172 STUDIES ON RESPIRATION. I 



tained f the reagent was then added and new measurements of respi- 

 ration were made in the same manner as before. 



The rate of respiration after the addition of the reagent is ex- 

 pressed in each case as per cent of the normal rate. The normal rate 

 is usually determined by taking the average of successive determina- 

 tions. The normal rate may be different in each experiment (on 

 account of differences in the quantity or kind of material used) , but it 

 is in all cases taken as 100 per cent. 



The relative rate was ascertained in all cases by comparing the 

 times required to produce a definite change of acidity and not by a 

 comparison of the changes in acidity produced in equal times. The 

 former method compares the reaction velocities directly, while the 

 latter may not.^ 



This method of comparison has the further advantage of making 

 it unnecessary to know what change in pH value is produced by 

 adding a definite amount of CO2. For if we always start the meas- 

 tirement from the same pH value and carry it to the same end- 

 point, we can be certain that the same amount of CO2 has been pro- 

 duced in each case, although we may not know what this amount is. 

 It is, in fact, quite unnecessary to know it if we are comparing the 

 times required to do equal amounts of work. 



An additional advantage of this method is that an error in the 

 buffer solutions does not affect the results, providing we use the same 

 buffer solutions throughout. For this purpose they are made up in 

 large quantities.* Two solutions are chosen, the pH value of one 

 being taken as the starting point (to which the experimental solution 

 is always brought when starting a measurement) and that of the 

 other as the end-point (to which the experimental solution comes as 

 the result of respiration). 



Certain precautions which are essential to accurate work may be 

 briefly mentioned, in addition to those described by Haas.^ It is 

 necessary to use the purest reagents and in particular to see that they 

 are neutral (or practically so); stock solutions may be kept in Pyrex 

 flasks or tubes to avoid alkalinity due to the glass. Slight departure 



^ Unless the normal rate was fairly constant the experiment was rejected. 



^Osterhout, Science, 1918, xlviii, 172. 



* Buffer solutions of phosphates or borates will keep for a long time. 



