ORGANIC ACIDS
Organic Chemistry. About a century ago, when the science of Chemistry
was still in its infancy, several substances were known which could then
only be obtained from animals or plants. The composition of these
substances was not understood, and they were not classified; moreover,
since none of them had ever been prepared artificially, it was supposed
that it was impossible to do this—the reason given was that “vital
force” was necessary for their production. In time, however, some of the
most typical animal and vegetable products were prepared in the
laboratory, and the belief in vital force disappeared.
In later times it was proved that substances like sugar, starch, urea,
indigo, and a great many more, all contain the element carbon. At the
present time, more than 100,000 compounds of this element are known; and
since they resemble one another, and at the same time differ in several
important respects from the compounds of other elements, it is both
natural and convenient that they should be classed together and studied
separately. This branch of Chemistry is called organic. It must not,
however, be supposed that all organic compounds are necessarily produced
by some living organism. A great many are, but there are many more which
are purely synthetic products.
Inorganic Chemistry includes all the other elements and their
derivatives. The _element_ carbon, and also some of its simpler
compounds, such as carbon monoxide, carbon dioxide, carbonic acid, and
carbonates, are more appropriately placed in the inorganic section.
The acids which have been considered up to this point are all inorganic
acids, and those which follow are organic. Sulphuric, nitric, and
hydrochloric acids are often distinguished as the mineral acids in
contradistinction to oxalic, citric, tartaric, and some others which
were first obtained from unripe fruits and therefore called vegetable
acids.
Organic acids have all the general properties of the class, but they are
much weaker than the mineral acids mentioned above. This is shown by
their solvent action on metals, oxides, and carbonates, which is in all
cases slight.
Vinegar is the trade name for what is essentially a dilute solution of
acetic acid which has been made by the acetous fermentation of
saccharine fluids containing weak alcohol. In addition to acetic acid,
vinegar contains minute quantities of a large number of compounds. Some
of these help to produce that agreeable flavour and aroma which
distinguishes vinegar from diluted acetic acid. The nature and quantity
of the flavouring constituents depend mainly upon the nature of the
alcoholic solution used.
Since the acetic acid in vinegar is always produced by fermentation, all
processes for the manufacture of vinegar are essentially arrangements
for promoting the vigorous growth and development of _Mycoderma aceti_,
the organism which produces the vinegar ferment.
Like all other plants, _Mycoderma aceti_ will flourish only under
certain favourable conditions. In the first place, it requires
nourishment, and therefore certain nitrogen compounds and salts must be
present in the alcoholic solution. These are contained in wines, beer,
cider, and malt liquors, but not in spirits of wine, which is pure
alcohol distilled from liquids which have undergone vinous fermentation.
If the plant is placed in dilute spirits of wine, only a very little
acetic acid is formed, and then the action ceases because the solution
does not contain the necessary food substances. Temperature also has a
very marked effect on growth, the most favourable range being between
68° and 95° F.
Alcohol is changed to acetic acid by the process of oxidation, and
therefore, in making vinegar, arrangements have to be made to bring
together weak alcohol and air in the presence of the plant. The ferment
which is secreted by the plant then causes an acceleration of the
reaction. There is a considerable amount of similarity between
fermentation and contact action. In this connection, it is interesting
to note that the conversion of alcohol into acetic acid can also be
brought about by exposing a mixture of alcohol vapour and air to the
action of platinum black; in fact, there is one process for making
vinegar in this way.
French Vinegar. New wine, especially that which contains a low
percentage of alcohol, is liable to many kinds of “sickness.” It may
turn bitter, it may turn sour, or it may undergo what is called lactic
fermentation. In either case, it becomes unsaleable as a beverage. Wine
which has turned sour is the best material for making vinegar, and when
this is done by the French or slow process, a product with a very fine
_bouquet_ is obtained.
The methods adopted are very simple. Formerly, the wine was poured into
barrels leaving the top portion empty, and providing for a current of
air over the surface. The barrels were often set up in rows in the open
air in an enclosure which was then known as a “vinegar field.” The
process of souring which had already begun went on naturally, and in the
course of a few months, nearly the whole of the alcohol was converted
into acetic acid.
The process now in use in some of the French factories is somewhat
similar. Large casks holding about 100 gallons are set up in a room, and
provision is made for keeping the temperature uniform. Each cask is
first acidulated by allowing strong vinegar to stand in it until the
vinegar plant has developed on the surface. The casks are then filled up
very gradually by adding a few gallons of wine every eight or ten days.
