_a._ From ACETATE OF COPPER:--AROMATIC V.[dagger]; SPIRIT OF
VERDIGRIS[dagger]; SPIRITUS VEN'ERIS[dagger], L.; ESPRIT DE VENUS, Fr.;
ACIDUM ACE'TICUM, (Ph. L. 1787.) _Process._ Carefully dry crystallised
verdigris (diacetate of copper) by a very gentle heat, and introduce it
into a large stoneware retort (see _engr._), the bottom of which has been
previously coated with a mixture of clay and horse-dung, to render it more
capable of standing the fire. Next place it in a suitable furnace, and
connect it, by an adapter, with 3 or 4 double tubulated globes, the last
of which must be furnished with a vertical tubulature, to which a double
Welter's safety-tube should be adapted; the other end being immersed in a
basin half-filled with distilled vinegar or water, while the funnel
portion communicates with the atmosphere. Then place each globe in a basin
of water, kept cool by a stream constantly passing through it; and cover
the upper portion with cloths kept continually wet with cold water. After
15 or 20 hours, fire may be applied, and must be so regulated that the
drops follow each other with considerable rapidity from the end of the
adapter, whilst the bubbles of air cause no inconvenience at the other end
of the apparatus. If otherwise, the fire must be damped a little. The
operation should be continued, and the fire gradually increased, until
vapour ceases to come over, known by the globes gradually cooling,
notwithstanding the heat of the furnace. The operation being concluded,
the whole may be allowed to cool, and the acid collected preparatory to
rectification. This may be effected in a similarly arranged apparatus,
except that it must be wholly of glass; and the retort should not be much
more than half-filled. The operation must now be very carefully
conducted, and discontinued before barely the whole of the acid has
distilled over; as the last portion is apt to injure the flavour and
colour of the rest. The first portions which come over are very weak, and
should be kept separate, until the sp. gr. reaches to about 1·372, when
the receiver should be changed, and the product collected in separate
portions, as noticed below.
_Obs._ Good diacetate of copper yields, by careful management, at a
temperature of 400° to 560° Fahr., fully one half its weight of a
greenish-coloured acid, of the sp. gr. of about 1·061, containing above
50% of hydrated acetic acid, or 43% of anhydrous acid. 20 lbs. of the
ordinary acetate yields 9-3/4 lbs. of this rough acid, leaving a residuum
of about 6-1/2 lbs. of metallic copper mixed with a little charcoal, in
the retort; the remainder (nearly 2/10ths of the acid in the acetate)
being decomposed by the heat, and lost. This 9-3/4 lbs. of crude acid
yields by rectification, and dividing the products, 1/2 lb. of acid of the
sp. gr. 1·023; 3 lbs. of the sp. gr. 1·042; and 6 lbs. of the sp. gr.
1·065; exclusive of a little acetone which comes over with it. In the
first distillation, the strongest acid is found in the third receiver, and
the weakest in the first. The acid obtained in this way is always
accompanied with a little fragrant pyro-acetic spirit; which renders it
preferable for aromatic vinegar and perfumery. It dissolves camphor,
resins, and essential oils with facility. This is one of the oldest
methods of obtaining glacial acetic acid, and the product is still
preferred for some purposes. It is the RADICAL VINEGAR of the alchemists,
and it is that which is preferred by the perfumers. Well-dried acetate of
lead, or of iron, as well as several other acetates, may be substituted
for acetate of copper in the above process; but are less economical and
convenient. In all cases, great care must be taken to avoid over-firing,
as thereby the quantity obtained is lessened, and the quality injured. The
residuum of the distillation is pyrophoric and frequently inflames
spontaneously, on exposure to the air. Due caution must be therefore
observed regarding it.
