Transfer a suitable aliquot of the solution prepared in 6.1 (containing 6.2.2 not more than 15 μg cobalt) to a 100 ml-beaker, add 15 ml sodium citrate solution (3.8), dilute to about 50 ml with water and adjust the pH to between 3 and 4 by adding 2 N hydrochloric acid solution (3.3). (A precipitate of ferric hydroxide may form but this can be dissolved by heating the solution). Cool to room temperature, add 10 ml hydrogen peroxide solution (3.5) and, after 5 minutes, 1 ml 2-nitroso-1-naphthol solution (3.7). Heat the solution to about 90°C and then allow to stand for 30 minutes at room temperature. Transfer the solution to a 125 ml separating funnel, add 10 ml toluene (3.2), shake vigorously for 2 minutes, allow the phases to separate and discard the lower aqueous phase.

To the toluene extract add 20 ml 2 N hydrochloric acid solution (3.3), shake for 1 minute and discard the lower aqueous phase. Add 20 ml 2 N sodium hydroxide solution (3.9), shake for 1 minute and again discard the aqueous phase. Repeat the washing with a further 20 ml of 2 N sodium hydroxide solution (3.9). Finally run off the toluene solution through a little anhydrous sodium sulphate (3.1) into a clean dry stoppered tube. Carry out a blank determination repeating the procedure, but omitting the sample. Measure the absorbance of the magenta coloured solutions at a wave length of 367 nm in the spectrophotometer (4.1) with toluene (3.2) as reference. Determine the quantity of cobalt in the solution by reference to the calibration curve (6.3).