When the cask is full, a fraction of the contents is drawn off and
replaced by wine. The process then becomes continuous, until it is
necessary to clean out the generator and start again.
In recent times, the manufacture of wine vinegar has been carried out on
more scientific principles. The vinegar plant is actually cultivated and
examined microscopically before being used, in order to make sure of the
absence of moulds and bacteria, which set up other fermentations,
producing substances which affect adversely the taste and aroma of the
finished product. The cultivated ferment is then added to the wine in
shallow vessels and the process is carried on as described above.
Malt Vinegar. A dilute solution of alcohol which is made from malt by
fermentation with yeast contains the nutritive substances necessary for
the growth of the vinegar plant, and can therefore be used as a
starting-point for the manufacture of vinegar. Sprouted barley or malt
is mixed with oats, barley, rice, or other starch-containing material.
The mixture is mashed with warm water and then fermented with yeast,
giving what is called “raw spirit.” This is converted into vinegar by
the “quick” process.
The vinegar generator (Fig. 10) is a large barrel divided into three
compartments by two perforated partitions. The lower disc is fixed about
one-third of the way up the barrel, and near it holes are bored to admit
air. The upper disc, fixed near the top of the barrel, is perforated
with a large number of small holes which are partially stopped up with
short threads or wicks, which hang from the under side. The space
between the two discs is packed with specially prepared beech shavings,
which have been left to stand in strong vinegar until they are covered
with the vinegar plant.
[Illustration: Fig. 10. QUICK VINEGAR PROCESS]
The weak spirit is delivered into the upper portion of the barrel and is
distributed in very small drops by the threads; it then passes slowly
over the vinegar plant, to which the air also has free access. When it
reaches the bottom, it overflows into a reservoir and is again passed
through the generator; this is repeated until the product contains the
desired amount of acetic acid.
The principle of the quick vinegar process is the same as that employed
in making wine vinegar. The speed of the reaction is, however, greatly
increased by having the ferment spread over a very large surface and by
the free circulation of air. It is possible to make wine vinegar by the
quick process, but it is not done, because the product is inferior in
taste and aroma to that made by the slow process.
Both wine vinegar and malt vinegar when freshly prepared have a
stupefying and unpleasant odour. Before the product is ready for the
market, it has to be matured in barrels. During this process, a small
quantity of alcohol which still remains in the vinegar combines slowly
with some of the acetic acid, producing acetic ester, a substance which
has a pleasant fruity odour.
The colour of wine vinegar is natural, but vinegar which is produced by
the quick process is colourless or only faintly coloured. Since the
public has a preference for vinegar which is brown in colour, the
product of the quick process is coloured artificially, either by adding
caramel or by preparing the weak spirit from malt which has been
slightly charred in drying.
Industrial Acetic Acid. The solutions of acetic acid dealt with above
would be too dilute for any industrial purpose; moreover, the acid can
be obtained much more cheaply by the distillation of wood. When wood is
subjected to a high temperature, it is converted into charcoal and, at
the same time, an inflammable gas, an acid liquid, and tar are given
off, and can be collected in suitable vessels. The following table, on
page 73, gives the relative amounts of the various substances obtained
from wood by dry distillation. The quantities are those derived from one
cord, that is, 125 cu. ft.
_Charcoal _Alcohol _Calcium _Tar in _Wood oil _Turpentine
in in acetate gallons._ in gallons._
bushels._ gallons._ in lbs._ gallons._
Hard 40-50 8-12 150-200 8-20
woods
Resinous 25-40 2-4 50-100 30-60 30-60 Heavy woods
woods 12-25
Light woods
2-10
Sawdust 25-35 2-4 45-75
The aqueous liquid that distils over contains methyl alcohol (wood
spirit), acetone, and acetic acid. The crude mixture is known as
pyroligneous acid. This is neutralized with milk of lime or soda ash,
which converts acetic acid into calcium or sodium acetate, but has no
action on the methyl alcohol and acetone which are also present. The
mixture is then distilled, when methyl alcohol, acetone, and water pass
over into the distillate, leaving the acetate in the retort.
To obtain the free acid from the acetate, the latter is well dried and
then distilled with concentrated sulphuric acid. Acetic acid, being the
more volatile of the two acids, distils over, and is nearly pure.
