252 OXY-ACIDS AND SALTS OF THE NON-METALS
Cover the crucible to prevent loss of particles of the salt by de-
crepitation, and heat gently until the charge just melts. Then
remove the cover and keep the melt just hot enough to main-
tain a brisk evolution of oxygen, but do not increase the tem-
perature when the mass shows a tendency to grow solid. At
the end of about 20 minutes the melt should begin to stiffen around
the edges and become more or less pasty or semi-solid throughout;
when this point is reached, let the contents of the crucible cool
completely, then cover it with 200 cc. of water, and let it stand
until it is entirely disintegrated. Collect the undissolved potas-
sium perchlorate on a suction filter and wash it with two successive
portions of 15 cc. of cold water (see Note 5 (a), page 9). Re-
dissolve the salt in hot water (see solubility table) and allow it to
recrystallize. About 30 grams of potassium perchlorate should be
obtained. A few crystals of the product should give no yellow
color (Cl 2 ) when treated with a few drops of 12 N hydrochloric
acid. The product should be entirely free from chloride (test with
silver nitrate).
QUESTIONS- Why is manganese dioxide added when oxygen is prepared
by heating potassium chlorate? - What is the reaction of hydrochloric acid with hypochlorous,
chloric, and perchloric acids, respectively? - What are the four oxy-acids of chlorine? Compare their
stability. - To what extent are hydrochloric, hypochlorous, chloric, and
perchloric acids ionized in dilute solution? - How could pure perchloric acid be prepared from potassium
perchlorate? - What is the solubility of silver chlorate and of silver per-
chlorate? How may preparations of chlorates and perchlorates
be tested for the presence of chlorides?
PREPARATION 40
SODIUM THIOSULPHATE, Na2S2O 3 -5H 2 O
Sodium sulphite is a salt of the lower oxide of sulphur, and may
thus be regarded as unsaturated with respect to oxygen; it is, in
fact, capable of slowly absorbing oxygen from the air and thereby