304 METHODS OF ANALYSIS [Chap. 



inches), fitted with a perforated cork, so as to hold the tube rigidly when in position. 

 Suspend the standard thermometer so that it can be used as a stirrer, and stir the 

 mass slowly until the mercury remains stationary for 30 seconds. Then allow the 

 thermometer to hang quietly, with the bulb in the center of the mass, and observe 

 the rise of the mercury column. The highest point to which it rises is regarded 

 as the titer of the fatty acids. 



Test the fatty acids for complete saponification as follows: 



Place 3 cc. in a test tube and add 15 cc. of 95% alcohol by volume. Bring the 

 mixture to a boil and add an equal volume of ammonium hydroxid (sp. gr. 0.96). 

 A clear solution should result. The titer must be made at about 20°C. for all fata 

 having a titer above 30°C., and at 10°C. below the titer for all other fats. 



14 Glycerol-Potassium Hydroxid Method. — Tentative. 



Heat 75 cc. of glycerol-potassium hydroxid solution (25 grams of potassium hy- 

 droxid in 100 cc. of high-test glycerol) to 150°C. in an 800 cc. beaker; then add 50 cc. 

 of the oil or melted fat, previously filtered if necessary to remove foreign substances. 

 Saponification often takes place almost immediately, but heating, with frequent 

 stirring, should be continued for 15 minutes, avoiding a temperature much above 

 150°C. When the saponification is complete, as indicated by the perfectly homo- 

 geneous solution, pour the soap into an 800 cc. casserole containing about 500 cc. 

 of nearly boiling water, add carefully 50 cc. of 30% sulphuric acid and heat the 

 solution, with frequent stirring, until the layer of fatty acids separates out perfectly 

 clear. Transfer the fatty acids to a tall separatory funnel, wash 3-4 times with 

 boiling water to remove all mineral acids, draw the fatty acids off into a small 

 beaker, and allow to stand on a steam bath until the water has settled out and the 

 acids are clear. Filter into a dry beaker and heat to 150°C. on a thin asbestos plate, 

 stirring continually with the thermometer, transfer to a titer tube, fill it to within 

 2.5 cm. of the top and take the titer as directed in 13. 



lODIN ABSORPTION NUMBER. 



Hubl Method.— Official. 



15 REAGENTS. 



(a) Htibl's iodin solution. — Dissolve 26 grams of pure iodin in 500 cc. of 95% alco- 

 hol by volume. Dissolve 30 grams of mercuric chlorid in 500 cc. of 95% alcohol 

 by volume. Filter the latter solution, if necessary, and mix the 2 solutions. Let 

 the mixed solution stand 12 hours before using. The solution loses strength with 

 age, but can be used so long as 35 cc. of N/10 thiosulphate are equivalent to 25 cc. 

 of the iodin solution. 



(b) N/10 sodium thiosulphate. — Standardize this solution as follows: Place in a 

 glass-stoppered flask 20 cc. of the N/10 potassium dichromate and 10 cc. of the 15% 

 potassium iodid solution. Add 5 cc. of strong hydrochloric acid. Dilute with 100 

 cc. of water and allow the N/10 sodium thiosulphate to flow slowly into the flask 

 until the yellow color of the liquid has almost disappeared, add a few drops of the 

 starch indicator and, with constant shaking, continue to add the N/10 sodium thio- 

 sulphate until the blue color just disappears. 



(C) Starch indicator. — Prepare as directed under VII, 3 (a). 



(d) 15% potassium iodid solution. 



(e) N/10 potassium dichromate. — The dichromate solution should be checked 

 against pure iron. 



