As the mixture becomes thick during the oxidation, it is very necessary
to use a stirrer which will keep the whole mass agitated by reaching
to the sides and bottom of the beaker.
If impure hydroquinone is used, a black, sticky precipitate will
usually appear after the addition of the sulfuric acid to the
hydroquinone solution. This should be removed, before the oxidation
is started, by filtration without suction through a fluted filter.
When technical sodium dichromate is used, the solution should be
filtered with suction, before it is added to the hydroquinone,
in order to remove any insoluble impurities.
In the laboratory it is convenient to make several small runs
of the size indicated, as far as the oxidation is concerned;
but the benzene extractions can be combined.
It is also possible to obtain good yields of quinone in the
following manner: 1500 cc. of water, 465 g. of concentrated
sulfuric acid and 300 g. of hydroquinone are mixed in a 3-l. beaker.
The mixture is cooled to 0'0, and 330 g. of sodium dichromate are added
in powdered form, the temperature being kept below 5'0 at all times.
This procedure requires a longer time and much more care in the control
of conditions than the method described above.