The water and acid are added a
little at a time, and warmed till solution is effected; then cooled, and
diluted to the required volume: 100 c.c. will equal 0.50 gram of
arsenic.
The _sodic acetate and acetic acid solution_ is made by dissolving 100
grams of sodic acetate in 500 c.c. of acetic acid, and diluting with
water to 1 litre. Five c.c. are used for each assay.
The solution of potassic ferrocyanide used as _indicator_ is made by
dissolving 10 grams of the salt in 100 c.c. of water.
To standardise the solution of uranium acetate, weigh up a quantity of
white arsenic (As_{2}O_{3}) which shall be about equivalent to the
arsenic contained in the assay (0.1 or 0.2 gram); transfer to a flask,
and dissolve in 10 c.c. of nitric acid with the aid of heat. Evaporate
to a small bulk (taking care to avoid the presence of hydrochloric
acid); dilute with water; add a small piece of litmus paper; render
faintly alkaline with ammonia; then add 5 c.c. of the sodic acetate
mixture; dilute to 150 c.c.; and heat to boiling.
Fill an ordinary burette with the uranium acetate solution, and run into
the assay a quantity known to be insufficient. Again heat for a minute
or two. Arrange a series of drops of the solution of ferrocyanide of
potassium on a porcelain slab, and, with the help of a glass rod, bring
a drop of the assay solution in contact with one of these.
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