Part 65

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.

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. 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. 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 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:

1. One-third normal bromin dissolved in carbon tetrachlorid:

2. One-tenth normal sodium thiosulfate:

3. One-tenth normal potassium hydroxid.

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. In the further study of the phenomena of bromin absorption, as indicated by McIlhiney, Hehner modified the method as indicated below. 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.

=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: 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.