50 ORGANIC AGRICULTURAL CHEMISTRY 



When acetic acid or the sodium salt of the acid is heated, 

 especially in the presence of a base such as lime, Ca(0H)2, or 

 sodium hydroxide, NaOH, the acid loses carbon dioxide, CO2, 

 and methane, the hydrocarbon next lower to ethane from which 

 the acetic acid is derived, is formed, as follows : 



CH3 - COOH -> CH4 4- CO2 



Acetic acid Methane 



The lime or sodium hydroxide absorbs the carbon dioxide. 

 This reaction is a general one for organic acids by which hydro- 

 carbons are obtained. The hydrocarbon in each case contains 

 one less carbon atom than the acid. This reaction occurs in 

 Experiment II in the preparation of methane. 



Vinegar. — Acetic acid produced by the natural fermenta- 

 tion process is known as vinegar. As vinegar may be produced 

 by the acetic fermentation of any alcoholic liquid, it will possess 

 characteristic properties depending upon its source. Naturally 

 this process is slow, the cider being allowed to stand for a long 

 time with access to the air. Industrially the process is much 

 hastened by allowing the weak alcoholic liquid to flow slowly 

 over beechwood shavings which are covered with the acetic 

 bacteria while the whole is kept at a temperature of about 33°, 

 well aerated. These processes all produce a vinegar which con- 

 tains from about 4.6 per cent (U.S.) to 8.0 per cent (France) of 

 pure acetic acid. 



Wood Vinegar. — Acetic acid of greater strength than this is 

 made by the destructive distillation of wood. This process 

 was described under the preparation of methyl alcohol (wood 

 alcohol). The distillate obtained from the wood and known as 

 pyroligneous acid contains about 4 to 8 per cent of pure acetic 

 acid. This is separated from the alcohol, acetone and other 

 substances present by conversion into its metallic salts (cal- 

 cium, sodium) and the distillation of the alcohol, acetone, etc. 

 The acid is again set free by treatment of the calcium acetate 

 with sulphuric acid and distilling off the acid. In this way 

 acid of about 90 per cent is obtained. 



