GRAVIMETRIC DETERMINATION.
The tungstic acid is dissolved, and separated as ammonic tungstate, and,
after evaporation, is gently ignited, the heat being increased towards
the end. The residual tungstic acid is fixed, so that when the ammonia
has been driven off it may be strongly heated without loss. It is a dark
yellow or brown powder whilst hot, which becomes a light yellow on
cooling. If any reduction has taken place it will be more or less
greenish. It is weighed when cold, and is the trioxide or "tungstic
acid" (WO_{3}), which contains 79.31 per cent. of tungsten. After its
weight has been taken its purity is checked by fusing with hydric
potassic sulphate, extracting with water, and treating the residue with
ammonic carbonate. Any silica present will be left undissolved; it
should be separated and weighed, and its weight deducted from that of
the tungstic acid found.
~Determination of Tungstic Acid in Wolfram.~--Take 2 grams of the
finely-powdered sample and boil with 50 c.c. of hydrochloric acid for
half an hour, adding 5 c.c. of nitric acid towards the end. Allow to
stand overnight and boil again for 15 or 20 minutes; dilute with an
equal volume of water, and filter. Wash with dilute hydrochloric acid,
dissolve in a few c.c. of warm dilute ammonia, and dilute to 200 c.c.
with distilled water; allow to settle, and filter.
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