Wolfram occurs associated with tin ores, the value of which is
consequently lowered. Both wolfram and scheelite are of considerable
importance as a source of tungstic acid for the manufacture of sodium
tungstate, which is used as a mordant and for some other purposes, and
as a source of metallic tungsten, which is used in steel-making.
The tungsten minerals have a high specific gravity (6 to 7.5). On
treatment with hydrochloric acid or aqua regia they are decomposed; the
yellow tungstic acid separates and remains insoluble.
Tungsten itself is insoluble in nitric acid or aqua regia; but is
converted into tungstic acid (WO_{3}) by prolonged and strong ignition
in air. Alloys containing tungsten leave tungstic acid after treatment
with nitric acid or aqua regia. Tungstic acid may be got into solution
after fusion with alkalies or alkaline carbonates. This solution gives
with hydrochloric acid a white precipitate of tungstic acid, which
becomes yellow on boiling, but the separation is not complete. Fusion
with bisulphate of potash gives a residue, which does not dissolve in
water, but is soluble in ammonic carbonate. For the assay of minerals
containing tungsten these reactions are only occasionally taken
advantage of for testing or purifying the separated tungstic acid.
~Detection.~--The minerals are easily recognised by their physical
characters, and the yellow tungstic acid separated by boiling with acids
is the best test for its presence; this, after decanting and washing,
immediately dissolves in a few drops of dilute ammonia.
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