=314. Solubility in Alcohol.=—As has already been noted, the glycerids
are freely soluble in ether, chloroform, carbon bisulfid, acetone,
carbon tetrachlorid, and some other less commonly used solvents. Their
solubility in absolute alcohol is variable and the determination of its
degree may often be useful in analytical work.
The method used by Milliau for determining the degree of solubility
is as follows:[267] The fatty matter is deprived of its free acids by
shaking for half an hour with twice its volume of ninety-five per cent
alcohol. After standing until the liquids are separated, the oil or fat
is drawn off and washed three times with distilled water. The sample
is deprived of water by filtering through a hot jacket filter and a
given weight of the dry sample is well shaken with twice its weight of
absolute alcohol. A weighed portion of the alcoholic solution obtained
is evaporated to remove the alcohol and the weight of the residual
fat determined. From the data obtained the percentage of solubility
is calculated. Olive oils, when treated as described above, show a
solubility of about forty-three parts per thousand of absolute alcohol,
cotton oil sixty-two parts, sesamé forty-one parts, peanut sixty-six
parts, colza twenty parts, and flaxseed seventy parts per thousand.
=315. Coloration Produced by Oxidants.=—When oils and fats are mixed
with oxidizing reagents, such as sulfuric and nitric acids, the
glycerids are partly decomposed with the production of colors which
have some analytical significance. The most simple method of applying
these tests is by the use of a thick porcelain plate provided with
small cup-shaped depressions for holding the few drops of material
required. Two or three drops of the oil under examination are placed
in each of the cups, a like quantity of the oxidizing reagent added,
and the mixture stirred with a small glass rod. The colors produced
are carefully noted and the mixture is allowed to remain at room
temperature for at least twelve hours in order that the final tint may
be observed. The sulfuric acid used for this reaction should have a
specific gravity of one and seven-tenths and the nitric acid should
have the usual commercial strength of the strongest acid. Pure lard,
when treated with sulfuric acid, as above described, shows but little
change of color while the vegetable oils mostly turn brown or black. In
addition to the reagents mentioned many others, including sulfuric and
nitric acids, sulfuric acid and potassium bichromate, chlorin, ammonia,
hydrogen peroxid, sodium hydroxid and aqua regia are used. Only a few
of these tests seem to have sufficient analytical importance to merit
any detailed description.[268]
=316. Coloration in Large Masses.=—Instead of applying the color test
in the small way just described, larger quantities of the fat may
be used, either in the natural state or after solution in petroleum
or other solvent. For this purpose about ten cubic centimeters of
the oil are shaken with a few drops of sulfuric acid or sulfuric and
nitric acids. Lard, when thus treated (five drops of sulfuric acid to
ten cubic centimeters of lard) shows practically no coloration. When
treated with an equal volume of sulfuric acid and shaken, the lard on
separating has a brown-red tint.[269]
Olive oil, with a few drops of sulfuric acid, gives a green color,
while cottonseed, peanut and other vegetable oils, when thus treated
with sulfuric and nitric acids, show brown to black coloration. The
delicacy of the reaction may be increased by first dissolving the fat
or oil in petroleum ether.
In the use of the coloration test with solvents, a convenient method
is to dissolve about one cubic centimeter of the fat in a test tube in
petroleum ether, add one drop of strong sulfuric acid and shake.
In the case of lard, the color does not change or becomes yellow
or red. Cottonseed oil, similarly treated, shows a brown or black
color.[270]
=317. Special Nitric Acid Test.=—A special nitric acid test for
cottonseed oil is made with nitric acid of exactly 1.375 specific
gravity at 15°. This test is especially valuable in detecting
cottonseed in olive oil. The operation is conveniently conducted by
shaking together equal volumes of the oil and acid in a test tube until
an intimate mixture or emulsion is secured. When any considerable
quantity of cottonseed oil is present an immediate brown coloration
is produced, from the intensity of which the relative proportion of
cottonseed oil in the case of a mixture may be roughly approximated.
When only a little cottonseed oil is present in the mixture, the test
tube containing the reagents should be set aside for several hours
before the final observation is made.
