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

METALS
When sample is dry and charred, place it in a temperature controlled muffle
furnace and raise temperature SLOWLY without ignition. Cover floor of
furnace with piece of asbestos board or SiO 2 plate so that sample receives
most of its heat by radiation and not by conduction. Ash the sample at
temperatures not exceeding 500ºC for about 8 to 10 hours. If ashing is not
complete, remove dish from furnace, cool and moisten the char with 2 ml of
ash aid solution. Dry contents of dish thoroughly and replace in furnace. If
ashing is not complete after 30 minutes, remove dish, cool and cautiously
add 2 to 3 ml HNO 3 . Dry again and place in furnace and continue ashing
until practically carbon-free ash is obtained.
When clean ash is obtained, cool, cover the dish with watch glass and
cautiously add 15 to 20 ml conc. HCl. Rinse down watch glass with water
and heat on steam bath. If clear solution is not obtained evaporate to dryness
and repeat addition of HCl. Dilute the clear solution with water to a definite
volume (filter solution if necessary, wash the insoluble material on filter
successively with few ml of hot HCl, hot hydrochloric acid citric acid
solution and hot 40% NH 4 OAC solution and make up to 100 ml). PREPARE
A SAMPLE BLANK SOLUTION using exactly the same amounts of
reagents including water and exposing the sample blank in furnace or on
steam bath for the same length of time and giving identical treatment as
given in case of sample.
Isolation and Determination of Lead:
(i)
Transfer suitable aliquot of ash solution of sample to a 250 ml
separator and add citric acid reagent equivalent to 10 g of citric acid
and mix well.
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