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

METALS
If first extraction indicates more than 5 µg of mercury, add stronger
concentrations of dithizone as indicated in Table given below until after 1
min vigrous shaking, CHCl 3 layer contains dithizone in marked excess.
Drain the CHCl 3 layer into second separator containing 0.1N HCl and again
extract sample solution with two 10 ml portions of 4 mg/l dithizone solution,
draining each successive extract into second separator.
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Hg range Dithizone Concentration Column of dithizone
µg ( mg/l) solution (ml)
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0 to 10 6 5
0 to 50 10 25
0 to 100 10 40
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Shake contents of second separator vigorously for 1 min and drain CHCl 3
layer into third separator containing 50 ml of 0.1N HCl. Extract solution in
second separator with 1 to 2 ml of CHCl 3 and transfer organic layer to third
separator.
To contents of third separator add 2 ml of Na 2 S 2 O 3 solution, shake
vigorously for 1 min. Let layers separate drain off CHCl 3 as completely as
possible and discard. Extract again with 1 to 2 ml of CHCl 3 drain carefully
and discard organic layer. Add enough solution of NaOCl reagent to furnish
175 mg available chlorine and shake vigorously for 1 min. Add 5 ml of
hydroxylamine hydrochloride reagent from pipette taking care to wet both
stopper and neck of separator. Shake vigorously for 1 min. Hold mouth of
separator in front of air vent and blow out any remaining gaseous chlorine.
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