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 |