=318. Coloration with Phosphomolybdic Acid.=—Among the color tests, one
which we have found of use is the coloration produced in certain oils,
mostly of a vegetable origin, by phosphomolybdic acid.[271]
The method of applying the test is extremely simple. A few cubic
centimeters of the oil or melted lard are dissolved in an equal volume
of chloroform, and a third volume of ten per cent phosphomolybdic acid
added. The mouth of the test tube is closed with the thumb, and the
whole is violently shaken. On being left in repose, the phosphomolybdic
acid gathers at the top, and the coloration produced therein is easily
observed. Cottonseed oil and peanut oil both give a beautiful green
when treated in this way, which is turned to a blue on the addition of
ammonia. Linseed oil gives a green color, but forms a kind of emulsion
which obscures the color to some extent. The pure lards rendered in
the laboratory give no coloration whatever to the reagent, but it
retains its beautiful amber color in every case. Mixtures containing
as little as ten per cent cottonseed oil and ninety per cent lard,
show a distinct greenish tint, while twenty per cent cottonseed oil
gives a distinct green. This reaction, therefore, may be considered
of great value, and on account of its easy application it should come
into wide use. But it is probable that different samples of cottonseed
oil, refined to different degrees or in different ways, vary in their
deportment with phosphomolybdic acid as they do with silver nitrate. In
other words, there may be some samples of cottonseed oil which will not
give the green color when treated as above, or so faintly as to have no
diagnostic value in mixtures.
This reaction shows itself with nearly all vegetable oils but those
which have been chemically treated either for the purpose of bleaching,
or for the removal of the acidity, do not respond to the test at all,
or else in a feeble manner, and that only after standing some time.
Lard, goose fat, tallow, deer fat, butter fat, etc., show no change in
color on being treated with this reagent, either with or without the
addition of alkali. The presence of a small quantity of vegetable oil
betrays itself by the appearance of the above mentioned coloration,
the intensity of which forms an approximate measure of the amount of
vegetable oil present in the sample. In experiments with suspected
lards, which deviated in their iodin absorption numbers from those of
genuine lard, the results were concordant, the color deepening as the
iodin figure rose. The mineral fats (paraffin, vaselin) are without
action on this reagent, and the only animal fat which reduces it is
codliver oil.
In like manner some samples of lard may be found which exhibit a
deportment with this reagent similar to that shown with vegetable oils,
and tallow and lard oil have been shown to give more distinct reactions
than some of the vegetable oils.[272]
The phosphomolybdic acid may be prepared by precipitating a solution
of ammonium molybdate with sodium phosphate and dissolving the washed
precipitate in a warm solution of sodium carbonate. The solution is
evaporated to dryness and the dry residue subjected to heat. If a blue
coloration be produced it may be discharged by adding a little nitric
acid and reheating. The residue is dissolved in water, acidified with
nitric and made of such a strength as to contain about ten per cent of
the substance.
=319. Coloration with Picric Acid.=—If to ten cubic centimeters of oil
a cold saturated solution of picric acid in ether be added and the
latter be allowed to evaporate slowly, the acid remains dissolved in
the oil, to which it communicates a brown color.
Pure lard, after the evaporation of the ether, appears of a
citron-yellow color; if cottonseed oil be present, however, the mixture
assumes a brown-red color.[273]
=320. Coloration with Silver Nitrate.=—A modification of Bechi’s
method of reducing silver nitrate, given further on, has been proposed
by Brullé.[274] The reagent employed consists of twenty-five parts
of silver nitrate in 1,000 parts of alcohol of ninety-five per cent
strength. Twelve cubic centimeters of the oil to be examined and five
of the reagent are placed in a test tube, held in a vessel containing
boiling water, and the ebullition continued for about twenty minutes.
At the end of this time an olive oil, even if it be an impure one,
will show a beautiful green tint. With seed oils the results are
quite different. Cotton oil submitted to this treatment becomes
completely black. Peanut oil shows at first a brown-red coloration and
finally a somewhat green tint, losing its transparency. Sesamé oil is
distinguished by a red-brown tint very pronounced and remaining red.
Colza oil takes on a yellowish green coloration, becomes turbid and is
easily distinguished in its reaction from olive oil. In mixtures of
olive oil with the other oils, any notable proportion of the seed oils
can be easily determined by the above reactions. Natural butter treated
with this reagent retains its primitive color. That containing margarin
becomes a brick-red and as little as five per cent of margarin in
butter can be detected by this test. With ten per cent the tint is very
pronounced.
=321. Coloration with Stannic Bromid.=—This reagent is prepared by
adding dry bromin, drop by drop, to powdered or granulated tin held in
a flask immersed in ice water, until a persistent red color indicates
that the bromin is in excess. In the application of this reagent three
or four drops of it are added successively to a little less than that
quantity of the oil, the mixture well stirred and set aside for a few
minutes. The unsaponifiable matters of castor oil give a green color
when thus treated, sandal wood oil a blood-red color and cedar oil a
purplish color.[275]
=322. Coloration with Auric Chlorid.=—The use of auric chlorid
for producing colorations in oils and fats was first proposed by
Hirschsohn.[276] One gram of auric chlorid is dissolved in 200 cubic
centimeters of chloroform and about six drops of this reagent added
to five cubic centimeters of the oil to be tested. In the case of
cottonseed oil a beautiful red color is produced.
