■J 6 PLANT RESPIRATION 



alcohol lowers the surface tension of water. The method is 

 being worked out by the writer and the pertinent investiga- 

 tions are not yet concluded. 



II. CHEMICAL METHODS 



I. Oxidation with Bichromate and Sulphuric Acid According 

 to Nicloux's Method.^ — This method is especially well-suited 

 to the determination of small amounts of alcohol, hence to be 

 recommended particularly for studies of anaerobic respiration. 

 The oxidation of the alcohol proceeds according to the equation : 



2K2Cr207 + 8H.2SO4 +3CH3— CH.2OH = 2K2S04 + 2Cr(S04)3 



+ 3CH3— COOH + 11H2O. 



Ethyl alcohol is thus oxidised to acetic acid. The amount of 

 alcohol is computed from the amount of oxygen used. 



Five cc. of the solution to be tested are placed in a test tube 

 of Jena glass. This solution may contain not more than 2% 

 alcohol, the necessary dilution of the distillate being found from 

 preliminary experiments if necessary. To it are added from a 

 burette 2 drops of potassium bichromate (9.5 g. in a liter) and 

 then 5-6 cc. of cone, sulphuric acid. If there is alcohol in the 

 liquid it turns blue-green in color as a result of the formation of 

 chromium sulphate. The solution of bichromate is now allowed 

 to run in from the burette, while the hquid is shaken and heated 

 to feeble boiling, until the color changes from blue-green to 

 yellow-green. The number of cubic centimeters of bichromate 

 used is then noted. The change in the color depends on the fact 

 that potassium bichromate is not decomposed on account of the 

 exhaustion of alcohol and the solution is colored yellow. The 

 titration is easier if a pair of comparison tubes are prepared 

 and the coloration of the liquid to be investigated is compared 

 with the color tone of these tubes after the change to yellow- 

 green. The alcohol content in per cent corresponds to half the 

 number of cubic centimeters of potassium bichromate used 

 and the whole analysis requires but a few minutes. Thus if n 

 is the number of cubic centimeters of bichromate used, the 

 alcohol is 0.05W cubic centimeters per 100 cc. of solution. The 



' Nicloux, M. Bull, de la soc. de chim.-biol. (III). 35= 330. 1906. 



