~Effect of Varying Bulk.~--In these experiments, 20 c.c. of arsenic
solution were taken, 2 grams of bicarbonate of soda and 2 c.c. of starch
solution added, and water supplied to the required bulk. The results
were:--
Bulk 50.0 c.c. 100.0 c.c. 250.0 c.c. 500.0 c.c.
"Iodine" required 20.0 " 20.0 " 20.0 " 20.0 "
Considerable variation in bulk does not interfere.
~Effect of Varying Bicarbonate of Soda.~--This salt must be present in
each titration in considerable excess, to prevent the interference of
free acid. The bicarbonate must be dissolved without heating, as neutral
carbonates should be avoided.
Bicarbonate added 1 gram 2 grams 5 grams 10 grams
"Iodine" required 20.1 c.c. 20.0 c.c. 20.1 c.c. 20.0 c.c.
These results show that large variation in the quantity of bicarbonate
has no effect.
~Effect of Free Acid.~--In these experiments, the arsenic taken, the
starch, and the bulk were as before, but no bicarbonate was added. In
one case the solution was rendered acid with 5 c.c. of acetic acid, and
in the other with 5 c.c. of hydrochloric acid; in both cases the
interference was strongly marked, and no satisfactory finishing point
could be obtained. This was much more marked with the hydrochloric acid.
~Effect of Foreign Salts.
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