URANIUM COMPOUNDS. 235
Uranyl Hydroxide, UO 2 (OH} 2.
The hydroxide of hexavalent uranium is amphoteric, that is, it has
acidic as well as basic properties. With acids it forms the uranyl salts, e.g.,
UO 2 (NO 3 ) 2 ; with bases it gives the difficultly soluble salts of pyro-uranio
acid, e.g., K 2 U 2 O 7. For this reason the free uranyl hydroxide cannot be
obtained by adding a solution of an alkali hydroxide to one containing a
uranyl salt; it is prepared by heating uranyl nitrate with anhydrous alcohol.Heat 20 g. of uranyl nitrate with 50 g. of absolute alcohol on the
water bath in such a way that the alcohol evaporates slowly,
but does not boil. After some time yellow uranyl hydroxide
separates; add more alcohol and evaporate further. Finally,
extract the precipitate with water, and dry it at a moderate
temperature.
Alkali Pyro-uranates. To a solution of uranyl nitrate, add
potassium hydroxide, sodium hydroxide, or ammonia until all of
the uranium is just precipitated. Drain the precipitate, wash it
carefully with water and dry it in the hot closet. Sodium pyro-
uranate is prepared industrially, and is used in the manufacture
of yellow-green fluorescent glass.
Ammonium Uranylcarbonate, UO 2 CO 3 • 2 (NHi)zC0 3. Add a
solution of ammonium carbonate carefully to a dilute solution
containing 20 g. of uranyl nitrate until precipitation is just com-
plete; an excess of the reagent dissolves the precipitate. Filter
off the small, light-yellow crystals, and wash them successively
with water, alcohol, and ether. On standing, the preparation
loses ammonium carbonate.
Uranium Trioxide, UO 3. Dry some uranyl ammonium carbon-
ate, or uranyl hydroxide, at 100°; then place it in a test-tube, and,
while shaking, heat it in an oil or paraffin bath at 250° to 300°
until the color has become brick-red.
Uranium Octo-oxide, U 3 OS. This oxide is obtained by ignit-
ing uranium trioxide (or any oxide of uranium), or, more con-
veniently, ammonium pyro-uranate, in a porcelain crucible; it is
a dark-green powder.Uranous Oxalaie, U(C 2 O 4 ) 2.
Uranyl salts are changed by reduction into uranous salts. The latter,
however, are easily oxidized again, although the difficultly soluble uranous
oxalate is relatively stable. A satisfactory reducing agent is the sodium salt
of hyposulphurous acid, which can be procured in the form of a paste contain-
ing 50% of the salt.