~--The process for getting the arsenic into
solution will exclude all metals except tin, but the solution will be
charged with sodium or ammonium salts in the process of neutralising, so
that it is only necessary to see if these cause any interference. The
alkaline hydrates, including ammonia, are plainly inadmissible, since no
free iodine can exist in their presence. Monocarbonates similarly
interfere, but to a much less extent; hence the necessity for rendering
the assay distinctly acid before adding the bicarbonate of soda.
With 20 c.c. of arsenic solution; and with bulk, soda, and starch as
before, the results obtained were:--
"Iodine" required.
With 20 grams of ammonic chloride 20.0 c.c.
" 20 grams of sodium chloride 20.0 "
" 20 grams of sodium acetate 20.0 "
" 0.050 gram of tin, as stannic chloride 19.6 "
Without any addition 20.0 "
The interference of the stannic salt is probably mechanical, the
precipitate carrying down some arsenious acid.
~Effect of Varying Arsenic.~--With bulk, starch, and soda as before, but
with varying arsenic, the results were:--
Arsenic added 1.0 c.c. 10.0 c.c. 20.0 c.c. 50.0 c.c. 100.0 c.c.
"Iodine" required 1.
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