f barium sulphate, calculate the percentage of
sulphur (S) in the sample.

[Note 1: This alkaline fusion is much employed to disintegrate
substances ordinarily insoluble in acids into two components, one
of which is water soluble and the other acid soluble. The reaction
involved is:

BaSO_{4} + Na_{2}CO_{3}, --> BaCO_{3}, + Na_{2}SO_{4}.

As the sodium sulphate is soluble in water, and the barium carbonate
insoluble, a separation between them is possible and the sulphur can
be determined in the water-soluble portion.

It should be noted that this method can be applied to the purification
of a precipitate of barium sulphate if contaminated by most of the
substances mentioned in Note 3 on page 114. The impurities pass into
the water solution together with the sodium sulphate, but, being
present in such minute amounts, do not again precipitate with the
barium sulphate.]

[Note 2: The barium carbonate is boiled with sodium carbonate solution
before filtration because the reaction above is reversible; and it is
only by keeping the sodium carbonate present in excess until nearly
all of the sodium sulphate solution has been removed by filtration
that the reversion of some of the barium carbonate to barium sulphate
is prevented. This is an application of the principle of mass action,
in which the concentration of the reagent (the carbonate ion) is
kept as high as practicable and that of the sulphate ion as low as
possible, in order to force the reaction in the desired direction (see
Appendix).]

DETERMINATION OF PHOSPHORIC ANHYDRIDE IN APATITE

The mineral apatite is composed of calcium phosphate, associated with
calcium chloride, or fluoride. Specimens are easily obtainable which
are nearly pure and leave on treatment with acid only a slight
siliceous residue.

For the purpose of gravimetric determination, phosphoric acid is
usually precipitated from ammoniacal solutions in the form of
magnesium ammonium phosphate which, on ignition, is converted into
magnesium pyrophosphate. Since the calcium phosphate of the apatite
is also insoluble in ammoniacal solutions, this procedure cannot be
applied directly. The separation of the phosphoric acid from the
calcium must first be accomplished by precipitation in the form of
ammonium phosphomolybdate in nitric acid solution, using ammonium
molybdate as the precipitant. The "yellow precipitate," as it is often
called, is not always of a definite composition, and therefo