152 HETEROPOLYACIDS AND THEIR SALTS.
Copaux assumes that two atoms of hydrogen can also act as the central
atom. In this way the hypothetical acid H 2 « or H^H^Oe) is obtained,from which metatungstic acid, Hio[H 2 (W207)6], is derived.
- Ammonium Phosphomolybdate, (NH 4 ) 3 PO4 • 13 MoO 3 • H 2 O.
Treat a concentrated solution of 24 g. ammonium molybdate
with concentrated nitric acid until the precipitate of molybdic acid
first appearing has redissolved and the solution remains clear even
on boiling. On adding a solution of 3.6 g. of sodium phosphate the
well-known, yellow precipitate is produced. Wash this and dry
it in the steam closet. Yield, about 20 g.
- Silicotungstic Acid, H 8 [Si(W2O 7 ) 6 ] • 39 H 2 O.
First prepare silicic acid hydrogel by dissolving 50 g. of com-
mercial, precipitated silicic acid in 12 g. of sodium hydroxide and
50 c.c. of water, diluting to 200 c.c, heating to about 100°, and
precipitating with hydrochloric acid. Dilute further with hot
water, drain the hydrogel on linen cloth in a large Biichner funnel
and wash with hot water. Stir up the hydrogel in a dish with
hot water and again drain it on the linen cloth.
Neutralize a solution of 50 g. pure, commercial sodium tungstate
in 200 c.c. of water with about 80 c.c. of 2-normal sulphuric acid,
using litmus paper as indicator. Have this solution boiling, add
the hydrogel above obtained, and boil the mixture until a filtered
sample no longer gives a yellow precipitate of tungstic acid when
acidified with hydrochloric acid. During the boiling add a little
2-normal sulphuric acid whenever the solution becomes alkaline
(in all about 20 c.c). This operation should take half to three-
quarters of an hour. Water should be added from time to time
to replace that lost by evaporation. Finally, filter the solution.
Cool the nitrate, which amounts to about 200 c.c. in volume, and
acidify it strongly by adding about 25 c.c. of concentrated sul-
phuric acid. Again cool and add ether, little by little with con-
stant shaking, until on settling in a separatory funnel three
layers are formed: on top an ethereal, in the middle an aqueous
solution, and underneath a thick, oily layer of a liquid compound
of silico-tungstic acid with ether. If this lowest layer does not
form, or forms but incompletely, more sulphuric acid must be
added. Allow the whole to stand over night, then draw off the