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:
| Substance. | Total bromin absorption in eighteen hours. | Bromin addition figure. | Bromin substitution 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]