Division 6 Execution of the reference procedure

When working with perchloric acid, which is strongly corrosive, necessary caution and preventive measures shall be taken. The samples shall be prepared as soon as possible after their arrival, in accordance with the following instructions:

1. (a)

   Preparing the sample

   The sample to be analysed is ground carefully using a meat grinder as described in point 5(a). An amount of 10 g ± 0,1 g of the ground sample is weighed out into a suitable container. This is mixed with 90,0 ml perchloric acid solution, homogenised for two minutes with a blender as described in point 5(b), and then filtered.

   The extract thereby obtained can be kept for at least seven days at a temperature of between approximately 2 °C and 6 °C;

2. (b)

   Steam distillation

   50,0 ml of the extract obtained in accordance with point (a) is put into an apparatus for steam distillation as described in point 5(e). For a later check on the extract's alkalinisation, several drops of phenolphtalein solution are added. After adding a few drops of silicone anti-foaming agent, 6,5 ml of sodium hydroxide solution is added to the extract and steam distillation begins immediately.

   The steam distillation is regulated so that around 100 ml of distillate is produced in 10 minutes. The distillation outflow tube is submerged in a receiver with 100 ml boric acid solution, to which three to five drops of the indicator solution have been added. After exactly 10 minutes, distillation is ended. The distillation outflow tube is removed from the receiver and washed out with water. The volatile bases contained in the receiver solution are determined by titration with hydrochloric acid standard solution.

   The pH of the end point must be 5,0 ± 0,1;

3. (c)

   Titration

   Duplicate analyses are required. The applied method is correct if the difference between the duplicates is not greater than 2 mg/100 g;

4. (d)

   Blank

   A blind test is carried out as described in point (b). Instead of the extract, 50,0 ml perchloric acid solution is used.