ELECTROLYTIC REDUCTION 



593 



chamber is closed by a well-fitting trebly bored rubber stopper, 

 the centre hole of which carries the solid lead cathode (c), while 

 the other holes are fitted with a thermometer (t) and a tube (d) 

 for leading off the evolved hydrogen. 



Connected in the same circuit is a second exactly similar 

 apparatus, which contains sulphuric acid only, and )w organic 

 substance. 



The evolved hydrogen from each cell is collected from time 

 to time in a specially constructed pneumatic trough during a 



Fig. I. 



period of one minute. The difference in the two volumes {v-h) 

 shows how much hydrogen has been used up by the process 

 of reduction, and is a direct measure of the course of the 

 reduction at that moment. 



If this difference {v-h) is graphically plotted against time, 

 a curve of the velocity of reaction is obtained. Fig. 2 shows a 

 typical curve taken in this case for caffeine. It will be seen that 

 the volume of hydrogen used diminishes regularly with the 

 amount of unreduced substance remaining in the solution. One 

 great advantage which such curves possess is that the area 

 enclosed by the curve and the axes OA'OB is a measure of 

 the total hydrogen used for the reduction. A comparison 



