PROCEDURE
Preparation of the solution for analysis
6.1.1 In the absence of organic matter
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(1) 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).
6.1.2 In the presence of organic matter
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(1) 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.)
Blank solution
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.
Determination
6.3.1 Preparation of sample and blank test solutions
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.
6.3.2 Preparation of the calibration solution
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.
Measurement
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.
Whatman 541 or equivalent.