INSECTICIDES AND FUNGICIDES

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contain 40 and 100 cc of water respectively. The flask D
contains 50 cc of water, which serves as a trap to prevent
entrance of air.

Determination of Total Arsenic: Distillation Method.—Prepare the following
reagents:

(a) Arsenous Acid.—Dissolve exactly 2 gm of arsenous oxide of known
purity (dried over calcium chloride for ten hours) by boiling with about
200 cc of water containing 10 cc of concentrated sulphuric acid. Cool and
transfer to a 500-cc volumetric flask, dilute to the mark and mix thoroughly.
Keep stoppered.

EC D

FIG. 61.—Apparatus for the determination of total arsenic by distillation.

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(?>) Starch Indicator.—Mix about 0.5 gm of starch with cold water to form
a thin paste; add about 100 cc of boiling water and stir thoroughly.
(c) Iodine Solution.—Dissolve 6.35 gm of iodine and 12.5 gm of potassium
iodide in about 100 cc of water, decant from any sediment, dilute to 1000 cc
and mix well. Standardize against solution (a) as follows:
Using a pipette, measure 50 cc of the arsenous acid solution into an
Erlenmeyer flask, dilute to about 400 cc and neutralize with sodium bicarbon-
ate, adding 4 to 5 gm in excess. Add the standard iodine solution from a
burette, rotating the flask continuously, until the yellow color disappears only
slowly, showing that the end point is near; then add 1 cc of the starch
solution and continue adding the iodine solution drop by drop until a per-
manent blue color is obtained. Calculate the value of the standard iodine
solution in terms of arsenous oxide (AsaOa). Keep the solution stoppered
and away from bright light. Even with this precaution the oxidizing value
changes and the solution should be standardized within a few hours of the
time when it is to be used.