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

METALS
dish and filter with several 5 to 10 ml portions of 0.1N HCl, cool and
dilute to volume with 0.1N HCl.
Isolation and determination:
To a suitable aliquot of ash solution of sample, add 2 drops of methyl red
indicator and 1 ml of CuSO 4 solution and neutralize with NH 4 OH. Add
enough HCl to make solution about 0.15N with respect to HCl
(approximately 0.5 ml excess in 50 ml solution is satisfactory). Adjust the
pH of this solution, as measured with glass electrode, to 1.9 to 2.1. Pass
stream of H 2 S into solution until precipitation is Complete. Filter through
fine paper {Whatman No. 42 or equivalent, previously washed with HCl
(1+6) followed by water}. Receive the filtrate in 250 ml beaker, wash flask
and filter with 3 or 4 small portions of water. Gently boil filtrate until odour
of H 2 S can no longer be detected. Add 5 ml of saturated bromine-water and
continue boiling until Br-free. Cool, neutralize to phenol red with NH 4 OH
and make slightly acid with HCl {excess of 0.2 ml of HCl (1+1)}. Dilute
resultant solution to definite volume. At this stage, for optimum conditions
of measurement, the solution should contain 0.2 to 1.0 µg of Zinc/ml.
To 20 ml aliquot of this prepared solution, in a 125 ml separator, add 5 ml of
ammonium citrate solution, 2 ml dimethylglyoxime solution and 10 ml of α-
nitroso-β- naphthol solution and shake for 2 min. Discard solvent layer and
extract with 10 ml of CHCl 3 . Discard solvent layer. (The extraction
procedure in this para eliminates nickel and cobalt present, if any, in the
solution).
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