_The Manipulation._—From a quarter to one gram of the fat, oil or
resin, is dissolved in ten cubic centimeters of carbon tetrachlorid
in a dry bottle of 500 cubic centimeters capacity, provided with a
well-ground glass stopper. An excess of the bromin solution is added,
the bottle tightly stoppered, well shaken and placed in the dark. At
the end of eighteen hours the bottle is placed in a freezing mixture
and cooled until a partial vacuum is formed. A piece of wide rubber
tubing an inch and a half long is slipped over the lip of the bottle so
as to form a well about the stopper. This well having been filled with
water the stopper is lifted and the water is sucked into the bottle
absorbing all the hydrobromic acid which has been formed. The well
should be kept filled with water, as it is gradually taken in until in
all twenty-five cubic centimeters have been added. The bottle is next
well shaken and from ten to twenty cubic centimeters of a twenty per
cent potassium iodid solution added.
The excess of bromin liberates a corresponding amount of iodin, which
is determined by the thiosulfate solution in the usual way, after
adding about seventy-five cubic centimeters of water. The total bromin
which has disappeared is then calculated from the data obtained,
the strength of the original bromin solution having been previously
determined. The contents of the bottle are next transferred to a
separatory funnel, the aqueous portion separated, filtered through a
linen filter, a few drops of thiosulfate solution added, if a blue
color persist, and the free hydrobromic acid determined by titration
with potassium hydroxid, using methyl orange as indicator. The end
reaction is best observed by placing the solution in a porcelain
dish, adding the alkali in slight excess, and titrating back with
tenth-normal hydrochloric acid until the pink tint is perceived.
From the number of cubic centimeters of alkali used the amount of
bromin present as hydrobromic acid is calculated, and this expressed
as percentage gives the bromin substitution figure. The bromin
substitution figure multiplied by two and subtracted from the total
absorption gives the addition figure.
Following are the data for some common substances:
Total bromin
absorption in Bromin addition Bromin substitution
Substance. eighteen hours. figure. figure.
Rosin 212.70 0.00 106.35
Raw linseed oil 102.88 102.88 00.00
Boiled ” ” 103.92 103.92 00.00
Salad cotton ” 65.54 64.26 0.64
Sperm ” 56.60 54.52 1.04
By the process just described it is possible to detect mixtures of
rosins and rosin oils with animal and vegetable oils. In this respect
it possesses undoubted advantages over the older methods.
=338. Method Of Hehner.=—The absorption of bromin which takes place
when unsaturated fats are brought into contact with that reagent was
made the basis of an analytical process, proposed by Allen as long
ago as 1880.[300] In the further study of the phenomena of bromin
absorption, as indicated by McIlhiney, Hehner modified the method as
indicated below.[301] From one to three grams of the sample are placed
in a tared wide-mouthed flask and dissolved in a little chloroform.
Bromin is added to the solution, drop by drop, until it is in decided
excess. The flask is placed on a steam-bath and heated until the
greater part of the bromin is evaporated, when some more chloroform is
added and the heating continued until all the free bromin has escaped.
The flask is put in a bath at 125° and dried to constant weight. A
little acrolein and hydrobromic acid escape during the drying and the
residue may be colored, or a heavy bromo oil be obtained. The gain
in weight represents the bromin absorbed. The bromin number may be
converted into the iodin number by multiplying by 1.5875.[302]
[Illustration: FIG. 100.—OLEIN TUBE.]
=339. Halogen Absorption and Addition of Fat Acids.=—Instead of
employing the natural glycerids for determining the degree of action
with the halogens the acids may be separated by some of the processes
of saponification hereafter described and used as directed for the
glycerids themselves. It is doubtful if any practical advantage arises
from this variation of the process. If the fat acids be separated,
however, it is possible to get some valuable data from the halogen
absorption of the fractions. Theoretically the stearic series of acids
would suffer no change in contact with halogens while the oleic series
is capable of a maximum absorptive and additive action. On this fact is
based a variation of the iodin process in which an attempt is made to
separate the oleic acid from its congeners and to apply the halogen to
the separated product.
