74 PLANT RESPIRATION 



lactic acid, acetaldehyde, and acetone deserve particular men- 

 tion, since they occur in plants and also pass over into the dis- 

 tillate with water vapor. 



3. The test of M. Berthelot^ which depends on the formation 

 of the ethyl ester of benzoic acid'is more reliable. The alcoholic 

 solution must be concentrated as much as possible by repeated 

 distillations. This solution is shaken with two drops of benzoyl 

 chloride and an excess of caustic potash solution. The charac- 

 teristic odor of the ethyl ester of benzoic acid is evolved in the 

 presence of alcohol. The surplus of benzoyl chloride must be 

 completely destroyed by continued shaking with caustic potash. 



4. By cautious oxidation the ethyl alcohol is changed to acetalde- 

 hyde with which various characteristic tests can then be made.- 

 For example, a cherry-red color is obtained with fuchsin-sul- 

 phurous acid. A deep blue color appears with sodium nitro- 

 prusside and diethylamine or pyridine. The characteristic 

 precipitate of acetaldehyde-p-nitrophenyl-hydrazone (melting 

 point 125° C.) is formed with an acetic acid solution of 

 p-nitrophenylhydrazine. 



Certainly the most rehable tests are those in which ethyl 

 alcohol changes to characteristic derivatives which can then be 

 conveniently analysed. Hence the following methods of identifi- 

 cation of ethyl alcohol are to be recommended. It must be 

 pointed out in this connection that all these reactions are best 

 carried out with isolated alcohol dehydrated with potassium 

 carbonate. If the quantity of alcohol is insufficient for isola- 

 tion the solution must at least be concentrated as much as 

 possible. 



I. Formation of Ethylene.^ — Ethyl alcohol is converted into 

 ethylene by cautious heating with concentrated sulphuric 

 acid in an atmosphere of COo. The ethylene is driven over 

 into an analysis tube filled with concentrated caustic potash 

 by means of a stream of CO2, and the C content of the gas is 

 then ascertained by gas analysis. The apparatus of Polowzow- 

 Richter (see above) with its explosion pipette is especially 

 suitable for this purpose. 



' Berthelot, M. Compt. rend. 73: 496. 1871. 



- Deniges, G. Bull, de la soc. de chim.-biol. (IV). 7: 951. 1910. 



'Berthelot, M. Compt. rend. 128: 1366. 1899. 



