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

METALS
When clean ash is obtained, boil the contents of the dish cautiously with
about 5 to 10 ml of H 2 O 2 over a burner with controlled flame, to remove the
nitrous fumes. This step has to be continued till no nitrous fumes are
evolved. The ash should essentially be free from every trace of nitrates.
After removing nitrates, cover dish with watch glass, add cautiously 5 ml of
conc. HCl, rinse down with water and heat on steam bath and evaporate to
near dryness. Add 20 ml of HCl (1+1); heat to dissolve the ash, transfer the
contents of the dish to a 100 ml volumetric flask and dilute to volume with
water.
Determination of Tin:
Transfer on aliquot of the ash solution to a reduction flask. Add 30 ml of
HCl (1+3) and about 0.3 g of aluminium foil. Fit the flask with delivery tube
dipping the other end of the tube into bicarbonate solution. Heat gently until
evolution of hydrogen has started and discontinue heating. Heat again when
the aluminium has nearly dissolved and finally boil until the liquid is
perfectly clear. Remove by gentle rotation/agitation, any metal particles
remaining at liquid gas interface. Cool flask in ice to below room
temperature. After cooling, wash down the inner walls of the flaks with
approximately 5 ml of iodide-carbonate solution (g), run in quickly, from a
5 ml pipette with the tip removed. Add a few drops of starch indicator and
titrate with KIO 3 working solution (f - ii) to the first appearance of blue
colour.
Calculate the Tin content of sample as follows:
297 x Titre volume (ml) x Volume of ash solution made (ml)
ppm of Tin = --------------------------------------------------------------------------
Aliquot of ash soln. taken (ml) x Wt. of sample taken for ashing (g)
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