~Dry Assay.~--There is no dry assay which is trustworthy. The following
method is sometimes used to find the proportion of arsenious oxide in
"crude arsenic":--Weigh up 5 grams of the dried sample, and place them
in a clean dry test-tube about 6 inches long. Tie a small filter-paper
over the mouth of the tube, so as to prevent air-currents. Heat the tube
cautiously so as to sublime off the white arsenic into the upper part of
the tube. Cut off the bottom of the test-tube by wetting whilst hot.
Scrape out the arsenic and weigh it. The weight gives an approximate
idea of the quantity, and the colour of the quality, of the white
arsenic obtainable from the sample. Some workers (sellers) weigh the
residue, and determine the white arsenic by difference. In determining
the percentage of moisture in these samples, the substance is dried on a
water-bath or in a water-oven.
WET METHODS.
~Solution.~--Where, as in crude arsenic, the substance is arsenious
oxide (As_{2}O_{3}) mixed with impurities, the arsenic is best got into
solution by warming with caustic soda, and neutralising the excess with
hydrochloric acid; it will be present as sodium arsenite. Metals and
alloys are acted on by means of nitric acid; or the arsenic may be at
the same time dissolved and separated by distilling with a strongly-acid
solution of ferric chloride, in the way described under _Volumetric
Methods_.
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