92 Biological Chemistry. 



formic acid can be obtained by treatment with hydrogen 

 sulphide gas. 5 * 



Formic acid when obtained anhydrous is a hygroscopic 

 liquid with a characteristic pungent odour. It differs 

 from the higher fatty acids of the series in the fact that 

 it is somewhat readily oxidized. Like the aldehydes, it 

 reduces ammoniacal silver solutions thus 



H-COOH + Ag 2 = 2Ag + C0 2 + H 2 



It forms a well-defined series of salts known as the 

 formates. Formic acid is now largely used as a solvent in 

 the laboratory. 



Acetic Acid, CH 3 -COOH. This important acid can 

 be obtained on a large scale by the destructive distilla- 

 tion of wood, and also from crude alcoholic liquors which 

 have been left exposed to air and allowed to become sour. 

 The reaction which takes place in the conversion of alcohol 

 into acetic acid may be represented by the equation 



C a H 6 + 2 = C a H 4 2 + C0 2 



It was found out by Pasteur that this reaction is not one 

 of simple oxidation, but is a fermentation ( process brought 

 about by the action of a living organism, the mycoderma 

 aceti. 



Vinegar thus produced is a dilute solution of acetic 

 acid, containing colouring matter and other substances 

 which give to the different preparations their characteristic 

 odours. Vinegars contain from 4 per cent, to 10 per cent, 

 of acetic acid. 



" Glacial," or anhydrous, acetic acid is made by treating 

 the anhydrous sodium salt with concentrated sulphuric 

 acid. The sodium salt can be obtained by neutralizing 

 dilute acetic acid preparations obtained in various com- 



* For details, refer to a text-book on practical organic chemistry. 

 f The action of ferments is discussed in greater detail in Chapter VI. 