The method of separation devised by Muter is carried out as
follows:[303] The separatory or olein tube consists of a wide burette
stem, provided with a lateral stopcock, and drawn out below to secure a
clamp delivery tube, and at the top expanded into a bulb closed with a
ground glass stopper, as shown in Fig. 100. Forty cubic centimeters of
liquid are placed in the tube and the surface is marked 0. Above this
the graduation is continued in cubic centimeters to 250, which figure
is just below the bulb at the top.
The process of analysis is conducted as follows: About three grams of
the oil or fat are placed in a flask, with fifty cubic centimeters of
alcoholic potash lye, containing enough potassium hydroxid to ensure
complete saponification. The flask is closed and heated on a water-bath
until saponification is complete. The pressure flask to be described
hereafter may be conveniently used. After cooling, the excess of alkali
is neutralized with acetic acid in presence of phenolphthalien and then
alcoholic potash added until a faint pink color is produced. In a large
porcelain dish place 200 cubic centimeters of water and thirty of a ten
per cent solution of lead acetate and boil. Pour slowly, with constant
stirring, into the boiling liquid the soap solution prepared as above
described, and allow to cool, meanwhile continuing the stirring. At the
end, the liquid remaining is poured off and the solid residue washed
with hot water by decantation.
The precipitate of lead salts is finally removed from the dish into a
stoppered bottle, the dish washed with pure ether, the washings added
to the bottle together with enough ether to make the total volume
thereof 120 cubic centimeters. The closed bottle is allowed to stand
for twelve hours with occasional shaking, by which time the lead oleate
will have been completely dissolved. The insoluble lead salts are next
separated by filtration, and the filtrate collected in the olein tube.
The washing is accomplished by ether and, to avoid loss, the funnel
is covered with a glass plate. The ethereal solution of lead oleate
is decomposed by dilute hydrochloric acid, using about forty cubic
centimeters of a mixture containing one part of strong acid to four
of water. The olein tube is closed and shaken until the decomposition
is complete, which will be indicated by the clearing of the ethereal
solution. The tube is allowed to remain at rest until the liquids
separate and the aqueous solution is run out from the pinch-cock at the
lower end. The residue is washed with water by shaking, the water drawn
off as just described, and the process continued until all acidity is
removed.
Water is then added until the separating plane between the two liquids
is at the zero of the graduation, and enough ether added to make the
ethereal solution of a desired volume, say 200 cubic centimeters. After
well mixing, the ethereal solution or an aliquot part thereof, _e.g._,
fifty cubic centimeters, is removed by the side tubulure and nearly
the whole of the ether removed from the portion by distillation. To
the residue are added fifty cubic centimeters of pure alcohol and the
solution is titrated for oleic acid with decinormal sodium hydroxid
solution. Each cubic centimeter of the hydroxid solution used is
equivalent to 0.0282 gram of oleic acid. The total quantity of oleic
acid contained in the amount of fat used is readily calculated from the
data obtained.
To determine the iodin absorption of the free acid another measured
quantity of the ethereal solution containing as nearly as possible half
a gram of oleic acid, is withdrawn from the olein tube, and the ether
removed in an atmosphere of pure carbon dioxid. To the residue, without
allowing it to come in contact with the air, fifty cubic centimeters
of Hübl’s reagent are added and the flask put aside in the dark for
twelve hours. At the end of this time thirty-five cubic centimeters of
a ten per cent solution of potassium iodid are added, the contents of
the flask made up to a quarter of a liter with water, fifteen cubic
centimeters of chloroform added, and the excess of iodin titrated
in the way already described. The percentage of iodin absorbed is
calculated as already indicated.
Lane has proposed a more rapid process for the above determination.[304]
The lead soaps are precipitated in a large erlenmeyer and cooled
rapidly in water, giving the flask meanwhile a circular motion which
causes the soaps to adhere to its walls. Wash with hot water, rinsing
once with alcohol, add 120 cubic centimeters of ether, attach a reflux
condenser, and boil until the lead oleate is dissolved, cool slowly, to
allow any lead stearate which has passed into solution to separate, and
filter into the olein tube. The rest of the operation is conducted as
described above. The percentage of oleic acid and its iodin absorption
in the following glycerids are given in the table below:
Cottonseed oil. Lard. Peanut oil.
