MONOBASIC FATTY ACIDS. 327 



acid. Apply heat and distil over about 35 c.c. Determine volumetrically the 

 amount of pure acetic acid in this liquid. 



Pure acetic acid, or glacial acetic acid, is solid at or below 15 C. 

 (59 F.); at higher temperatures it is a colorless liquid having a 

 characteristic, penetrating odor, boiling at 118 C. (224 F.), and 

 causing blisters on the skin ; its specific gravity is 1.056 ; it is misci- 

 ble with water, alcohol, and ether, is strongly acid, forming salts 

 known as acetates, which are all soluble in water. 



Vinegar is dilute acetic acid (about 6 per cent.), containing often 

 other substances, such as coloring matter, compound ethers, etc. 

 Vinegar was formerly obtained exclusively by the oxidation of fer- 

 mented fruit-juices (wine, cider, etc.), the various substances present 

 in them imparting a pleasant taste and odor to the vinegar ; to-day 

 vinegar is often made artificially by adding various coloring and 

 odoriferous substances to dilute acetic acid. Vinegar should be tested 

 for sulphuric and hydrochloric acids, which are sometimes fraudu- 

 lently added. 



Acidum aceticum, Acidum aceticum dilutum, and Acidum aceticum 

 glaciate are the three official forms of acetic acid. The first-named 

 acid contains 36 per cent., the second 6 per cent., the third at least 99 

 per cent, of pure acetic acid. 



Acetic acid shows an exceptional behavior in regard to the specific 

 gravity of its aqueous solutions. The highest specific gravity of 

 1.0748 belongs to an acid of 78 per cent., which is equal to an acid 

 containing one molecule of water and one of acetic acid, or C 2 H 4 O 2 .H 2 O. 

 The addition of either acetic acid or of water causes the liquid to 

 become lighter. For instance, the specific gravity of an acid contain- 

 ing 95 per cent, is equal to that containing 56 per cent, of pure acid, 

 both solutions having a specific gravity of 1.066. 



The specific gravity of dilute acetic acid cannot, therefore, be used 

 as a means of determining the amount of pure acid ; this is done by 

 exactly neutralizing a weighed portion of the acid with an alkali ; 

 from the quantity of the latter used, the quantity of actual acid present 

 may be easily calculated. (See also volumetric methods in Chapter 

 37.) 



Analytical reactions. 

 (Sodium acetate, NaC 2 H 3 O 2 , may be used.) 



1. Any acetate heated with sulphuric acid evolves acetic acid, 

 which may be recognized by its odor. 