I have found that even pure lards give a trace of color sometimes with
this reagent, and therefore the production of a slight red tint cannot
in all cases be regarded as conclusive of the presence of cottonseed
oil.[277]
In general, it may be said that the color reactions with fats and oils
have a certain qualitive and sorting value, and in any doubtful case
they should not be omitted. Their value can only be established by
comparison under identical conditions with a large number of fats and
oils of known purity. The analyst must not depend too confidingly on
the data found in books, but must patiently work out these reactions
for himself.
=323. Thermal Reactions.=—The measurement of the heat produced by
mixing glycerids with reagents which decompose them or excite other
speedy chemical reactions, gives valuable analytical data. These
measurements may be made in any convenient form of calorimeter. The
containing vessel for the reagents should be made of platinum or some
other good conducting metal not affected by them.
The heat produced is measured in the usual way by the increment in
temperature noted in the mass of water surrounding the containing
vessel. The determination of the heat produced in chemical reactions is
a tedious and delicate operation requiring special forms of apparatus
for different substances. The time element in these operations is
a matter of importance, since it is necessary to work in rooms
subject to slight changes of temperature and to leave the apparatus
for some time at rest, in order to bring it and its contents to a
uniform temperature. For these reasons the more elaborate methods of
calorimetric examination are not well suited to ordinary analytical
work, and the reader is referred to standard works on thermal
chemistry for the details of such operations.[278] For our purpose here
a description of two simple thermal processes, easily and quickly
conducted, will be sufficient, while a description of the method of
determining the heat of combustion of foods will be given in another
place.
=324. Heat of Sulfuric Saponification.=—Maumené was the first to
utilize the production of heat caused by mixing sulfuric acid with a
fat as an analytical process.[279] In conducting the process a sulfuric
acid of constant strength should be employed inasmuch as the rise of
temperature produced by a strong acid is much greater than when a
weaker acid is employed. The process is at best only comparative and it
is evident that the total rise of temperature in any given case depends
on the strength of the acid, the character, and purity of the fat or
oil, the nature of the apparatus and its degree of insulation, the
method of mixing and the initial temperature. For this reason the data
given by different analysts vary greatly.[280] For some of the methods
of conducting the operation the reader may consult the work of Allen,
cited above, or other authorities.[281]
In this laboratory the process is conducted as follows:[282] The initial
temperature of the reagents should be a constant one. For oils 20° is a
convenient starting point and for fats about 35°, at which temperature
most of them are soft enough to be easily mixed with the reagent. The
acid employed should be the pure monohydrated form, specific gravity at
20°, 1.845.
The apparatus used is shown in Fig. 98.
[Illustration: FIG. 98.—APPARATUS FOR DETERMINING RISE OF TEMPERATURE
WITH SULFURIC ACID.]
The test tube which holds the reagents is twenty-four centimeters in
length and five in diameter. It is provided with a stopper having three
perforations, one for a delicate thermometer, one for a bulb funnel
for delivering the sulfuric acid, and one to guide a stirring rod bent
into a spiral as shown. The thermometer is read with a magnifying
glass. Fifty cubic centimeters of the fat are placed in the test tube
and ten of sulfuric acid in the funnel and the apparatus is exposed
at the temperature desired until all parts of it, together with the
reagents, have reached the same degree. The test tube holding the oil
should be placed in a vacuum-jacket tube, such as will be described in
paragraph =316=. The oil is allowed to run in as rapidly as possible
from the funnel and the stirring rod is moved up and down two or three
times until the oil and acid are well mixed. Care must be exercised
to stir no more than is necessary for good mixing. The mercury is
observed as it ascends in the tube of the thermometer and its maximum
height is noted. With the glass it is easy to read to tenths, when
the thermometer is graduated in fifths of a degree. When oils are
tested which produce a rise of temperature approaching 100°, in the
above circumstances, (cottonseed, linseed and some others) either
smaller quantities should be used or the oil diluted with some inert
substance or dissolved in some inert solvent of high boiling point.
For a study of the variations produced in the rise of temperature when
varying proportions of oil and acid are used, the work of Munroe may be
consulted.[283]
The thermélaeometer described by Jean is a somewhat complicated piece
of apparatus and does not possess any advantage over the simple form
described above.[284]
Instead of expressing the data obtained in thermal degrees showing the
rise of temperature, Thompson and Ballentyne refer them to the heat
produced in mixing sulfuric acid and water.[285]
The observed thermal degree of the oil and acid divided by that of
the water and acid is termed the specific temperature reaction.