IV. _From Wood_, by _dry distillation_. See PYROLIGNEOUS ACID. The
preparation of the purified acid, by converting it into an acetate, and
subsequent distillation with a strong acid, is noticed _above._
[Illustration]
V. _From Alcohol._ (ALCOHOL VINEGAR, GERMAN ACETIC ACID.) In a bell-glass,
or an oblong glass case, perforated shelves are arranged, a few inches
apart, one above another, on which are placed a number of small flat
dishes of porcelain, earthenware, or wood. These dishes are filled with
spirit of wine or dilute alcohol; and over each is suspended a watch-glass
or capsule containing a portion of platinum-black; the whole being
arranged so that the platinum-black and the surface of the alcohol are not
more than 1-1/2 to 2 inches apart. Strips of porous paper are next so hung
in the case, that their bottom edges are immersed in the spirit, to
promote evaporation; and lastly, the apparatus, loosely covered, is set in
a light place at a temperature of from 70° to 90° Fahr.--the sunshine,
when convenient. In a short time the temperature of the platinum rises,
and the formation of acetic acid begins; and the condensed vapour trickles
down the sides of the glass and collects at the bottom of the case, whence
it is removed once or twice a day. (See _engr._) The product of a case of
twelve cubic feet content, with 7 or 8 oz. of platinum-powder, is capable
of producing daily, if well managed, nearly 1·31 lb. of hydrated acetic
acid from 1 lb. of absolute alcohol; 25 lbs. of platinum-powder and 300
lbs. of alcohol will, in like manner, furnish a daily supply of nearly 350
lbs. of pure acid, and of other strengths in proportion. Theoretically,
the product should be 130 parts of the hydrated acid for every 100 parts
of alcohol consumed; but this is never quite obtained in practice, owing
to a small portion of the alcohol mixing with the newly formed acid, and
escaping decomposition; and from another small portion of both the
alcohol, and of the newly formed aldehyd, being carried off by the air
that permeates the apparatus. The platinum-powder does not waste, and the
most inferior spirit may generally be employed.
_Rationale._ In this process, the alcohol (as in other cases of
acetification) is first converted into aldehyd; and this, as rapidly as
formed, absorbs oxygen and passes into hydrated acetic acid. The
simultaneous formation of aldehyd during the oxygenation of that already
formed, may be detected by its odour.
_Obs._ During the mutual action of the platinum-black and the vapour of
alcohol, the temperature increases, and continues to do so until all the
oxygen contained in the air enclosed in the case is consumed, when the
acetification stops. On opening the case for a short time, to admit of a
fresh supply of air, the operation recommences, thus showing its
dependence on the oxygen of the atmosphere. For this transmutation, 100
grains of alcohol require 71 grains (equal to 200 cubic inches) of oxygen,
or about 1000 cubic inches of atmospheric air. To render the process
continuous and rapid, a fresh supply of air must, therefore, be constantly
provided. This may be effected by either having a loosely covered opening
at the top of the case, and several much smaller ones near its lower part;
or (and preferably) by means of two small glass tubes passing through the
lid or cover, one of which terminates just below the point of insertion,
whilst the other divides into branches which reach to within a short
distance from the bottom, as shown in the _engraving_. In this way a very
slow current of fresh air will always be kept up in the apparatus.
[Illustration]
In practice, we find, that by loosely spreading the platinum-black on
pieces of platinum-gauze, and supporting these on small tripods or bars of
glass or porcelain (or even wood), the watch-glasses and their troublesome
suspension may be dispensed with; as also may be the strip of porous
paper, provided a temperature of not less than 90° Fahr. be maintained in
the case or acetifier, which may easily be done by the application of
artificial heat in the absence of sunshine. On the large scale, a case of
wood with a glass roof, or even a well-seasoned cask or vat may be
employed, in which case the temperature of the apparatus must be kept up
either by means of steam-pipes or flues, or by the supply of warm air. On
the small scale, a hand bell-glass placed on a dish, with a single
watch-glass or piece of platinum-gauze, and a single capsule containing
alcohol, may be used, provided the bell-glass be supported on three very
small wedges, to admit of a supply of air. A modification of this is
sometimes employed, in which the alcohol is supplied, in drops, to the
platinum-black, by means of a long, tubular funnel passing through the
mouth of the bell-glass, and having its lower extremity drawn to a very
fine point, as shown in the _engr._ To ensure success, the platinum-black
should be either fresh-prepared, or recently washed and very gently
heated, before placing it in the acetifier. Spongy platinum, though
ordered by many chemical compilers, does not answer well for this process.
By the above elegant and economical process, perfectly pure acetic acid of
considerable strength may be produced from even impure alcohol; but it is
impossible in this way to obtain a concentrated acid without a subsequent
operation, because the action of platinum-black on absolute alcohol, or
even on strong alcohol, is so violent that the platinum soon begins to
glow, and inflammation ensues. Unfortunately the revenue laws of this
country, until lately, stood in the way of the adoption of this beautiful
process, unless duty-paid alcohol or methylated spirit be employed; but
there is no statute that prevents an individual employing pure spirit, of
any strength, on the small scale, for private consumption. In Germany, and
in the United States of America, vinegar is manufactured on this plan, and
from the low price of crude alcohol there, it will no doubt prove
ultimately to be the cheapest source of both pure acetic acid and culinary
vinegars.