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

METALS
(6) Repeat extraction by shaking cyclohexane layer with 3 ml of sodium
hydroxide solution (h – ii) and add the aqueous layer into the content
of the 50 ml beaker mentioned in step 5.
(7) Wash the cyclohexane layer retained in the separatory funnel with 5
ml of NaOAC solution (k) by cautiously tilting the funnel and forcing
the contents to flow forth and back six times.
(8) Decolourise the iodine present in the acidified aqueous solution (step
6) by dropwise addition of ascorbic acid solution (i). Add 2 ml of
catechol violet solution (j) and mix. Add to this solution, the sodium
acetate washing solution (obtained in step 7) and adjust the pH of
solution to 3.8 ± 0.1 with ammonia solution (1) and/or HCl solution
(e), using pH meter.
(9) Quantitatively transfer the solution (after adjusting the pH to 3.8 ± 0.1
into a 25 ml volumetric flask, add 2 ml of absolute ethanol, make up
to the mark with water and mix. Set the flask aside for 45 min at room
temperature. Measure the absorbance ‘A’ of the solution using water
as reference.
Carry out a blank determination starting with 50 ml of 4.5M sulphuric acid
solution (f) and carrying out steps from 2 to 9, the optical density of the
blank should not exceed 0.070.
Convert the absorbance ‘A’ to µg of Tin from calibration graph and
calculate Tin content of sample as follows:
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