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

METALS
Reagents:
(a)
Reducing solution:
Mix 50 ml H 2 SO 4 with approximately 300 ml water. Cool to room
temperature and dissolve 15 g NaCl, 15 g hydroxylamine sulphate and
25 g SnCl 2 in solution. Dilute to 500 ml.
(b)
Diluting solution:
To 1000 ml volumetric flask containing 300 to 500 ml water add 58
ml HNO 3 and 67 ml H 2 SO 4 . Dilute to volume with water.
(c)
Magnesium perchlorate:
Drying agent placed in filter flask (Fig. 1.1). Replace as needed
(Caution: Mg(ClO 4 ) 2 is explosive when in contact with organic
substances).
(d)
Mercury standard solution:
(i) Stock Solution (1000 µg/ml): Dissolve 0.1354 g HgCl 2 in 100
ml water.
(ii) Working solution (1 µg/ml): Dilute 1 ml stock solution to 1000
ml with 1N H 2 SO 4 . Prepare fresh daily.
Preparation of Sample:
Weigh 5.0 g of sample into digestion flask, add 25 ml of 18N H 2 SO 4 , 20 ml
of 7N HNO 3 , 1 ml of 2% sodium molybdate solution and 5-6 boiling chips.
Connect condenser (with water circulating through it) and apply gentle heat
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