Page 162 - Analytical method for food addtives

E249–50: Nitrites 107

Grind the test sample by means of an appropriate device. Mix the ground mass
quickly and, if possible, grind a second time and again mix thoroughly. If the
sample cannot be ground, mix it thoroughly by intensive stirring and kneading.
Transfer the test sample to an airtight sample container to await the determination
which should be carried out soon after grinding. If a delay is unavoidable, take all
precautions to ensure proper conservation of the test sample and to prevent
condensation of moisture on the inside surface of the container. Ground cheese
showing unwanted mould growth or beginning to deteriorate should, however, not
be examined. Clean the device after grinding of each sample.

Preparation of test sample: whey cheese
Prepare the test sample as specified.

Preparation of the copperised cadmium column
Transfer an amount of cadmium granules of approximately between 40 g and 60 g
for each column into a 250 mL conical flask. Add sufficient of hydrochloric acid
working solution to cover the cadmium. Swirl for a few minutes. Decant the
solution. Wash the cadmium in the conical flask with water, until it is free from
chloride (i.e. until reaction with silver nitrate is negative).
Copperise the cadmium granules by adding the copper (II) sulphate solution of
which about 2.5 mL copper solution per g of cadmium is needed and swirl for 1
min. Decant the solution and wash the copperised cadmium immediately with
water, taking care that the cadmium is continuously covered with water. Terminate
the washing when the wash water is free from precipitated copper.
Fit a glass wool plug to the bottom of the glass column intended to contain the
2
copperised cadmium (see Fig. 1 ). Fill the glass column with water. Transfer the
copperised cadmium into the glass column with minimum exposure to air. The
height of the copperised cadmium should be 15 cm to 20 cm. Avoid trapping air
bubbles between the copperised cadmium granules. The level of the liquid should
not fall below the top of the copperised cadmium.
Condition the newly prepared column by running through it a mixture of
750 mL of water, 225 mL of the standard potassium nitrate solution, 20 mL of the
buffer solution and 20 mL of the EDTA solution at a flow rate not exceeding 6 mL/
min. Wash the column with 50 mL of water.

Checking the reducing capacity of the column
Check the column at least twice a day, but also at the beginning and at the end of
a series of determinations for its capacity.
Pipette 20 mL of the standard potassium nitrate solution into the reservoir on
top of the column. Immediately add 5 mL of the buffer solution to the contents of
the reservoir. Run the contents of the reservoir through the column at a flow rate
not exceeding 6 mL/min. Collect the eluate in a 100 mL volumetric flask. When
the reservoir of the reduction column is nearly empty, wash the walls of the
reservoir with about 15 mL of water. Repeat the washing with another 15 mL of
water after the water has run off. When the second washing has run into the