c. of water, and is cooled by being
placed in a jar or large beaker of cold water. The water used for
cooling should be renewed for each assay.
Pour on the assay in the flask 50 c.c. of a "ferric chloride mixture,"
made by dissolving 600 grams of calcium chloride and 300 grams of ferric
chloride in 600 c.c. of hydrochloric acid, and making up to 1 litre with
water.
Firmly cork up the apparatus, and boil over a small Bunsen-burner flame
for fifteen or twenty minutes, but avoid evaporating to dryness.
Disconnect the flask, and pour away its contents at once to prevent
breakage of the flask by their solidification. The arsenic will be
condensed in the ~U~-tube, together with the greater part of the
hydrochloric acid; transfer the distillate to a beaker washing out the
tube two or three times with water; add a small piece of litmus paper;
neutralise with ammonia; render faintly _acid_ with dilute hydrochloric
acid; add 2 grams of bicarbonate of soda in solution; and dilute to 250
c.c. The solution is now ready for titrating.
The arsenic comes over in the early part of the distillation, as will be
seen from the following experiment, made on 1 gram of copper
precipitate; in which experiment the distillate was collected in
separate portions at equal intervals, and the arsenic in each portion
determined:--
Time Iodine Equivalent to Arsenic
Distilling.
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