118 OXY-HALOGEN COMPOUNDS.
volume, add 20 c.c. of a 20% manganous sulphate solution to pre-
vent the hydrochloric acid from interfering with the titration, and
titrate in the cold with 0.1-normal potassium permanganate. A
slight correction can be applied to allow for the chromate present.
Determine in a similar manner the percentage of KC10 3 in the
crystals obtained; then calculate the entire quantity of potassium
chlorate and the current yield. It is to be remembered that
according to the equation for its formation, 96,540 ampere-seconds
yield but 1/6 of a mol. of KCIO3. The current yield amounts to
about 70%, and about 85% of all the chlorate is obtained in the
crystals. A further loss occurs when the crude, solid product is
purified by recrystallization from hot water. The yield of puri-fied chlorate actually obtained in an experiment carried out
according to the above directions was 23 g.
The chlorate dissociates into a potassium ion and a chlorate ion.
Test the purity of the preparation by dissolving a little in water,
acidifying with nitric acid and adding silver nitrate; there should
be no precipitate of silver chloride.
To illustrate the decomposition of chlorates by heat, melt a little
of the preparation in a test-tube and test for oxygen with a glowing
splinter (see the next preparation).- Potassium Percblorate.
Potassium chlorate on being heated to about 400° decomposes in two
different ways: - 4 KCIO3 = KC1 + 3 KCIO4.
- KCIO3 = KC1 + 3 0.
If the vessel is clean and the potassium chlorate pure, the decomposition
proceeds essentially according to equation 1. If, on the other hand, cataly-
zers are present, such as manganese dioxide or ferric oxide, or if it is heated
to a higher temperature, then the reaction takes place principally according
to equation 2. It is on this account that a mixture of manganese dioxide
with potassium chlorate is used, rather than the pure chlorate, in preparing
oxygen.
Heat 50 g. of potassium chlorate in a new 100 c.c. porcelain cru-
cible until the salt just melts. Without increasing the heat keep
the temperature as uniform as possible, so that oxygen barely
escapes while the melt gradually becomes more viscous and pasty.
If at the end of 10 or 15 minutes the mass has become uniformly
semi-solid, allow it to cool, then cover it with 50 c.c. of cold water
and allow it to stand until fully disintegrated. Collect the undis-