26 PROVISIONAL METHODS FOR ANALYSIS OF FOODS. 



performance of the blank experiments should be just as described. These blank 

 experiments must be made each time the iodin solution is used. 



Example blank determinations: Forty cc iodin solution required 62.05 cc of sodium 

 thiosulphate solution. Forty cc iodin solution required 62.15 cc of sodium thiosul- 

 phate solution. Mean, 62.1 cc. 



Per cent of iodin absorbed: 



Weight of fat taken grams. . 1. 0479 



Quantity of iodin solution used cubic centimeters. . 40. 



Thiosulfate equivalent to iodin used do 62. 1 



Thiosulfate equivalent to remaining iodin do 30. 2 



Thiosulfate equivalent to iodin absorbed do 31. 9 



Per cent of iodin absorbed, 31. 9x0.0124x100-=-!. 0479=37.75. 



5. DETERMINATION OF SAPONIFICATION NUMBER AND SOLUBLE AND INSOLUBLE ACIDS. a 



The saponification number, and soluble and insoluble acids, are determined in one 

 sample by the following method: 



(a) PREPARATION OF REAGENTS." 



Standard sodium hydroxid solution. A decinormal solution of sodium hydroxid is 

 used. Each cubic centimeter contains 0.0040 gram of sodium hydroxid and neu- 

 tralizes 0.0088 gram of butyric acid (C 4 H 8 O 2 ). 



Alcoholic potash solution. Dissolve 40 grams of good potassium hydroxid in 1 liter 

 of 95 per cent redistilled alcohol. The solution must be clear and the potassium 

 hydroxid free from carbonates. 



Standard add solution. Prepare accurately a half normal solution of hydrochloric 

 acid. 



Indicator. Dissolve 1 gram of phenol phthalein in 100 cc of 95 per cent alcohol. 



(b) WEIGHING OF SAMPLE. 



The saponification is carried on in a wide-mouth Erlenmeyer flask holding from 

 250 to 300 cc. These are cleaned by thoroughly washing with water, alcohol, and 

 ether, wiped perfectly dry on the outside, and heated for one hour at the tempera- 

 ture of boiling water. The flasks are then placed on a tray, covered with a silk 

 handkerchief, and allowed to cool. They must not be w r iped with a silk handkerchief 

 within fifteen or twenty minutes of the time they are weighed. 



About 5 grams of the melted fat, which has been filtered, is run in by m ^ans of a 

 pipette, and after cooling the flask and contents are again weighed. d 



(c) KOETSTORFER OR SAPONIFICATION NUMBER. e 



Measure 50 cc of the alcoholic potash solution into the flask by means of a burette 

 or pipette, which is allowed to drain a definite time. Connect the flask with a 

 reflux f condenser and boil for thirty minutes, when the fat is completely saponified. 

 Cool the flask and titrate with half-normal hydrochloric acid, using phenolphthalein 

 as indicator. The Koetstorfer number (milligrams of potassium hydroxid required 

 to saponify 1 gram of fat) is obtained by subtracting the number of cubic centi- 

 meters of hydrochloric acid used to neutralize the excess of alkali after saponification 



U. S. Dept. of Agr., Div. of Chem., Bui. 46 revised, p. 47. 



b See appendix, p. 150. 



c The alcohol should be redistilled from potassium hydroxid on which it has been standing for 

 some time, or with which it has been boiled for some time, using a reflux condenser. 



d See appendix, p. 150. 



Chiefly of value in oil work in the detection of rape-oil, resin, and paraffin products. 



'Almost any sort of a reflux condenser will do. A small funnel placed in the mouth of the flask is 

 perfectly satisfactory and very convenient. 



