

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If method 26c has been used:

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where:

• Zn is the quantity of zinc expressed as a percentage of the fertiliser;

• x_s is the concentration in μg/ml of the test solution;

• x_b is the concentration in μg/ml of the blank solution;

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

• 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 Methods 26a or 26b.

Calculation of the dilution factor D: where (a₁), (a₂), (a₃), . . . , (a_i) and (a) are successive aliquot portions and (v₁), (v₂), (v₃), . . . , (v_i) and (100) are the volumes in ml corresponding to their respective dilutions, the dilution factor D will be:

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