594 



SCIENCE PROGRESS 



between the total hydrogen used and that theoretically required 

 by an equation gives valuable assistance in following the course 

 of the reduction and in knowing what products may be expected. 

 Thus, from the curve in fig. 2 the enclosed area when measured 

 up represents a volume of 2,250 cc. while the equation 

 CsHioO.N, + 4H = C8Hi,ON4 + H,0 

 Caffeine Desoxycaflfeine 



requires (for 10 grams of caffeine) 2,303 cc. hydrogen— the differ- 

 ence between the observed and calculated amounts being well 

 within the limits of experimental error. 



It has already been mentioned that the cathode material affects 

 the reduction of aromatic nitro-compounds. With difficultly 



reducible substances, such as caffeine and uric acid, a reduction 

 at any other cathode than mercury or lead is seldom possible. 



The cause of this important fact is that the hydrogen from 

 a lead cathode is evolved at a higher potential, and therefore 

 possesses greater energy than the hydrogen evolved, say from 

 a platinum cathode. This is intimately connected with the 

 high "super-tension" of lead. Caspari^ showed that the tension 

 at various cathodes at which gaseous hydrogen visibly escapes 

 differs widely. For lead and mercury this tension is highest — for 

 platinum lowest. The excess of potential over that of a reversibly 

 working hydrogen-oxj^gen-sulphuric acid element Caspari called 

 the " super-tension " of the metal in question. The following are 

 the values for the " super-tension " of a few metals : 



Mercury 078 volt. Lead o"64. Copper 0*23. 



Silver o"i5. Gold 0-02. Platinum 0*005. 



* Caspari, Zeitschr.f. phys. Chetn. 30 (1899), 89. 



