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

METALS
Determination:
Pipette suitable aliquot of sample digest (containing not more than 50 µg of
copper) into a short stem separator, add 2N H 2 SO 4 to make total volume of
25 ml and add 10 ml of citrate EDTA reagent. Add two drops of thymol blue
indicator and 6N NH 4 OH dropwise until solution turns green or blue-green.
Cool and add 1 ml of carbamate solution and 15 ml of CCl 4 . Shake
vigorously for 2 min. Let layers separate and drain CCl 4 through cotton
pledget into glass stoppered tube. Determine absorbance ‘A’ at 400 nm.
To test for Bi and Te, return CCl 4 solution to separator, add 10 ml 5.0%
KCN solution and shake for 1 min. If CCl 4 layer becomes colourless Bi and
Te are absent. If test is positive, develop colour in another aliquot of digest
solution as above (without KCN). Drain CCl 4 layer into second separator,
add 10 ml of 1N NaOH and shake for 1 min. Let layers separate and drain
CCl 4 into third separator. Again wash CCl 4 extract with 10 ml of 1N NaOH.
Determine absorbance of CCl 4 layer and convert to µg of Cu.
Preparation of Standard Curve:
Transfer 0, 1, 2.5, 5, 10 and 25 ml of copper standard solution (2 µg/ml) to
separators and add 2N H 2 SO 4 to make total volume of 25 ml. Add 10 ml
citrate EDTA reagent and proceed as for sample as given in ‘Determination’
beginning’ Add two drops of thymol blue indicator.
Plot absorbance ‘A’ against µg of copper. Since there is usually some
deviation from linearity read sample values from smoothened curve.
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