Glucose and Mannitol hy B. coli communis. 



81 



estimation of carbon. In the absence of such an apparatus an accurate 

 method is to mix the sample with acid and carry off the disengaged gas in a 

 current of C02-free air (the CO2 produced in the experiment being absorbed 

 in standard barimn hydroxide). When once a solution free from CO2 has 

 been obtained all the subsequent estimations of CO2 are simple titrations. 



To determine the amount of carbon dioxide evolved during the fermentation, 

 the sodium hydroxide solution of the bottles C and D is transferred to a 

 graduated flask and diluted to a definite volume. Of this solution a sample 

 is mixed with standard barium hydroxide solution, and after filtration 

 from the precipitate of barium carbonate the diminution of alkahnity is 

 determined by titration with standard acid. An example of an estimation 

 may be given. 



250 c.c. of the COa-free sodium hydroxide solution was diluted to 1 litre. 

 Of this solution 20 c.c. corresponded to 54'88 c.c. of N/IO H2SO4. 



Another 250 c.c. of the same solution was diluted with water in the bottles 

 C and D for the absorption of the CO2 evolved in an experiment, and the 

 solution was ultimately diluted to 1 litre ; of this solution 20 c.c. was mixed 

 with 50 c.c. of standard barium hydroxide solution and the mixture filtered ; 

 20 c.c. of the filtrate neutrahsed 33-50 c.c. of N/10 H2SO4. 



20 c.c. of the diluted NaOH corresponds to 54-88 N/10 H2SO4 



50 c.c. of barium hydroxide corresponds to 102-80 N/10 H2SO4 



157-68 



Alkalinity after absorption 33-5 x 7/2 = 117-25 



» 



Carbon dioxide in 20 c.c. of NaOH solution = 40-43 N/10 

 Total carbon dioxide evolved 202-15 c.c. normal. 



The Estimation of Alcohol. — The most accurate and at the same time the 

 simplest method of estimating alcohol is that of Martin.* According to the 

 original method, the fermented solution is distiUed directly into the mixture 

 of bichromate and sulphuric acid, but this cannot be done in the presence of 

 acids wliich could themselves reduce the bichromate. A previous distillation 

 from an acid solution must be followed by a distillation from an alkaline 

 solution. It is not necessary to distil the alcohol directly into the bichromate 

 mixture, but the dilute solution of alcohol may be added gradually from a 

 pipette to the oxidising mixture and the solution maintained at the 

 temperature of the water- Ijath for 10 minutes. The excess of bichromate is 

 titrated with ferrous sulphate solution in the usual way. 



* Martin, 'Mod. Sci. Quesn.,' 1901. * 



