The drying is very tedious, and is likely to leave behind more
water than is allowed for in the formula. In a series of determinations
in which the arsenic was weighed in both forms, the results were:--
Ammonic-magnesic Arsenic Magnesium Pyrarsenate Arsenic
Arsenate in grams. in grams. in grams. in grams.
0.0080 0.0032 0.0065 0.0031
0.0400 0.0158 0.0330 0.0160
0.0799 0.0316 0.0633 0.0306
0.1600 0.0632 0.1287 0.0623
0.4000 0.1580 0.3205 0.1551
0.7990 0.3156 0.6435 0.3114
VOLUMETRIC METHODS.
There are two methods: one for determining the arsenic in the lower, and
the other in the higher state of oxidation. In the first-mentioned
method this is done by titrating with a standard solution of iodine; and
in the latter with a solution of uranium acetate. Where the arsenic
already exists as arsenious oxide, or where it is most conveniently
separated by distillation as arsenious chloride, the iodine method
should be used; but when the arsenic is separated as ammonic-magnesic
arsenate or as sulphide, the uranium acetate titration should be
adopted.
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