The Fertilisers (Sampling and Analysis) Regulations 1996 No. 1342

SCHEDULE 2 METHODS OF ANALYSIS

PART I

25h. DETERMINATION OF IRON IN FERTILISER EXTRACTS BY ATOMIC ABSORPTION SPECTROMETRY

1.    SCOPE

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

3.    PRINCIPLE

After suitable treatment and dilution of the extract, the iron content is determined by atomic absorption spectrometry.

4.    REAGENTS

Hydrochloric acid solution, about 6M.

See Method 25d (4.1).

Hydrochloric acid solution, about 0.5 M

See Method 25d (4.2).

Hydrogen peroxide solution (30% H₂O₂ p = 1.11 g/ml) free from trace element.

Lanthanum salt solutions (10 g of La per litre)

See Method 25d (4.3).

Iron calibration solutions

4.5.1. Iron stock solution (1,000 μg/ml)

In a 500 ml beaker, weigh to the nearest 0.1 mg, 1 g of pure iron wire, add 200 ml of 6 M hydrochloric acid (4.1) and 15 ml of hydrogen peroxide solution (4.3). Heat on a hotplate until the iron is completely dissolved. When cool, transfer quantitatively to a 1 litre volumetric flask. Make up to volume with water and mix thoroughly.

4.5.2. Iron working solution (100 μg/ml)

Place 20 ml of the stock solution (4.5.1) in a 200 ml volumetric flask. Make up to volume with the 0.5 M hydrochloric acid solution (4.2) and mix thoroughly.

5.    APPARATUS

Atomic absorption spectrometer: see Method 25d (5). The instrument must be fitted with a source of rays characteristic of iron (248.3 nm).

6.    PREPARATION FOR THE SOLUTION TO BE ANALYSED

Iron extract solution

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

Preparation of the test solution

See Method 25d (6.2). The test solution must contain 10% (v/v) of a lanthanum salt solution.

7.    PROCEDURE

Preparation of blank solution

See Method 25d (7.1). The test solution must contain 10% (v/v) of the lanthanum salt solution used in 6.2.

Preparation of calibration solutions

See Method 25d (7.2).

For an optimum determination range of 0 to 10 μg/ml of iron, place 0, 2, 4, 6, 8 and 10 ml respectively of working solution (4.5.2) in a series of 100 ml volumetric flasks. If necessary adjust the hydrochloric acid concentration as closely as possible to that of the test solution. Add 10 ml of the lanthanum salt solution used in 6.2. Make up to volume with 0.5 M hydrochloric acid solution (4.2) and mix thoroughly. These solutions contain 0, 2, 4, 6, 8 and 10 μg/ml respectively of iron.

Determination

See Method 25d (7.3). Prepare the spectrometer (5) for measurement at a wavelength of 248.3nm.

8.    EXPRESSION OF RESULTS

See Method 25d (8).

The percentage of iron in the fertiliser is given by:

Fe% =  [ (x_s - x_b) × V x D ]  / M x 10₄)

If Method 27d is used:

Fe% =  [ (x_s - x_b) × V x 2D ]  / M x 10₄)

where:

• Fe is the quantity of iron expressed as a percentage of the fertiliser;

• x_s is the concentration in μg/ml of Fe in the test solution (6.2);

• x_b is the concentration in μg/ml of Fe in the blank solution (7.1);

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

• D is the factor of the dilution carried out in 6.2;

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

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

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