One hundred c.c. will equal one gram of copper.
The starch solution is made by mixing 1 gram of starch into a thin paste
with cold water, pouring it into 200 c.c. of boiling water, and
continuing the boiling for a minute or so. The solution must be cold
before use, and about 2 c.c. is used for each assay. It should not be
added until the bulk of the iodine has been reduced.
To standardise the "hypo," weigh up 0.3 or 0.4 gram of pure copper,
dissolve in 5 c.c. of dilute nitric acid, boil off nitrous fumes, and
dilute with an equal bulk of cold water. Add "soda" solution until a
permanent precipitate is obtained, and then 1 c.c. of acetic acid. This
should yield a clear solution. Fill an ordinary burette with the "hypo."
Add 3 grams of potassium iodide crystals to the copper solution, and,
when these are dissolved, dilute to 100 c.c. with water. Run in the
"hypo" solution rather quickly until the brown colour is nearly
discharged--_i.e._, to within 3 or 4 c.c. of the finish. Add 2 c.c. of
the starch solution, and continue the addition of the "hypo" a few drops
at a time until the tint suddenly changes to a cream colour. The blue
colour must not return on standing three or four minutes. Calculate the
standard in the usual way.
In assaying ores, the copper is dissolved and separated with
sulphuretted hydrogen as in the other processes, but the sulphide should
be washed more completely to ensure the absence of iron salts.
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