230 COMPOUNDS OF THE RARE ELEMENTS.
Wash the insoluble residue, consisting of silicic acid, unat-
tacked wolframite, and yellow tungstic acid, first with water con-
taining a little hydrochloric acid, and then with pure water;
transfer the powder to a beaker, and warm it on the water bath
with enough 2-normal ammonia so that all of the tungstic acid
passes into solution. Then filter and treat the residue with
acids, etc., exactly as at first, in order to make certain that no
large amount of the original mineral remains unattacked.
Finally, unite all the ammoniacal extracts, filter the solution
again, and concentrate it until crystals of ammonium tungstate
begin to form. Then add 10 c.c. of concentrated ammonia and
allow the solution to cool. Drain the colorless crystalline meal
on a filter, and work up the mother liquor until only a very little
remains. Dry the preparation in the hot closet. Yield, about 90 g.
A solution of ammonium tungstate gives a light-yellow pre-
cipitate with stannous chloride. The precipitate dissolves partly
or wholly in concentrated hydrochloric acid, and, on heating the
latter solution, a precipitate is thrown down which at first is
dirty blue and later becomes pure dark blue.{b) Tungstic Acid and Tungsten Trioxide.
Yellow Dibasic Tungstic Acid, H^WO^. Boil 3 g. of ammo-
nium tungstate gently for five minutes in an evaporating dish
with a mixture of 10 c.c. of concentrated hydrochloric acid
and 5 c.c. of concentrated nitric acid. Allow the liquid to cool
and dilute it with water. Collect the solid residue on a suction
filter, wash it with hot water until the washings show a neutral
reaction to blue litmus, and dry it in the hot closet. By igniting
the tungstic acid it loses one molecule of water and is transformed
into tungsten trioxide. Determine the loss in weight.
White Tungstic Acid, HJVO^I). Dissolve 3 g. of ammonium
tungstate in 30 c.c. of water with the addition of a few drops of
ammonia. Then add to the solution at the room temperature an
equal volume of 10% hydrochloric acid. White tungstic acid is
thereby precipitated. After it settles, collect the solid and wash it
with hot water containing some hydrochloric acid. If the filtrate
is opalescent by reflected light, it contains colloidal tungstic acid
which can be precipitated by gentle heating. Dry the prepara-
tion at a moderate temperature on top of the hot closet.