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Page 89
By the second sublimation the whole of the benzoic acid is not
volatilized. What remains in the resin may be separated by boiling it
with caustic lime, and precipitating the acid from the resulting
benzoate of lime with hydrochloric acid. Benzoic acid can be obtained
also in the wet way, and the resin yields a greater product in this
process than in the former; yet it has a less perfumery value, because
it is free from the volatile oil which, as above stated, gives it its
peculiar odor. The wet method devised by Scheele is as follows:--Make
one ounce of freshly-burnt lime into a milk with from four to six ounces
of hot water. To the milk of lime, four ounces of powdered benzoin and
thirty ounces of water are to be added, and the mixture boiled for half
an hour, and stirred during this operation, and afterwards strained
through linen. The residue must be a second time boiled with twenty
ounces of water and strained, and a third time with ten ounces; the
fluid products must be mixed and evaporated to one-fourth of their
volume, and sufficient hydrochloric acid added to render them slightly
acid. When quite cold, the crystals are to be separated from the fluid
by means of a linen strainer, upon which they are to be washed with cold
water, and pressed, and then dissolved in hot distilled water, from
which the crystals separate on cooling. When hydrochloric acid is added
to a cold concentrated solution of the salts of benzoic acid, it is
precipitated as a white powder. If the solution of the salts of this
acid is too dilute and warm, none or only a portion of the benzoic acid
will be separated. However, the weaker the solution is, and the more
slowly it is cooled, the larger will be the crystals of this acid. In
the preparation of this acid in the wet way, lime is to be preferred to
every other base, because it forms insoluble combinations with the
resinous constituents of the benzoin, and because it prevents the
gum-resin from conglomerating into an adhesive mass, and also because an
excess of this base is but slightly soluble.
Stoltze has recommended a method by which all the acid can be removed
from the benzoin:--The resin is to be dissolved in spirit, to which is
to be added a watery solution of carbonate of soda, decomposed
previously by alcohol. The spirit is to be removed by distillation, and
the remaining watery solution, from which the resin has been separated
by filtration, treated with dilute sulphuric acid, to precipitate the
benzoic acid. This method gives the greatest quantity of acid, but is
attended with a sacrifice of time and alcohol, which renders it in an
economical point of view inferior to the above process of Scheele. It
is so far valuable, that the total acid contents of the resin can be
determined by it.
Dr. Gregory considers the following process for obtaining benzoic acid
the most productive. Dissolve benzoin in strong alcohol, by the aid of
heat, and add to the solution, whilst hot, hydrochloric acid, in
sufficient quantity to precipitate the resin. When the mixture is
distilled, the benzoic acid passes over in the form of benzoic ether.
Distillation must be continued as long as any ether passes over. Water
added towards the end of the operation will facilitate the expulsion of
the ether from the retort. When the ether ceases to pass over, the hot
water in the retort is filtered, which deposits benzoic acid on cooling.
The benzoic ether and all the distilled liquids are now treated with
caustic potash until the ether is decomposed, and the solution is heated
to boiling, and super-saturated with hydrochloric acid, which
afterwards, on cooling, deposits, in crystals, benzoic acid.
Benzoic acid, as it exists in the resin, is the natural production of
the plant from which the resin is derived. It may also be produced
artificially. Abel found that when cumole (C_{18}H_{12}) was treated
with nitric acid, so dilute that no red vapors were evolved for several
days, this hydro-carbon was converted into benzoic acid. Guckelberger
has, by the oxidation of casein with peroxide of manganese and sulphuric
acid, obtained as one of the products benzoic acid. Albumen, fibrin, and
gelatin yielded similar results when treated as above. W�hler has
detected benzoic acid in Canadian castor, along with salicin. It is also
formed by the oxidation of the volatile oil of bitter almonds. Benzoate
of potash results when chloride of benzoyle is treated with caustic
potash. Benzoic acid in the animal economy is converted into hippuric
acid, which may by the action of acids, be reconverted into benzoic
acid.
Benzoic acid should be completely volatile, without leaving any ash or
being carbonized when heated. When dissolved in warm water, to which a
little nitric acid has been added, nitrate of silver and chloride of
barium should produce no precipitates. Oxalate of potash should give no
turbidity to an ammoniacal solution of this acid. When heated with an
excess of caustic potash it should evolve no smell of ammonia,
otherwise, it has been adulterated with sal ammoniac. In spirit, benzoic
acid is easily soluble, and requires 200 parts of cold and 20 parts of
boiling water to dissolve one part of it.
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