284 CONSIDERATION' OF CARBON COMPOUNDS. 



hydrate and distilled, when methyl alcohol, water, etc., evapo- 

 rate and a solid residue is left, which is an impure calcium 

 acetate. From this latter, acetic acid is obtained by distilling 

 with sulphuric (or hydrochloric) acid, calcium sulphate (or chlo- 

 ride) being formed and left in the retort, whilst acetic acid distils 

 over. 



Pure acetic acid is solid at or below 16, at higher tempera- 

 tures it is a colorless liquid having a characteristic, penetrating 

 odor, boiling at 118, and causing blisters on the skin ; its spe- 

 cific gravity is 1.056 ; it is miscible with water, alcohol, and 

 ether, is strongly acid, forming salts known as acetates, which 

 are all soluble in water. 



Vinegar is a diluted acetic acid (about six per cent.), con- 

 taining often other substances, such as coloring matter, com- 

 pound ethers, etc. Vinegar was formerly obtained exclusively 

 by the oxidation of fermented 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 a 

 diluted acetic acid. Vinegar should be tested for sulphuric and 

 hydrochloric acids, which are sometimes fraudulently added. 



Acidum aceticum, Acidum aceticum dilutam, and Acidum aceticwn 

 glaciale are the three officinal 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 77 per cent., which is 

 equal to an acid containing one molecule of water and one of 

 acetic acid, or C 2 H 4 2 .H 2 0. The addition of either acetic acid 

 or of water causes the liquid to become lighter, so that, for 

 instance, the specific gravity of an acid containing 95 per cent. 

 is equal to that containing 56 per cent, of pure acid, both solu- 

 tions having a specific gravity of 1.066. 



The specific gravity of a diluted 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. 