For convenience in writing, this quotient is multiplied by 100.
The respective quantities of acid and water are ten and fifty
cubic centimeters. This method of calculation has the advantage of
eliminating to a certain degree the variations which arise in the use
of sulfuric acid of differing specific gravities. In the following
table are given the comparative data obtained for some common oils.[286]
Acid of 95.4 Acid of 96.8 Acid of 99
per cent. per cent. per cent.
/-----------------\ /----------------\ /--------------\
Rise of Specific Rise of Specific Rise of Specific
temp. temp. temp. temp. temp. temp.
with reaction. with reaction with reaction.
Kind of oil. the oil. the oil. the oil.
0° 0° 0° 0° 0° 0°
Olive oil 36.5 95 39.4 85 44.8 96
Rapeseed oil 49.0 127 37.0 89 58.0 124
Castor oil 34.0 88
Linseed oil 104.5 270 125.2 269
=325. Method of Richmond.=—The rise of temperature produced by
mixing an oil and sulfuric acid is determined by Richmond in a simple
calorimeter, which is constructed by fitting a small deep beaker inside
a larger one with a packing of cotton. The heat capacity of the system
is determined by adding to ten grams of water, in the inner beaker,
at room temperature, twenty-five grams of water of a noted higher
temperature and observing the temperature of the mixture. The cooling
of the system, during the time required for one determination of heat
of sulfuric saponification, does not exceed one per cent of the whole
number of calories produced.[287] Between the limits of ninety-two per
cent and one hundred per cent the rise of temperature observed is
directly proportional to the strength of the acid.
_Relative Maumené Figure._—The total heat evolved per mean molecule is
called by Richmond the relative maumené figure. It is calculated as
follows:
Let _x_ = percentage of sulfuric acid in the acid employed;
_h_ = heat capacity of calorimeter in grams of water;
_R_ = observed rise of temperature (twenty-five grams of
oil, five cubic centimeters sulfuric acid);
_K_ = potash absorbed for saponification (19.5 per cent of
potassium hydroxid, standard of comparison);
_M_ = relative maumené figure:
21.5 20 + _h_ 19.5
Then _M_ = _R_ × -----------× --------- × -----.
_x_ - 78.5 20 _K_
=326. Heat of Bromination.=—The rise of temperature caused by mixing
fats with sulfuric acid has long been used to discriminate between
different fats and oils. Hehner and Mitchell propose a similar reaction
based upon the rise of temperature produced by mixing bromin with
the sample.[288] The action of bromin on unsaturated fatty bodies is
instantaneous and is attended with a considerable evolution of heat.
Since the action of bromin on many of the oils is very violent it is
necessary to dilute the reagent with chloroform or glacial acetic acid.
Owing to its high boiling point the acetic acid has some advantage
over chloroform for this purpose. The tests are conveniently made in
a vacuum-jacket tube. In such a tube there is no loss of heat by
radiation. The bromin is measured in a pipette having at its upper end
a tube filled with caustic lime held between plugs of asbestos. The
bromin sample to be tested and the diluent employed are kept at the
same temperature before beginning the trial. They are quickly mixed
and the rise of temperature noted. The oil is first dissolved in the
chloroform and the bromin then added.
A somewhat constant relation is noticed between the rise of temperature
and the iodin number when one gram of oil, ten cubic centimeters of
chloroform and one cubic centimeter of bromin are used.
If the rise in temperature in degrees be multiplied by 5.5 the product
is approximately the iodin number of the sample. Thus a sample of lard
gave a rise in temperature of 10°.6 and an iodin number of 57.15. The
number obtained by multiplying 10.6 by 5.5 is 58.3.
In like manner the numbers obtained for some common oils are as follows:
Material. Rise of temperature Iodin No. Calculated
with bromin. Iodin No.
Butter fat 6.6 37.1 36.3
Olive oil 15.0 80.8 82.5
Maize ” 21.5 122.0 118.2
Cotton ” 19.4 107.1 106.7
Castor ” 15.0 83.8 82.5
Linseed oil 30.4 160.7 167.2
Codliver ” 28.0 144.0 140.0
=327. Modification of the Heat of Bromination Method.=—The method
described above by Hehner and Mitchell presents many grave difficulties
in manipulation, on account of the inconvenience of handling liquid
bromin. The process is made practicable by dissolving both the oil or
fat and the bromin in chloroform, or better in carbon tetrachlorid, in
which condition the bromin solution is easily handled by means of a
special pipette.[289]
In order to make a number of analyses of the same sample ten grams of
the fat may be dissolved in chloroform or carbon tetrachlorid and the
volume completed with the same solvent to fifty cubic centimeters.
