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

METALS
3.0 SINGLE ELEMENT METHODS
3.1 DETERMINATION OF ARSENIC IN FOODS BY
COLORIMETRIC MOLYBDENUM BLUE METHOD
Principle:
The sample is digested with nitric acid and sulphuric acid. After the
digestion/oxidation is complete, the digest is treated with saturated
ammonium oxalate solution to remove yellow coloration due to nitrocompounds,
fats, etc. Arsine is generated from digest using zinc and HCl and
trapped in NaOBr solution and is treated with ammonium molybdate to form
a blue compound which absorbs as 845 nm.
Apparatus:
(i) Generators and absorption tubes (Fig. 2.1):
Use about 100 ml wide mouth bottles of uniform capacity and design
as generators. Fit each of them by means of perforated stopper with
glass tube of 1 cm diameter and 6 to 7 cm long, with additional
constricted end to facilitate connection. Place small pad of glass wool
in constricted bottom end of tube and add 3.5 to 4.0 g sand (same
amount should be added in each tube) Moisten sand with 10% lead
acetate solution and remove excess by light suction. Clean sand when
necessary by treatment with nitric acid followed by water rinse and
suction and treat with lead acetate solution (for cleaning sand, the
sand should not be removed from tube). If sand has dried, clean and
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