100 MASS. EXPERIMENT STATION BULLETIN 166. 



reflux condenser and the solution brought to boil on a water bath to insure 

 solution of the free fatty acids. The boiling should not be prolonged, as. 

 esterification is liable to result. The flask is then placed in a water bath 

 at 00° C, and the solution after cooling to that temperature titrated with 

 N/10 alkah, using 1 cubic centimeter of phenolphthalein as indicator, to 

 the appearance of a pink tint. N/2 alkali is preferable for high percent- 

 ages of free acids, preventing unnecessary dilution and cooling of the' solu- 

 tion which otherwise might cause partial hydrolysis of the resulting soap ^ 

 if the alcoholic strength fell below 40 per cent. - In practical work, how- 

 ever, at least 50 per cent, is advisable. As the change in color with phen- 

 olphthalein is gradual in many instances, without a sharply defined end 

 point, cotton blue (1 cubic centimeter) offers certain advantages in sucli 

 cases, as it yields a pronounced red and is more decisive. The coloration 

 is not permanent with either indicator because of the saponification of 

 neutral esters and the decolorizing action of carbonic acid absorbed from 

 the air on shaking. Thorough shaking during titration, however, is essen- 

 tial, although the color persists for only a short time. Several blank 

 determinations should be run on the alcohol with every series of tests, and 

 deducted. Redistilled alcohol should be practically neutral, or can be 

 readily made so by the addition of alkali. 



1 cubic centimeter of N/10 alkah is equivalent to 5.6108 milligrams of 

 potassium hydroxide. 



Limit of error, 0.10 milhgram. 



Synopsis of Reaction. 



RCOOH +KOH = RCOOK +H2O 



Supplementary Notes. — Koettstorfer expresses the acidity in cubic- 

 centimeters of N potassium hydroxide required for 100 grams of fat as 

 "degrees of acidity." Stockmcier^ reports "degrees of rancidity" in the 

 same manner. Ten grams of fat with N/10 alkali are, however, a more 

 convenient amount with which to work. 



1° rancidity = .56108 acid number 

 1 acid number = 1.78228° rancidity 



The amount of free acids in lubricating oils is sometimes reported m 

 terms of sulfuric anhj^dride (SO3) . 



The acid number of oils and fats varies with the purity, age and amount 

 of hydrolj^sis and of oxidation they have undergone. Contact with fer- 

 menting or decaying matter, such as animal tissue, casein of butter and 



' On diluting and cooling a clear hot solution of neutral alkali palmitate or stearate (RCOOK), a 

 precipitate containing more than one equivalent of fatty acids to one of alkali is produced, and the 

 ratio tends to increase with greater dilution. Neutral oleate acts in a like manner, although 

 investigators differ as to its relative stability. Holde claims that 80 per cent, alcohol is necessary- 

 to prevent hydrolysis of the oleate. 



2 Kanitz, Ber. Deut. Chem. Gesell. 1903, p. 400. (From Lewkowitsch.) 

 ' Abstract, Vrtljschr. Chem. Nahr. u. Genussmtl. 4, pp. 428-429 (1889). 



