502 CONSIDERATION OF CARBON COMPOUNDS. 



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 

 glaciale 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. 

 When the acid and water are mixed in this proportion, a maximum 

 rise in temperature and contraction in volume take place, which fact 

 indicates the existence of ortho-acetic acid, CH 3 C(OH) 3 , some ethereal 

 salts of which are known. The addition of either acetic acid or of 

 water causes the liquid to become lighter. For instance, the specific 

 gravity of an acid containing 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 39.) 



The vapor density of absolute acetic acid at just a little above its boiling- 

 point is twice as great as that corresponding to the formula, C 2 H 4 O 2 . At 

 200 C. or above, the vapor density is normal. This kind of behavior has 

 been observed in the case of other substances. 



While vinegar is used in our diet, it should be remembered that acetic acid 

 acts as an irritant and corrosive, having caused in some instances perforation 

 of the stomach, and death in 6 to 15 hours. Milk of magnesia should be given 

 as an antidote with a view of neutralizing the acid. 



