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Saponification and
esterification
[professional translation]
French text
Direct access
to
Measurements
(ester index, saponification
index, iodine index)
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Saponification is the following chemical reaction:
ester + water
or base
yield
acid + alcohol or
phenol
The reverse reaction is called
esterification:
acid + alcohol or phenol
yield
ester + water
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The proportion of esters produced
in a given period of time depends on the type of alcohol: with a mono-alcohol (ethanol),
it approximates 70%, with a di-alcohol (glycol),
60%, and with a triol (glycerol),
10%. The type of alcohol function is
also relevant. Indeed, the more complex the alcohol, the more complex, and
therefore the slower the reaction. Over-sized molecules are formed more easily
with polyalcohols whereas the probabilities for reactions of esterification are
lower.
Both chemical reactions – saponification and esterification – can form a cycle
that can be interrupted only by evaporation and/or because one of the two
reactions is slower than the other (usually esterification). This cycle can
sometimes start again in a moist environment or on contact with alkaline
elements if there are esters remaining or in the presence of alcohol if there
are acids remaining, and above all if the properties of the chemical elements
produced afford it.
This process has been known since the second half of the 19th century only, and
was not scientifically proved until WWI.
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Classic example of esterification: the
formation of a
triglyceride
On the one hand we have the
alcohol functions
CH2OH + CHOC + CH2OH,
i.e. a
glycerine (glycerol)
And on the other we have three organic acids (see
carbo-esterase)
r1-COOH + r2-COOH + r3-COOH.
This yields
CH2OCO-r1 + CHOCO-r2 + CH2OCO-r3 (triglyceride) + 3 H2O
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As far as
soaps are concerned, note that these are produced with
potash or sodic soda and one or several oils,
usually vegetable oils.
Some paints, which use acids, bases, and alcohols, pose a problem of
reversibility, which can be solved by
proportioning and choosing balanced and well-thought-out products yielding a
reasonable and adapted result through careful conservation. Other paintings (or
products) are not reversible. Saponification cannot be “restarted” by ordinary
means as soon as water has evaporated.
E.g., the
combination phtalic acid/glycerol, once it is dry,
yields a very hard-wearing substance, which is nothing but
alkyd paint. This ester makes a hard layer
that no longer reacts to water and therefore cannot be saponified, at least in
theory.
This is in theory only, because a very alkaline
substrate (wet concrete, wet plaster, lime) will always tend to attack ordinary
paints (oil, acrylic, alkyd, etc.), which produce esters. In decorative
painting, the term “saponification”, for that matter, refers to the breaking
down of binding agents, which occurs when applying painting on this kind of
substrates.
The environment
and proportioning of all these chemical bodies is always decisive for a given
use and context.
Equally, some “saponified” binding agents (e.g. saponified waxes) are, in the
end, very stable once they are dry. There is no general rule for preserving
beyond the properties of each binding agent. A powerful
alkali like ammonia,
for example, triggers a reaction of radical dissolution, even on oil paintings
that have been dry for years, whereas a mere shower will be enough to wash out a
gouache left outside. Each binder, each painting medium, and more generally each
ester behaves in a specific way. All this can be measured (see
below).
Notes
Under certain conditions – in the presence of metals
belonging to the alkaline earths – a
saponification/esterification might create drosses,
mineral deposits, bleaching, and
other not so desirable pictorial effects.
The mechanical elimination of esters, acids, and alcohols is nothing but
a cleaning operation; and why not consider it for original artistic prospects in
which the paint would be lathered up, in the proper sense.
Imperative: Salts and their formation,
fat, soap,
glue/oil/sugar soap
emulsion, sugar soap in glossary,
soda, caustic soda
and potash in glossary.
Measurements
(ester index, saponification index, and iodine index)
The different properties of an ester can be measured
up.
* The ester index indicates the amount of
potash (an alkaline matter used as a reference)
necessary to get a reaction of saponification. It represents somehow the
“solidity” of an ester.
* The saponification index is equally expressed by
an amount of potash, which, here, corresponds to the yielding of a neutral
pH.
* Finally, the iodine index refers directly to the
saturation of the ester. It is very important for
an artist to know if the oil paint he wants to use will be “siccative”.
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