manual of methods of analysis of foods - Ministry of Health and ...

METALS
3.4 DETERMINATION OF COPPER IN FOOD
BY COLORIMETRIC CARBAMATE METHOD (IUPAC METHOD)
Principle:
The sample is digested with HNO 3 and H 2 SO 4 . Copper is isolated and
determined calorimetrically at pH 8.5 as diethyl dithiocarbamate in presence
of chelating agent EDTA. Bi and Te also give coloured carbamates at pH
8.5, but are decomposed with 1N NaOH. Range of colour measurement is 0
to 50 µg.
Reagents:
(a) Sodium diethyldithio carbamate solution: (1.0% w/v)
Dissolve 1.0 g of the salt in water and dilute to 100 ml with water and
filter. Store in refrigerator and prepare weekly.
(b) Citrate-EDTA solution:
Dissolve 20 g of dibasic ammonium citrate and 5 g Na 2 EDTA in
water and dilute to 100 ml. Remove traces of copper by adding 0.1 ml
of carbamate solution and extracting with 10 ml CCl 4 . Repeat
extraction until CCl 4 extract is colourless.
(c) Ammonium hydroxide – 6N:
Purify as in (b) above.
(d) Copper Standard solution:
(i) Stock solution:
Place 0.2500 g of Cu wire or foil in 125 ml Erlenmeyer flask.
Add 15 ml of HNO 3 (1+4), cover the flask with watch glass and
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