310 METHODS OF ANALYSIS [Chap. 



As it is very difficult to dry the unsaturated acids without very serious oxidation, 

 it is just as satisfactory to determine the weight of insoluble acids by the following 

 method: 



Wash the insoluble soap left on the filter into a flask, decompose with hydro- 

 chloric acid and heat until the fatty acids are melted. Fill the flask with hot water, 

 cool, pour off the water and again wash the solidified fatty acids. Dissolve in hot 

 95% alcohol by volume, transfer to a weighed dish, remove the alcohol by evapo- 

 ration, dry, weigh and calculate the percent of solid fatty acids. 



30 FREE FATTY ACIDS.— OFFICIAL. 



Weigh 20 grams of fat, or oil, into a flask, add 50 cc. of 95% alcohol by volume 

 which has been neutralized with dilute sodium hydroxid solution, using phenol- 

 phthalein as an indicator, and heat to boiling. Shake the flask thoroughly in order 

 to dissolve the free fatty acids as completely as possible. Titrate with N/10 alkali, 

 shaking thoroughly until the pink color persists after vigorous shaking. 



Express the results either as percentage of oleic acid, as acid degree (cc. of N/1 

 alkali required to neutralize the free acids in 100 grams of oil or fat), or as acid 

 value (mg. of potassium hydroxid required to saturate the free acids in 1 gram of fat 

 or oil). 



One cc. of N/10 alkali is equivalent to 0.0282 gram of oleic acid. 



ACETYL VALUE. 



31 Benedikt-Leiokoioitsch Method^. — Tentative. 



Boil the oil or fat with an equal volume of acetic anhydrid for 2 hours, pour the 

 mixture into a large beaker containing 500 cc. of water and boil for 30 minutes. 

 To prevent bumping, pass a slow current of carbon dioxid into the liquid through a 

 finely drawn out tube reaching nearly to the bottom. Allow the mixture to sepa- 

 rate into 2 layers, siphon off the water, and boil the oily layer with fresh water 

 until it is no longer acid to litmus paper. Separate the acetylated fat from the water 

 and drj^ and filter in a drying oven. 



Weigh 2-4 grams of the acetylated fats into a flask and saponify with alcoholic 

 potash as in 20. If the distillation process is to be adopted, it is not necessary to 

 work with a standardized alcoholic potassium hydroxid solution, but in the fil- 

 tration method, which is much shorter, the alcoholic potassium hydroxid solution 

 must be measured exactly. In either case evaporate the alcohol after saponification 

 and dissolve the soap in water. Then either distil or filter as follows: 



(1) Distillation. — Acidify with sulphuric acid (1 to 10) and distil as in 25. As 

 several hundred cc. must be distilled, either run a current of steam through or add 

 portions of water from time to time (500-700 cc. of distillate will be sufficient). 

 Filter the distillates to remove any insoluble acids carried over by the steam and 

 titrate with N/10 potassium hydro.xid, using phenolphthalein as an indicator. Mul- 

 tiply the nmnber of cc. of alkali employed by 5.61 and di^dde by the weight of sub- 

 stance used to obtain the acetyl value. 



(2) Filtration. — Add to the soap solution a quantity of standard sulphuric acid 

 exactly corresponding to the amount of alcoholic potassium hydroxid solution 

 added, warm gently, filter off the free fatty acids which collect on top, wash with 

 boiling water until the washings are no longer acid and titrate the filtrate with N/10 

 potassium hydroxid, using phenolphthalein as an indicator. Calculate the acetyl 

 value as directed under (1). 



