alies, and of ferric compounds; and, since in this
analysis ammonium nitrate has resulted from the neutralization of the
excess of the nitric acid added to oxidize the iron, it is essential
that this should be destroyed by repeated evaporation with a
relatively large quantity of hydrochloric acid. During evaporation a
mutual decomposition of the two acids takes place, and the nitric acid
is finally decomposed and expelled by the excess of hydrochloric acid.

Iron is usually found in the precipitate of barium sulphate when
thrown down from hot solutions in the presence of ferric salts. This,
according to Kuster and Thiel (!Zeit. anorg. Chem.!, 22, 424), is due
to the formation of a complex ion (Fe(SO_{4})_{2}) which precipitates
with the Ba^{++} ion, while Richards (!Zeit. anorg. Chem.!, 23, 383)
ascribes it to hydrolytic action, which causes the formation of a
basic ferric complex which is occluded in the barium precipitate.
Whatever the character of the compound may be, it has been shown that
it loses sulphuric anhydride upon ignition, causing low results, even
though the precipitate contains iron.

The contamination of the barium sulphate by iron is much less in the
presence of ferrous than ferric salts. If, therefore, the sulphur
alone were to be determined in the ferrous ammonium sulphate, the
precipitation by barium might be made directly from an aqueous
solution of the salt, which had been made slightly acid with
hydrochloric acid.]

[Note 4: The precipitation of the barium sulphate is probably complete
at the end of a half-hour, and the solution may safely be filtered at
the expiration of that time if it is desired to hasten the analysis.

As already noted, many precipitates of the general character of this
sulphate tend to grow more coarsely granular if digested for some time
with the liquid from which they have separated. It is therefore well
to allow the precipitate to stand in a warm place for several hours,
if practicable, to promote ease of filtration. The filtrate and
washings should always be carefully examined for minute quantities of
the sulphate which may pass through the pores of the filter. This is
best accomplished by imparting to the filtrate a gentle rotary motion,
when the sulphate, if present, will collect at the center of the
bottom of the beaker.]

[Note 5: A reduction of barium sulphate to the sulphide may very
readily be caused by the reducing action of the burning carbon of the
filter