In like manner twenty cubic centimeters of the bromin are dissolved
in one of the solvents named and the volume completed to 100 cubic
centimeters therewith.
For convenience of manipulation the solutions are thus made of such a
strength that five cubic centimeters of each represent one gram of the
fat and one cubic centimeter of the liquid bromin respectively.
[Illustration: FIG. 99. APPARATUS FOR DETERMINING HEAT OF BROMINATION.]
The apparatus used for the work is shown in the accompanying figure.
The pipette for handling the bromin solution is so arranged as to be
filled by the pressure of a rubber bulb, thus avoiding the danger of
sucking the bromin vapor into the mouth. The filling is secured by
keeping the bromin solution in a heavy erlenmeyer with a side tubulure
such as is used for filtering under pressure. The solutions are
mixed in a long tube, held in a larger vessel, from which the air is
exhausted to secure a minimum radiation of heat. A delicate thermometer
graduated in tenths serves to register the rise of temperature. The fat
solution is first placed in the test tube, with care not to pour it
down the sides of the tube but to add it by means of a pipette reaching
nearly to the bottom. The whole apparatus having been allowed to come
to a standard temperature the bromin solution is allowed to run in
quickly from the pipette. No stirring is required as the liquids are
sufficiently mixed by the addition of the bromin solution. The mercury
in the thermometer rapidly rises and is read at its maximum point by
means of a magnifying glass. With a thermometer graduated in tenths, it
is easy to read to twentieths of a degree.
It is evident that the rise of temperature obtained depends on
similar conditions to those mentioned in connection with sulfuric
saponification. Each system of apparatus must be carefully calibrated
under standard conditions and when this is done the comparative rise
of temperature obtained with various oils and fats will prove of
great analytical use. It is evident that the ratio of this rise of
temperature to the iodin number must be determined for every system of
apparatus and for every method of manipulation employed, and no fixed
factor can be given that will apply in every case.
With the apparatus above described and with the method of manipulation
given the following data were obtained for the oils mentioned:
Rise of temperature.
Olive oil 20°.5
Refined cottonseed oil 25°.7
Sunflowerseed oil 28°.4
Calycanthusseed oil 29°.0
Bromin and chloroform, when mixed together, give off heat, due to
the chemical reaction resulting from the substitution of bromin for
hydrogen in the chloroform molecule and the formation of hydrobromic
acid. For this reason the data obtained, when chloroform is used as a
solvent, are slightly higher than with carbon tetrachlorid. The use of
the latter reagent is therefore to be preferred.
=328. Haloid Addition Numbers.=—Many of the glycerids possess the
property of combining directly with the haloids and forming thereby
compounds in which the haloid, by simple addition, has become a part of
the molecule. Olein is a type of this class of unsaturated glycerids.
The process may take place promptly as in the case of bromin or move
slowly as with iodin. The quantity of the haloid absorbed is best
determined in the residual matter and not by an examination of the fat
compound. By reason of the ease with which the amount of free iodin in
solution can be determined, this substance is the one which is commonly
employed in analytical operation on fats.
In general, the principle of the operation depends on bringing the fat
and haloid together in a proper solution and allowing the addition
to take place by simple contact. The quantity of the haloid in the
original solution being known, the amount which remains in solution
after the absorption is complete, deducted from that originally
present, will give the quantity which has entered into combination with
the glycerid.
=329. Hübl’s Process.=—In determining the quantity of iodin which
will combine with a fat, the method first proposed by Hübl, or some
modification thereof, is universally employed.[290] In the determination
of the iodin number of a glycerid it is important that it be
accomplished under set conditions and that iodid be always present
in large excess. It is only when data are obtained in the way noted
that they can be regarded as useful for comparison and determination.
Many modifications of Hübl’s process have been proposed, but it is
manifestly impracticable to give even a summary of them here. As
practiced in the chemical laboratory of the Agricultural Department and
by the Association of Official Agricultural Chemists, it is carried out
as follows:[291]
(1) PREPARATION OF REAGENTS.
(_a_). _Iodin Solution._—Dissolve twenty-five grams of pure iodin in
500 cubic centimeters of ninety-five per cent alcohol. Dissolve thirty
grams of mercuric chlorid in 500 cubic centimeters of ninety-five per
cent alcohol. The latter solution, if necessary, is filtered, and then
the two solutions mixed. The mixed solution should be allowed to stand
twelve hours before using.
