0 c.c. 20.2 c.c. 19.3 c.c. 18.2 c.c.
In the 5 grams experiment, when the finishing point had been apparently
reached the colour slowly returned; but as the results generally on
titrating were not satisfactory a repetition of the experiment was made
with the addition of 5 c.c. of acetic acid, which gave an equally bad
result.
~Effect of Foreign Salts.~--The conditions of these experiments were the
same as before. The salts were added and dissolved before the addition
of the potassium iodide. Using 5 grams (or in the case of the acids, 5
c.c.), the results were as follows:--
dilute Acetic
Salt added -- H_{2}SO_{4} acid NaAc NaCl
"Hypo" required 20.0 c.c. 20.0 c.c. 20.1 c.c. 19.3 c.c. 20.1 c.c.
Salt added KNO_{3} Na_{2}SO_{4} AmCl Am_{2}SO_{4}
"Hypo" required 20.2 c.c. 18.7 c.c. 20.0 c.c. 19.9 c.c.
The low result with the sulphate of soda was evidently due to the
formation of a sparingly soluble double salt, which removed copper from
the solution; on adding a little acetic acid the full amount of "hypo"
was required. The effect of the presence of certain metals is important.
The method of determining it was to add the substance to the solution
containing the copper, and partly precipitate with soda solution; then
treating with 1 c.
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