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

METALS
(ii)
(iii)
Intermediate solution: (100 µg /ml)
Dilute 100 ml of stock solution to 100 ml.
Working solution: (2.0 µg /ml)
Transfer 20 ml intermediate solution to 1000 ml volumetric
flask, add about 15 ml HCl and dilute to volume with water (the
acidity of final solution should be approximately 0.2N).
Preparation of the Sample:
Digest a suitable quantity of sample equivalent to 5 to 10 g of product,
calculated on dry basis, with 10 ml of H 2 SO 4 (1+1) and HNO 3 as needed. If
sample tends to char rather than to oxidize evenly, add 5 or 10 ml of
additional H 2 SO 4 . Continue digestion, adding HNO 3 as required, until
digestion is complete and SO 3 is evolved. Cool, add 15 ml saturated
ammonium oxalate solution and again heat to fumes.
Fat in biological material such as kidney and liver may cause bumping and
frothing during digestion. If comparatively large sample of such materials
are available, make partial digestion with warm HNO 3 until only fat remains
undissolved. Cool, filter free of solid fat, wash residue with water, make
combined filtrate to suitable volume and digest appropriate aliquot as above.
Determination:
Dilute digest with 25 ml water, filter free of excessive insoluble matter if
present and transfer to separator marked 125 ml, using additional 10 ml
portions of water for rinsing and completing transfer. Add 1 to 2 g of citrate
reagent (a) and 1 ml of thymol blue indicator and adjust pH to
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