(_b_). _Decinormal Sodium Thiosulfate Solution._—Dissolve 24.8 grams
of chemically pure sodium thiosulfate, freshly pulverized as finely as
possible and dried between filter or blotting paper, and dilute with
water to one liter, at the temperature at which the titrations are to
be made.
(_c_). _Starch Paste._—One gram of starch is boiled in 200 cubic
centimeters of distilled water for ten minutes and cooled to room
temperature.
(_d_). _Solution of Potassium Iodid._—One hundred and fifty grams of
potassium iodid are dissolved in water and the volume made up to one
liter.
(_e_). _Solution of Potassium Bichromate._—Dissolve 3.874 grams of
chemically pure potassium bichromate in distilled water and make the
volume up to one liter at the temperature at which the titrations are
to be made.
(2). DETERMINATION.
(_a_). _Standardizing the Sodium Thiosulfate Solution._—Place twenty
cubic centimeters of the potassium bichromate solution, to which have
been added ten cubic centimeters of the solution of potassium iodid,
in a glass stopper flask. Add to this mixture five cubic centimeters
of strong hydrochloric acid. Allow the solution of sodium thiosulfate
to flow slowly into the flask until the yellow color of the liquid
has almost disappeared. Add a few drops of the starch paste, and with
constant shaking continue to add the sodium thiosulfate solution until
the blue color just disappears. The number of cubic centimeters of
thiosulfate solution used multiplied by five is equivalent to one gram
of iodin.
_Example._—Twenty cubic centimeters of potassium bichromate solution
required 16.2 sodium thiosulfate; then 16.2 × 5 = 81 = number cubic
centimeters of thiosulfate solution equivalent to one gram of iodin.
Then one cubic centimeter thiosulfate solution = 0.0124 gram of iodin:
(Theory for decinormal solution of sodium thiosulfate, one cubic
centimeter = 0.0127 gram of iodin.)
(_b_). _Weighing the Sample._—About one gram of butter fat is placed
in a glass stopper flask, holding about 300 cubic centimeters, with
the precautions to be mentioned for weighing the fat for determining
volatile acids.
(_c_). _Absorption of Iodin._—The fat in the flask is dissolved in ten
cubic centimeters of chloroform. After complete solution has taken
place thirty cubic centimeters of the iodin solution (1) (_a_) are
added. The flask is now placed in a dark place and allowed to stand,
with occasional shaking, for three hours.
(_d_). _Titration of the Unabsorbed Iodin._—One hundred cubic
centimeters of distilled water are added to the contents of the flask,
together with twenty cubic centimeters of the potassium iodid solution.
Any iodin which may be noticed upon the stopper of the flask should
be washed back into the flask with the potassium iodid solution. The
excess of iodin is taken up with the sodium thiosulfate solution, which
is run in gradually, with constant shaking, until the yellow color of
the solution has almost disappeared. A few drops of starch paste are
added, and the titration continued until the blue color has entirely
disappeared. Toward the end of the reaction the flask should be
stoppered and violently shaken, so that any iodin remaining in solution
in the chloroform may be taken up by the potassium iodid solution in
the water. A sufficient quantity of sodium thiosulfate solution should
be added to prevent a reappearance of any blue color in the flask for
five minutes.
(_e_). _Setting the Value of the Iodin Solution by the Thiosulfate
Solution._—At the time of adding the iodin solution to the fat, two
flasks of the same size as those used for the determination should be
employed for conducting the operation described above, but without the
presence of any fat. In every other respect the performance of the
blank experiments should be just as described. These blank experiments
must be made each time the iodin solution is used.
_Example of Blank Determinations._—Thirty cubic centimeters of
iodin solution required 46.4 cubic centimeters of sodium thiosulfate
solution: Thirty cubic centimeters of iodin solution required 46.8
cubic centimeters of sodium thiosulfate solution: Mean, 46.6 cubic
centimeters.
Weight of fat 1.0479 grams
Quantity of iodin solution used 30.0 cubic centimeters
Thiosulfate equivalent to iodin used 46.6 ” ”
Thiosulfate equivalent to remaining
iodin 14.7 ” ”
Thiosulfate equivalent to iodin absorbed 31.9 ” ”
Percent of iodin absorbed, 31.9 × 0.0124 × 100 ÷ 1.0479 = 37.75.
=330. Character of Chemical Reaction.=—The exact nature of the chemical
process which takes place in this reaction is not definitely known.
