Paragraph 7 PROCEDURE

If the fertiliser has a declared magnesium (Mg) content of more than 6% (i.e. 10% as MgO), take 25 ml (V₁) of the extraction solution (6). Transfer into a 100 ml graduated flask, and make up to volume with water and mix. The solution factor is D₁ = 100/V₁.

Using a pipette, take 10 ml of the extraction solution (6) or the solution (7.1). Transfer into a 200 ml graduated flask. Make up to the volume with the 0.5 M hydrochloric acid solution (4.2) and mix. The dilution factor is 200/10.

Dilute this solution (7.2) with the 0.5 M hydrochloric acid solution (4.2) so as to obtain a concentration in the optimum working field of spectrometer (5.1). V₂ is the volume of the sample in 100 ml. The dilution factor is D₂ = 100/V₂.

Prepare a blank solution by repeating the whole procedure from the extraction (method 15 or 17), omitting only the test sample of fertiliser.

By diluting the standard solution (4.3) with 0.5 M hydrochloric acid, prepare at least five calibration solutions in increasing concentrations within the optimum measuring range of the apparatus (5.1).

• These solutions should contain 10% v/v of the strontium chloride solution (4.4).

Set up the spectrometer (5.1) at a wavelength of 285.2 nm.

• Spray, successively, the calibration solutions (7.5), the sample solution (7.3) and the blank solution (7.4), washing the instrument through with the solution to be measured next. Repeat this operation three times. Plot the calibration curve using the mean absorbances of each of the calibration solutions (7.5) as the ordinates and the corresponding concentration of magnesium in μg/ml as the abscissae. Determine the concentration of magnesium in the sample (7.3), x_s and blank (7.4), x_b, by reference to the calibration curve.