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

METALS
Preparation of sample:
Sample preparation is done by following any one of the two procedure given
below:
(A)
Wet Ashing:
Accurately weigh, into 300 or 500 ml Kjeldahl flask, representative
sample of about 25 g (containing about 100 µg of Zinc). Evaporate the
liquid sample to small volume. Add conc. HNO 3 and heat cautiously
until first vigorous reaction subsides and then add 2 to 5 ml conc.
H 2 SO 4 . Continue heating, adding more HNO 3 in small portions ad
needed to prevent charring, until solution is clear and almost
colourless. Continue heating until dense fumes of H 2 SO 4 are evolved
and all HNO 3 has been removed. Cool, dilute with approximately 25
ml of water filter, if necessary through pre-washed fast filter paper and
dilute the filtrate to 100 ml with water.
(B)
Dry Ashing:
Accurately weigh, into a clean platinum or silica dish, a representative
portion of sample (about 25 g). Char the sample and ash at
temperatures not exceeding 500ºC. Raise the temperature of the
muffle furnace slowly to avoid ignition. When ash is carbon-free,
dissolve ash under watch glass, in minimum volume of HCl (1+1).
Add about 20 ml of water and evaporate to near dryness on team bath.
Add 20 ml 0.1N HCl and continue heating for 5 minutes. Filter
through pre-washed fast filter paper into 100 ml volumetric flask. Wash
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