Hübl supposed that the products formed were chloro-iodid-additive
compounds, and he obtained a greasy product from oleic acid, to which
he ascribed the formula C₁₈H₃₄IClO₂. By others it is thought that
chlorin alone may be added to the molecule.[292]
In general, it may be said that none of the glycerids capable
of absorbing halogens is able to take on a quantity equivalent
to theory.[293] While the saturated fatty acids (stearic series)
theoretically are not able to absorb iodin some of them are found to
do so to a small degree. It is evident, therefore, that it is not
possible to calculate the percentage of unsaturated glycerids in a fat
from their iodin number alone. According to the data worked out by
Schweitzer and Lungwitz both addition and substitution of iodin take
place during the reaction.[294] This fact they determined by titration
with potassium iodate and iodid according to the formula 5HI + HIO₃ =
6I + 6H₂O. The authors confess that whenever free hydriodic acid is
found in the mixture that iodin substitution has taken place and that
for each atom of hydrogen eliminated from the fat molecule two atoms
of iodin disappear, one as the substitute and the other in the form
of hydriodic acid. When carbon bisulfid or tetrachlorid is used as a
solvent no substitution takes place and pure additive compounds are
formed.
The following process is recommended to secure a pure iodin addition
to a glycerid: About one gram or a little less of the oil or fat is
placed in a glass stopper flask, to which are added about seven-tenths
of a gram of powdered mercuric chlorid and twenty-five cubic
centimeters of a solution of iodin in carbon bisulfid. The stopper is
made tight by smearing it with powdered potassium iodid, tied down,
and the mixture is heated for some time under pressure. By this method
it is found that no hydriodic acid is formed, and hence all the iodin
which disappears is added to the molecule of the glycerid. The additive
numbers obtained for some oils are appended:
Time of Per cent Per cent
Oil. heating. Temperature. iodin added. hübl number.
Lard oil 30 minutes. 50°.0 73.0 78.4
Cottonseed oil 2 hours. 50°.0 103.0 106.5
Oleic acid 2 ” 65°.5 93.8
=331. Solution in Carbon Tetrachlorid.=—Gantter has called attention
to the fact that the amount of iodin absorbed by fat does not depend
alone upon the proportion of iodin present but also upon the amount of
mercuric chlorid in the solution.[295] Increasing amounts of mercuric
chlorid cause uniformly a much greater absorption of the iodin.
For this reason he proposes to discard the use of mercuric chlorid
altogether for the hübl test and to use a solvent which will at the
same time dissolve both the iodin and the fat. For this purpose he uses
carbon tetrachlorid. The solutions are prepared as follows:
_Iodin Solution._—Ten grams of iodin are dissolved in one liter of
carbon tetrachlorid.
In the preparation of this solution the iodin must not be thrown
directly into the flask before the addition of the tetrachlorid. Iodin
dissolves very slowly in carbon tetrachlorid and the solution is made
by placing it in a sufficiently large weighing glass and adding a
portion of the carbon tetrachlorid thereto. The solution is facilitated
by stirring with a glass rod until the added tetrachlorid is apparently
charged with the dissolved iodin. The dissolved portion is then poured
into a liter flask, new portions added to the iodin and this process
continued until the iodin is completely dissolved, and then sufficient
additional quantities of the tetrachlorid are added to fill the flask
up to the mark.
=332. Sodium Thiosulfate Solution.=—Dissolve 19.528 grams of pure
sodium thiosulfate in 1000 cubic centimeters of water. For determining
the strength of the solution by titration, the solution of iodin in
carbon tetrachlorid and a solution of sodium thiosulfate in water are
each placed in a burette. A given volume of the iodin solution is
first run into a flask with a glass stopper and afterward the sodium
thiosulfate added little by little until, after a vigorous shaking,
the liquid has only a little color. Some solution of starch is then
added and shaken until the mixture becomes deep blue. The sodium
thiosulfate solution is added drop by drop, with vigorous shaking after
each addition, until the solution is completely decolorized. If both
solutions have been correctly made with pure materials they will be of
equal strength; that is, ten cubic centimeters of the iodin solution
will be exactly decolorized by ten cubic centimeters of the sodium
thiosulfate solution.
=333. Method of Conducting the Absorption.=—The quantity of the fat
or oil employed should range from 100 to 200 milligrams, according to
the absorption equivalent. These quantities should be placed in flasks
with glass stoppers in the ordinary way. In the flasks are placed
exactly fifty cubic centimeters of the iodin solution equivalent to
500 milligrams of iodin, and the flask is then stoppered and shaken
until the fat or oil is completely dissolved. In order to avoid the
volatilization of the iodin finally, sufficient water is poured into
the flask to form a layer about one millimeter in thickness over the
solution containing the iodin and fat. The stopper should be carefully
inserted and the flask allowed to stand at rest for fifty hours.
