Thoria which has been ignited is not readily soluble in acids.
ZIRCONIA.
The oxide of zirconium, ZrO_{2}, is found in the mineral zircon, a
silicate of zirconia, ZrSiO_{4}. When heated intensely it becomes very
luminous, and is used on this account for incandescent lights.
In the ordinary course it is thrown down by ammonia with the other
earths, from which it is thus separated:--The hydrates precipitated in
the cold, and washed with cold water, are dissolved in hydrochloric
acid, nearly neutralised with soda, and precipitated by boiling with
hyposulphite of soda. Dissolve; and from the hydrochloric acid solution
precipitate the thoria (if any) with ammonium oxalate. To the filtrate
add carbonate of ammonia, which will precipitate any titanium present.
The zirconia will be in solution, and is recovered by precipitating with
potassium sulphate, or by evaporating the solution and igniting. It is
separated from alumina by taking advantage of its insolubility in
potassic hydrate.
It is estimated in zircons in the following way:--The powdered substance
is fused with bisulphate of potash, and extracted with dilute sulphuric
acid. The residue is fused with caustic soda and extracted with water.
The portion not dissolved, consisting of zirconate of soda, is dissolved
in hydrochloric acid. The solution is diluted, filtered if necessary,
and treated with ammonia in excess.
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