The methods employed up to the present in examination of fats, animal and
vegetable, are mere reactions lacking general application; scattered
throughout the literature, and doubtful with regard to reliability, they
are of little or no value to the experimenter--an approximate quantitative
examination even of a simple mixture being exceedingly difficult if not
impossible, since the qualitative composition of fatty substances is the
same, and the separation of the nearer components impracticable. The
object of analysis consisted in estimating the accompanying impurities of
fat, as, resin, albuminoids, and pigments. The nature of these substances
depends on the mode of extraction and preservation of the fat, and are
subject in the course of time to alteration. The only reaction based upon
the chemical constitution of fat is produced by treatment of oleic or
linoleic acid with nitrous acid, which therefore is of some value in the
examination of drying oils. Of general application are the methods which
correspond to the chemical constitution of fats, and thus determine the
relative quantity of the components; advantage can then be derived from
qualitative reactions, inasmuch as they further affirm the result of the
quantitative test, or dispel any doubt with regard to the correctness of
the result. The principal methods which comply with these demands have
been carefully studied by Hueble for the purpose of discovering a process
of general application; methods founded on the determination of density,
freezing, and melting point were compared with those dependent on the
solubility of fatty substances in glacial acetic acid or a mixture of
alcohol and acetic acid; also the method of Hehner for testing of butter,
the determination of glycerine and oleic acid, and at length the process
of saponification. Nearly all fats contain members belonging to one of the
three series of fatty acids, _e.g._, acids of the type of acetic acid
(stearic and palmitic acids); such as are derivatives of acrylic acid
(oleic and erucic acids); and such as are homologues of tetrolic acid
(linoleic acid). It is likely that the relative quantity of each of these
acids is variable, with regard to the same fat, within definite limits,
and changes with the nature of the fatty substance. The groups of fatty
acids are distinguished by a characteristic deportment toward halogens;
while members of the first series are indifferent to haloids, those of the
second and third class combine readily, without suffering substitution,
with two respectively four atoms of a haloid. In view of this behavior the
first series is termed saturated, the second and third that of unsaturated
acids. Addition of halogen to one of the unsaturated acids yields on
subsequent examination an invariable quantity of the former, representing
two or four atoms, according to one or the other of unsaturated groups;
and as the molecular weights of fatty acids are unequal, the percentage
quantity of halogen will be found varying with regard to members belonging
to the same series. The amount of iodine absorbed by some of the fatty
acids is illustrated by the following items:
Hypogallic acid, C_{16}H_{30}O_{2}, combines with 100.00 grammes. iodine.
Oleic acid, C_{18}H_{34}O_{2} " " 90.07 " "
Erucic acid, C_{22}H_{42}O_{2} " " 75.15 " "
Ricinoleic acid, C_{18}H_{34}O_{3} " " 85.24 " "
Linoleic acid, C_{16}H_{28}O_{2} " " 201.59 " "
Of the halogens employed in the examination, iodine is preferable to
either chlorine or bromine; it acts but slowly at ordinary, but
energetically at elevated temperatures. The reagents are solution of
mercury iodo-chloride prepared by dissolving of 25 grms. iodine, 500 c.c.
alcohol of 95 per cent., and of 30 grms. mercury chloride in an equal
measure of the same solvent; both liquids are filtered and united; a
standard solution of sodium hyposulphite produced by digestion of 24 grms.
of the dry salt with 1 liter water and titration with iodine solution;
solution of potassium iodide of 1:10; chloroform, and finally a solution
of starch. The above solution of mercury iodo-chloride acts on both free
unsaturated acids and glycerides, producing addition products. For testing
a sample of 0.2 to 0.4 grm. of a liquid, and from 0.8 to 1.0 grm. of a
solid fat being used, which is dissolved in 10 c.c. chloroform and treated
with 20 c.c. mercury iodo-chloride solution run into it from a burette, if
the liquid appear opalescent a further measure of chloroform is
introduced, while the amount of mercury iodo-chloride must be such as to
produce a brownish coloration of the chloroform for two subsequent hours.
The excess of iodine is determined, on addition of from 10 to 15 c.c.
potassium iodide solution and 150 c.c. distilled water, by means of
caustic soda. From a burette divided into 0.1 c.c. a solution of caustic
soda is poured with continual gyration of the flask into the tinged
liquid, and the percentage of combined iodine ascertained by difference;
for this purpose 20 c.c. of mercury iodo-chloride are tested, on
introduction of a solution of potassium iodide and starch, previously to
its use as reagent. Adulteration of solid or semi-liquid fats, especially
lard, butter, and tallow, with vegetable oils are readily detected by this
method, since the latter yield on examination a high percentage of iodine.
Animal fats, absorb comparatively less halogen than vegetable fats, and
the power to combine with iodine increases with the transition from the
solid to the liquid state, and attains its maximum with vegetable
oils--the method being adapted to the examination of fat mixtures
containing glycerides and free saturated fatty acids, provided that
substances which under similar conditions combine with iodine are absent.
These conditions are fulfilled with regard to the examination of animal
fats and soap. Ethereal oils are also acted upon by iodine; the reaction
proceeds similar to that observed in ordinary fat mixtures. Alcoholic
mercury iodo-chloride can probably be used with success in synthetical
chemistry, as it allows determination of the free affinities of the
molecule and conversion of unsaturated compounds into saturated
chlorine-iodo addition products.--_Rundschau._
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