IN a 2.5-l. beaker, 100 g. of hydroquinone are dissolved
in 2000 cc. of water heated to about 50'0. After the solid
is completely dissolved, the solution is cooled to 20'0, 100 g.
of concentrated sulfuric acid are slowly poured in, and the mixture
is again cooled to 20'0. A concentrated solution of technical sodium
dichromate is prepared by dissolving 140 g. in 65 cc. of water.
This solution is then added gradually to the hydroquinone solution,
with the use of a mechanical stirrer (see notes), the mixture being
cooled so that the temperature never rises above 30'0. At first
a greenish-black precipitate forms, but upon further addition of
the sodium dichromate solution, the color changes to yellowish green.
As soon as this color remains permanent (a slight excess
of sodium dichromate does no harm) the reaction is complete.
This requires about one-half to three-quarters of an hour;
90 to 110 cc. of sodium dichromate solution is necessary.
The mixture is then cooled to about 10'0 and filtered with suction.
As much water as possible is pressed out of the crystals.
The filtrate is extracted twice, 150 cc. of benzene being used
for each extraction. The precipitate of quinone is transferred
to a 1-l. beaker, and 500 cc. of benzene, including the 300 cc.
used to extract the filtrate, are added, The mixture is now heated
with stirring on a steam-bath, and as soon as most of the quinone
has dissolved the benzene layer is decanted into another beaker.
It is dried while hot by stirring a short time with a little
calcium chloride, and then filtered through an ordinary funnel
into a 1-l. distilling flask before it cools. There is a certain
amount of quinone which does not go into the 500 cc. of benzene,
so that the residue is extracted a second time with about 100 cc.
of benzene, which is dried and filtered with the first extract.
During these extractions, the benzene should not be at the boiling point,
as this will cause a considerable volatilization of the quinone.
The distilling flask is now attached to a condenser set
for downward distillation, and the benzene is distilled.
As soon as the quinone starts to separate, the residue in the flask
is transferred to a beaker and cooled in an ice bath. The precipitate
is filtered off with suction and the product spread out for a short
time to dry. The product is yellow in color and weighs 75 to 80 g.
(76-81 per cent of the theoretical amount). Material made in this way
will hold its yellow color over long periods of time, provided it
is protected from light.
The benzene distillate is yellow and contains some quinone.
This, as well as the benzene from the final filtration of
the quinone crystals, may be used in a subsequent run and thus
raises the yield of the subsequent runs to about 85-90 g.
(85-90 per cent of the theoretical amount).