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1. This method describes a procedure for determining iron in fertiliser extracts.
2. 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. After suitable treatment and dilution of the extracts, the iron content is determined by atomic absorption spectrometry.
4.—4.1 See Method 25d (4.1).
4.2 See Method 25d (4.2).
4.3 Hydrogen peroxide solution (30% H2O2 ρ = 1.11 g/ml) free from trace element.
4.4 See Method 25d (4.3).
4.5.1In 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.
4.5.2Place 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. Atomic absorption spectrometer: see Method 25d (5). The instrument must be fitted with a source of rays characteristic of iron (248.3 nm).
6.—6.1 See Methods 25a and/or 25b and, if appropriate, 25c.
6.2 See Method 25d (6.2). The test solution must contain 10% (v/v) of a lanthanum salt solution.
7.—7.1 See Method 25d (7.1). The test solution must contain 10% (v/v) of the lanthanum salt solution used in 6.2.
7.2 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.
7.3 See Method 25d (7.3). Prepare the spectrometer (5) for measurement at a wavelength of 248.3 nm.
8. See Method 25d (8).
The percentage of iron in the fertiliser is given by:
If Method 27d is used:
where:
Fe is the quantity of iron expressed as a percentage of the fertiliser;
xs is the concentration in μg/ml of Fe in the test solution (6.2);
xb 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), . . . , (ai) and (a) are aliquot portions and (v1), (v2), (v3), . . . , (vi) and (100) are the volumes in ml corresponding to their respective dilutions, the dilution factor D is given by:
D = (v1 / a1) × (v2 / a2) × (v3 / a3)x . . . x (vi / ai) × (100 / a).
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