Acetic acid is added to
dissolve the precipitate, and when this is effected more of the acid is
poured on so as to render the solution strongly acid. To this potassium
iodide crystals are added in the proportion of ten parts of iodide to
each one part of copper supposed to be present. The solution is then
titrated with "hypo" as usual.
For the examination of technical products experiments made in sulphuric
acid solutions have no value, since arsenic acid, which is generally
present to a greater or less extent, affects the end reaction. In such
solutions bismuth may also interfere.
The solution best suited for the assay is one containing acetate of soda
and free acetic acid. The presence of acetate of soda counteracts the
interference of arsenic and of bismuth.
The return of the blue colour after titration is due to the excessive
dilution of the assay, or to an insufficiency of potassium iodide, or to
the presence of nitrous fumes. The interference of an excess of sodium
acetate is avoided by adding more iodide crystals to the extent of
doubling the usual amount.
The interference of lead can be avoided by the addition of sulphuric
acid or of phosphate of soda to the acid solution containing the copper,
and before neutralising with carbonate of soda. The end reaction is,
however, with care distinguishable without this addition.
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