_a._ From ACETATE OF SODA:--
1. Commercial acetate of soda (_i.e._, the 'pure acetate' of the
pyroligneous acid works), in crystals, is put into the body of a stout
copper still, and a deep cavity being made in the centre of the mass,
about 35% of sulphuric acid of a sp. gr. of not less than 1·84 is poured
in; the walls of the cavity are then thrown in upon the acid, and the
whole briskly agitated, for a very short time, with a large wooden
spatula; the head of the still is next luted on, and the distillation
conducted at a gentle heat, the receiver being changed as soon as the
distillate begins to acquire a slight empyreumatic odour. The product,
when the process is well managed, is an almost colourless acid of the sp.
gr. of fully 1·05, containing about 40% of glacial acid, or between 34%
and 35% of anhydrous acid. Any trace of colour or empyreuma is removed by
agitation with some well-washed and recently ignited vegetable charcoal,
or with a very small quantity of recently ignited purified animal
charcoal, and subsequently passing it through a prepared calico
bag-filter; or by allowing it to stand, for about a fortnight, in barrels
containing some beech-wood chips; after which it is ready for sale, either
as the ordinary acetic acid or pure pyroligneous acid of commerce, or (on
dilution, &c.) as vinegar.
2. The acid of sp. gr. 1·05 (obtained as above) is distilled with fused
chloride of calcium, the distillate being run into a refrigerator; the
crystals that form are drained at a temperature below 40° or 45° Fahr.,
and after removal to a warmer temperature, where they liquefy, and
agitation with a little peroxide of lead, are submitted to a second
distillation, as before; and this is repeated until the whole of the acid
crystallises at 51° Fahr. The product is the glacial acetic acid of
commerce.
_Obs._ The above are the processes usually adopted, on the large scale, in
this country.
3. (M. Mollerat's process--without distillation.) Pure commercial acetate
of soda, in coarse powder, is placed in a hard glazed stoneware or glass
pan or receiver set in a cool situation, and 35% or 36% of concentrated
sulphuric acid, of the sp. gr. 1·843, added, in such a manner that the
acid may flow under the powder, and little heat be generated by the
operation; the whole is then allowed to remain in contact (covered) for
some hours, when crystalline grains of sulphate of soda are found covering
the bottom and sides of the vessel, and hydrated acetic acid, partly
liquid and partly in crystals, the upper portion. The temperature being
now slightly raised to a point just sufficient to cause the liquefaction
of the crystals of acetic acid (_i.e._, to from 62° to 65° Fahr.), the
fluid is poured off, and a very small quantity of pure acetate of lime
added to it gradually, until it ceases to yield any trace of free
sulphuric acid on evaporation. After sufficient repose it is carefully
decanted for use. An excellent commercial strong acetic acid is thus
obtained, without distillation, owing to the insolubility of sulphate of
soda in acetic acid; and from which glacial acid may be procured by
refrigeration. If, however, the process be badly managed, or the
proportions of the ingredients be not carefully observed, the product will
be contaminated with either a little sulphuric acid or saline matter. It
is also important to the success of this process that it be performed in a
cool apartment, and in well-cooled vessels. Perfectly pure acetic acid may
easily be obtained by rectification from this acid. The above plan of
superseding a troublesome distillation is one of the greatest improvements
yet introduced into the manufacture of acetic acid.
4. (Liebig's process.) Pure acetate of soda, thoroughly dried and finely
powdered, 3 parts, is placed in a capacious retort, and pure concentrated
sulphuric acid, 9·7 parts, poured over it through the tubulature. One
eighth of the acetic acid passes over by the heat developed by the
reaction of the ingredients. The heat of a sand bath is next applied and
continued until the contents of the retort become quite liquid. The
distillate, carefully rectified, yields two parts of pure acid, containing
only 20 per cent. of water. On exposing the latter portion which comes
over in a closed vessel to a temperature below 40° Fahr., crystals of
hydrated acetic acid are deposited. The weaker, or liquid portion, being
poured off, the crystals are again melted and re-crystallised by cooling.
The crystals of the last operation, separated from the liquid, and
carefully drained in a cool and closed vessel, are perfectly pure hydrated
acetic acid.
_Obs._ The above is an excellent process for obtaining a chemically pure
acid. The excess of sulphuric acid left from the process may be recovered
by distillation; or the whole residuum may be employed in a second
distillation with fresh acetate.
[Illustration: (_C._) A Liebig's Condenser. (The other _reference letters_
are self-explanatory.)]
Although a retort is recommended by Liebig for the distillation, and is
usually adopted, on the small scale, for the purpose, a flask closed by a
cork perforated by two tubes, as exhibited by the _engr._, will be found
more convenient and safe; as the product is then less likely to be
contaminated by the 'spirting' of the ingredients over the brim of the
vessel. The heat of a diffused gas-flame may also be often advantageously
substituted for a sand bath.
_b._ From ACETATE OF POTASH:--
1. Acetate of potash (fused and powdered) is placed in a still, or other
suitable vessel, and 50% of the strongest sulphuric acid ('oil of vitriol'
of fully 1·84 sp. gr.) being added, the mixture is distilled to dryness,
as before. The product is 50 to 51% of the weight of the acetate employed,
with a sp. gr. of about 1·0735 to 1·074, containing about 66% of anhydrous
acetic acid, or nearly 80% of ordinary glacial acid. By rectification from
a little dried acetate of lead a perfectly pure acid of almost any
strength may be obtained. The ingredients are nearly in equiv.
proportions.
_c._ From ACETATE OF LEAD:--
1. (Ure.) Take of dried acetate of lead, 4 parts; strongest oil of
vitriol, 1 part. Distil slowly to dryness. Nearly equal to the last.