~Effect of Alkalies.~--On theoretical grounds the presence of these is
known to be inadmissible. A solution rendered faintly alkaline with
ammonia required only 11.2 c.c. of "hypo;" and another, with 0.5 gram of
caustic soda, required 4.0 c.c. instead of 20.4 c.c. as in neutral
solutions.
~Effect of nearly Neutralising Hydrochloric Acid Solutions with
Ammonia.~--Provided care is taken not to add excess of ammonia, this
has a good effect, counteracting the interference of excess of acid.
Thus 20 c.c. of iodine (as before) required 20.4 c.c. of "hypo;" with 15
c.c. of hydrochloric acid 20.7 c.c. were required, but with 15 c.c. of
acid, nearly neutralised with dilute ammonia 20.4 c.c. were used.
~Effect of the Addition of Starch.~--The addition of varying quantities
of starch has no effect, provided it is added when the titration is
nearly finished, as the following experiments show:--
Starch added 1.0 c.c. 5.0 c.c. 10.0 c.c. 50.0 c.c.
"Hypo" required 20.4 " 20.4 " 20.4 " 20.5 "
But if the starch is added before the titration, the results are liable
to error.
Starch added 1.0 c.c. 50.0 c.c.
"Hypo" required 20.4 " 24.0 "
The starch should be used fresh, and is best made on the day it is used;
after four days the finishing point is not so good.
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