(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]