=334. Estimation of the Iodin Number.=—This is determined in the usual
way by titration of the amount of iodin left in excess after the
absorption as above described. The iodin number is to be expressed
by the number of milligrams of iodin which are absorbed by each 100
milligrams of fat.
_Example._—One hundred and one milligrams of flaxseed oil were
dissolved in fifty cubic centimeters of the carbon tetrachlorid
solution of iodin and allowed to stand as above described for fifty
hours. At the end of this time, 42.3 cubic centimeters of the sodium
thiosulfate solution were required to decolorize the excess of iodin
remaining.
_Statement of Results._—Fifty cubic centimeters of the sodium
thiosulfate equal 500 milligrams of iodin; therefore, 42.3 cubic
centimeters of the thiosulfate solution equal 423 milligrams of iodin.
The difference equals seventy-seven milligrams of iodin absorbed by 101
milligrams of the flaxseed oil. Therefore, the iodin number equivalent
and the milligrams of iodin absorbed by 100 milligrams of flaxseed oil
equal 76.2.
It is evident from the above determination that the iodin number of
the oil, when obtained in the manner described, is less than half that
secured by the usual hübl process. Since the solvent employed, however,
is more stable than chloroform when in contact with iodin or bromin,
the proposed variation is one worthy of the careful attention of
analysts.
McIlhiney has called especial attention to the low numbers given by the
method of Gantter, and from a study of the data obtained concludes that
iodin alone will not saturate glycerids, no matter what the solvents
may be.[296]
It is clear, therefore, that the process of Gantter cannot give numbers
which are comparable with those obtained by the usual iodin method. Any
comparative value possessed by the data given by the process of Gantter
must be derived by confining it to the numbers secured by the carbon
tetrachlorid process alone.
=335. Substitution of Iodin Monochlorid for Hübl’s Reagent.=—Ephraim
has shown that iodin monochlorid may be conveniently substituted for
the hübl reagent with the advantage that it can be safely used at once,
while the hübl reagent undergoes somewhat rapid changes when first
prepared. The present disadvantage of the process is found in the fact
that the iodin monochlorid of commerce is not quite pure and each new
lot requires to be titrated for the determination of its purity.
The reagent is prepared of such a strength as to contain 16.25 grams
of iodin monochlorid per liter. The solvent used is alcohol. The
operation is carried out precisely as in the hübl method, substituting
the alcoholic solution of iodin monochlorid for the iodin reagent
proposed by Hübl.[297] If the iodin monochlorid solution, after acting
on the oil, be titrated without previous addition of potassium iodid a
new value is obtained, the chloriodin number. In titrating, the sodium
thiosulfate is added until the liquid, which is made brown by the
separated iodin, becomes yellow. At this point the solution is diluted,
starch paste added, and the titration completed.
=336. Preservation of the Hübl Reagent.=—To avoid the trouble due to
changes in the strength of Hübl’s reagent, Mahle adds hydrochloric acid
to it at the time of its preparation.[298] The reagent is prepared as
follows: Twenty-five grams of iodin dissolved in a quarter of a liter
of ninety-five per cent alcohol are mixed with the same quantity of
mercuric chlorid in 200 cubic centimeters of alcohol, the same weight
of hydrochloric acid of 1.19 specific gravity added and the volume of
the mixture completed to half a liter with alcohol. After five days
such a solution gave, on titration, 49.18 instead of 49.31 grams per
liter of iodin.
It will be observed that this solution is double the usual strength,
but this does not influence the accuracy of the analytical data
obtained. It appears that the hübl number is not, therefore, an iodin
number, but expresses the total quantity of iodin, chlorin and oxygen
absorbed by the fat during the progress of the reaction.
=337. Bromin Addition Number.=—In the process of Hübl and others an
attempt is made to determine the quantity of a halogen, _e.g._, iodin,
which the oil, fat or resin will absorb under certain conditions.
The numbers obtained, however, represent this absorption only
approximately, because the halogen may disappear through substitution
as well as absorption. Whether or not a halogen is added, _i. e._,
absorbed or substituted, may be determined experimentally.
The principle on which the determination depends rests on the fact that
a halogen, _e. g._, bromin, forms a molecule of hydrobromic acid for
every atom of bromin substituted, while in a simple absorption of the
halogen no such action takes place. If, therefore, bromin be brought
into contact with a fat, oil or resin, the determination of the
quantity of hydrobromic acid formed will rigidly determine the quantity
of bromin substituted during the reaction. If this quantity be deducted
from the total bromin which has disappeared, the relative quantities of
the halogen added and substituted are at once determined. In the method
of McIlhiney[299] bromin is used instead of iodin because the addition
figures of iodin are in general much too low.
_The Reagents._—The following solutions are employed: