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

METALS
Determination:
Take a suitable aliquot of the blank, standard or sample solution in the
reaction vessel. Add the required amount of 10% nitric acid to maintain a
volume of 10 ml. Add 2 ml of stannous chloride solution (20% w/v in 10%
HCl) and stopper the reaction vessel immediately. Switch on the magnetic
stirrer and stir vigorously for about 5 min. After adjusting ‘0’ and 100% T,
start the pump and allow the mercury free air to purge through the reaction
vessel. The air is rendered mercury free by passing t through a trap
containing 20 ml of 1.0% permanganate solution in 10% H 2 SO 4 . Note the
absorbance as early as possible (within one minute) in the ‘Hold’ mode of
operation and switch back to ‘Normal’ mode. Switch off the pump and the
magnetic stirrer. Adjust 0% and 100% T just before each measurement.
Before reaching the absorption cell the air along with the mercury vapour
from the reaction vessel is first passed through a trap containing 4 ml of
20% w/v NaOH and then through a trap containing 4 ml of 1:1 H 2 SO 4 to
absorb acid vapour and moisture respectively. Air along with mercury
vapour leaving the absorption cell is passed through a trap containing 20 ml
of 1.0% KMnO 4 in 10% H 2 SO 4 to absorb the mercury vapour and thereby
avoid contaminating the surrounding with mercury vapour.
Preparation of Standard Curve:
Repeat the measurement for standards 30, 60, 90, 120 and 150 µg Hg and
draw calibration graph by plotting absorbance versus concentration of Hg.
Concentration of mercury in the sample is calculated from the calibration
curve.
(Ref:- Manual Methods of Analysis for Adulterants and Contaminants in
Foods I.C.M.R 1990 Page 142)
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