POTASSIUM NITRATE 57larger crystals that would be obtained by slow cooling. Separate
the KNO3 crystals (C) from the cold liquor by means of the suction
filter (observing the last sentence of Note 3, page 5). Press the
mass of crystals in the filter to squeeze out as much of the solution
as possible. The thin layer of solution left adhering to the surfaces
of the crystals is saturated with NaCl. To remove the greater
part of this, stop the suction, pour carefully over the surface of the
crystals 20 cc. of ice-cold distilled water, let this water penetrate
into the mass of crystals for perhaps 30 seconds, then apply suction
and drain these washings into the rest of the nitrate. Pour the
nitrate (D) into the casserole in which the batch of NaCl crystals
(A) is reserved. Bring this mixture to the boiling point and boil
gently 5 minutes with the casserole uncovered, thus allowing a
part of the water to escape by evaporation. Then filter at the
boiling temperature exactly as in the first instance. Cool the
nitrate, and collect and rinse a second crop of KN0 3 crystals
(F) in exactly the same manner as the first crop (C). Reserve the
nitrate in a flask labeled " mother liquors " (G). Add together the
two crops of KNO3 crystals (C and F), and test them for chloride
by dissolving about 0.1 gram in 2 cc. of water and adding 5 drops of
dilute HNO3 acid and 1 drop of AgNO 3 solution. A considerable
white precipitate will be seen, indicating the presence of chloride,
and the crystals must be purified by recrystallization. Weigh
the crystals while they are still moist, add to them in a beaker one-
half this weight of distilled water, and warm until solution is
complete.*
Cool the solution to 10° with stirring, and pour the mixture
on to the suction filter. Some of the nitrate, which is saturated
with KNO3, may be poured back into the beaker to rinse the last
of the crystals on to the filter. Drain the crystals with suction,
pressing them down compactly in order to squeeze out as much of
the solution as possible. Stop the suction. Pour carefully over
the crystals 15 cc. of ice-cold distilled water. Let it permeate the
whole mass, then again apply suction and drain and collect all the
* If the solution is not perfectly clear at this point it must be filtered.
Dilute it with 50 cc. of water so that it will not " freeze " on the filter. Pour
it at the boiling temperature without suction through a filter, and then pour
20 cc. of boiling water around the upper edge of the filter, letting it run through
into the filtrate to carry the last of the KNO3. The solution will have to be
evaporated to bring it back to its original volume.