IODINE PROCESS.
This is based on the fact that sodium arsenite in a solution containing
an excess of bicarbonate of soda is indirectly oxidised by iodine to
sodium arsenate,[103] and that an excess of iodine may be recognised by
the blue colour it strikes with starch. The process is divided into two
parts--(1) the preparation of the solution, and (2) the titration.
~Preparation of the Solution.~--For substances like crude arsenic, in
which the arsenic is present as arsenious oxide, the method is as
follows:--Take a portion which shall contain from 0.25 to 0.5 gram of
the oxide, place in a beaker, and cover with 10 c.c. of sodic hydrate
solution; warm till dissolved, put a small piece of litmus paper in the
solution, and render acid with dilute hydrochloric acid. Add 2 grams of
bicarbonate of soda in solution, filter (if necessary), and dilute to
100 c.c. The solution is now ready for titrating.
[Illustration: FIG. 68.]
Where the arsenic has to be separated as arsenious chloride, the process
is as follows:[104]--Weigh up 1 gram of the finely-powdered ore (metals
should be hammered out into a thin foil or be used as filings), and
place in a 16-ounce flask provided with a well-fitting cork, and
connected with a ~U~-tube, as shown in the drawing (fig. 68). The
~U~-tube should contain 2 or 3 c.
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