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

METALS
approximately 8.8 by adding NH 4 OH slowly while cooling intermittently,
until solution changes from yellowish green to greenish blue. Dilute to 125
ml mark with water. Extract vigorously with 5 ml portions of dithizone in
chloroform solution until chloroform layer remains green. Then extract with
3 ml of chloroform.
Transfer all chloroform extracts to second separator previously wetted with
2 to 3 ml CHCl 3 . Add 40 ml of 0.2N HCl to combined extracts, shake
vigorously for more than a minute and let layers separate. Carefully drain
chloroform phase and discard. Remove remaining droplets of dithizone by
extracting with 1 to 2 ml CCl 4 layer. CONDUCT THE DRAINING
OPERATIONS SO THAT NO ACID ENTERS BORE OR STEM OF
SEPARATOR. Discard CCl 4 layer.
Adjust aqueous phase to 5.0% alkalinity by adding 10 ml of NaOH solution
(g). Extract cadmium with 25 ml of dithizone solution, by shaking
vigorously for more than or equal to one min and transfer to third separator
previously wetted with 2 to 3 ml of same dithizone solution. Repeat
extraction with additional 10 ml portions of dithizone solution until CCl 4
layer becomes colourless. Amounts of Cd upto 100 µg are completely
removed by third extraction.
To verify assumption that pale pink persisting after third extraction is due to
Zn, transfer a questionable extract to another separator containing 5.0%
NaOH solution. Add several ml of dithizone solution and shake vigorously.
If CCl 4 layer becomes colourless, original pink was due to Zn and no further
extractions are necessary. If however, pink persists, indicating the presence
of Cd, and extract to contents of third separator and continue extraction.
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