SILVER NITRATE. 123
Dissolve 190 g. of crude Chile saltpeter in 200 c.c. of boiling
water in a previously weighed flask. To the boiling solution
add 150 g. of powdered potassium chloride and boil for half an
hour longer, replacing any water lost by evaporation. The con-
tents of the flask should at the end weigh 520 to 540 g. While still
hot, filter rapidly through a Biichner funnel and rinse the residue
with a test-tubeful of hot water. Cool the filtrate rapidly while
shaking, whereby a crystalline meal of potassium nitrate is formed.
Evaporate the mother-liquor; remove the sodium chloride by
filtering while hot; and cool the
1
filtrate rapidly to obtain more
potassium nitrate. If sufficient mother-liquor still remains, work
it up in the same manner to obtain a further yield. Unite all of
the crystals of potassium nitrate and purify them by recrystalliza-
tion until they are free from chloride. The yield is from 60 to
70% of the calculated.- Silver Nitrate.
The nitrates of the alkali metals break down into nitrites when heated;
those of all the other metals dissociate into metallic oxide and nitric anhy-
dride or its decomposition products:
Cu(NO 3 )2 = CuO + N 2 O 4 + O.
This dissociation begins to take place at very different temperatures with
the various metals. Silver nitrate can be melted without decomposition,
while, at the same temperature, the nitrates of metals with a higher valence,
for example, copper nitrate, are decomposed; thus by melting a mixture of
these two nitrates and dissolving the fusion, silver nitrate can be obtained
free from copper.
By carefully regulating the temperature and repeating the process, mix-
tures of very closely related nitrates can be separated; the "nitrate method"
for separating the metals of the rare-earths depends upon this principle.
A completely analogous behavior is shown by the sulphates, as in the
Ziervogel process for obtaining silver from argentiferous pyrite. By roasting,
the sulphates of the metals are first formed, all of which, however, with the
exception of silver sulphate, decompose at a somewhat higher temperature
into metal oxide and sulphur trioxide; by leaching, the silver is obtained in
the solution.
Dissolve a silver coin in 30% nitric acid, evaporate the solution
to dryness, and transfer the residue to a porcelain crucible. Place
this crucible upon a wire triangle inside a larger crucible, andgradually heat the outer crucible to a dull red heat. When the
decomposition is completed, extract the black residue with water,
concentrate the filtrate and test it for copper. Should any copper