The method of removing the last traces of water depends upon the fact
that acetic acid solidifies at 17° C. The acid, which is nearly, but not
quite, free from water, is cooled until a portion solidifies. The part
which still remains liquid is poured away, and the process is repeated
until a residue is obtained which solidifies as a whole. This is glacial
acetic acid, so called because it is a mass of glistening plates which
look like newly-formed ice.
The Acetates
Aluminium Acetate, made by dissolving alumina in acetic acid, is the
“red liquor” which is used as a mordant in dyeing. It is a colourless
liquid, but is called “red liquor” because it is used with dyes which
give a red colour.
Ferrous Acetate, made in a similar way from scrap iron and acetic acid,
is the “black liquor” used in dyeing.
Verdigris, or basic copper acetate, is a valuable pigment. It is made by
interposing cloths soaked in vinegar between plates of copper. After the
action has been allowed to go on for a long time, the plates are washed
with water and the verdigris is scraped off. The finest verdigris is
made in France in the wine-producing district around Montpellier. Here,
instead of cloths soaked in vinegar, the solid residue from the wine
presses is spread in layers between the copper plates. The product made
in this way is called _vert de Montpellier_.
[Illustration: Fig. 11. DUTCH PROCESS FOR WHITE LEAD]
Verdigris, like all the copper compounds, is extremely poisonous. It is
very liable to be formed on the surface of copper utensils used for
cooking purposes.
Lead Acetate, or sugar of lead, is used in large quantities in the
colour industry for making various reds and yellows. It is prepared by
dissolving the metal or its oxide (litharge) in acetic acid.
The slow action which acetic acid vapour has upon the metal lead finds a
very interesting application in what is known as the Dutch process for
the manufacture of white lead[4] for paint. The metal is cast into grids
or spirals, which are placed on the shoulders of the specially made pots
sketched in Fig. 11. A little dilute acetic acid is poured into each of
the pots, which are then arranged side by side on a thick layer of tan
bark, stable manure, or other material which will heat by fermentation.
The first layer of pots is then boarded over; another layer of pots is
placed upon this, and so on, tier upon tier, until the shed is quite
full. The heat developed by the fermenting material vaporizes the acetic
acid, and this vapour corrodes the lead, forming basic lead acetate. The
carbon dioxide which is also produced during fermentation converts the
acetate into the carbonate, which falls as a heavy white powder into the
pots.
Future Supply of Acetic Acid. When all the operations involved in the
production of acetic acid from wood, from the felling of the tree to the
final separation of the glacial substance, are taken into consideration,
it will be readily understood how it is that this acid has never been
cheap when compared with other acids used on an equally large scale. In
addition to this, the competition for wood for paper-making and for the
very numerous cellulose industries is rapidly increasing. It is,
therefore, not surprising to learn that chemists have turned their
attention towards the discovery of newer and cheaper methods of making
acetic acid.
Such a process seems to have been worked out in Germany. The
starting-point is acetylene gas made by the action of water on calcium
carbide. When this gas is passed through sulphuric acid containing
suspended mercuric oxide or dissolved mercury salt, the acetylene is
oxidized first to aldehyde and then to acetic acid.
If this process should prove to be successful, it will form the
starting-point of a new and important industry, for, apart from the
large amount of acetic acid which is used in commerce, there is the
production of the very important solvent known as acetone, which can be
made from acetic acid by a very simple operation.
Tartaric Acid. Grape juice contains a large quantity of potassium
hydrogen tartrate dissolved in it; when the liquid is fermented and
alcohol is formed, this salt crystallizes out because it is not soluble
in alcohol. After the new wine has been poured off, the salt is found as
a brownish crystalline residue adhering to the sides of the vat. Also
the salt goes on crystallizing after the wine is put into barrels, and
forms an incrustation on the sides. This is called the _lees_ or
sediment of wine. In commerce, the substance is known as _argol_
(sometimes spelt _argal_), and also _tartar_ of wine.
Crude argol is purified by dissolving it in water and destroying the
colour by boiling with animal charcoal. When the clear liquid obtained
from this mixture by filtration is evaporated, a white crystalline
substance separates out. This is potassium hydrogen tartrate or _cream
of tartar_.
Tartaric acid is obtained from cream of tartar. The salt is dissolved in
water and nearly neutralized with milk of lime. Insoluble calcium
tartrate is precipitated, and potassium tartrate remains in solution. A
further quantity of calcium tartrate is obtained by adding calcium
chloride to the solution just mentioned. The two precipitates of calcium
tartrate are then mixed and decomposed by dilute sulphuric acid, and
after the calcium sulphate is filtered off, tartaric acid is obtained as
a solid by evaporating the clear liquid.
