Part 64

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. 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:

(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.

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. 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. 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. 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: