224 COMPOUNDS OF THE RARE ELEMENTS.
Determine the actual amount of the oxides present by igniting
0.6 g. of the powder in a platinum crucible to constant weight.
For each gram of the anhydrous oxides, 2.5 g. of concentrated
hydrofluoric acid and 0.5 g. of potassium carbonate are to be
used. The hydrofluoric acid must be pure and above all free
from fluosilicic acid. Test for the presence of the latter with
potassium salt solution in a platinum crucible.
First dissolve the oxides in the hydrofluoric acid, and pour the
solution cold through a plaited filter placed in a funnel which
has been coated with paraffin. Allow the filtrate to run into a
platinum dish containing a filtered solution of the required amount
of potassium carbonate. Dissolve the resulting precipitate by
boiling and adding sufficient water, then evaporate the solution
until crystals begin to deposit. On cooling, fine needles of potas-
sium tantalum fluoride are obtained. After twelve hours collect
the crystals on a plaited filter in a paraffined funnel and wash
them with cold water. On further evaporation of the filtrate the
remainder of this salt (which is but very little) can be obtained.
Finally, concentrate the solution to a much smaller volume
until, instead of the fine needle-like crystals, larger thin plates of
potassium columbium oxyfluoride are obtained. Several further
crops of these crystals should be obtained from the mother-liquor.
Recrystallize both of the salts from water containing some
hydrofluoric acid. Yield, usually about 1.5 g. K 2 [TaF 7 ] and 30 g.
K 2 [Cb0F 6 ]; but the relative quantities of the two salts vary
according to the composition of the original material.- Molybdenum Compounds from Molybdenite.
Molybdenum occurs in nature to some extent in the form of molybdates
(wulfenite, PbMoO^ and powellite, CaMoOO, but the most important ore is
the sulphide (molybdenite, MoS 2 ).
(a) Ammonium Molybdate, 5(NH'
Grind 50 g. of molybdenite
1
and sift it through fine wire gauze;grind again all the powder that will not pass through the sieve,
and continue the process until nothing remains behind. Roast
(^1) Since molybdenite is difficult to pulverize the expedient recommended
long ago by Scheele may be used. Cut it into small pieces with scissors and
mix it with potassium sulphate crystals, the sharp edges of which under the
pestle tear the flakes of the molybdenite apart; finally dissolve out the potas-
sium sulphate with water.