1. This method is applicable to all fertilisers.
2. The sample is ashed and dissolved in dilute hydrochloric acid or, if it contains no organic substances, it is dissolved directly in dilute hydrochloric acid. The solution is diluted and the copper content is determined by atomic absorption spectrophotometry.
3.—3.1 Hydrochloric acid (d = 1. 18 g/ml).
3.2 Hydrochloric acid, 6 N solution.
3.3 Hydrochloric acid, 0.5 N solution.
3.4 Hydrogen peroxide, approximately 100 volume, 30% by weight.
3.5.1Copper solution(1) (stock):
weigh to the nearest 0.001 g, 1 g pure copper, dissolve in 25 ml 6 N hydrochloric acid solution (3.2), add 5 ml hydrogen peroxide (3.4) and dilute to 1 litre with water. 1 ml of this solution = 1,000 μ of copper (Cu).
3.5.2Copper solution (dilute):
dilute 10 ml of stock solution (3.5.1) to 100 ml with water and then dilute the resulting solution, 10 ml to I 00 ml with water. 1 ml of the final dilution = 10μ of copper (Cu).
4.—4.1 Atomic absorption spectrophotometer with a copper lamp (324.8 nm).
5. See Method 1.
6.1.1Weigh to the nearest 0.001 g, 5 g of the prepared sample, place it in a 400 ml beaker, add carefully 5 ml hydrochloric acid (3.1) (there may be a vigorous reaction due to carbon dioxide formation). Add more hydrochloric acid, if necessary. When effervescence has stopped, evaporate to dryness on a steam bath, stirring occasionally with a glass rod. Add 15 ml 6 N hydrochloric acid solution (3.2) and 120 ml water. Stir with the glass rod, which should be left in the beaker, and cover the beaker with a watch glass. Boil the solution gently until dissolution appears complete and then filter through a filter paper(2) into a 250 ml graduated flask. Wash the beaker and filter with 5 ml hot 6 N hydrochloric acid solution (3.2) and twice with boiling water. Cool and make up to the mark with water (the hydrochloric acid concentration of this solution should be about 0.5 N).
6.1.2Weigh to the nearest 0.001 g, 5 g of the prepared sample into a silica or platinum crucible and place the crucible into a cold mute furnace. Close the furnace and gradually raise the temperature to 450-475°C over about 90 minutes. Maintain this temperature for at least 16 hours and then open the furnace and allow the crucible to cool. Moisten the ash with water and transfer it into a 250 ml beaker. Wash the crucible with about 5 ml hydrochloric acid (3. 1) and add the latter slowly and carefully to the beaker (there may be a vigorous reaction due to carbon dioxide formation). If necessary, add more hydrochloric acid (3.1) with stirring, until all effervescence has stopped. Evaporate the solution to dryness, occasionally stirring with a glass rod. Add 15 ml 6 N hydrochloric acid solution (3.2) and 120 ml 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 filter paper(2) into a 250 ml graduated flask. Wash the beaker and filter with 5 ml of hot 6 N hydrochloric acid solution (3.2) and twice with boiling water. Cool and make up to the mark with water. (The hydrochloric acid concentration of this solution should be about 0.5 N.)
6.2 Prepare a blank solution from which only the sample has been omitted and allow for this in the calculation of the final results.
6.3.1Dilute the sample solutions (6.1.1 or 6.1.2) and the blank test solution (6.2) with 0.5 N hydrochloric acid solution (3.3) to a concentration within the optimal measuring range of the spectrophotometer.
6.3.2By diluting the standard solution (3.5.2) with 0. 5 N hydrochloric acid solution (3.3) prepare at least 5 standard solutions corresponding to the optimal measuring range of the spectrophotometer.
6.4 Set up the spectrophotometer (4.1) at a wavelength of 324.8 nm using an oxidising air-acetylene flame. Spray successively, in triplicate, the standard solution (6.3.2), the sample solution and the blank test solution (6.3.1), 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 copper μ/ml as the abscissae.
Determine the concentration of copper in the final sample and blank solution by reference to the calibration curve.
7. Calculate the copper 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.
Commercially available standard copper solution may be used.
Whatman 541 or equivalent.