The Fertilisers (Sampling and Analysis) (Amendment) Regulations 1991

Regulation 4(2)(d)

SCHEDULE 4 PROVISIONS TO BE INSERTED IN PART II OF SCHEDULE 2 TO THE PRINCIPAL REGULATIONS

8.    DETERMINATION OF ZINC

1.    Scope and field of application

This method is for the determination of zinc.

2.    Principle

The sample is dissolved directly in hydrochloric acid. The element is determined by atomic absorption spectrophotometry.

3.    Reagents

3.1 Hydrochloric acid (d = 1.18g/ml).

3.2 Hydrochloric acid solution, 6N.

3.3 Hydrochloric acid solution, 0.5N.

3.4 Zinc solution (stock): weigh to the nearest 0.001g, 1g pure zinc, dissolve in 25ml 6N hydrochloric acid (3.2) and make up to 1,000ml with water. 7ml of this solution = 1000 μg zinc (Zn).

3.4.1 Zinc solution (dilute): dilute 10.0ml of stock solution (3.4) to 1,000ml with water.

1 ml of this solution = 10 μg zinc (Zn).

NOTE:commercially prepared standard solutions of zinc may also be used.

3.5 Lanthanum chloride solution: dissolve 12g lanthanum oxide in 150ml water, add 100ml 6N hydrochloric acid (3.2) and dilute to 1,000ml with water.

4.    Apparatus

Atomic absorption spectrophotometer fitted with zinc lamp set at 213.8nm.

5.    Procedure

5.1 Preparation of the solution for analysis.

Weigh to the nearest 0.001g, 5g of the prepared sample into a 250ml beaker, add 5ml hydrochloric acid (3.1) (take suitable precautions if there is a vigorous reaction due to carbon dioxide formation). If necessary add more hydrochloric acid (3.1), stirring until all effervescence has stopped. Evaporate the solution to dryness, occasionally stirring with a glass rod. Add 15ml 6N hydrochloric acid solution (3.2) and 120ml water. Stir with the glass rod, which should be left in the beaker, and cover with a watch glass. Boil the solution gently until dissolution appears complete and filter through a Whatman 541 (or equivalent) filter paper into a 250ml graduated flask. Wash the beaker and filter with 5ml of hot 6N hydrochloric acid solution (3.2) and with boiling water. Cool and make up to the mark with water. (The hydrochloric acid concentration of the solution should be about 0.5N.)

Dissolve the residue in 2ml hydrochloric acid (3.1), evaporate to dryness and add 5ml 6N hydrochloric acid solution (3.2). Heat, filter the solution into the 250ml graduated flask and make up to the mark with water. (The hydrochloric acid concentration of the solution should be about 0.5N.)

5.2 Blank test.

Carry out a blank determination repeating the procedure but omitting the sample.

5.3 Determination.

5.3.1 Preparation of the sample and blank test solutions.

Dilute the sample solutions prepared in 5.1 and the blank test solution (5.2) with 0.5N hydrochloric acid solution (3.3) to a concentration within the working range of the spectrophotometer (4). Each final solution must contain 10 % (V/V) of the lanthanum chloride solution (3.5).

5.3.2 Preparation of the calibration solutions.

By diluting the standard solution 3.4.1 with 0.5N hydrochloric acid solution (3.3) prepare at least 5 standard solutions of increasing concentrations corresponding to the optimal measuring range of the spectrophotometer. The final solutions must contain 10 % (V/V) lanthanum chloride solution (3.5).

5.4 Measurement.

Set up the spectrophotometer for the determination of zinc (213.8nm) using an oxidising air-acetylene flame. Spray successively, in triplicate, the standard solutions (5.3.2), the sample solutions and the blank test solutions (5.3.1), washing the instrument through with distilled water between each spraying. Plot the calibration curve, using the mean absorbances as the ordinates and the corresponding concentrations of zinc in μ/gml as the abscissae. Determine the concentration of the zinc in the final sample and blank solutions by reference to the calibration curves.

6.    Expression of the results

Calculate the zinc content of the sample taking into account the weight of the test sample and the dilutions carried out in the course of the analysis. Express the result either as a percentage or as mg/kg.”