Paragraph 6 PROCEDURE

Place an aliquot portion of the solution containing not less than 20 mg Cu in a 500 ml Erlenmeyer flask.

• Drive off any excess oxygen present by boiling briefly. Make up to volume of about 100 ml water. Add 5 ml of nitric acid (4.1), bring to boiling and allow to boil for about half a minute.

• Remove the Erlenmeyer flask from the heating apparatus, add about 3 g of urea (4.2) and resume boiling for about half a minute.

• Remove from the heating apparatus and add 200 ml of cold water. Where necessary, cool the contents of the Erlenmeyer flask to ambient temperature.

• Gradually add ammonium hydroxide solution (4.4) until the solution becomes blue, then add 1 ml in excess.

• Add 50 ml of ammonium bifluoride solution (4.3) and mix.

• Add 10 g of potassium iodide (4.6) and allow it to dissolve.

Place the Erlenmeyer flask on a magnetic stirrer. Insert the rod into the Erlenmeyer flask and adjust the stirrer to the desired speed.

• Using a burette, add standard sodium thiosulfate solution (4.5) until the brown colour of the iodine released from the solution becomes less intense.

• Add 10 ml of the starch solution (4.8).

• Continue to titrate with the sodium thiosulfate solution (4.5) until the purple colour has almost disappeared.

• Add 20 ml of the potassium thiocyanate solution (4.7) and continue titration until the violet blue colour has completely disappeared.

• Note the volume of thiosulfate solution employed.