The general properties of tartaric acid are well known. It is soluble in
water, giving a solution which has a pleasantly acid taste.
Citric Acid. The sharp flavour of many unripe fruits is due to the
presence of citric acid; the juice of lemons contains 5-6 per cent. of
the acid. The free acid is obtained in a manner precisely similar in
principle to that described for tartaric acid.
Oxalic Acid. Oxalic acid and its salts, the oxalates, are very widely
distributed in the vegetable kingdom. These compounds are present in
wood sorrel (_Oxalis acetosella_), in rhubarb, in dock, and in many
other plants. The acid is made on a large scale by mixing pine sawdust
to a stiff paste with a solution containing caustic soda and potash. The
paste is spread out on iron plates and heated, care being taken not to
heat the mixture to the point at which it chars. The mass is then
allowed to cool, and is mixed with a small quantity of water to dissolve
out the excess of alkali. This is recovered and used again.
Sodium oxalate, which is the main product of the reaction described
above, is dissolved in water and treated with milk of lime, whereby
insoluble calcium oxalate is obtained, which is subsequently decomposed
with sulphuric acid, yielding oxalic acid.
Potassium hydrogen oxalate is sometimes called _salts of sorrel_, and
potassium quadroxalate, _salts of lemon_. The most familiar use of the
latter substance is in the removal of ink stains.
Oxalic acid and its salts are poisonous. The free acid has sometimes
been mistaken for sugar with fatal results.
Formic Acid (_L. formica_, an ant) is found both in the vegetable and in
the animal kingdom. If the leaf of a stinging nettle is examined with a
microscope, it is seen to be covered with long pointed hairs having a
gland at the base. This gland contains formic acid. When the nettle is
touched lightly, the fine point of the hair punctures the skin, and a
subcutaneous injection of formic acid is made, which quickly raises a
blister.
The inconvenience which arises from the stings of bees and wasps, also
from the fluid ejected by ants when irritated, is due to formic acid.
The remedy in each case is the same; the acid must be neutralized as
quickly as possible with mild alkali, such as washing soda.
Formic acid was first made by distilling an infusion of red ants. It is
now made from glycerine and oxalic acid.
The Fatty Acids. Animal fats and vegetable oils are similarly
constituted bodies. They are composed mainly of three chemical compounds
known as stearin, palmitin, and oleïn. Of these, stearin and palmitin
are solids at ordinary temperatures, while oleïn is a liquid. Hard fats
like those of mutton and beef are composed mainly of stearin; fats of
medium hardness contain stearin, palmitin, and some oleïn; while oils
such as cod-liver oil and olive oil are nearly pure oleïn.
Stearin, palmitin, and oleïn are analogous in composition to salts.
Their proximate constituents are glycerine and certain organic acids,
stearic, palmitic, and oleïc respectively.
In order to obtain the fat free from tissue which it contains in its
natural state, it is tied up in a muslin bag and heated in boiling
water. The fat is squeezed out through the meshes of the fabric and
floats on the surface of the water as an oil which solidifies on
cooling. This clarified fat is called tallow.
All fats and vegetable oils can be resolved into their two constituents,
the acid and the glycerine. This can be brought about by heating the fat
with water to about 200° C. This operation must be carried out in a
vessel capable of withstanding pressure and closed with a safety valve;
otherwise, the requisite temperature could not be obtained. After this
treatment, there is left in the vessel an oily layer which solidifies on
cooling and an aqueous layer which contains the glycerine. The
solidified oily layer is the fatty acid. In the case of mutton or beef
tallow, it would be mainly a mixture of stearic and palmitic acids. This
mixture is used to make “stearin” candles. The acids themselves are
wax-like solids without any distinctive taste. Stearic acid melts at 69°
C. and palmitic at 62° C. They have no perceptible action on the colour
of litmus, neither have they any solvent action on metals or carbonates.
We should not recognize these substances as acids at all were it not for
the fact that they combine with alkalis, forming salts.
The salts of the fatty acids are called soaps. To make soap, the fat is
boiled with caustic alkali or caustic lye, as it is more often called.
This breaks the fat up primarily into the acid and glycerine; but in
this case, instead of obtaining the acid as the final product as we did
above by heating with water under pressure, we get the sodium or
potassium salt of the acid according to the alkali used. When caustic
soda is used, the product is a hard soap; when caustic potash is used,
it is a soft soap. The treatment of fats in this way with caustic
alkalis is called “saponification.”