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

METALS
carried out such that the final solution for determination of tin is
approximately 4.5M in H 2 SO 4 .
(ii)
Add successively 50 ml HNO 3 (b), 12.5 ml of H 2 SO 4 (a) and three
glass beads and mix thoroughly.
(iii) Heat the contents of the flask to boiling on soft flame of burner and
keep contents boiling during digestion. Rotate the flask occasionally
to prevent caking of sample on glass exposed to flame.
(iv)
(v)
(vi)
Maintain oxidizing conditions during digestion by adding small
amounts of HNO 3 (b) whenever contents turn brown or darkens.
Continue step (iv) till all organic matter is destroyed. After digestion
is complete, heat the contents of the flask till copious fumes of
sulphuric acid are evolved and continue heating for 5 more minutes.
Now the solution remains colourless or pale straw coloured.
Cool the solution to room temperature, add 1 ml of HClO 4 (c) and reheat
till copious white fumes appear. Continue heating for 5 minutes.
(vii) Cool the solution again, add 1 ml of H 2 O 2 (d). Heat to fuming and
continue heating for 5 minutes more.
(viii) Repeat H 2 O 2 treatment twice, each time-re-heating to fuming which is
continued for 5 min.
(ix)
(x)
Cool, rinse the neck of the flask with approximately 5 ml of water and
re-heat t fuming.
Cool the solution.
62