Paragraph 7 PROCEDURE

Prepare a blank solution by repeating the whole procedure from the extraction stage, omitting only the test sample of fertiliser.

Prepare a series of at least six calibration solutions of increasing concentration corresponding to the optimum response range of the spectrometer.

• For the interval 0-12.5 μg molybdenum, place 0, 1, 2, 3, 4 and 5 ml, respectively, of the working solution (4.7.3) in the separating funnels (5.2). Make up to 50 ml with 6 M hydrochloric acid (4.1). The funnels contain, respectively, 0, 2.5, 5, 7.5, 10 and 12.5 mg molybdenum.

To each separating funnel (6.2, 7.1 and 7.2), add in the following order:

• — 10 ml of the copper solution (4.2);

• — 20 ml of the ascorbic acid solution (4.3);

• mix thoroughly and wait for two or three minutes. Then add:

  • — 10 ml of n-butyl acetate (4.4), using a precision pipette;

  • — 20 ml of the thiocyanate solution (4.5).

• Shake for one minute to extract the complex into the organic phase; allow to separate; after the separation of the two phases, draw off the entire aqueous phase and discard it; then wash the organic phase with:

  • — 10 ml of the stannous chloride solution (4.6).

• Shake for one minute. Allow to separate and draw off the entire aqueous phase. Remove the organic phase in a test tube; this will make it possible to collect the drops of water in suspension.

Measure the absorbencies of the solutions obtained at 7.3 at a wavelength of 470 nm using the 0 mg/ml molybdenum calibration solution (7.2) as a reference.