Paragraph 5 PROCEDURE

Weigh 25 grams, to the nearest 0.001 gram, of the sample into a 400 ml beaker, add carefully 20 ml of 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 M hydrochloric acid solution (3.2) and 120 ml of 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 is complete and then cool.

• Transfer the solution quantitatively into a 250 ml graduated flask, by washing the beaker with 5 ml 6 M hydrochloric acid (3.2), and twice with 5 ml of boiling water, cool and make up to the mark with 0.5 M hydrochloric acid (3.3) and mix carefully.

• Filter through a copper-free filter paper^f1, discarding the first 50 ml.

Prepare a blank solution from which only the sample has been omitted and allow for this in the calculation of the final result.

Preparation of sample and blank test solutions

Preparation of the calibration solutions

Set up the spectrometer (4) at a wavelength of 324.8 nm and use an oxidising air-acetylene flame. Spray, in triplicate, the calibration solutions (5.3.2), the sample solution and the blank solution (5.3.1), washing the instrument through with distilled water between each spraying. Plot the calibration curve using the mean absorbances of every standard used as the ordinates and the corresponding concentrations of copper in μg/ml as the abscissae.

• Determine the concentration of copper in the final sample and blank solutions by reference to the calibration curve.