Per cent oleic acid 75.16 64.15 79.84
Per cent iodin absorbed 141.96 99.48 114.00
=340. Saponification.=—In many of the analytical operations which are
conducted on the glycerids it is necessary to decompose them. When this
is accomplished by the action of a base which displaces the glycerol
from its combination with the fat acids, the resulting salts are known
as soaps and the process is named saponification. In general use the
term saponification is applied, not only strictly, as above defined,
but also broadly, including the setting free of the glycerol either by
the action of strong acids or by the application of superheated steam.
In chemical processes the saponification of a glycerid is almost
always accomplished by means of soda or potash lye. This may be in
aqueous or alcoholic solution and the process is accomplished either
hot or cold, in open vessels or under pressure. It is only important
that the alkali and glycerid be brought into intimate contact. The
rate of saponification is a function of the intimacy of contact, the
nature of the solvent and the temperature. For chemical purposes, it
is best that the decomposition of the glycerid be accomplished at a
low temperature and for most samples this is secured by dissolving the
alkali in alcohol.
In respect of solvents, that one would be most desirable, from
theoretical considerations, which acts on both the glycerids and
alkalies. In the next rank would be those which dissolve one or the
other of the materials and are easily miscible, as, for instance,
carbon tetrachlorid for the glycerid and alcohol for the alkali.
As a rule, the glycerid is not brought into solution before the
saponification process is commenced. Instead of using an alcoholic
solution of sodium or potassium hydroxid the sodium or potassium
alcoholate may be employed, made by dissolving metallic sodium or
potassium in alcohol. It is probable, however, that a little water is
always necessary to complete the process.
If a fat be dissolved in ether and treated with sodium alcoholate,
a granular deposit of soap is soon formed and the saponification is
completed in twenty-four hours. As much as 150 grams of fat can be
saponified with ten grams of metallic sodium dissolved in 250 cubic
centimeters of absolute alcohol.[305] For practical purposes the
alcoholic solution of the hydroxid is sufficient.
The chemical changes which fats undergo on saponification are of a
simple kind. When the process is accomplished by means of alkalies,
the alkaline base takes the place of the glycerol as indicated in the
following equation:
Triolein 884. Potassium hydroxid 168.
C₃H₅(O.C₁₈H₃₃O)₃ + 3KOH =
Potassium oleate 960. Glycerol 92.
(KO.C₁₈H₃₃O)₃ + C₃H₅(OH)₃.
The actual changes which take place in ordinary saponification are
not so simple, however, since natural glycerids are mixtures of
several widely differing fats, each of which has its own rate of
decomposition. Palmitin and stearin, for instance, are saponified more
readily than olein and some of the saponifiable constituents of resins
and waxes are extremely resistant to the action of alkalies. The above
equation may be regarded as typical for saponification in aqueous or
alcoholic solutions in open dishes or under pressure. If the alkali
used be prepared by dissolving metallic sodium or potassium in absolute
alcohol (sodium alcoholate or ethoxid) the reaction which takes place
is probably represented by the equation given below:
C₃H₅(O.C₁₈H₃₃O)₃ + 3C₂H₅.ONa = C₃H₅(ONa)₃ + 3C₁₈H₃₃O.O.C₂H₅,
in which it is seen that complete saponification cannot occur without
the absorption of some water, by which the sodium glyceroxid is
converted into glycerol and sodium hydroxid, the latter compound
eventually uniting with the ethyl ether of the fat acid.[306]
Glycerids are decomposed when heated with water under a pressure of
about sixteen atmospheres or when subjected to a current of superheated
steam at 200°. The reaction consists in the addition of the elements
of water, whereby the glyceryl radicle is converted into free glycerol
and the fat acid is set free. The chemical change which ensues is shown
below: