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

METALS
1.1.3 Reagents :
It is necessary to use reagents and distilled water of suitably low metal
content taking into consideration that the concentrated mineral acids are
generally used in amounts several times more than the sample. Even when
these reagents are used, reagent blank determination shall be necessary.
Blanks must be prepared with the same quantities of the reagents as are used
in the test.
In expressions like (1+2), (1+3) etc, used with the name of a reagent, the
first numeral indicates (volume/weight) of (liquid/solid) reagent and second
numeral indicates volume of water. For example HCl (1+2) means reagent
prepared by mixing one volume of HCl with two volumes of water.
All chemicals that are used in these procedures, should be of highest purity
i.e AR grade. The chemicals should not be transferred to other bottles If any
chemical to be used has any kind of impurity, then it should be purified.
Procedures for purification of some frequently used chemicals are given
below:
(i)
(ii)
Adsorbent cotton (metal free): If traces of metals are present, remove
them by digesting cotton several hours with 0.2N HCl, filtering on
Buchner and finally washing with water until acid free.
Ammonium hydroxide: Distil ordinary reagent into ice cold redistilled
water. Concentration of re-distilled NH 4 OH can be determined by
specific gravity or titration.
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