THE PLANT OXIDASES 247 



free titration flask S. The contents of H are then titrated 

 by running in ^ o sulphuric acid from T. As indicator, 

 phenolphthalein, followed by Congo-red, is used, and from 

 the difference in their end points the carbon dioxide 

 absorbed may be calculated. 



Since the volume of the apparatus, the temperature, and 

 the initial and final pressures, are known, the actual weight 

 of oxygen used up can be calculated when desired. 



The most considerable source of error was found to be 

 that occasioned by an initial rise of pressure on closing 

 the stopcock and beginning to shake. This appears to be 

 due to an incomplete state of saturation of the air in the 

 flask. It can be allowed for by blank experiments. 



THE EFFECT OF VARIABLE FACTORS ON THE TOTAL 

 OXYGEN ABSORPTION. 



Before arriving at the quantities set forth previously, 

 Bunzel tested directly the results of using widely divergent 

 amounts of the reagents, and of otherwise altering the 

 conditions. He ascertained the limits of concentration 

 of the pyrogallol for accurate work to lie between about 

 0-25 and 10 per cent. The age of the solution was found 

 to be immaterial. Absorption of oxygen was shown to be 

 at least approximately proportional to the amount of 

 oxidase solution used. The effects of altering the concen- 

 tration of alkali, rate of shaking, and of small temperature 

 variations, were also tested. Shaking of the potato-juice 

 for fifteen to thirty minutes before the addition of the 

 oxidizable substance reduces its oxidizing power to about 

 half its original value, but when prolonged further little 

 additional alteration can be detected. 



